metallic corrosion - sept'10

8/8/2019 Metallic Corrosion - Sept'10

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the bodhi tree the bodhi tree

MetallicCorrosionMetallic

Corrosion

S E P T E M B E R 2 0 1 0

S h a r i n g K n o w l e d g e

All materials have an expected life and its replacement over a period of time is inevitable. In general, any material wou perform to its expected lifespan only when the conditions it is exposed to, are favourable, else it would corrode. It may be theresafely assumed that all materials will eventually suffer from some form of deterioration or the other.

Reasons for material deterioration may vary; however, the most common reason for a metal failure is its eventual corrosion

Corrosion may be defined as destruction or deterioration of a material due its reaction with the surrounding environment, whto some degree is always corrosive.

In order to understand the potential causes of unsatisfactory material performance, one should have a high degree of awarenregarding the material design, specification and installation combinations. Also, along with these aspects, awareness regardcorrosion is also important.

This issue of Bodhi Tree, therefore, is an attempt to understand the basics of the metallic corrosion.

Project Name ICI Vietnam, Location Vietnam


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Most metals occur naturally in chemical combination with other elements and are usually chemically stable but may not besuitable for commercial usages. Extracted metal/alloy, whenexposed to oxygen and other oxidizing agents, try to revert totheir natural state. This natural reversal is Metallic Corrosion.Man made materials change into its constituents due to chemicalor electrochemical reactions with its surroundings, starting at itssurface. Many structural alloys corrode merely from exposure tomoisture in the air, but the corrosion rate (Fig. 1) is stronglyaffected by certain factors/substances in the environment.

All metals exhibit a tendency to corrosion; some corrode moreeasily than others. A tabulation of the relative strength of thistendency is called the Galvanic Series (Please refer Table 1).The position of a metal in the series also suggests its potentialusefulness for structural and other applications.

Table 1: Galvanic Series of Metals (and Alloys) in Sea Water

NOBLE (CATHODIC)

GoldPlatinumTitaniumGraphiteSilver Stainless steel*Copper-Nickel*Bronzes*Copper

Brasses*TinLeadCast IronCarbon SteelCadmium

AluminiumZincMagnesium AlloysMagnesium

ACTIVE (ANODIC)

*- Different alloy chemistry may result in different position in theGalvanic Series table.

Corrosion TypesGalvanic/Bi-metallic Corrosion : Galvanic Corrosion is of major interest where water is in contact with metal. For GalvanicCorrosion, an electrically conductive path (two metals inelectrical contact) and a path for movement of ions (immersionin an electrolyte) are required. As the name Bi-metallicsuggests, this type of corrosion will always involve 2 dissimilar non-compatible metals. In the process, more active metal (as inGalvanic Series) corrodes at higher rate than the noble metal.Galvanic corrosion is often utilised in sacrificial anodes e.g.

usage of Zinc as a sacrificial anode for steel structures (pipelinesor docked naval ships).

Pitting Corrosion : Pitting corrosion is usually found whereresistance against general corrosion is present, except at therusting points. Pitting corrosion occurs due to localisedconditions such as high concentrations of salt/ localisedbreaking of passive film, which are favourable for corrosion e.g.chlorides in contact with stainless steel/aluminised steel. Thesetiny local fluctuations may degrade the protective oxide film.Corrosion at these points gets greatly enhanced and holes aredeveloped. It may be noted that, although pitting occurs due tosevere local conditions, it may continue, even after theseconditions are not there.

Corrosion is usually defined as the loss of a metal or its properties, caused due to a reaction with its surrounding environment. In other words, corrosion is the wearing

out of the metal due to a chemical reaction. Corrosion occurs in various forms ranging from a generalised attack over the entire surface to a localised one. Corrosion

in case of materials other than metals, such as ceramics or polymers, may be termed as degradation.

Time of Wetness

Corrosion Rate Airborne SalinitySO

Special Factors

Metallic Corrosion

Fig.1-Factors influencing corrosion rate.

Active discharge from Copper tubecausing rusting of coated steel sheet. Stainless steel screw causingcorrosion of metallic coated steel


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Crevice Corrosion : Crevice corrosion is another localised formof corrosion. It occurs in spaces where the access of the workingfluid from the environment is limited and varying oxygenconcentration is present. These spaces are generally calledcrevices. Examples of crevices are gaps and contact areas

between parts, under gaskets or seals, seams/lap joints etc. Aluminised steel/Aluminium is prone to Crevice Corrosion inmarine atmosphere.

Microbial Corrosion : Microbial corrosion or bacterial corrosionis caused or promoted by microorganism. It can happen to bothmetallic and non-metallic materials, irrespective of the presenceor lack of oxygen. In the presence of oxygen, some bacteriadirectly oxidize iron to iron oxides and hydroxides, while other bacteria oxidises Sulphur and produces Sulphuric Acid, causingcorrosion. Non-metals, nylon etc. are degraded by plastic eatingbacteria.

High Temperature Corrosion : This non-galvanic form of corrosion can occur when a metal is subjected to a hightemperature atmosphere that contains oxygen, sulphur or other gases. For example, materials used in Aerospace, Power Generation and even in car engines get exposed to anatmosphere containing corrosive products of combustion at hightemperature.

At times, the by-products of high temperature corrosion can alsoprove to be advantageous. The formation of oxides on stainlesssteels/carbon steel for example, provides a protective layer thatprevents further atmospheric attack at both room and hightemperature.

Intergranular Corrosion : Metal is made up of a large number of "crystal grains" with regularity in atoms. Between the twograins lies the grain boundary. Misalignment amongst atoms of one grain to other may occur at these grain boundaries.

Intergranular corrosion is a form of corrosion that occurs at thegrain boundaries. It occurs due to thermal processes causing

change in constituents at grain boundaries within themicrostructure of the metal. For example: depletion of Chromiumat grain boundaries in stainless steel during welding/ improper heat treatment, renders it less resistant to corrosion at thesepoints.

Rusting : Rusting is the most common form of corrosion. Metals

may undergo corrosion, but the resulting oxides are notcommonly referred to as rust. This is because, rusting is a termused for corrosion of iron and steel products only. Iron/steelproducts react with oxygen in the presence of water or air moisture and form hydrated iron oxides, commonly known asrust. There are also other forms of rust, such as the green rustwhich is a result of the reaction of iron with chlorine. White rustis corrosion of zinc in the coating on steel. Generally rust has a

much higher volume than that of the original mass of metal, andits build-up may also cause failures.

Steel is relatively unaffected by pure water or by dry oxygenalone. As in case of hot rolled/blued steel, a tightly adheringoxide coating- passivation layer, protects the bulk steel from

further oxidation. However, when in contact with water andoxygen, or other strong oxidants and/or acids, steel will rust.Similarly in the presence of salt, for example, near sea, it tendsto rust more quickly due to the electro-chemical reactions. Other degrading solutions are sulphur dioxide / carbon dioxide inwater. Under these corrosive conditions, hydrated oxides areformed. And these oxides (rust) are not passivating becausethese do not adhere to the bulk metal. Rust is permeable to air and water, therefore, the interior iron/steel continues to corrodeuntil all of the steel or oxygen or water in the system is removedor consumed.

Crevice corrosion at lock seam

White rust Red rust

Intergranular corrosion

Project Name Inter Auto, Location India


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Materials most resistant to corrosion are those for whichcorrosion is unfavourable. Any corrosion products of gold &platinum tend to decompose spontaneously into pure metal,which is why these elements can be found in metallic form.Some metals have naturally slow reaction kinetics, even thoughtheir corrosion is thermodynamically favourable. These includemetals such as Zinc, Magnesium and Cadmium. While corrosionof these metals is continuous and ongoing, it happens at anacceptably slow rate.

More common metals or alloys such as steel need to beprotected. In most cases, it is impossible or economicallyimpractical to completely arrest the corrosion process. However,it is usually possible to control the process to acceptable levels.This can be achieved by various ways for different material withdifferent results in different environments.

Barrier Protection : Corrosion can be controlled with the help of impervious coatings e.g. coatings produced by painting/plating/anodising/ bluing, etc. These coatings isolate the metal from theenvironment. They work by providing a barrier of corrosion-resistant material between the damaging environment and thematerial (Fig.2). At places that are not covered or with damagedcoating, corrosion may set in.

Passivation : Passivation is another type of barrier protectionprovided by a very thin film. It could be achieved by self passivation of material or by an externally applied solution. Inself passivation, a thin film of corrosion products on a metalsurface acts as a barrier to prevent further corrosion. When thisthin film stops growing at less than a micrometre in thickness,the phenomenon is known as passivation. In case of external

passivation, a thin film of external solution passivates thematerial e.g. chrome passivation of metallic coated steel.

Dehumidification : Another method to avoid corrosion is bycontrolling the environment. Controlling the humidity, if possible,below a certain threshold level can help reduce the rate of corrosion process.

Design Considerations : Corrosion can also be controlled byproper design. Some points for consideration towards the sameare:

Unwashed Areas : In a building design, there may be certainareas which will not be exposed to general rain washing. Theseunwashed areas have a build up of dust and dirt, whichincreases the time of wetness on the material surface. The

Mecha nica l Bar rie r be twee nst eel a nd atmosp here

Br eaks in coa ting r es ultin cor r osi on t o ba se s tee l

O Pa int

Rus t

Base Metal (Fe)

O O

Paint

Base Metal (Fe)

Corrosion Resistance / Prevention / Protection

Fig.2-Barrier protection mechanism

Project Name - Sungai Buloh Hospital, Location Malaysia


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time of wetness is an important factor governing the rate of corrosion. An increase in the time of wetness will increase therate of corrosion. In case there is a presence of salts/SO2 inthese deposits, the corrosion will worsen. The underside of eaves, fascia linings, vertical walls protected by overhangingsections, the top sections of garage doors, etc. are all typical

examples of such unwashed areas. Minimisation of these areaswill lead to increased life.

End Lapping : Metal sheets are manufactured by a continuousprocess. Therefore, it is possible to have sheet lengthsmanufactured (at the building site) such that there is no end lap.

However, when shorter sheet lengths are to be used, certainprecautions are required to prevent the early deterioration at thelapped joint. This potential problem can be avoided byappropriate use of neutral cure silicone rubber sealant.

Slip Sheets : Sometimes slip sheets may be used at the pointof corroded old roof. However, it must be noted that coated steelslip sheets will corrode when in contact with rusted or bare steel.

Pitch Transition Flashing : In all instances where a change of pitch occurs, the appropriate change of pitch flashing should bechosen in order to correctly flash the joint. Care should be takenthat while waterproofing a transition joint, the overlapping sheetshould not be simply fastened firmly down onto the underlyingsheet. If it is done so, thermally induced stresses will remove thecoating on the underlying sheet at the point of contact and willresult in rapid corrosion due to the loss of sacrificial metalliccoating.

Low Pitched Roofing : Laying of sheet at a pitch lower thanrequired for the respective profile type causes a possibility of water movement through capillary action, penetrating the sidelaps. This not only results in damage of the ceiling structure dueto water but also corrosion of the roof sheets.

Expansion Joints : Expansion joints are provided in order toovercome longitudinal thermal movement encountered on a longroof, in order to avoid structural damage to the roof sheeting.Care must be taken while designing and installing the expansion

joint, so as to adequately flash it and prevent water and dirt fromentering into the lapped joint.

Supporting Steel Structure : Coated (both pre-painted &metallic coated) steel roofing and walling must be installed onstructure coated with an appropriate compatible material. This isessential since, the corrosion of uncoated steel will also causecorrosion of the coated steel in contact.

Ponding : Ponding causes deterioration of the sheet coatingand leads to perforation. Care must be taken not to block anyvalleys or trays that would prevent the water from draining.Gutters are most affected due to ponding of water.

Use Of Compatible Materials : Bi-metallic corrosion isobserved when non-compatible materials come in directcontact or when overflow of water (e.g. from Copper to Zncontaining coating) occurs.

In case of corrosion due to overflow, the noble metal corrodesslightly and dissolves in water which subsequently flows over aless noble material and may then deposit on it forming a bi-metallic contact. A typical example of this can also be seen withstainless steel screws causing corrosion of metallic (bare/pre-painted) coated steel around the screw. This is further enhancedif the washer contains carbon black (highly conductive) even inthe absence of a noble or cathodic screw. Thus, use of washersfree of carbon black, is advised.

Sacrificial /Cathodic/Galvanic ProtectionThis is the most commonly used method of corrosion protection.

It is achieved by placing the metal in contact with another, moreeasily corroding metal (Fig.3).

This leads to increased corrosion of the more active metal anddecreased corrosion of the more noble metal. Cathodicprotection continues till the sacrificing metal is present. Thebiggest advantage of sacrificial protection is the protection of areas which are not directly covered e.g. scratches/cut

ends/piercings. Sacrificial anodes are made from material with amore negative electrode potential than the cathode (e.g. zincacts as anode to iron/steel).

Anod ic Reaction

M2+ + 2e -2M

Zn 2+ + 2e -Zn

e -

M2+

Cathodic Reaction

O2+ 2H

2O+ 4 e - 4OH -

OH -

Metal

Steel

Fig.3-Sacrificial protection mechanism

Improper end lapping


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Our endeavour is to make The Bodhi Tree, a know ledge-sharing platform.Please send us your feedback and comments to help make the next issue more interesting and meaningful.

We look forward to he aring from you at our dedicated email id'thebodhitree@ tatabluescopesteel.com'.

It may be concluded that corrosion reactions are generallygoverned by the available moisture and oxygen in theenvironment. It may be delayed but cannot be avoided. Whiledesign and installation practices play an important role in theprocess of corrosion, the use of compatible materials helps

reduce corrosion. Metallic corrosion and its rate is influenced byproperties of metal, time of wetness and the constituents of environment it is exposed to. Further contamination of theenvironment by corrosive products (may exist or be derivedfrom industrial or marine condition), accelerate corrosion of the

material. Galvanic series is a tool to select the sacrificialprotecting metal/alloy. For steel, corrosion is due to iron trying toreturn to the original hydrated iron oxide or iron oxide state.

Although many other metals form their oxides when corrosionoccurs, rusting is a term reserved only for the corrosion of Iron

and Steel. Zinc & Zinc containing e.g. 55% Al-Zn alloy coatingprotects steel by way of barrier as well as sacrificial protection.The 55% Al-Zn alloy coating provides up to 4 times morecorrosion protection to the steel as compared to Zinc coating of equal thickness, in a near neutral environment.

Conclusion

Project Name BSC Suzhou Factory, Location China


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Tata BlueScope Steel Limited

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metallic corrosion - sept'10

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