8/2/2019 Ch_13_and_14

1/28

Lecture notes on Chapt. 13 and 14

Corrosion EngineeringCorrosion Engineering is the engineering design of corrosion control methodsand the solution of in-service problems, including:

Corrosion preventionMaterial selectionPaints and coatingsChemical treatments

Corrosion results in billions of dollars. The problem is largely avoidable byproper maintenance and protection methods. Corrosion prevention and controlprograms can help reduce the high cost of corrosion.

8/2/2019 Ch_13_and_14

2/28

Economics are a key consideration in corrosion engineering.

Develop and implementation of corrosion prevention and control maintenanceprocedures

Implement a monitoring program for early detection of incipient corrosionproblems.Reduce the impact of corrosion and develop cost-effective corrosion controlprograms

A corrosion Engineer has to provide technical services in four areas:

Prediction,Assessment and Diagnosis,TestingRemediation/Mitigation.

A Corrosion engineer should be able to:

Set up test programs,Analyzes information acquired from different sourcesConstruct profiles of corrosion problems.Suggest operating or maintenance schemes, create test programs forselecting new materials or altering operating conditions, and deviseremedial action plans for corrosion problems.

8/2/2019 Ch_13_and_14

3/28

8/2/2019 Ch_13_and_14

4/28

Active corrosion protectionThe aim of active corrosion protection is to influence the reactions which

proceed during corrosion, it being possible to control not only the packagecontents and the corrosive agent but also the reaction itself in such amanner that corrosion is avoided. Examples of such an approach are thedevelopment of corrosion-resistant alloys and the addition of inhibitors tothe aggressive medium.

Passive corrosion protectionIn passive corrosion protection, damage is prevented by mechanicallyisolating the package contents from the aggressive corrosive agents, forexample by using protective layers, films or other coatings. However, thistype of corrosion protection changes neither the general ability of thepackage contents to corrode, nor the aggressiveness of the corrosive agentand this is why this approach is known as passive corrosion protection. Ifthe protective layer, film etc. is destroyed at any point, corrosion may occurwithin a very short time.

8/2/2019 Ch_13_and_14

5/28

Permanent corrosion protectionThe purpose of permanent corrosion protection methods is mainly to provide protection at theplace of use. The stresses presented by climatic, biotic and chemical factors are relatively slight inthis situation. Machines are located, for example, in factory sheds and are thus protected fromextreme variations in temperature, which are frequently the cause of condensation. Examples ofpassive corrosion protection methods are:

Tin plating Galvanization Coating Enameling Copper plating

Temporary corrosion protectionThe stresses occurring during transport, handling and storage are much greater thanthose occurring at the place of use. Such stresses may be manifested, for example,as extreme variations in temperature, which result in a risk of condensation.Especially in maritime transport, the elevated salt content of the water and air in so-called salt aerosols (salt spray) may cause damage, as salts have a stronglycorrosion-promoting action. The following are the main temporary corrosionprotection methods:

http://www.tis-gdv.de/tis_e/verpack/korrosio/schutz/schutz.htmhttp://www.tis-gdv.de/tis_e/verpack/korrosio/schutz/schutz.htmhttp://www.tis-gdv.de/tis_e/verpack/korrosio/schutz/schutz.htm

8/2/2019 Ch_13_and_14

6/28

Protective coating method, Desiccant method and VCI method

1. Protective coating methodThe protective coating method is a passive corrosion protection method. The protective coating isolates the metallic surfacesfrom the aggressive media, such as moisture, salts, acids etc..

The following corrosion protection agents are used:

Solvent-based anticorrosion agentsVery high quality protective films are obtained.Once the anticorrosion agent has been applied, the solvent must vaporize so that the necessary protective film is formed.Depending upon the nature of the solvent and film thickness, this drying process may take as long as several hours. The thickerthe film, the longer the drying time. If the drying process is artificially accelerated, there may be problems with adhesionbetween the protective film and the metal surface.Since protective films are very thin and soft, attention must always be paid to the dropping point as there is a risk at elevatedtemperatures that the protective film will run off, especially from vertical surfaces.

Since solvent-based corrosion protection agents are often highly flammable, they may only be used in closed systems forreasons of occupational safety.

Water-based anticorrosion agentsWater-based anticorrosion agents contain no solvents and thus do not require closed systems.Drying times are shorter than for solvent-based anticorrosion agents.Due to their elevated water content, water-based anticorrosion agents are highly temperature-dependent (risk of freezing orincreased viscosity).The advantage of this method is that the protective film is readily removed, but the elevated water content, which may increaserelative humidity in packaging areas, is disadvantageous.

Corrosion-protective oils without solventCorrosion-protective oils without solvent produce only poor quality protective films. Good quality protection is achieved byadding inhibitors. Since these corrosion-protective oils are frequently high quality lubricating oils, they are primarily used forproviding corrosion protection in closed systems (engines etc.).

Dipping waxesT he protective layer is applied by dipping the item to be packaged into hot wax. Depending upon the type of wax, the temperature

may have to be in excess of 100C. Removal of the protective film is relatively simple as no solid bond is formed between the

wax and metal surface. Since application of dipping waxes is relatively complex, its use is limited to a few isolated applications.

http://www.tis-gdv.de/tis_e/verpack/korrosio/schutz/schutz.htmhttp://www.tis-gdv.de/tis_e/verpack/korrosio/schutz/schutz.htmhttp://www.tis-gdv.de/tis_e/verpack/korrosio/schutz/schutz.htmhttp://www.tis-gdv.de/tis_e/verpack/korrosio/schutz/schutz.htmhttp://www.tis-gdv.de/tis_e/verpack/korrosio/schutz/schutz.htmhttp://www.tis-gdv.de/tis_e/verpack/korrosio/schutz/schutz.htm

8/2/2019 Ch_13_and_14

7/28

2. Desiccant methodIntroduction

Desiccant bags are intended to protect the package contents from humidity during transport andstorage in order to prevent corrosion, mold growth and the like".The desiccant bags contain desiccants which absorb water vapor, are insoluble in water and arechemically inert, such as silica gel, aluminum silicate, alumina, blue gel, bentonite, molecularsieves etc. Due to the absorbency of the desiccants, humidity in the atmosphere of the packagemay be reduced, so eliminating the risk of corrosion. Since absorbency is finite, this method isonly possible if the package contents are enclosed in a heat sealed barrier layer which isimpermeable to water vapor. This is known as a climate-controlled or sealed package. If thebarrier layer is not impermeable to water vapor, further water vapor may enter from outside such

that the desiccant bags are relatively quickly saturated, without the relative humidity in thepackage being reduced.

Desiccants are commercially available in desiccant units."A desiccant unit is the quantity of desiccant which, at equilibrium with air at 23 2C, adsorbs thefollowing quantities of water vapor:min. 3.0 g at 20% relative humiditymin. 6.0 g at 40% relative humidity

The number of desiccant units is a measure of the adsorption capacity of the desiccant bag."Desiccants are supplied in bags of 1/6, 1/3, 1/2, 1, 2, 4, 8, 16, 32 or 80 units. They are available inlow-dusting and dust-tight forms. The latter are used if the package contents have particularrequirements in this respect.

8/2/2019 Ch_13_and_14

8/28

Barrier filmsBarrier films are available in various forms, for example as a polyethylene film or as a composite filmswith two outer polyethylene layers and an aluminum core. The composite film performs far better withregard to water vapor permeability (WVP), achieving WVP values of below 0.1 (g/md). In thecomposite film, the barrier layers are arranged so as to bring about a considerable reduction inpermeability in comparison with a single layer.

In accordance with current standards, water vapor permeability is always stated for both 20C and40C. According to information from the manufacturer, it may be concluded that water vaporpermeability rises with increasing temperature and falls with increasing thickness. This problemoccurs most particularly with polyethylene films, while aluminum composite films are largelyinsensitive to rises in temperature.

Placement of desiccant bagsThe desiccants should be suspended from strings in the upper part of the climate-controlled packageto ensure good air circulation around them.It is essential to avoid direct contact between the desiccant bag and the package contents as themoist desiccant would promote corrosion.It is advisable to use numerous small bags rather than fewer large ones, as this increases theavailable surface area of the desiccant and so improves adsorption of the water.

In order to ensure the longest possible duration of protection, the barrier film must be heat sealedimmediately once the desiccant bags have been inserted.

Desiccant bags are always supplied in certain basic package sizes which, depending upon thedesiccant unit size, may contain a single bag (of 80 units) or up to 100 bags (of 1/6 unit). The basicouter package should only be opened directly before removal of a bag and must immediately be heatsealed again.

8/2/2019 Ch_13_and_14

9/28

Comparison of advantages and disadvantages of the desiccant methodAdvantages

Desiccants provide excellent corrosion protection to both metallic and nonmetallicitemsRemoval of the desiccant on delivery to the receiver is straightforward, unlike theremoval of protective films in the protective coating method. The package contentsare immediately available.No particular occupational hygiene requirements apply as the desiccant is non-hazardous.

DisadvantagesPlacement of the desiccant bags and heat sealing of the barrier films are relativelylabor-intensive.The slightest damage to the barrier layer may negate the effectiveness of corrosionprotection.Calculating the required number of desiccant units is not entirely simple and it is easyto over-calculate. However, too much protection is better than too little.

Humidity indicators inside the package are not very reliable as they are only valid forcertain temperature ranges.

8/2/2019 Ch_13_and_14

10/28

3. VCI (Volatile Corrosion Inhibitor) methodMode of action and use

Inhibitors are substances capable of inhibiting or suppressing chemical reactions. They may be

considered the opposite to catalysts, which enable or accelerate certain reactions.Unlike the protective coating method, the VCI method is an active corrosion protection method, aschemical corrosion processes are actively influenced by inhibitors.In simple terms, the mode of action is as follows: the substance (applied onto paper supports or in apowder or spray formulation) passes relatively continuously into the gas phase and is deposited asa film onto the item to be protected. This change of state proceeds largely independently of ordinarytemperatures or humidity levels. The inhibitor inhibits corrosion in the aggressive, corrosive medium,suppressing either the anodic or cathodic half-reactions. Under certain circumstances, the period ofaction may extend to two years.

The mode of action dictates how VCI materials are used. At item to be protected is, for example,wrapped in VCI paper. The metallic surfaces of the item should be as clean as possible to ensurethe effectiveness of the method. The VCI material should be no further than 30 cm away from theitem to be protected. Approximately 40 g of active substances should be allowed per 1 m of airvolume. It is advisable to secure this volume in such a manner that the gas is not continuouslyremoved from the package due to air movement. This can be achieved by ensuring that thecontainer is as well sealed as possible, but airtight heat sealing, as in the desiccant method, is notrequired.The VCI method is primarily used for items made from steel, iron, nickel, chromium, aluminum and

copper, for which it provides good protection. The protective action or compatibility of inhibitors withspecific alloys must be clarified with the manufacturer. Ordinary commercial VCI materials provideno protection to zinc, cadmium, tin, tungsten or lead.

8/2/2019 Ch_13_and_14

11/28

Comparison of advantages and disadvantages of the VCI method

Advantages Since the gas also penetrates holes and cavities, these areas also receive

adequate protection. The period of action may extend to two years. The wrapping need not be provided with an airtight heat seal. On completion of transport, the packaged item need not be cleaned, but is

immediately available.

Disadvantages The VCI method is not suitable for all metals. It may cause considerable

damage to nonmetallic articles (plastics etc.). Most VCI active substances may present a hazard to health, so it is

advisable to have their harmlessness confirmed by the manufacturer and toobtain instructions for use.

8/2/2019 Ch_13_and_14

12/28

Different Corrosion protection methodsAnodizingAdvantagesA tough surface layer which has very good corrosion protectionproperties and very good adhesion to the surface.DisadvantagesMust be applied after welding or brazing if the joining areas are to beprotected. This can be complicated for large structures. Can not be doneon site.When to useFor protection against weathering and scratching/abrasion.

http://www.alu-info.dk/Html/alulib/modul/A00199.htmhttp://www.alu-info.dk/Html/alulib/modul/A00190.htmhttp://www.alu-info.dk/Html/alulib/modul/A00199.htm

8/2/2019 Ch_13_and_14

13/28

Conversion coatingAdvantagesA non costly and simple protection of the aluminum surface, which in additionincreases the adhesion of lacquers and adhesives.

DisadvantagesHas limited resistance to mechanical and thermal influence.When to usePrimarily used as a pre-treatment before lacquering or adhesive bonding.

LacqueringAdvantagesA high quality lacquering system has very good corrosion protectionproperties.Disadvantages

The performance of the lacquering system is very dependent on the qualityof the pre-treatment and application work.Relatively expensive.When to use lacqueringWhere appearance and/or corrosion performance is very important

http://www.alu-info.dk/Html/alulib/modul/A00197.htmhttp://www.alu-info.dk/Html/alulib/modul/A00198.htmhttp://www.alu-info.dk/Html/alulib/modul/A00198.htmhttp://www.alu-info.dk/Html/alulib/modul/A00197.htm

8/2/2019 Ch_13_and_14

14/28

InhibitorsAdvantages

Can be tailored to give excellent protection in specificenvironments.DisadvantagesExpensive to use with large amounts of liquid.May cause increased corrosion if incorrectly used.When to use inhibitors

For protection against internal corrosion in closed systems,circulating or non-circulating

Protective adhesive tapes

AdvantagesPrevents galvanic contact. Grease filled tapes will seal crevices.DisadvantagesCostly to apply. May need to be supported in place.When to useBuried pipelines

http://www.alu-info.dk/Html/alulib/modul/A00200.htmhttp://www.alu-info.dk/Html/alulib/modul/A00200.htm

8/2/2019 Ch_13_and_14

15/28

8/2/2019 Ch_13_and_14

16/28

Application of CP

Engineering and Design, Cathodic Protection, requires both cathodic protection(CP) and coatings, regardless of soil or water resistivity, for the followingburied or submerged ferrous metallic structures:

Natural gas and propane piping Liquid fuel piping

Oxygen piping Underground storage tank (UST) systems Fire protection piping Steel water tank interiors Ductile or cast iron pressurized piping under floor (slab on grade) in soil Underground heat distribution & chilled water piping in ferrous metallic

conduit in soils Other structures with hazardous products

8/2/2019 Ch_13_and_14

17/28

Schematic of an ImpressedCurrent CP System

In the impressed current CP,the large electrochemical isformed between an anode andthe structure to be protected bya power supply that iscontrolled by reading areference electrode close tothe structure.

8/2/2019 Ch_13_and_14

18/28

Impressed Current Cathodic Protection System

The system depicted above shows one way by which cathodic protection may be applied. In this system,power is drawn from the national grid and converted into a dc current by means of a transformer-rectifier. This is not the only method by which the dc current which is required may be supplied. In remoteareas, or parts of the world where a mains supply is not available, the driving force for the current is oftenprovided by a diesel generator, solar cell, ..

8/2/2019 Ch_13_and_14

19/28

Protection of underground tanks

8/2/2019 Ch_13_and_14

20/28

The basic principle of cathodic protection (CP) is simple. A metal dissolutionis reduced through the application of a cathodic current. Cathodic protection

is often applied to coated structures, with the coating providing the primaryform of corrosion protection. The CP current requirements tend to beexcessive for uncoated systems. The first application of CP datesback to 1824, long before its theoretical foundation was established.Cathodic protection has probably become the most widely used method forpreventing the corrosion deterioration of metallic structures in contact withany forms of electrolytically conducting environments, i.e. environmentscontaining enough ions to conduct electricity such as soils, seawater andbasically all natural waters.

Cathodic protection basically reduces the corrosion rate of a metallicstructure by reducing its corrosion potential, bringing the metal closer to animmune state. The two main methods of achieving this goal are by either:

Using sacrificial anodes with a corrosion potential lower than the metal tobe protected (see the seawater galvanic series) Using an impressed current provided by an external current source

http://www.corrosionsource.com/technicallibrary/corrdoctors/Modules/CP/cp-history.htmhttp://www.corrosionsource.com/technicallibrary/corrdoctors/Modules/CP/cp-history.htmhttp://www.corrosionsource.com/technicallibrary/corrdoctors/Modules/CP/cp-history.htmhttp://www.corrosionsource.com/technicallibrary/corrdoctors/Modules/CP/cp-history.htmhttp://www.corrosionsource.com/technicallibrary/corrdoctors/Modules/CP/cp-history.htm

8/2/2019 Ch_13_and_14

21/28

Design Procedures

1. Area to be protected

2. Polarized Potential

3. Current Demand

4. Anode Consumption 5. Anode # and distribution

6. Anode resistance

7. design output current

Use FEM and Laplace Eqn: d2V/dx2=0

8/2/2019 Ch_13_and_14

22/28

8/2/2019 Ch_13_and_14

23/28

Impressed current systems:

An alternative method of providing the current to protect a system isto use some sort of external power supply. As with the sacrificialsystem, the structure to be protected is made the cathode; thedifference being that the driving force behind the current is not thedifference in potential between the anode and cathode of the systembut from the power supply.

As the anode need not be less noble than the structure, the choiceof materials is wider. Examples of different anode materials are:PlatinumTitaniumGraphiteHigh Silicon Cast Iron

8/2/2019 Ch_13_and_14

24/28

Stray Current CorrosionWhenever a metallic structure is placed in the electric field between the

structure and the anode, it provides an alternative route for the electroncurrent path. Thus current can enter a foreign structure at one point andleave it at another location. At the interface on the foreign structure wherethe electrons move away from, corrosion is enhanced. This is known asstray current corrosion. This may be easily demonstrated in thelaboratory and may be explained using a modified galvanic corrosion

polarization diagram. Stray currents in soils could originate as well fromDC electrified rail tracks. A pipeline buried nearby could suffer straycurrent corrosion. The influence of high AC voltage overhead power lineson the corrosion of nearby structures is subject to substantialinvestigation.

Stray current corrosion refers to corrosion damage resulting fromcurrent flow other than in the intended circuit(s). For larger structures thisterm usually alludes to corrosion damage caused by extraneouscurrent(s) flowing through soil and / or water.

8/2/2019 Ch_13_and_14

25/28

8/2/2019 Ch_13_and_14

26/28

Consequences of OverprotectionIt is possible during cathodic protection to supply excess direct current topolarize a structure below the recommended protection potential. This state

of affairs is termed 'overprotection'. There are two main consequences ofoverprotection, namely, waste of current and more seriously the violation ofthe structural integrity of the metal. The waste of current is due to thepolarization of the metal below its equilibrium potential with the excesscurrent being used to evolve hydrogen. The gas produced could cause thedetachment of organic coatings and the removal of calcareous deposits in

offshore structures. Hydrogen production has also adverse effects on boththe corrosion fatigue life and hydrogen embrittlement properties of structuresespecially those made of high strength materials.During overprotection large amounts of hydroxyl ions are also produced. Onbare surfaces immersed in seawater, these could have a beneficial effectsince the hydroxyl species may passivate and /or enhance the formation of

calcareous deposits which in turn will reduce the current demand. Howeverfor organically coated surfaces the strong alkali condition at the metal surfacemay result in loss of adhesion for the paint. This phenomenon is known ascathodic disbonding.

8/2/2019 Ch_13_and_14

27/28

Introduction to Stray Current Corrosion

Stray currents which cause corrosion may originate from direct-current distribution lines,

substations, or street railway systems, etc., and flow into a pipe system or other steel structure.Alternating currents very rarely cause corrosion. The corrosion resulting from stray currents(external sources) is similar to that from galvanic cells (which generate their own current) butdifferent remedial measures may be indicated. In the electrolyte and at the metal-electrolyteinterfaces, chemical and electrical reactions occur and are the same as those in the galvaniccell; specifically, the corroding metal is again considered to be the anode from which currentleaves to flow to the cathode. Soil and water characteristics affect the corrosion rate in thesame manner as with galvanic-type corrosion.

However, stray current strengths may be much higher than those produced by galvanic cellsand, as a consequence, corrosion may be much more rapid. Another difference betweengalvanic-type currents and stray currents is that the latter are more likely to operate over longdistances since the anode and cathode are more likely to be remotely separated from oneanother. Seeking the path of least resistance, the stray current from a foreign installation maytravel along a pipeline causing severe corrosion where it leaves the line. Knowing when straycurrents are present becomes highly important when remedial measures are undertaken sincea simple sacrificial anode system is likely to be ineffectual in preventing corrosion under suchcircumstances.

8/2/2019 Ch_13_and_14

28/28

Consider how to protect a steel structure:

It has been empirically determined that the corrosion protectionfor mild steel is -840mV with reference to a copper/coppersulfate reference electrode.

What is needed is a metal that is less noble than steel to affordthis form of protection. Different applications favor differentmaterials, for example:

Submersed Marine Structures Zinc or Aluminum Buried Pipelines Magnesium

The amount of anode material used and the positioning of theanodes is determined by the individual application.

http://www.cp.umist.ac.uk/lecturenotes/JDS_Notes/rig.htmhttp://www.cp.umist.ac.uk/lecturenotes/JDS_Notes/pipe.htmhttp://www.cp.umist.ac.uk/lecturenotes/JDS_Notes/pipe.htmhttp://www.cp.umist.ac.uk/lecturenotes/JDS_Notes/pipe.htmhttp://www.cp.umist.ac.uk/lecturenotes/JDS_Notes/pipe.htmhttp://www.cp.umist.ac.uk/lecturenotes/JDS_Notes/pipe.htmhttp://www.cp.umist.ac.uk/lecturenotes/JDS_Notes/rig.htmhttp://www.cp.umist.ac.uk/lecturenotes/JDS_Notes/rig.htm