Atmospheric Corrosion of Materials.pdf

7/28/2019 Atmospheric Corrosion of Materials.pdf

1/24

Emirates Journal for Engineering Research, 11 (1), 1-24 (2006)

(Review Paper)

1

ATMOSPHERIC CORROSION OF MATERIALS

S. SYED

Corrosion Research Group, Atomic Energy Research Institute, King Abdulaziz City for Science and Technology,

P.O. Box 6086, Riyadh-11442, Saudi Arabia. Email: [email protected]

(Received January 2006 and accepted May 2006)

..

.

In recent years, atmospheric corrosion of materials have attracted materials community for it

accounts for more failures on both a tonnage basis and cost basis than any other type ofenvironmental corrosion. Tremendous amounts of materials in industries, automobiles, bridgesand buildings are exposed to the atmosphere and attacked by pollutant and water. In this reviewpaper attention is paid to the atmospheric corrosion of various materials, influence of exposureparameters and basics theory of atmospheric corrosion.

Keywords: Materials, atmospheric corrosion, corrosion processes

1. INTRODUCTION

The word corrosion is derived from the latin corrosuswhich means eaten away or consumed by degrees; an

unpleasant word for an unpleasant process[1].Corrosion is defined as the destruction of materialscaused by chemical or electrochemical action of thesurrounding environment. This phenomenon isexperienced in day to day living. The most commonexamples of corrosion include rusting, discolorationand tarnishing[2]. Corrosion is an ever occurringmaterial disease. It can only be reduced it cannot beprevented because thermodynamically it is a spontane-ous phenomena.

In fact, economy of any country would bedrastically changed if there were no corrosion. For

example, automobiles, ships, underground pipelinesand house-hold appliances would not require coatings.The stainless steel industry would disappear andcopper would be used for electrical applications.Although corrosion is inevitable, its cost could bereduced.

Corrosion can be fast or slow. Sensitized 18-8stainless steel is badly attacked in hours by polythionicacid. Railroad tracks usually show slight rusting notsufficient to affect their performance over many years.The famous iron Delhi Pillar in India was made almost2000 years ago and is almost as good as new. Itsheight is 32 feet and dia 2 feet. It should be noted

however, that it has been exposed mostly to aridconditions [3].

1.1. Classification of Corrosion Process

Corrosion process can be conveniently classified as

follows[4]

:

Corrosion process

Chemical corrosion Electrochemical corrosion

Direct oxidation, corrosion Immersion Underground Atmosphericby liquid metals, fused corrosion corrosion corrosionhalides, non-aqueous

solution, etc.

Reaction of metals with dry air or oxygen isconsidered as a chemical corrosion. High temperatureoxidation of metals and tarnishing of metals likecopper, silver etc. fall in this category. Of late this isalso considered to be an electrochemical process withthe diffusion of oxygen (inwards) and metal ions(outwards) through the oxide layer, the electromotiveforce at metal-oxide interface being the driving force.Electrochemical corrosion occurs in the presence ofelectrolyte. The reaction is considered to take place atthe metal-solution interface with the creation of localcathodic and anodic sides on the metal surface [5].


2/24

S. Syed

2 Emirates Journal for Engineering Research, Vol. 11, No.1, 2006

1.2. Atmospheric Corrosion

The term atmospheric corrosion comprises theattack on metal exposed to the air as opposed to metalimmersed in a liquid. Atmospheric corrosion is themost prevalent type of corrosion for common metals[6].Atmospheric corrosion is a subject of global concernbecause of its importance to the service life ofequipment and durability of the structural materials.While there is a general agreement on the possibletypes of parameters that may lead to corrosion, thesestudies suffer severely from the lack of generality inthe sense that their predictive capability is extremelypoor.

Conventional atmospheric parameters that maylead to metal corrosion comprise of weathering factorssuch as temperature, moisture, rainfall, solar radiation,wind velocity, etc. Air pollutants such as sulphurdioxide, hydrogen sulphide, oxides of nitrogen,

chlorides have also been found to contribute toatmospheric corrosion[7].The complexity and diverse nature of the

atmospheric pollutants make the prediction of theatmospheric corrosion difficult. The synergisticinteraction of the variables must also be considered inthe model for arriving at a definite solution. A directapproach to the problem is to measure the observedcorrosion rates and the participating atmosphericparameters and correlate them. The correlationequations, thus derived, are known as damagefunctions and they have been found to be extremelyuseful, though in a restricted manner, as the results are

not easily transferable from one place to another[8].Predictability of atmospheric corrosion, in

principle, should be based upon the completeunderstanding of the corrosion process andinterdependence of the contributing parameters.Extensive data have been collected all over the worldon atmospheric corrosion of metals exposed atdifferent locations. Empirical and semi-empiricalrelationships have been developed to generalize theseobservations. Most prominent of these relationshipshave been the linear and exponential dependence ofcorrosion rate with relative humidity, pollutant levelsand temperature [9]. Grossman [10] has investigated theatmospheric factors which determined the time ofwetness of the outdoor structures. A thermodynamicperspective of copper tarnishing by SO2 in thepresence of moisture was reported by Chawla andPayer[11]. Walters [12-13] carried out some exhaustivestudies on the laboratory simulation of atmosphericcorrosion by SO2 detailing the apparatus,electrochemical techniques and example results. Theeffect of pollutants such as SO2, NaCl, dust, etc., onthe critical humidity for the rusting to occur was welldocumented by Vassie [14]. A statistical evaluation ofthe atmospheric corrosion of stainless steel was

undertaken by Blank and Lherbier[15]

. The atmosphericcorrosion rates of mild steel and low alloy cast steelswere studied by Thomas and Alderson; Briggs [16,17],

respectively. The damage function describing theatmospheric deterioration of materials due to acidicdeposition was studied in detail by Lipfert [18]. Stilesand Edney studied the potential damage to galvanizedsteel by the dissolution of zinc into thin aqueous filmsas a function of residence time, acidic species and

pH[19]

. The corrosion product Zn2+

correlated linearlywith incident H+ concentration. Some developments inthe atmospheric corrosion testing were carried out byPourbaix and Pourbaix[20] and the assessed thecorrosion behaviour of different types of steel in bothnatural and laboratory simulated conditions.

Atmospheric corrosion can further be convenientlyclassified into dry, damp and wet categories. Dryoxidation takes place in the atmosphere with all metalsthat have a negative free energy of oxide formation.The damp moisture films are created at a certaincritical humidity level (largely by the adsorption ofwater molecules), while the wet films are associated

with dew, ocean spray, rainwater, and other forms ofwater splashing. By its very nature, atmosphericcorrosion has been reported to account for morefailures in terms of cost and tonnage than any otherform of corrosion.

The atmospheric environments are classified asrural, urban, industrial, marine, or combinations ofthese. These types of atmospheres have been describedas follows [21-23].Rural: Rural environments are usually free ofaggressive agents (deposition rate of SO2 and NaCllower than 15 mg m-2 day-1). Their principal corrosives

consist of moisture, relatively small amounts of sulfuroxides (SOX), and carbon dioxide (CO2) from variouscombustion products. Ammonia (NH3), resulting fromthe decomposition of farm fertilizers, may also bepresent. Rusting becomes pronounced when therelative humidity exceeds a certain value. For clean airthis value is about 70 percent. Rural environmentsgenerally are not aggressive towards metals. This typeof atmosphere is generally the least corrosive andnormally does not contain chemical pollutants, butdoes contain organic and inorganic particulates. Aridand tropical types are especially extreme cases in therural category.

Urban: Urban atmosphere is similar to ruralatmosphere where there is little industrial activity,characterized by pollution composed mainly of SOxand NOx variety, from motor vehicles and domesticfuel emissions which, with the addition of dew or fog,generate a highly corrosive wet acid film on exposedsurfaces (deposition rate of SO2 higher than 15 mg m

-2day-1 and that of NaCl lower than this value).Industrial: The most potent causes of corrosion inindustrial environments are the sulfur oxides (SOX)and nitrogen oxides (NOX) produced by the burning ofautomotive fuels and fossil fuels in power stations.The critical relative humidity, above which metalscorrode, drops to about 60 percent when theseairborne pollutants are deposited on the metal surface.


3/24

Atmospheric Corrosion of Materials

Emirates Journal for Engineering Research, Vol. 11, No.1, 2006 3

These atmospheres are also associated withconcentrations of chlorides, phosphates, hydrogensulphate, ammonia and its salts.Marine: The corrosiveness of a marine environmentdepends on the topography of the shore, wave actionat the surf line, prevailing winds and relative humidity.

While the corrosiveness decreases rapidly withincreasing distance from the shore, severe storms cancarry salt spray inland as much as 15 km. Salt isdeposited on steel surfaces by marine fog and wind-blown spray droplets(deposition rate of NaCl higherthan 15 mg m-2 day 1). This contamination inducessevere corrosion at relative humidities exceedingabout 55%. This environment is characterized byproximity to the ocean and salt laden air that canproduce very severe corrosion damage on manystructural materials, enhance galvanic corrosion, andaccelerate deterioration of protective coating systems.Marine atmospheres are usually highly corrosive. Theprincipal culprit in marine atmospheres is the chloride(CL-) ion derived from sodium chloride.

2. BASIC OF ATMOSPHERIC

CORROSION

2.1. Theory of Atmospheric Corrosion

A fundamental requirement for electrochemicalcorrosion processes is the presence of an electrolyte.Thin-film invisible electrolytes tend to form onmetallic surfaces under atmospheric exposureconditions after a certain critical humidity level isreached. It has been shown that for iron, the criticalhumidity is 60 percent in an atmosphere free of sulfurdioxide. The critical humidity level is not constant anddepends on the corroding material, the tendency ofcorrosion products, surface deposits to absorbmoisture and the presence of atmospheric pollutants.

In the presence of thin-film electrolytes,atmospheric corrosion proceeds by balancing anodicand cathodic reactions. The anodic oxidation reactioninvolves the dissolution of the metal, while thecathodic reaction is often assumed to be the oxygenreduction reaction. It should be noted that corrosive

contaminant concentrations can reach relatively highvalues in the thin electrolyte films, especially underconditions of alternate wetting and drying. Oxygenfrom the atmosphere is also readily supplied to theelectrolyte under thin-film corrosion conditions.

The cathodic process: If it is assumed that thesurface electrolyte in extremely thin layers is neutralor even slightly acidic, then the hydrogen productionreaction (Eq.) can be ignored for atmosphericcorrosion of most metals and alloys.

2H+ + 2e- H2

Exceptions to this assumption would include

corrosive attack under coatings, when the productionof hydrogen can cause blistering of the coating, and

other crevice corrosion conditions. The reduction ofatmospheric oxygen is one of the most importantreactions in which electrons are consumed. In thepresence of gaseous air pollutants, other reductionreactions involving ozone and sulfur and nitrogenspecies have to be considered [24]. For atmospheric

corrosion in near-neutral electrolyte solution, theoxygen reduction reaction is applicable (Eq.)

O2 + 2H2O + 4e- 4O H-

Two reaction steps may actually be involved, withhydrogen peroxide as an intermediate, in accordancewith (Eqs).

O2 + 2H2O + 2e- H2O2 + 2

H2O2 + 2e- 2

If oxygen from the atmosphere diffuses throughthe electrolyte film to the metal surface, a diffusion-limited current density should apply. It has been

shown that a diffusion transport mechanism foroxygen is applicable only to an electrolyte-layerthickness of approximately 30 m and under strictlyisothermal conditions [25]. The predicted theoreticallimiting current density of oxygen reduction in anelectrolyte-layer thickness of 30 m significantlyexceeds practical observations of atmosphericcorrosion rates. It can be argued, therefore, that theover-all rates of atmospheric corrosion are likely to becontrolled not by the cathodic oxygen reductionprocess, but rather by the anodic reaction(s).

The anodic process: Equation represents thegeneralized anodic reaction that corresponds to therate-determining step of atmospheric corrosion.

MMn+ + ne-

The formation of corrosion products, the solubilityof corrosion products in the surface electrolyte, andthe formation of passive films affect the overall rate ofthe anodic metal dissolution process and causedeviations from sample rate equations. Passive filmsdistinguish themselves from corrosion products, in thesense that these films tend to be more tightly adherent,are of lower thickness, and provide a higher degree ofprotection from corrosive attack. Atmospheric

corrosive attack on a surface protected by a passivefilm tends to be of a localized nature. Surface pittingand stress corrosion cracking in aluminum andstainless alloys are examples of such attack.

Relatively complex reaction sequences have beenproposed for the corrosion product formation andbreakdown processes to explain observed atmosphericcorrosion rates for different classes of metals.Fundamentally, kinetic modeling rather thanequilibrium assessments appears to be appropriate forthe dynamic conditions of alternate wetting and dryingof surfaces corroding in the atmosphere. A frameworkfor treating atmospheric corrosion phenomena on a

theoretical basis, based on six different regimes, hasbeen presented by Graedel [26]. The regimes in this so-


4/24

S. Syed


called GILDES-type model are the gaseous region(G), the gas-to-liquid interface (I), the surface liquid(L), the deposition layer (D), the electrodic layer (E),and the corroding solid (S).

For the gaseous-layer effects, such as entrainmentand detrainment of species across the liquid interface,

chemical transformations in the gas phase, the effectsof solar radiation on photosensitive atmosphericreactions, and temperature effects on the gas phase,reaction kinetics are important. In the interface regime,the transfer of molecules into the liquid layer prior totheir chemical interaction in the liquid layer is studied.Not only does the liquid regime receive species fromthe gas phase, but species from the liquid are alsovolatilized into the gas phase. Important variables inthe liquid regime include the aqueous film thicknessand its effect on the concentration of species, chemicaltransformations in the liquid, and reactions involvingmetal ions originating from the electrochemical

corrosion reactions.In the deposition zone, corrosion products will

accumulate, following their nucleation on thesubstrate. The corrosion products formed under thinfilm atmospheric conditions are closely related to theformation of naturally occurring minerals. Over longperiods of time, the most thermodynamically stablespecies will tend to dominate. The nature of corrosionproducts found on different metals exposed to theatmosphere is shown in Table 1. The solution knownas the inner electrolyte can be trapped inside orunder the corrosion products formed. The deposited

corrosion product layers can thus be viewed asmembranes, with varying degrees of resistance toionic transport. Passivating films tend to representstrong barriers to ionic transport.

Table 1. Nature of corrosion products formed on four metals [27].

Common species Rarer species

AlAl(OH)3

Al2 O3, Al2 O3, 3H2OAlOOH, Alx (OH)Y (SO4)Z,

AlCl(OH)2. 4H2O

FeFe2O3, FeOOH,FeSO4, 4H2O

Fex (OH)YClz,FeCO3

CuCu2O1,Cu4SO4 (OH)6,Cu4 SO4 (OH)6, 2H2O,

Cu3 SO4 (OH)4

Cu2Cl(OH)3,Cu2 CO3 (OH)2,

Cu2 NO3 (OH)3

ZnZnO, Zn5 (OH)6 (CO3)2,

ZnCO3Zn(OH)2, ZnSO4,

Zn5 Cl2 (OH)8. H2O

Any corroding surface has a complex chargedistribution, producing in the adjacent electrolyte amicroscopic layer with chemical and physicalproperties that differ from those of the nominalelectrolyte. This electrodic regime influences theoverall reaction kinetics in atmospheric corrosionprocesses. In the solid regime, the detailed mechanisticsteps (sequences) in the dissolution of the solid andtheir kinetic characteristics are relevant.

2.2. Corrosion rate Expressions

Mostly the rates of corrosion of metals are expressedas mpy or mmpy. The relative scale for corrosion ofmetal is given as [28,3].

Safe: Less than 5mpy or 0.125 mmpyModerate: 5 mpy to 50 mpy or 0.125 mmpy to1.25mmpy.Severe: Greater than 50 mpy or 1.25 mmpy.

The rate of corrosion of metal is usually measuredeither by gravimetric method or by electrochemicalmethods. The conversion factors for the two methodsare. Gravimetric method:

87.6 x weight loss (mg)Corrosion rate (mmpy ) = --------------------------------

Area (cm2) x time (hrs) x Density

Electrochemical method:Eq.wt

Corrosion rate (mmpy) = 3.2 x 1corr(mA/cm2) x ------

-Density

Some times the corrosion rate is also given1.44

= mmd x ---------- = mpyDensity

where mdd is mg per squre decimeter per day, mpy ismils per year Also the formula for calculating thecorrosion rate is given as:

534W

mpy = ----------DATwhere W=weight loss, mg; D=density of specimen,g/cm3; A= Area of specimen. Sq.in and T= exposuretime, hr

3. ATMOSPHERIC CORROSION IN

VARIOUS MATERIALS

Carbon steel (hot and cold rolled) is the most widelyused metal for outdoor applications although largequantities of galvanized steel, stainless steel 304,aluminum, brass and copper are also used. Metals

customarily used for outdoor installations arediscussed.

3.1. Carbon Steel (hot and cold rolled)

Outstanding in its drawability and weldability, hotrolled carbon steel is extensively used in automobileframes, wheels, special vehicles, building structures,bridges, general structures and ships. Cold rolledcarbon steel is excellent in low temperature toughness,hydrogen-induced crack resistance, fracture resistance,high weldability and formability. This steel is used incold rolled products such as CR, GI, colour plates,structural pipes, general pipes, special pipes, pipes for

machines, high pressure gas cylinders and oil wellpipes [29]. Its corrosion studies are of immense interest


5/24



due to its wide spread use. The atmospheric corrosionof carbon steel has always been of prime interest forelectrochemist, and corrosion engineers because it isone of the most widely used materials.

In polluted atmospheres, chlorides and SO2 are thecommon pollutants influencing metallic corrosion.

Though chlorides come from natural airborne salinity,they are considered to be a significant pollutant as aconsequence of their strong action on metals duringatmospheric exposure. Relationships between chlorideconcentration in corrosion products, or atmosphericsalinity, and corrosion rates have been reported byCorvo and Morcillo et al., [30-31]. The concentration ofSO2 in the atmosphere also plays an important role indetermining the magnitude of atmospheric steelcorrosion. The adsorbed SO2 is postulated to acidifythe moisture layer at the metal interface and produceFeSO4 which undergoes hydrolysis reactions to formoxyferric hydroxides and probably regenerate H2SO4

again and further attack carbon steel. It has also beensuggested [32] that once some FeSO4 and rust havebeen formed, the conditions become favourable to anelectrochemical cycle, which operates much fasterthan the acid regeneration cycle. However, theexistence of competitive adsorption processes betweenchloride and sulphur compounds in atmospheres hasalso been reported by several authors [31, 33-34, 8- 9].

Other pollutants, such as NOx and O3, as well asclimatological and geographical parameters can alsoaffect the atmospheric corrosion of carbon steel [9,11,35].Organic acids, such as acetic and formic acids, also

play an important role in the atmospheric corrosion ofcarbon steel, even when in small concentrations. Theorigin of these organic acids is mainly wood, plasticsand paints and they cause the degradation of carbonsteel near by. This is a common occurrence, forexample, in products that are stocked or packedtemporarily in places where those substances exist.The presence of acetic acid and formic acid has beenalso detected in the rain by Galloway and Likens,Graedel et al.[36-37], where they increase the acidity.Atmospheres containing 0.5 and 10 ppm acetic acid ata relative humidity (RH) of 100 % will show corrosionbehavior on carbon steel [38].

Particulate matter present in the atmosphere alsoplays a vital role in undermining materials resistanceto atmospheric corrosion. Aggressive anions such asCl- and SO4

2- are renowned culprits for inducinglocalized attack. Sea-salt is a further importantatmospheric contaminant, especially for the corrosionof carbon steel structures. The primary sources of sea-salt in the atmosphere are the oceans. Normal sea windmay carry an average of 10 to 100 lb (4.5-45 kg) ofsea-salt per cubic mile (4.17 km3) of air. Brierly [39]showed that sea-salt fallout may range from anextremely high level of 3000-4000 lb acre-1 y-1 (0.3-0.45 kg m-2 y-1) on oceanic islands and coastal areas to

3-5 lb acre-1 y-1 (3.4 x 10-4-5.6 x 10-4 kg m-2 y-1) in aridareas. Evans[40] demonstrated that the presence of

hygroscopic magnesium chloride in sea-salt or seamist enables corrosion to take place on carbon steel atmuch lower relative humidity than if only sodiumchloride is present. Ericsson [41] showed that sodiumchloride particles on a carbon steel surface can causecorrosion at relative humidities which have been

considered too low to start SO2 induced corrosion. Hereported that the synergistic effect of sodium chlorideand SO2 at 90% RH increased the corrosion rate ofcarbon steel by about 14 times than caused by sodiumchloride alone. Among the climatic factors, time ofwetness (TOW), temperature and rainfall are widelyreported by Evans, Brown and Masters, Morcillo et al.and Feliu et al., [34,9,35,42]. Less information is availableabout the influence of wind [34,36] or the height ofexposure sites[43].

Among the factors affecting the type and amountof atmospheric corrosion products, the main role isplayed by the reactivity of carbon steel. This

characteristic depends on several properties such astheir chemical composition (which depends onmanufacturing procedures and finishing treatments)and the design and types of structures and joints. Thecorrosion rate also depends on atmosphericaggressiveness, which is a function of meteorologicaland pollution parameters.

The most common characteristic of metallicatmospheric corrosion is the localized character of itsnucleation. Preferential nucleation sites depend on themetal structure and are associated to the presence ofdifferent phases or environmental pollutants on

discrete areas of the metallic surface. In the case ofcarbon steel, nucleation starts with the formation ofsmall protuberances of corrosion products at isolatedpoints on the metallic surface, followed by theformation of a large number of corrosion productnuclei that can cover the entire surface after relativelyshort exposure times.

The corrosion rate after 1 year of exposuresupplies critical information about the interactionbetween the metallic surface and environmentalparameters. The wide range of values obtained is aconsequence of the exposure of the base metal inatmospheres presenting different combinations of

corrosive agents, depending on the location of the siteand the time of year. The weight losses of metals afterlonger exposure times provide information about theprotective character of the carbon steel corrosionproducts layers (SCPLs), which once sufficientlydeveloped, attenuate the effects of meteorological andpollution variables. The barrier effect of SCPLsdepends on their thickness, uniformity, porosity,solubility, adhesion and other characteristics, and cansignificantly affect the carbon steel corrosion rate andattack morphology through different mechanisms, ashas been shown for other metals.

The lowest atmospheric corrosion rates and attack

nucleation densities can be found in rural atmospheres,where corrosion products also usually present


6/24

S. Syed


relatively smaller structures. However, as atmosphericcorrosion nucleation depends on the time of wetness(TOW) of metallic surfaces, soil pollution andbackground atmospheric pollution, corrosion rates inrural atmospheres present a relatively wide range ofvalues for each metal [44].

3.2. Galvanized Steel

Galvanized steel is used in telecommunicationindustry, power transmission lines, thermal powerplant, automotive industry, roofing, siding andfencing[45-47]. The corrosion behaviour of galvanizedsteel in a wide variety of environments has beenthoroughly investigated by Mardar and Goodwin [48-49]. In areas where SO2 is present in any appreciablequantity galvanized surface will be attacked.Particulate species in the atmospheres can acceleratecorrosion of galvanized steel by increasing theconductivity of the surface layer after dissolution ofsoluble ions from the particulate. It has been reportedon the effect of particles of (NH4)2 SO4 on thecorrosion of galvanized steel [50].

Atmospheric corrosion of galvanized steelstructures begins through an electrochemical processwhen the Zn surface becomes wet with rain, mist, ordew [51-52]. A film of basic zinc carbonate, 2ZnCO33Zn(OH)2,forms on galvanized steel structures in longexposures, defined as exposures exceeding one year[53-54]. This film tends to inhibit further Zn corrosion;however, there are environmental conditions in whichfilm removal processes compete with film formation.

Haynie

[55-56]

, described Zn corrosion in steady-state, long-exposure conditions as linear functions ofthese processes.

C = A/B + Btwhere C = total Zn corrosion, mt = exposure time, y;B = dissolution rate of the film, m Zn/y; andA = diffusivity of corrosive species through the film,

m2/y.

The total Zn corrosion (C) is the sum of the Zncontained in the film (A/B) and the Zn dissolved fromthe film by precipitation (B). Diffusivity (A) can beaffected by atmospheric species combining with thefilm and forming various mineral phases. Thedissolution rate (B) is determined by the delivery rateof acidic pollutants to the film and by the mineralphases present. Dry and wet deposition deliverymechanisms provide a means for describing the ratecontrolling mass transfer processes affecting corrosionof the Zn coating.

Spence, et al. [46] developed a model for predictingthe corrosion of galvanized steel structures based ontwo competing mechanisms: the formation anddissolution of the basic zinc carbonate film that forms

on zinc surfaces. The model consists of a diffusivityterm that describes film growth and a dissolution termthat describes the rate of film removal. Dissolution

becomes the rate-determining process for predictingthe long-term corrosion behaviour of galvanized steelstructures. Components of the dissolution term wereevaluated with data collected from field exposureexperiments that were designed to separate the effectsof wet and dry acidic deposition from the effects of

normal weathering of galvanized steel specimens. Themodels dissolution term predicted the long-termcorrosion of galvanized steel with reasonableaccuracy. For further evaluation, the dissolution modelwas applied to historical, long-term corrosion data ofgalvanized steel products, taking into account theirsizes and shapes. The field data used in this evaluationwas consistent with corrosion rates predicted by themodel, within the limits of uncertainty of theenvironmental data. Thus, the model can be used withreasonable confidence to predict corrosion behavior ofdifferent structures if environmental conditions can beproperly described.

3.3. Stainless Steel 304

Stainless steels 304 was first introduced intocommercial use about 70 years ago [57]. It is used indairy equipment, fruit juice industry, food processingapplications such as in mills, bakeries , slaughter andpacking houses, dye tanks, pipeline buckets, dippers,railroad cars, fermentation vats and haulingequipment. A considerable amount of information hasaccumulated about their atmospheric corrosionbehavior.

3.4. Aluminum

Aluminum is one of the most abundant elements innature. Its low density, high elastic modulus, thermaland electrical conductivity, its corrosion resistance,and its capacity to form alloys with many elementsmakes it one of the most useful materials ofconstruction [58]. Aluminum and its alloys undergoblack to grey staining when exposed to humidatmosphere due to condensation of moisture or rain onthe surfaces [59-61]. The degree of staining does notdepend on the composition of water and the stainingrate is mainly controlled by the rate of diffusion ofoxygen into the thin film of water condensed [62]. The

stained area is mostly bayerite (Al2O3.3H2O) with thethickness of 2500-5000 [59]. Of all the aluminumalloys aluminum-magnesium alloys are highlysusceptible to water staining due to formation ofmagnesium oxide film [63].

The corrosion of aluminum in the atmosphere hasmainly been investigated through field studies [64-67].Few laboratory investigations in controlledenvironments have been published. Besides a stronghumidity dependence it is generally agreed thatdeposition of SO2 and chlorides and the pH in rain aremajor factors that determine the corrosion rate ofaluminum. In the presence of SO2, oxidizing agents

such as O3 and H2O2 may also play a role in theatmospheric corrosion of aluminum [64].


7/24



Compared to other metals, aluminum corrodesrather slowly under atmospheric conditions, because itforms an insulating amorphous oxide film of lowsolubility in air and aqueous solutions over the pHrange from 4 to 8.6. However, enhanced corrosionoccurs in marine environments and in some urban

areas[68]

. Common corrosion products found underthese conditions are basic aluminum sulfates andamorphous aluminum sulfate hydrate [68-69]. Indoors,sulfate is the most abundant anion found on aluminumsurfaces [70]. The presence of SO4

2- on these surfacesmay be due to SO2 induced corrosion or sulfatesassociated with particle deposition. The accumulationof inorganic ionic substances is primarily due toparticle deposition [71]. Ammonium and sulfate ionsare the most abundant ions in fine dust particlescommonly found in urban environments and may,therefore, play a dominant role in the corrosionprocess. This is especially true for atmospheres

containing solid, ionic contaminants like ammoniumsulfate. Salt spray tests, which are a common testmethod for industrial applications, are not suitable forstudying the effect of dry deposits, since the initialperiod, when the particles may start to absorb watervapor from the atmosphere, cannot be simulated.

Patterson and Wilkinson [73] investigated the effectof NaCl and NH4Cl particles on the corrosion ofaluminum at 293 K and 80% relative humidity (RH).The weight increase due to NaCl particles was linearwith time and was small compared to the weightincrease due to NH4Cl particles. With NH4Cl, Al

showed an initial phase of slow weight increase, andafter 5 days, a sudden rapid weight gain. Also theappearance of the specimen changed after 5 days fromthe initial uniform deposit to a granular unevenproduct. The corrosion products were not identified. Ina later study, Wilkinson and Patterson [74] investigatedthe effect of RH (70 to 90%) on the corrosion at 293 Kusing NH4Cl, the weight gain of Al after 90 days wasseven times greater at 80% RH than at 70%. Thecorrosion products observed at 80% RH were muchbulkier than at 70% RH which appeared after a fewdays. At 90% RH the bulky corrosion productsappeared on the first day and continued growing.

Sanyal and Bhadwar[75]

investigated the effect ofNaCl, Na2SO4, NH4Cl, and (NH4)2 SO4 particles onthe atmospheric corrosion of aluminum at 313 K.Corrosion rates increased with increasing humidity,temperature and period of exposure, and weregenerally much higher than under immersedconditions in the corresponding electrolytes.

The estimated minimum NO2 concentrationinducing aluminum corrosion is believed to be 30g/m3 [76]. For a urban-industrial Cuban atmosphere itwas obtained diary NO2 concentrations over 31 g/m

3only in a 10% of the measurements [77]. Rural andcoastal atmospheres do not report values over 30g/m3. Therefore, the influence of this pollutant on theatmospheric corrosion may be negligible.

3.5. Brass

Brasses are widely used engineering materials andhave found their applications in electrical, airconditioning, marine, construction and fabricationindustries. 70/30 brass has phase structure and is

resistant to many organic and inorganic reagents.Anodic reactions for the corrosion of brass is thedissolution of both Cu and Zn, but in some corrodentsthere is a preferential dissolution of zinc leading todezincification of the alloy and results in the loss oftensile strength of the alloy. In acidic medium thecathodic reactions are the discharge of hydrogen ion toform nascent hydrogen and reduction of dissolvedoxygen to form water which may further give nitrousacid which may combine with H+ to form nitrosoniumion. Reduction of nitrosonium ion may result in theformation of nitrous acid. The formation of nitrosylion and nitrous acid [78] represent the limiting stage in

the overall cathodic reaction of the reduction ofnitrous acid. Nitrous acid is an active species for thecorrosion of brass and its concentration controls thedissolution of 70/30 brass in HNO3 acid

[79-80].

3.6. Copper

Copper is on one hand, a natural component in mostecosystems and on the other, a metal that always hasfound many applications in old and modern societies.Todays society relies on the electrical conductivity ofa small number of metals to convey power and signalinformation to electrical and electronic equipment,architectural and artistic. One of the most important

metals in this respect is copper. Due to the widespreaduse of copper in many applications besidesconductors, e.g., as roofing material, for decorativestructures, and statues, there is a vast knowledge of thelong time behavior of the atmospheric corrosion ofunsheltered copper outdoors. For instance, copperpatina formation, its stages, and most importantconstituents, together with mechanistic deliberations,have been thoroughly examined [81-83].

The general sequence of atmospheric corrosion ofcopper is well known. Initially oxygen and water reactwith a fresh copper surface forming a sequentialstructure consisting of Cu

2O/CuO/[Cu(OH)

2or CuO.

xH2O], the main component being Cu2O, cuprite [82].This is later followed by reaction with pollutantspresent as gases (e.g., SO2, NO2, O3, Cl2, HCl, andH2S) as ionic constituents of aerosol particles or asions in precipitation. Eventually a patina of severaldifferent compounds forms on top of the initiallyformed cuprite layer. Important copper compoundsfound as patina constituents are Cu2Cl(OH)3,atacamite, Cu4SO4 (OH)6. H2O, posnjakite, andCu4SO4 (OH)6, brochantitie, in rural and urban areas.In urban areas Cu2SO4 (OH)4, antlerite, andCu2CO3(OH)2, malachite, are also found

[82]. When

copper is used for electronic signal transmission, verysmall defects on contact surfaces due to atmosphericcorrosion can cause partial or complete failure of the


8/24

S. Syed


device. Even a relatively benign environment or shortexposures in more polluted environments can causedamage. Thus, it is very important to investigate minorattacks preceding actual patina formation.In the literature, information on corrosion processes oncopper can be found in studies conducted in the

laboratory, in indoor field exposures, and in shelteredand unsheltered outdoor exposures. Since thepioneering work by Vernon [84], the effect of differentcombinations of gaseous pollutants has beeninvestigated in several laboratory studies [85-89]. Vernonalso realized the importance of aerosol particles ascorrosion accelerators [84, 90]. Current concentrationsof aerosol particles and their impact on reliability ofelectronics have been studied by Sinclair, Sinclair andPsotakelty and Sinclair et al. [91-93,72] in several fieldexposures. These investigations later inspired multi-analytical laboratory studies of the influence ofammonium sulfate on the corrosion of copper [94].

Field studies combining pollutant monitoring andcopper corrosion measurements have been pursuedboth indoors [85,95-96] and in sheltered outdoorenvironments [97-99]. For atmospheric corrosion tooccur, the presence of water on the metal surface isalso essential [84-85, 94]. The amount of water sorbed isdependent on relative humidity and on surfacedeposits [100].

4. INFLUENCE OF EXPOSURE

PARAMETERS

4.1. Humidity

Atmospheric air is a mixture of dry air and watervapor. In industrial and sea shore places, gases such asSO2, Cl2 and H2S and particulates of NaCl and othersalts are present. The air humidity is characterized bythe indices RH, absolute humidity, moisture contentand specific air humidity [4].

4.2. Critical Relative Humidity

The primary value of the critical relative humiditydenotes that humidity below which no corrosion of themetal in question takes place. However, it is importantto know whether this refers to a clean metal surface or

one covered with corrosion products. In the latter casea secondary critical humidity is usually found at whichthe rate of corrosion increases markedly [84]. This isattributed to the hygroscopic nature of the corrosionproduct. In the case of iron and steel it appears thatthere may even be a tertiary critical humidity [101].Thus at about 60% RH rusting commences at a veryslow rate (primary value) [102] at 75-80% RH there is asharp increase in corrosion rate probably attributableto capillary condensation of moisture within therust[84,103]. At 90% RH there is a further increase inrusting rate [101], corresponding to the vapour pressureof saturated ferrous sulphate solution[104], ferrous

sulphate being identifiable in rust as crystallineagglomerates[105]. The primary critical RH foruncorroded metal surfaces seems to be virtually thesame for all metals, but the secondary values varyquite widely.

It has been found, that at high relative humidities,

aluminum and iron show no SO2 + NO2 synergism[106], and that for steel in negligible [107-108]. It wasreported that a thick layer of water on the metalsurface seems to act as a sink for SO2, but as a barrierfor NO2

[108]. For metals with a protecting oxide film,NO2 may even act as an inhibitor; otherwise, thereseems to be synergistic effects [106]. It has been showedby several authors that the SO2 + NO2 synergism oncopper corrosion is only active at high relativehumidity (90%) [109], and steel corrosion at lowrelative humidity [110-111, 108,106]. Kucera et al. [128]

reported that the synergistic effect of SO2 + O3 can beboth stronger than SO2 + NO2, as for copper, and

weaker, as for nickel. In other works no synergisticeffects of simultaneous interaction of SO2 and NO2with either nickel or copper have been observed [112].This dry deposition is in most cases dominating andSO2 exerts the strongest corrosive effect

[106]. The roleof NO2 has not yet been clarified and its strongsynergistic effect with SO2, shown for many materialsin different laboratory studies, has not been observedin the field exposure and may be due to the strongcorrelation between SO2, NO2 and O3 concentrations[106].

4.3. Specific AtmospHeric Corrodents (Pollutants)

The electrolyte film on the surface will contain variousmaterials deposited from the atmosphere or originatingfrom the corroding metal. The composition of theelectrolyte is often the factor which determines the rateof corrosion.

Joel and Karl [186] reports the combined effects offire, corrosive smoke, and particulate as firecorrosivity. While the effects of fire corrosivity arewell known, little quantitative information is availableconcerning the mechanisms involved and the degree towhich materials, particularly metals, are susceptible.In this investigation, the deleterious effects of

combustion products generated during fires on normalconstruction materials are documented. Table 2summarizes the depth of corrosion data correspondingto post-test exposure in a humidity chamber.

Karlsson, et al. [147] reported the serious corrosivedamage to carbon steel can be caused by fume gascontaining 50-500 ppm SO2, 1-2 g m

_3 of alkalichloride dust and 8-15% oxygen at temperatures of 3-500oC. It is confirmed by experiments, that a liquidphase may exist in a system containing Fe2O3, alkalichlorides, SO2 and O2, and that the melting point is aslow as 310oC. The liquid phase is very corrosive andattacks stainless steel 304 and low alloyed steel at

approximately the same rate.


9/24



Table 2. Depth of corrosion data corresponding to post-testexposure in a humidity chamber following combustionproduct exposure [186].

Post-test exposure 27oC (80oF) 75%RH corrosion value (micro inch)

Material ULIdentification

1 day 2 days

3

days

4

daysStainlesssteel (304)

SS1 35.5 53.5 70.5 79.0

Stainlesssteel (304)

SS2 31.0 39.0 46.5 52.0


SS3 33.0 53.5 66.5 73.0


SS4 0.0 0.0 0.0 0.0

Allam, et al. [187] have used the energy dispersiveX-ray mico-analysis, X-ray diffraction andfluorescence, Auger, X-ray photo-electron

spectroscopy and Fourier transform infraredspectroscopy to characterize corrosion products oncarbon steel after atmospheric exposure for periods upto 12 months in an industrial environment near thewest coast of the Arabian Gulf. The results indicatethat atmospheric corrosion starts by the formation ofsmall blisters at discrete locations on the metalsurface, presumably the anodic sites. The blistercovers are very rich in iron chlorides and contain ironoxyhydroxides, oxides, sulphates and possiblyhydroxide. The formation of iron chlorides as theprimary corrosion product is only limited to the earlystages of blister formation due to the aggressive nature

of chloride ions. Chloride formation during laterstages may be partially impaired since it requires theinward transport of fresh chloride ions through thethen thick rust layer. In contrast, the formation of ironsulphates at the rust-metal interface continues by theacid regeneration mechanism (which leads to theelectrochemical mechanism); therefore it is lessdependent on the supply of fresh sulphate ions fromthe surface electrolyte through the growing rust layer.

Chawla and Payer [188] studied the early stage ofcorrosion of copper by moist air containing 0.5%sulfur dioxide (75% relative humidity at 25oC).Scanning electron microscopy, Auger electron

spectroscopy, and transmission electron microscopyhave been used to examine the surface topography,surface chemistry and microstructure of copper foilsbefore and after exposure to the corrosive atmosphere.The analysis shows that in the initial stage of corrosiona mixture of copper oxide and copper sulfide forms onthe surface. Reduction of sulfur dioxide to sulfide onthe metal surface indicates that sulfur dioxide is acathodic depolarizer in the early stages of corrosion.The primary contaminants in the air that lead toatmospheric corrosion are SOx, NOx, chlorides, carbondioxide, hydrogen per oxide, hydrogen chloride,

ozone, oxygen, hydrogen sulphide, organic acids andsaline particles etc.

4.3.1. Sulfur dioxide

Sulfur dioxide, a product of the combustion of sulfur-containing fossil fuels, plays an important role inatmospheric corrosion in urban and industrialatmospheres. It is adsorbed on metal surfaces, has ahigh solubility in water, and tends to form sulfuricacid in the presence of surface moisture films. Sulfateions are formed in the surface moisture layer by theoxidation of sulfur dioxide in accordance with Eq.

SO2 + O2 + 2e- SO4

2-

The required electrons are thought to originatefrom the anodic dissolution reaction and from theoxidation of ferrous to ferric ions. It is the formationof sulfate ions that is considered to be the maincorrosion accelerating effect from sulfur dioxide. Foriron and steel, the presence of these sulfate ionsultimately leads to the formation of iron sulfate

(FeSO4). Iron sulfate is known to be a corrosionproduct component in industrial atmospheres and ismainly found in layers at the metal surface. The ironsulfate is hydrolyzed by the reaction expressed by Eq.

FeSO4 +2H2O FeOOH + SO42- + 3H+ + e-

The corrosion-stimulating sulfate ions are liberatedby this reaction, leading to an auto-catalytic type ofattack on iron [25,26,113]. The acidification of theelectrolyte could arguably also lead to acceleratedcorrosion rates, but this effect is likely to be ofsecondary importance because of the buffering effectsof hydroxide and oxide corrosion products. In

nonferrous materials such as zinc, sulfate ions alsostimulate corrosion, but the auto-catalytic corrosionmechanism is not easily established. Corroding zinctends to be covered by stable zinc oxides andhydroxides, and this protective covering is onlygradually destroyed at its interface with theatmosphere. In moderately corrosive atmospheres,sulfates present in zinc corrosion products tend to bebound relatively stronger, with limited watersolubility. At very high levels of sulfur dioxide,dissolution of protective layers and the formation ofmore soluble corrosion products is associated withhigher corrosion rates.

It is well known that SO2 pollutant substantiallyenhances the corrosion rates of metals exposed in theatmosphere. Rozenfeld [114] has suggested that,because of its greater solubility (SO2 is about 2600times more soluble than oxygen), it might be reducedat cathodic sites more rapidly than oxygen,consequently increasing anodic dissolution rates. Insolution, electro-chemical reduction of SO2-3 competeswith its oxidative conversion to SO2-4. However,Seinfeld, [115] states that, in the absence of catalysts,solution phase oxidation of SO2-3 by dissolved oxygenis slow. Under these circumstances SO2 may persistfor a sufficient length of time to act as a cathodicdepolarizer in the manner as suggested by Rozenfeld.


10/24

S. Syed


4.3.2. Nitrogen compounds

Nitrogen oxide emissions originate from combustionprocesses other than those emitting SOx. Road trafficand energy production are the primary sources. Mostof the nitrogen oxides are emitted as NO incombustion processes. In the atmosphere oxidation toNO2 takes place successfully according to

2NO + O2 2NO2

As the pollutant moves further from the source it isfurther oxidized by the influence of ozone:

NO + O3 NO2 + O2

Near the emission source nitrogen dioxide isconsidered to be the primary pollutant. The NO2/NOratio in the atmosphere varies with time and distancefrom the source. Allowed enough time the NOx maybe further oxidized according to the reaction

2NO + H2O + 3/2 O2 2HNO3Since this reaction occurs at a very slow rate, the

amounts of HNO3 and nitrates in the vicinity of thesource are very low.

In urban atmosphere NO2 levels show a lesspromising trend. This has led to some interest in thecorrosive effect of NO2 on copper. Thus Eriksson andJohansson [116] exposed copper to humid air containingNO2. Simon, et al.,

[117] studied the corrosion productsformed on copper exposed to humid air containingNO2 identified cuprite (Cu2O) and basic copper nitrate(Cu2(OH)3 NO3) on the surface using XPS. The

general conclusion of these studies is that NO2 in theppm range has very slight corrosive effects on copper.

4.3.3. Chlorides

Atmospheric salinity distinctly increases atmosphericcorrosion rates. Apart from the enhanced surfaceelectrolyte formation by hygroscopic salts such asNaCl and MgCl2, direct participation of chloride ionsin the electrochemical corrosion reactions is alsolikely. In ferrous metals, chloride anions are known tocompete with hydroxyl ions to combine with ferrouscations produced in the anodic reaction. In the case ofhydroxyl ions, stable passivating species tend to be

produced. In contrast, iron-chloride complexes tend tobe unstable (soluble), resulting in further stimulationof corrosive attack. On this basis, metals such as zincand copper, whose chloride salts tend to be lesssoluble than those of iron, should be less prone tochloride-induced corrosion damage [25] and this wasconsistent with practical experience.

Chloride ions is the most common and importantatmospheric corrosive agent, as has been reported bydifferent authors all over the world [42,118,107,119].However, different works concerning the influence ofchloride ion on metallic corrosion have beenreported[42,107].

4.3.4.Carbon dioxide

The concentration of carbon dioxide in the atmosphereis about 350 ppm [120]. The effect of CO2 on theatmospheric corrosion of zinc was investigated byfalk, et al. and lindstrom, et al. [121-122]. They reportedthat ambient concentrations of CO

2inhibit the NaCl-

induced corrosion of zinc. This effect is important forunderstanding zinc corrosion in cases where thesupply of CO2 is limited, eg., in crevices and underpaint films. In the case of zinc and copper, carbonate-containing corrosion products are often reported fromthe field [68,123]. In contrast, aluminum carbonate hasnot been identified as a corrosion product in theatmosphere [64]. A laboratory study of the effect ofCO2 on the atmospheric corrosion of aluminum isreported. The samples were exposed to pure air with95% relative humidity and the concentration of CO2was


11/24



iron (III) ions to the more reactive iron (II) form, sincethe reaction.

HSO-3 + H2O2 HSO-4 + H2O

Thus, in the summer months when atmosphericchemistry is vigorous and H2O2 concentrations are

high[135]

it is generally true that in water films on steelsurfaces (H2O2) > [Fe3+], rust and dissolution of the

steel is inhibited. In the winter months, or in very highsulfur environments, [Fe3+] > [H2O2], the reduction ofiron (III) to iron (II) is promoted while surfacepassivation is inhibited, and steel dissolution proceedsrapidly. In recent years the sulfur emissions to theatmosphere in some parts of the world, particularlyNorth America, has decreased or remained relativelyconstant, while increased emissions of oxides ofnitrogen and hydrocarbons have resulted in increase inatmospheric H2O2. The consequence of these changeswould be an expected decrease in the corrosion rate of

steel, especially in the summer; unfortunately, thehistorical data needed to examine this conjecture is notavailable.

4.3.6. Hydrogen chloride

Very little work has been reported on the effect of HClon the degradation of materials in the environment.This is probably because HCl, which is presentoutdoors in markedly reduced concentrations whencompared with SO2, has not been considered tocontribute to significant degradation of materials. Themajor natural source of HCl is from volcanicfumaroles; however, atmospheric transformations ofchlorinated hydrocarbons also contribute to outdoorlevels, [136] as do emissions from waste incinerationespecially of PVC and chlorinated solvents[137].Although the concentration of chloride species inrainwater has been measured frequently, it is generallydifficult to distinguish between chloride derived fromcombustion with that derived from sea-salt aerosol or,in specific locations, road dering salt.

The first major study of atmospheric degradationof metals by HCl was carried out by Feitnecht [138] whoexposed zinc, iron and copper to HCl vapours atvarying humidities between 50% and 95% RH. He

found that HCl reacted with metals only when acritical relative humidity was exceeded, the value ofwhich was approximately that of the vapor pressureover a saturated solution of the metal chloride formedduring corrosion.

Barton and Bartonova [139] carried out a much moreextensive investigation of the corrosive effect of HClgas at concentrations between 7 and 10 ppm on zinc,mild steel and copper at temperatures between 20 and50oC and at relative humiditys of 70 and 95%. Twodistinct stages were seen in the behavior. The firststage was characterized by a non-linear increase inweight loss with time, which Barton and Bartonova

termed the indication period for steady-state corrosion.The second stage, after about 16 days exposure,

showed steady-state corrosion with a linear increase inweight-loss with time. The primary corrosion productsfound on iron were FeO(OH), Fe3O4 and FeCl2, whilstthose found on zinc were 4Zn (OH)2. ZnCl2, Zn (OH)2and ZnO. The amount of chloride in the corrosionproduct tended to decrease slowly with time, during

the initial period of exposure. After the steady statecorrosion region had been reached, the composition ofthe corrosion product remained unchanged.The corrosion rate was measured at differenttemperatures in the steady state region. For zinc, thecorrosion rate decreased as the temperature increased;for iron, the corrosion rate increased with temperaturesup to 40oC, but decreased at 50oC. Barton andBartonova proposed the following mechanisms toexplain their observations.Iron: Initially, a uniform film of FeO (OH) and FeCl2forms, presumably by reaction with the air-formedsurface film on iron. Subsequently, the FeCl2 reacts as

follows.

FeCl2 + H2O + O2 = FeO (OH) (1)

The metal then reacts with HCl released:

Fe + 2HCl + O2 = FeCl2 + H2O (2)

The steady-state can be represented by a combinationof these two reactions. The rate of the reactions didnot appear to depend on the diffusion of HCl to thesurface since the corrosion rate was similar in flowingand stationery atmospheres. The implication is that thecorrosion rate is dependent on chemical reaction rate.

Zinc: During the initial period, the following reactionsmay occur:

Zn + 2HCl + O2 = ZnCl2 + H2O (3)

ZnCl2 + 4H2O + 2O2 + 4Zn = ZnCl2. 4Zn (OH)2 (4)

In the steady state, reactions (3) and (4) aresignificant; a reaction involving the destruction ofZnCl2. 4Zn (OH)2 is also important :

ZnCl2.4Zn(OH)2+2HCl =2ZnCl2+3Zn (OH)2+2H2O (5)

The steady-state is represented by a combination ofreactions (3), (4) and (5). The kinetics of corrosion iscontrolled by the transfer of HCl to the corrosionproduct atmosphere interface, its adsorption and thesubsequent production of soluble ZnCl2. The corrosionrate also depends on the hydroxide / chloride ratio inthe corrosion product as the hydroxides are moreprotective than the chlorides. No literature is availableon the combined corrosive effects of gaseous SO2 andHCl pollutant.

Corrosion of iron and zinc in HCl has been carriedout under a range of conditions simulating naturalatmospheres. The corrosion rate of zinc did notsignificantly increase upon exposure to HCl atpresentation rates typical of the highest found in an

urban area (2.5 x 10-6

mg cm-2

s-1

). This is explainedby the formation of protective basic zinc chloride by


12/24

S. Syed


reaction with pre-existing zinc hydroxide. At higherlevels of pollutant (representative of episodicindustrial atmospheres) reaction of HCl with zinchydroxide proceeds further to form soluble zincchloride. Consequently, the protective ability of thecorrosion product is lost. Under these conditions,

corrosion rate of zinc is controlled by the availabilityof HCl at the metal surface. In contrast, the corrosionrate of mild steel samples exposed to HCl presentationrates typical of the highest encountered in an urbanatmosphere was 18 times than found in an unpollutedatmosphere.

Increasing the HCl presentation rate further did notresult in a significant increase in corrosion rate. Theseresults are explained by the reaction of HCl at themetal surface and the subsequent formation of FeCl2,which is oxidized to FeO(OH), liberating HCl whichcan initiate further corrosion. The reaction sequenceforms a cycle and is, therefore, apparently independent

of incoming HCl [140].

4.3.7. Ozone

In Urban environment where O3 sometimes reachesextreme values, sulphate and oxide formation wereaccelerated more than expected [99].

A few laboratory exposures have recognized O3 asa potential agent for atmospheric corrosion of copper.The general oxidative power of O3, increases theoxidation of H2S to sulphur and SO2 to SO3

[141]. LaterGraedel et al. [142] unambiguously showed that O3enhances the atmospheric sulphadation of copper.

Laboratory experiments involving SO2 and O3 asgaseous pollutants are scarce, Svensson andJohansson[143] showed that O3 increases the corrosionattack on galvanized steel by oxidizing SO2 muchmore efficiently than NO2. Similar evidence wasprovided for copper by Eriksson [144], who found abasic copper sulphate, Cu2..5 SO4 (OH)3, 2H2O, afterone day of exposure to SO2+O3 at 90% relativehumidity. Copper samples are exposed at 75% relativehumidity to explore the possible influence of ozone onthe atmospheric corrosion rate of copper, variouscombinations of the gaseous pollutants such as sulphurdioxide, nitrogen dioxide and ozone were added.

Ozone promotes the oxidation of SO2 to sulphatemore efficiently than NO2 does. A synergism betweenSO2 and O3 is suggested. This synergism includes boththe oxidation of Sulphur dioxide by ozone and thecapability of ozone to form oxides, hydroxides orother oxygen containing reaction products in thepresence of smaller amounts of SO2. The synergisticeffect possibly can explain the unexpectedly highcorrosion rates of copper found at rural sites. The ruralsites are characterized by low SO2 and NO2concentrations, and by high ozone concentrations [145].

It has been reported that the presence of ozone inthe atmosphere may lead to an increase in the sulfur

dioxide deposition rate. While the accelerating effectof ozone on zinc corrosion appears to be very limited,

both aluminum and copper have been noted toundergo distinctly accelerated attack in its presence[24]. Role of ozone on the SO2

induced atmosphericcorrosion of copper increases the formation rate ofboth Cu2O and CuSO4. x H2O all over the surface

[146].

4.3.8. Oxygen

Oxygen is a natural constituent of air and is readilyabsorbed from the air into the water film on the metalsurface, which may be considered saturated, thuspromoting any oxidation reactions. Serious corrosivedamage to iron alloys can be caused by 8-15% ofoxygen at temperature of 3-500oC. It is confirmed byexperiments [147].

4.3.9. Hydrogen sulphide

Trace amount of hydrogen sulphide is present in somecontaminated atmospheres. Hydrogen sulphide isknown to be extremely corrosive to most metals andalloys. The tarnishing of copper in a test atmosphereconsisting of air and hydrogen sulphide has beendescribed. Initially, linear growth is observed and thetarnished film is made up of cuprous oxide. In a laterstage the film growth becomes parabolic and cuproussulphide is formed [148]. Cuprous oxide protects copperfrom further attack by sulphur compounds in a dryatmosphere. In the presence of water, present as anadsorbed film, cuprous oxide reacts with hydrogensulphide and a sandwiched reaction layer (Copper/cuprous oxide/cuprous sulphide) is formed [149].

In humid air (no hydrogen sulphide present) thin

copper films is oxidized to cuprous oxide

[150,151]

.According to Leest [152] and Abbott [153] when copper isexposed to H2S atmosphere the corrosion productsformed was Cu2S. Lenglet, et al.

[154] reports that whencopper exposed to H2S atmosphere the corrosionproducts are Cu2S and sulphate, most likely asposjnakite, Cu4SO4 (OH)6. H2O.

4.3.10. Organic acids

Organic acids, such as acetic and formic acids, alsoplay an important role in the atmospheric corrosion ofmetals, even when in small concentrations. The originof these organic acids is mainly from woods, plastics,

certain paints, rubbers, resins, and other materialslikely to be found alongsidepackaged metal items [159-166], and they cause the degradation of metals nearby.This is a common occurrence, for example, inproducts that are stocked or packed temporarily inplaces where those substances exist. The presence ofacetic acid and formic acids has been detected in therain [36,37], where they increase the acidity.

The influence of organic acid vapors on copperstructures after prolonged exposure in a townatmosphere was studied by Vernon [155]. The presenceof these acids in outdoor atmospheres is a source offree acidity in precipitation in industrial areas [156-

158,128,194,96]. Acetic vapor is also present in industrialatmospheres, e.g., from vinegar in the food processing


13/24



industry and from the decomposition of raw materialsin the paper industry. Organic acid anions constituteabout 0.1 to 1% of the total ion concentration in thecorrosion-products (patina) on copper exposed to theoutdoor atmosphere for extended periods [37]. Animportant aspect of acetic acid vapor is that it causes

metal corrosion at very low concentrations[167-170]

.Atmospheres containing 0.5 and 10 ppm acetic acid ata relative humidity (RH) will show corrosion behavioron mild steel [38].

A study was made for the copper corrosion rateand corrosion products originated by the action ofacetic acid vapor at 100% relative humidity. Copperplates were exposed to an acetic acid contaminatedatmosphere for a period of 21 days. Five acetic vaporconcentration levels were used. The copper corrosionrate was in the range of 1 to 23 mg/dm2 day. Some ofthe compounds identified were cuprite (Cu2O), copperacetate hydrate [Cu(CH3COO)2. 2H2O] and copper

hydroxide acetate [Cu4(OH)(CH3COO)7. 2H2O]. Thislast compound was also characterized. The thicknessof the patina layers was 4 to 8 nm for amorphouscuprite, 11 to 48 nm for cuprite, and 225 nm forcopper acetate. The patina, in which the cementationprocess of different corrosion-product layers plays animportant role, is formed by the reaction of aceticvapor with copper through porous cuprite paths [171].

4.3.11. Saline particles

These are of two main types. The first is ammoniumsulphate, the effect of submicron-sized particles of

(NH4)2SO4 on the corrosion of copper in air at 373 K.This study was undertaken because ammonium andsulfate ions are usually the most abundant ions in finedust particles commonly found in urban environments.These particles can lead to accelerated corrosion ofelectronic materials, e.g., copper, which is used inmicroelectronic devices, circuit boards and connectors[172-173, 72]. It was found that copper reacts with(NH4)2 SO4 particles only if the critical relativehumidity (CRH) of (NH4)2 SO4 is exceeded. The CRHis that humidity at which the salt starts to absorbsignificant amounts of water. At relative humidities(RH) above the CRH, copper was heavily corroded

leading to a thick Cu2O-layer overgrown by crystals ofbasic copper sulfates. A corrosion mechanism thatexplains these results was proposed [94].

The corrosion mechanism of copper at 373 and300 K in the presence of submicron (NH4)2 SO4particle deposits has been investigated. Several in situtechniques have been used to monitor the corrosionprocess in real time. At and above the critical relativehumidity of (NH4)2 SO4, dissolution of Cu is followedby formation of Cu2O, oxidation of Cu(I) ions toCu(II) ions and precipitation of antlerite[Cu3(SO4)(OH)4], brochantite [Cu4(SO4)(OH)6], orposnjakite [Cu4(SO4)(OH)6. H2O]. The amount of

corrosion product formed increases with the amount of

(NH4)2 SO4, particles, relative humidity (RH) andtemperature [94].

The effect of (NH4)2 SO4 particles on theatmospheric corrosion of aluminum has beeninvestigated at 300 and 373 K at various relativehumidities (RH) levels. Aluminum reacts with (NH4)2

SO4 particles only at or above the critical relativehumidity (CRH) at either temperature. The corrosionrate increases with increasing RH and temperature.Above the CRH of (NH4)2 SO4, droplets are formed onthe particles at both temperatures, makingelectrochemical reactions possible. The (NH4)2 SO4decomposes and ammonia evaporates from thedroplets. At 373 K mixed ammonium-metal-sulfatesare formed, followed by basic metal sulfates; oxideformation is enhanced at 373 K compared to 300 K.At 300 K no solid corrosion products containing Alare found, but it was shown that Al dissolves in thedroplets. A corrosion mechanism has been proposed

that explains the experimental observations, includingpH and corrosion potential changes with time [174]. Thesecond is marine salt, mainly sodium chloride butquite appreciable quantities of potassium, magnesiumand calcium ions are analyzed in rainfall [175].

4.3.12. Other atmospheric contaminant and

airborne particles

The corrosive effects of gaseous chlorine in thepresence of moisture tend to be stronger than those ofchloride salt anions because of the acidic characterof the former species [25].

Airborne particles are divisible into two groups: a-The inert non-absorbent particles, usually siliceous,which can only affect corrosion by facilitatingdifferential aeration processes at points of contact, andb- The absorbent particles such as charcoal and sootare intrinsically inert but have surfaces orinfrastructures that adsorb SO2 by either co-adsorptionof water vapor or condensation of water within thestructure and catalyse the formation of a corrosive acidelectrolyte solution. Dirt with soot assists theformation of patinae on copper and its alloys byretaining soluble corrosion products long enough forthem to be converted to protective, insoluble basic

salts [176].4.4. Temperature

The effect of temperature on atmospheric corrosionrates is also quite complex. An increase in temperaturewill tend to stimulate corrosive attack by increasingthe rate of electrochemical reactions and diffusionprocesses. For a constant humidity level, an increasein temperature would lead to a higher corrosion rate.Raising the temperature will, however, generally leadto a decrease in relative humidity and more rapidevaporation of surface electrolyte. When the time ofwetness is reduced in this manner, the overall

corrosion rate tends to diminish [27].


14/24

S. Syed


For closed air spaces, such as indoor atmospheres,it has been pointed out that the increase in relativehumidity associated with a drop in temperature has anoverriding effect on corrosion rate [177]. This impliesthat simple air conditioning that decreases thetemperature without additional dehumidification will

accelerate atmospheric corrosion damage. Attemperatures below freezing, where the electrolytefilm solidifies, electrochemical corrosion activity willdrop to negligible levels. The very low atmosphericcorrosion rates reported in extremely cold climates areconsistent with this effect.

5. RELATED PREVIOUS EXPERIENCEAND WORK DONE WORLDWIDE

Natesan, et al. [178] reported that the loss due tocorrosion is often compared with that of othercalamities such as earthquake and cyclone. In fact

similar to earthquake and cyclone, corrosion is anatural process, only difference being that its impact isinvariably indirect. In the case of earthquake, mappingof seismic zones is already in practice. In the case ofcyclone also, weather prediction is available on aglobal level. Different countries are independentlypreparing their own corrosivity maps confined to theregion of their interest. With the on goingliberalization and globalization of the industries, thereis an urgent need for preparing corrosion maps on aglobal level. The different organizations which areinvolved are to be brought together and a centralizeddata bank created. The data generated are presentedand discussed in the light of the global data. Collectionof data on the corrosivity of atmospheres in differentlocations has been going on in different countries.Certain interesting observations are highlighted here.Spain: Researchers from Centro Nacional deInvestigaciones Metalurgicas, Ciudad Universitaria,Madrid have taken into consideration three clearlydifferentiated meteorological areas in Spain: thecentral, north western and southern areas. Table 3shows the data on corrosion (m) of aluminium andtheir respective durability factors [DF] obtained atthree different locations. It can be seen that the

durability factor of aluminium varies not only fromlocation to location but also from station to station.Durability factor for aluminium varies from 34 to 144.USA: Laque center for corrosion is a pioneeringinstitution involved in carrying out atmosphericcorrosion studies. This center had ranked thecorrosivity of a number of sites in Canada as well asUSA using the mass loss technique. The data hadindicated that short term mass loss data can exhibitwide variations because of uncontrolled environmentalfactors in natural atmospheric environments andseasonal effects. Thus longer exposure (e.g. 1-2 years)is intended to average out the influence of large

fluctuations in short term (e.g. 1 month) environmentalvariables [178].

Table 3. Corrosion (m) and durability factors (DF) of aluminum [178].

Area Stati on Period 1982-83 Period 1983-84

m DF m DF

South 1 0.37 51 0.24 -2 0.41 39 0.32 433 0.33 34 0.29 41

4 0.10 144 0.12 93North West 1 0.81 45 0.63 52

2 0.49 76 0.5 773 0.31 75 0.17 1254 0.33 79 0.19 134

Central 1 - - 0.18 662 - - 0.11 983 - - 0.13 824 - - 0.18 53

Table 4. Durability factors for sites at which carbon steel,galvanized steel and Aluminium exposed[178].

Sites Carbon steelGalvanized

steel

Alumin ium

Paraparaumu 1 26 400Flock House 1 38 69Levin 1 49 323Kairaga 1 75 677Tiwai point 1 22 341Invercargill 1 63 843Gore 1 53 121Tapanui 1 43 73

New Zealand: Is a group of islands in the southwestpacific. The climate is warm and humid withprevailing westerly winds depositing large amounts of

sea salt far inland, condition thought to pose severeatmospheric corrosion hazard. Results of anatmospheric corrosivity survey of New Zealand arereported. Carbon steel, aluminium and galvanized steelare exposed for one year at 168 sites locatedthroughout New Zealand. One year corrosion ratesranged between 18 4800 gm-2 per year for carbonsteel and 0.7 1417 gm-2 per year for galvanized steel.Results for aluminium were significantly greater thanzero at a number of severe marine sites. Maximumcorrosion rate of 2.6 gm-2 per year was found. And atone industrial site a rate of 1.3 gm-2 per year wasrecorded.

A clear correspondence between corrosion ratesand proximity to the coast is evident in these results,inferring that atmospheric corrosion rate in NewZealand are related to levels of chloride deposition.Table 4 shows the durability factors for carbon steel,galvanized steel and aluminium. The durability factorsfor galvanized steel and aluminium varies from placeto place. Durability factor for galvanized steel andaluminium varies from 22 to 75 and 69 to 843respectively.Germany: The results of long time examinations ofthe corrosivity of the atmosphere demonstrated that inthe period from 1979 to 1989 on the territory of theformerly GDR no changes could be established.


15/24



Table 5. Typical durability factor based on relative corrosion ratesfor galvanized steel and aluminium (one year data) [178].

Location Galvanized steel Aluminium

SVRECT, Surat 10.75 6.95MPT, Mormugoa 2.46 41.26NIO, Goa 13.27 120.96NMPT, Mangalore 16.16 21.25IOC, Mumbai 18.89 27.2INS Naval Base, Kochi 24.29 180Kayamkulam - 84CECRI Unit, Tuticorin 3.43 3.09Mandapam Camp 8.69 155.79Nagapattinam 89.75 V.HighCuddalore 12.21 44.15INS Naval Chennai 44.94 60.93Near Nellore - 551.72CECRI Unit Kochi 31.64 502.78Mettupalayam - 33.33MPL, Manali 25.22 81.56Tirupur - 6

LPSC Mahendragiri 3.86 2.52Coimbatore - 4Portblair 1.4 9.5

The comparison of these results with thoseobtained at the same territory in the period 1989-1994shows a significant decrease of corrosivity againstmetallic materials which is caused by a lower deposit-ion rate of the corrosion pollutant SO2. This positivedevelopment can be explained by an improvement ofthe situation which occurs due to sequence of changesof the industrial structure as well as by active measureof environmental protection in the new countries ofFRG after the political changes in the year 1989 [178].India: It is almost 32 years since the first corrosionmap of India was brought out. Over these years, lot ofenvironmental changes has occurred due toindustrialization, population growth and enormousvehicle population. Durability data clearly indicatedthat non ferrous viz galvanized steel and aluminiumhave better durability factors. However the factorsvary from location to location.

If durability factor and cost factor are takentogether it can be clearly seen that aluminium hasappreciable cost benefit ratio. At certain locations

galvanized steel may prove to be a cost effectivecandidate material. Table 5 shows typical durabilityfactor based on relative corrosion rates for galvanizedsteel and aluminium (one year data).

Vashi, et al. [179] reports the corrosion rate ofaluminium as well as the sulphation rate wasdetermined under outdoor exposure at Ankleswar,South Gujarath representing an industrial atmosphere.Monthly corrosion rate of aluminium vary from 4 to30 (1 to 5 m/y) mg/sq.dm. the values for yearly ratesbeing 65 to 126 (1 to 15 m/y) mg/sq.dm. Aluminiumor aluminium coated sheets would, therefore, givebetter performance in comparison with mild steel or

zinc.

05

10

15

20

25

Rainy Summer

Season

Winter

SEASON

mg/sq.dm.

Figure 1. Average seasonal corrosion rate (in mg/sq.dm.) of

different months.

The corrosion rate of aluminium in rainy months(22.7 mg/sq.dm.) was higher than the rate inwintermonths (6.5 mg/sq.dm.) and summer months (10.5mg/sq. dm) (Fig. 1) [179].

Odnevall, et al.[180]

summarized the results from anextensive field exposure program implemented tostudy possible seasonal dependencies of coppercorrosion rates and runoff rates. Two year exposuresin one urban and one rural environment wereperformed at four different starting seasons. Anextensive multi-analytical approach was undertaken ofall exposed samples.

Seasonal differences in corrosion productformation was observed during the first month ofexposure and attributed mainly to differences inrelative humidity conditions. Seasonal differences incorrosion rate at the rural site could be discerned

throughout the whole two-year exposure, again,mainly attributed to differences in relative humidity.No seasonal effect could be observed at the urban siteindicating that other parameters influenced thecorrosion kinetics at this site. While corrosion ratesexhibit a continuous decrease with exposure time, theyearly runoff rates are independent of time. Dependingon starting months the yearly copper runoff ratesranged from 1.1 to 1.7 gm 2y 1 for the urban site, andfrom 0.6 to 1.0 gm 2y1 for the rural site. Theseseasonal variations were primarily attributed todifferences in precipitation quantity and environmental

characterist-ics. Runoff rates are significantly lowerthan corrosion rates as long as the adhering copperpatina is growing with exposure time.

Almeida, et al. [44] summarizes the results obtainedin the MICAT project for carbon steel specimensexposed for 1 to 4 years in 22 rural and urbanatmospheres in the Ibero-American region. Test sitecharacterization, chemical and morphologicaldetermination of the steel corrosion product layers(SCPLs) contributed to understanding the corrosionphenomena involved. It was observed how someclimatological factors could affect steel corrosion ratesand SCPL properties. Although the studied

atmospheres were classified into different ISO groups,steel corrosion rates did not differ significantly


16/24

S. Syed


Table 6. MICAT S0P0,S0P1 and S0P2 site characteristics[44].

Carbon steelcorrosion productsName

Depositionrate(mgm-2d-1)

Cl-SO2

Timeofwetness(TOW

)

ISOClass(**)

Carbonsteelcorrosionrate(myear_

1)

1year4years

ISOClass(***)

1year

2years

3years

Solublesalts(mgm-2)

Cl-

SO42-

Cuzco (*) (*) 4 - 1.4 0.8 C2 - - - 66 47

Arties 1.7 9.0 3 C2-C3 3.9 3.3 C2 L.G L.G L.G 101 213

San Juan (*) (*) 3 - 4.9 1.9 C2 - - - 96 341

Iguazu (*) (*) 5 - 5.7 2.8 C2 L L - 38 104

Trinidad 1.5 0.7 4 C3 6.7 5.2 C2 - - - 35 133

Riobamba 1.1 1.2 4 C3 8.4 - C2 L - - - -

Granada (*) 6.2 3 - 8.5 5.2 C2 L.G L.G L.G 91 411

Brasflia (*) (*) 4 - 12.9 8.7 C2 L L.G - - -Cuernavaca (*) 7.9 3 - 13.4 6.6 C2 L.G - - 65 522

Pucallpa (*) (*) 5 - 14.3 8.0 C2 L.G - - 87 94

V. Martelli (*) 9.0 4 - 14.7 7.7 C2 L - - 52 245

Arequipa (*) (*) 2 - 15.4 7.8 C2 L - - 308 811

San Pedro (*) 0.6 5 - 17.0 6.2 C2 L.G - - 53 113

Belem (*) (*) 5 - 19.4 - C2 L.G - - 57 0

Cotove (*) 0.3 4 - 19.6 6.1 C2 L.G - - 61 128

Tortosa (*) 5.3 4 - 20.2 10.7 C2 L.G L.G L.G 41 259

Leon (*) (*) 3 - 20.8 12.9 C2 L.G L.G L.G 46 487

Guayaquil 1.5 3.0 4 C3 22.6 - C2 L - - 249 487

La Plata (*) 6.9 4 - 28.1 15.3 C3 - - - 34 294

Mexico (*) 13.6 3 C2-C3 9.7 6.6 C2 - - - 84 518

S.L.Potosi (*) 18.9 3 C2-C3 31.1 20.3 C3 - - - 49 1282

Sao Paulo (*) 57.8 5 - 20.6 8.3 C2 L.G L.G L.G 270 1020

(*) Apparently unpolluted atmosphere ( < 3 mg Cl- m-2 d-1 and < 10 mg SO2 m-2 d-1

(**) Based on climatological and pollution parameters;(*** ) Based on 1st year corrosion rate;

- Not available ;C2 - Low;C3 - Medium;L lepidocrocite ( -FeOOH);G goethite ( - FeOOH);

Table 7. Copper and aluminium corrosion rates (g/m2

a + standard deviation) [181]

.Exposure time (months)

Rural Coastal Urban - industrial

Outdoor Sheltered Ventilated shed Closed space Outdoor Sheltered Ventilated shed Outdoor Sheltered

6 27.4 + 1.6 9.8 + 0.8 3.5 + 0.2 0.2 + 0.04 5.3 + 0.7 44.5 + 0.5 19.8 + 4.5 23.9 + 1.8 6.8 + 1.512 19.4 + 0.4 11.5 + 0.5 1.9 + 0.3 0.2 + 0.02 34.6 + 0.04 54.1 + 2.6 47.2 + 3.7 19.8 + 0.04 8.5 + 0.418 14.3 + 0.6 8.6 + 0.7 1.9 + 0.3 2.0 + 1.2 30.3 + 2.0 72.4 + 3.5 33.1 + 0.5 15.0 + 0.5 7.5 + 0.7

Aluminium6 0.3 + 0.04 1.1+ 0.14 0.2 + 0.03 0.4 + 0.08 4.3 + 0.2 4.8 + 0.1 2.2 + 0.1 0.7 + 0.05 1.1 + 0.0412 0.3 + 0.02 1.2+ 0.14 0.3 + 0.01 0.3 + 0.10 3.2 + 0.03 4.0 + 0.4 1.6 + 0.3 0.6 + 0.05 1.7 + 0.1318 0.2 + 0.01 0.8 + 0.11 0.2 + 0.00 0.3 + 0.04 2.2 + 0.4 3.6 + 0.2 1.4 + 0.1 0.4 + 0.00 1.8 + 0.1


17/24



between them. The only common characteristic ofthese atmospheres was an increase in SCPLsprotectiveness with exposure time. Table 6summarizes the experimental results corresponding tocarbon steel surfaces exposed in rural and urbanatmospheres during the MICAT project.

Antonio, et al.,[181]

reports the atmosphericcorrosion of copper and aluminium exposed at threetest sites indoors and outdoors (coastal, urban-industrial and rural) under different exposureconditions up to 18 months based on the influence ofenvironmental parameters and main pollutants (SO2and chlorides) on the atmospheric corrosion of metalsat Cuba. The interaction between the chloridedeposition rate with the time of rainfall (outdoors)and with the time of wetness at temperature between5oC and 25oC (indoors) were found to be the mostsignificant variables influencing the corrosion of thetwo metals investigated; although other variables

appeared to be important in the corrosion processdepending on the metal nature. A classification of theatmospheric corrosion aggressivity of the test sitesbased both on environmental data and corrosion ratemeasurements was made according to ISO 9223. Thecorrosion aggressivity prognostic of this standard isnot always in agreement with the results obtained inCuban atmospheric conditions. Table 7 reported theaverage corrosion values for copper and aluminium,which corroded uniformly.

Indira, et al.[182] studied the atmospheric corrosionbehavior of galvanized steel and aluminium in marine

environment at Kochi, India for a period of one year(from August `98 to July `99). Monthly corrosionrates of these metals were determined by mass lossmethod. The pollutants like chloride and sulphurdioxide present in atmosphere have been estimatedperiodically. These values were correlated withcorrosion rate values. The rate of galvanized steel andaluminium are in the range of 0.0025 to 0.0314 mpyand 0 to 0.0014 mpy respectively. The durabilityvalues clearly indicated that non ferrous metals viz.,galvanized steel and aluminium have betterdurability. Table 8 gives the monthly corrosion ratesof galvanized steel and aluminium.

Lopez-Delgado et al.[171]

studied the coppercorrosion rate and corrosion products originated bythe action of acetic acid vapor at 100% relativehumidity. Copper plates were exposed to an aceticacid contaminated atmosphere for a period of 21 daysat Madrid, Spain. Five acetic vapor concentrationlevels were used. The copper corrosion rate was inthe range of 1 to 23 mg/dm2 day. Some of thecompounds identified were cuprite (Cu2O), copperacetate hydrate (Cu(CH3COO)2 2H2O), and copperhydroxide acetate (Cu4 OH(CH3COO)7. 2H2O). Thislast compound was also characterized. The thicknessof the patina layers was 4 to 8 nm for amorphous

cuprite, 11 to 48 nm for cuprite, and 225 nm forcopper acetate.

0

2

4

6

8

10

Winter Summer Rainy Winter

SEASON

1994-1995 1995-96

Figure 2. Average seasonal corrosion rate (mg/dm2) of aluminium.

Table 8. Atmospheric corrosion rates of galvanized steel andaluminum at Kochi marine site[182].

Corrosion rate is mpyS. No. Month Galvanized steel Aluminium

1 January 0.00466 Negligible2 February 0.0061 -do-3 March 0.0098 0.00058

4 April 0.0314 0.00145 May 0.0229 0.001326 June 0.0233 0.001217 July 0.0291 0.00128 August 0.007362 Nil9 September 0.00821 Nil10 October 0.00827 Nil11 November 0.0044 Nil12 December 0.0025 Nil

The patina, in which the cementation process ofdifferent corrosion-product layers plays an importantrole, is formed by the reaction of acetic vapor withcopper through porous cuprite paths.

Vashi and Patel[65] studied and determined thecorrosion rate of aluminium as well as the sulphationrate under outdoor exposure at Baroda, India(Petrochemical Complex Area, Central Gujarat)representing an industrial atmosphere. Monthlycorrosion rate of aluminium vary from 0.6 to 4.9mg/sq.dm (0.3 to 2.2 m/y). The values for yearlyrates being 25.5 to 37.6 mg/sq.dm (0.9 to 1.4m/y).Aluminium or aluminium coated sheets would,therefore, give better performance. Figure 2 showsaverage seasonal corrosion rate of aluminium.

Mohan, et al.,[183] studied the proper corrosion

preventive methods. Design engineers requirecorrosivity data of an area and the pollutants presentin atmosphere. The intensity of corrosion of variousmetals and the pollution data of different locations inthe southern region is reported. Table 9 shows thecorrosion rates of aluminium, copper and stainlesssteel 304 for 3 months, 6 months, 9 months and oneyear. Corrosion rate at Site 1, 2 and 3 which are oneastern coast of India are higher than the sites 4 and5, which are on western coast of India. Uniformlycorroding metal copper shows a slight decrease incorrosion rate during prolonged exposure whereas inthe case of aluminium and stainless steel 304 the

initial corrosion rate is very low and increase rapidlywith increase in time with the formation of pits.


18/24

S. Syed


Table 9. Corrosion rate of metals at various locations [183].

Corrosion rate in mm/yPeriod exposureAluminium Copper Stainless steel 304

Site I3 months 0.0007 0.03000 0.001506 months 0.007 0.02000 0.00500

9 months 0.0026 0.01800 0.00550One year 0.0029 0.01500 0.00500

Site II3 months - - -6 months - - -9 months - - -One year - - -

Site III3 months 0.0110 0.25500 0.000606 months 0.0092 0.01460 0.000609 months 0.0017 0.00512 0.00027One year 0.0020 0.00870 0.00040

Site IV3 months - - -

6 months 0.0001 - -9 months 0.0004 - -One year 0.0005 - -

Site V3 months 0.0002 - -6 months 0.0001 - -9 months 0.0004 - -One year 0.0014 - -

Vashi and Patel [184] reports the corrosion rates ofaluminium as well as the sulphation rate have beendetermined under outdoor conditions of exposure atSurat (South Gujarat) representing an industrialatmosphere. The monthly rates of outdoor corrosionof aluminium vary from 1 to 40 mg/sq.dm (0.44 to17.8 m/y), the values for yearly rates being 24 to 55mg/dq.dm (0.87 to 2.01 m/y). Aluminium oraluminium coates sheets would, therefore, give betterperformance. Figure 3 shows average seasonalcorrosion ra

Atmospheric Corrosion of Materials.pdf

Documents

Transcript of Atmospheric Corrosion of Materials.pdf