Corrosion in Various Environments and Their Role

8/3/2019 Corrosion in Various Environments and Their Role

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Various Corrosive

Environments And Their Role

In Corrosion

Prepared By

Sajan Brahmbhatt

Roll No. 936

B.E. IV-Metallurgy


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There are various environments in which deterioration of material takesplace some of the prominent environments are listed and discussedbelow:

Atmospheric Corrosion

Corrosion in Fresh Water and Other Waters

Corrosion in Seawater

Corrosion in Soils

Corrosion in Concrete

Atmospheric Corrosion

Environmental Factors and Their Effects:

Atmospheric corrosion is electrochemical corrosion in a system that consists of a metallic material,corrosion products and possibly other deposits, a surface layer of water (often more or lesspolluted), and the atmosphere. The general cathodic reaction is reduction of oxygen, whichdiffuses through the surface layer of water and deposits.

The three main factors,i.e. time of wetness, composition of electrolyte, and temperature, depend for one thing on the typeof atmosphere:

1. Rural (inland): dry environment with little or no pollution2. Marine: on and by the sea, with high humidity and chlorides

3. Urban: polluted by exhaust, smoke and soot4. Industrial: highly polluted by industry smoke and precipitate


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Climatic conditions are also important. For instance, the corrosion conditions may be severe in thetropical zone due to high temperature and particularly at the coast high humidity.

If the metal surfaces are completely clean and smooth, and there is no pollution of theatmosphere, significant corrosion does not occur at a relative humidity below 100%.

In reality, however, materials like steel, copper, zinc and nickel corrode when the relativehumidity exceeds 5070%. This is partly due to corrosion products.

If the surface in addition is contaminated by chloride, soot or dust particles, which arehygroscopic, the corrosion rate is considerable at lower relative humidity than when itexceeds 60%.

Industrial and urban atmospheres contain more or less solid particles consisting of carbon,soot, sand, oxides, and salts, e.g. chloride and sulphate.

Many of these substances attract moisture from the air; some of them also attract polluting

and corrosive gases.

The salts cause high conductivity, and carbon particles can lead to a large number of smallgalvanic elements because the particles act as efficient cathodes after deposition on thesurface.

Rain affects the corrosion rate in different and opposite ways. Some effects stimulatecorrosion, but rain may also wash away pollutants and thus reduce the corrosionrate.

The most significant pollutant in the form of a gas is SO2, originating from combustion of oil,gas or coal containing some sulphur. Together with water, SO2 forms sulphuric acid,H2SO4. Therefore, metallic materials that corrode strongly in acids are not suitable without acoating in industrial atmospheres with high contents of SO2.

In marine atmospheres measured corrosion rates are related to the rates of chloridedeposition.

Automobile corrosion worries a huge number of people and is of large economicsignificance. Internal cavities in doors and nearly closed sections are critical places.This is due to varying temperature and bad ventilation, leading to formation and

conservation of condensate, i.e. long time of wetness.

Atmospheric Corrosion on Different Materials

o The corrosion rate of steeldepends strongly on the type of atmosphere.

o Some alloying elements also have a large effect on the corrosion rate.

o For instance, small amounts of Cu, Cr and Ni cause the rust layer formed during service tobe denser, so that the corrosion rate becomes relatively low after some time. Steels alloyed

in this way are the so-called weathering steels, of which Cor-Ten steel is a well-knownproduct. The rust layer is often decorative with nice colors, and such steels are used in frontpanels for buildings.


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o Several aluminum alloysshow very good corrosion resistance in various atmospheres.

o Extensive galvanic corrosion may occur on Aluminum in contact with copper, mild steel (inmarine atmosphere) and graphite, less in contact with stainless steel, while aluminum iscompatible with Zinc.

o Stainless steelbehaves mainly in accordance with its name under atmospheric conditions.

o For comparison, corrosion rates in different waters are also shown in the table. The lowrates of corrosion on zinc are due to protecting surface films of basic zinc chloride andcarbonate.

o Copper and copper alloysare highly resistant to atmospheric corrosion because of surfacefilms mainly composed of basic copper salts.

o Tinas well as nickeland nickel alloysalso corrode at similar rates.

o Leadpossesses excellent corrosion resistance in atmospheres due to surface protecting

films (insoluble sulphate, sulphide, carbonate and oxide).

The most important methods for preventing atmospheric corrosion are

1) To select a corrosion-resistant structural material,

2) To use a corrosion-resistant coating (metallic, inorganic, organic or composite),

3) To close internal cavities in order to avoid access of humid air, or conversely,

4) To make sure that there is ample ventilation and drainage,

5) To supply oil, grease, or special substances

Corrosion in Fresh Water and Other Waters

This group of environments comprises primarily fresh water in lakes, rivers and brooks, rain andground water. In addition there are waters contaminated in someway, or with some matter added.A common condition for corrosion in natural waters is the presence of oxygen.

o The rate-determining step is often diffusion of oxygen to the metal surface. This isparticularly the case for corrosion of unalloyed or low-alloy steels. At least partly it appliesalso to zincwhen pH < 89, where reduction of hydrogen may play an additional role. Onthe other hand, materials like aluminumand stainless steels(and Zn at higher pH values)are under anodic control (passivation).


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o Contrary to atmospheric corrosion, corrosion in natural waters is hardly dependent on thegrade of iron, unalloyed and low alloy steel, because composition and treatment of the steelin this case have little effect on the barrier properties of surface layers of corrosionproducts.

o In the pH ranges < 4 and > 10, however, the composition of the steel or iron is significantbecause the corrosion process involves hydrogen evolution and passivation, respectively.

o At low pH values, the corrosion rate increases with increasing contents of C, N, P and S,with cold working.

o The reason is depolarization, partly of the hydrogen reduction and partly of the anodicdissolution reaction. The effects of some other elements, such as As, Mn, Cu, Cr and Ni,depend on the concentration.

o The properties of steel and other metallic materials in acidic environments are of interestwhen applied to ground water with sulphate-reducing bacteria and water contaminated byH2S.

o Oxygen concentration, temperature and flow velocityhave large effects on oxygen transportto the metal surface and consequently on the corrosion rate in natural waters. In addition,variation in corrosion rate is also caused by the composition of the water, since this maystrongly affect the barrier properties of surface layers.

o Natural waters contain different amounts of Ca and Mg salts. If the salt content is high, thewater is called hard, in the opposite case it is soft.

o At pH < 4, iron corrodes faster because of hydrogen reduction, while the lower corrosion

rate at pH > 10 may partly be due to diffusion-limiting deposition (see below) and partly topassivation.

o Hard waters are known to be less corrosive than soft ones.o The reason for this is that, when the salt content becomes high enough, deposits

particularly CaCO3 are formed, which hinder the oxygen diffusion to the metal surface.

o When the measured pH is higher than pHs, i.e. the saturation index SI is positive (the wateris supersaturated with CaCO3), CaCO3 is deposited.

o Good correlation has been found between SI and corrosively of the water.

o Acid salts, i.e. salts that hydrolyze and give acid solutions, lead to corrosion undercombined hydrogen evolution and oxygen reduction to the extent that may be expected atthe actual pH. The oxygen reduction reaction may be more efficient than usual.Examples of such salts are AlCl3 and FeCl3.

o Ammonium salts(e.g. NH4Cl) are also acidic, and they give higher corrosion rates thanother salts at the same pH. This applies particularly to ammonium nitrate, which in thepresence of NH3 may give extreme corrosion rates.

o Alkaline salts(e.g. Na3PO4, Na2SiO3, and Na2CO3), which hydrolyze and give alkaline

solutions, act as corrosion inhibitors by passivation of iron and steels under access byoxygen. Oxide is formed, but in some cases one get a more or less passivizing salt layer inaddition (phosphate, silicate).


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o Oxidizing saltsmay either lead to rapid corrosion by providing an extra cathodic reaction(as do FeCl3, and CuCl2), or to inhibition and passivation.

Organic matter in the water may affect corrosion in different ways. Substances that existsuspended or dissolved may be deposited and form more or less voluminous layers in regionswith low flow velocity. This may give a reduced average corrosion rate, but increased andsometimes heavy localized corrosion, i.e. deposit corrosion.In hot tubes, such layers may also result in local overheating and crack formation.

In pipes with smaller diameters, such deposits may reduce the effective crosssection stronglyand sometimes block the pipe completely. Deposit corrosion takes place underneath the tubercles.Several cases of corrosion attacks in which bacteria have played a significant role have beenexperienced, e.g. in cooling water systems and heat exchangers.

Corrosion in public water piping systems is responsible for large economic loss.In pipes of cast iron, steel and other metallic materials,Corrosion may be prevented by use of coatings or by water treatment (addition of calciumcompounds, alkalization or carbonation).

Corrosion in Seawater

The importance of seawater as a corrosive environment has increased during the last fewdecades because of offshore exploration of oil and gas. There is also an increasing application ofseawater for production of fresh water in parts of the world where this is in short supply.

Corrosion is a major problem in desalination plants. Seawater has a chloride content

that gives maximum corrosion rate.

Mechanism:

o The corrosion rate is usually not particularly large because the water contains Ca and Mgsalts. The bulk pH in seawater is commonly 88.3, but due to cathodic production of OH,the pH value at the surface increases sufficiently for deposition of CaCO3 and possibly to asmaller extent some Mg(OH)2together with iron hydroxide.

o The deposits form a surface layer that reduces oxygen diffusion so much that the average

corrosion rate on steel in seawater (0.1

0.15 mm/year) is less than in soft fresh water.

Because the corrosion rate depends on the supply of oxygen, it will be highest in the splash zonewhere a thin water film exists a major proportion of the time, and where parts of the corrosionproduct layer frequently are washed away. On offshore structures the splash zone is the mostdifficult zone to protect satisfactorily, because it is not possible to make use of cathodic protection.Thick organic coatings (e.g. Neoprene) orMonel sheathing have been used in many cases.

Metals in seawater are exposed to marine growth (fouling). The fouling tendency is verydifferent for different metals and increases in the following order:

1. Copper and 9010 coppernickel.2. Brass and bronze.3. 7030 coppernickel, Aluminum, Bronze and Zinc.4. Nickelcopper.5. Carbon steel, low-alloy steels, stainless steels, nickel alloys and titanium.


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Some Important methods for preventing sea water corrosion

o Chlorination is frequently carried out, primarily by adding sodium hypochlorite. In stainlesssteel systems, amounts corresponding to 12 ppm Cl2 are often added continuously.

o By this, slime formation is prevented, but at the same time the potential is raised, and therisk for initiation of crevice corrosion is increased.

o Recent research shows that the resistance to crevice corrosion initiation on high-alloystainless steel increases with the time in chlorinated seawater.

o If moderate chlorination is applied in the beginning, with increasing addition of sodiumhypochlorite after some time, the risk for initiation of corrosion will be lower than if fullchlorination was applied from the start.

o Corrosion in seawater may also occur on metallic materials that originally are protected bythinner or thicker organic coatings.

o The reason for this is that water, oxygen and chlorides diffuse through the coating.Undermining corrosion starts and the coating will slowly be broken down.

o One example of this is corrosion on sea cables. The outer layers of a sea cable consist of ametallic reinforcement usually steel wires with hot-dip zinc coating and an external layerof jute and asphalt or polypropylene and asphalt.

Corrosion in Soils

Soils can be classified on the basis of particle size.

o Gravel contains the coarsest Particles (> 2 mm) and clay the finest ones (< 0.002 mm), withsand and silt in between.

o Soils containing the finest particles, with ample distribution of small particle sizes are verydense and prevent supply of oxygen (but not of water), while gravel allows oxygen to betransported easily.

Most metallic materials that are used in soils corrode under cathodic control, i.e. under control ofoxygen transport. Thus, the density of the soilis important.

The relationship is, however, somewhat complicated: the corrosion rate is often lower inporous than in more compact soil, because preventive deposits may be formed more easilyin the former case.

Where the density varies along the metal surface, differential aeration cells may be establishedwith concentrated corrosion at areas with a dense soil.

There are also other important environmental factors: other conditions affecting water

and oxygen supply, resistivity, dissolved salts, pH, microbiological activity and strayelectric currents.

Generally, aggressive soils have low resistivity. In corrosive sediments it may be up to20004000 ohm cm, in little or non corrosive 40006000 ohm cm or more.


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Various biological factors affect corrosion in soils.

o Organic acids originating from humus are relatively corrosive versus steel, zinc, lead andcopper.

o In certain soil types, particularly acidic clay, very high corrosion rates have beenexperienced under anaerobic conditions. This is commonly caused by sulphate-reducingbacteria, which is indicated by ferrous sulphide found as part of the corrosion products.

o On the critical surface area of the pipeline, the current runs from metal to environment byanodic dissolution of the metal, giving high corrosion rates under unfavorable conditions.

o Stray currents may originate from railways, electrolysis plants, electric cables and largeelectric plants. Generally, direct current causes the greatest problem. Alternating Straycurrent has usually little effect on steel, but it may attack lead to a great extent. Straycurrent originating from a cathodic protection system may cause localized corrosion onsteel pipelines.

Composition of carbon steel and low-alloy steel is of little significance for corrosion in soils,as is the case for corrosion in seawater and other waters.

A problem of particularly large economic significance is corrosion on buried pipelines, andthere are special reasons to worry about the external corrosion.

o Examples from some communities show that a major part of the water in the public freshwater pipeline system gets lost due to leakage, mainly because of corrosion.

o The leakages cause damage to other property, the capacity demand of the waste watersystem increases, costly repair is needed, and such work may conflict with other activities

and traffic. To prevent this, one of the external coating systems that have beenrecommended for water pipelines in soil is 25 m zinc + asphalt or Bitumen.

o Generally, the most important measures for corrosion prevention in soils are use of organiccoatings (coal tar, plastics or rubber (6 mm), wound tape), inorganic coatings (cementmortar or enamel, but these are brittle and may easily be damaged mechanically), fiber-reinforced cement mortar, metallic coatings (hot-dip or thermal spray zinc), filling bylimestone or other calcium-containing matter, and cathodic protection.

o The combination of reinforced coal tar coating and cathodic protection has been usedextensively to protect pipelines and tanks.

Corrosion in Concrete

The development and building of large concrete structures for offshore oil and gas production inthe 1970s called for increased attention to corrosion of steel reinforcement, and of seawater-

exposed steel parts in metallic contact with the reinforcement.Concrete constitutes an alkaline environment for the embedded steel reinforcement, which leadsto passivation of the steel. As passivized steel is liable to localized corrosion in environments withsufficient concentration of an aggressive substance like chloride.


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o A crucial corrosion mechanism is therefore the transport of chloride and oxygen through theouter layer of concrete, which covers the reinforcement.

o At an early stage of the actual research it was assumed and shown by laboratoryexperiments that intensive corrosion might occur on

i) reinforcement steel bars at cracks in theconcrete cover,ii) On seawater-exposed steel parts that were metallically connected to the reinforcement,because these cases most often implied a large area ratio between the reinforcement(cathode) and the exposed steel (anode). Sites without cracks but with extensive penetration ofchloride through the concrete cover were also assumed to be strongly liable to corrosion due tolocal breakdown of the passive oxide layer on the steel bars. There is still some uncertainty asregards risk for galvanic corrosion on exposed steel parts. This problem may exist, particularlydue to efficient aeration of the concrete in the splash zone.

In harbor structures, bridge structures and buildings, many cases of extensive damage due tocorrosion of reinforcement have occurred during recent years.

For structures exposed to the atmosphere there is, in addition to possible localized breakdown dueto chloride, a second important mechanism: carbonation of the concrete, which reduces the pH of

the solution in pores and leads to general breakdown of passivity.

Besides localized or general surface corrosion of the reinforcing bars, stress corrosion cracking orcorrosion fatigue may take place, which emphasizes the severity of the attacks. A major cause ofcorrosion in these structures is that they have not been subject to the same strict rules as offshorestructures with respect with concrete quality and cover. One of the problems is connected withconcrete decks on road bridges, where salt containing a high proportion of chlorides may originatefrom de-icing or from the sea.In addition to mechanical repair (replacement of old concrete and application of surface coatings),electrochemical methods are used for rehabilitation of concrete structures nowadays.Electrochemical methods comprise chloride removal, re-alkalization and cathodic protection.

Advancement is that improved concrete with low permeability has been developed in recent years.In addition to the mentioned protection methods, i.e. use of high-quality concrete, sufficient cover,external organic coating or sealing, and cathodic protection, possible methods for prevention ofreinforcement corrosion in new concrete structures include also the application of a protectionmeasure directly on to the reinforcement:

i) hot-dip zinc coating, with varying experience as to the effectiveness,ii) Powder epoxy coating, e.g. for bridge decks,iii) Reinforcing barsiv) Corrosion inhibitors, primarily in high quality concrete with a thick cover.

Reference

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