1.05 Basic Concepts of Corrosion - Elsevier

1 Concepts of CorrosionL. L. Shreir{This article is a revision of the Third Edition article by L. L. Shreir, volume 1, pp 1:3 1:15, 2010 Elsevier of of Approach to Corrosion as a Chemical Reaction at a Metal Environment of of A Classification of Corrosion Processes96 Existing Classifications96 Dry Corrosion97 Wet Corrosion98 Corrosion in Organic Solvents98 Suggested Classification and IntroductionModern technology has at its disposal a wide range ofconstructional materials metals and alloys, plastics,rubber, ceramics, composites, wood, etc.}

2 , and the selec-tion of an appropriate material for a given application isthe important responsibility of the design engineer. Nogeneral rule governs the choice of a particular materialfor a specific purpose, and a logical decision involves aconsideration of the relevant properties, ease of fabri-cation, availability, relative costs, etc. of a variety ofmaterials; often, the ultimate decision is determined byeconomics rather than by properties, and ideally, thematerial selected should be the cheapest possible thathas adequate properties to fulfill the specific metals are involved, mechanical, physical,andchemicalproperties must be considered, and in thisconnection, it should be observed that while mechani-cal and physical properties can be expressed in terms ofconstants.

3 The chemical properties of a given metal aredependent entirely on the precise environmental con-ditions prevailing during service. The relative impor-tance of mechanical, physical, and chemical propertiesdepends, in any given case, on the application of themetal. For example, for railway lines, elasticity, tensilestrength, hardness, and abrasion resistance are of majorimportance, whereas electrical conductivity is of pri-mary significance in electrical transmission. In the caseof heat-exchanger tubes, good thermal conductivityis necessary, but this may be outweighed in certainenvironments by chemical properties in relation tothe aggressiveness of the two fluids involved thus,although the thermal conductivityof copper is superiorto that of aluminum brass or the cupronickels, thealloys are preferred when high-velocity seawater isused as the coolant.

4 As copper has very poor chemicalproperties under these a metal or alloy may be selected largely onthe basis of its mechanical or physical properties, thefact remains that there are very few applications wherethe effect of the interaction of a metal with its envi-ronment can be completely ignored, although theimportance of this interaction is of varying signifi-cance according to circumstances; for example, theslow uniform wastage of a steel structure of massivecross-section (such as railway lines or sleepers) is of farless importance than the rapid perforation of a buriedsteel pipe or the sudden failure of a vital stressed steelcomponent in sodium hydroxide effect of the metal environment interaction onthe environment itself is often more important than theactual deterioration of the metal.

5 For instance, leadpipes cannot be used for conveying plumbo-solvent{ , since a lead level of> ppm is toxic; similarly,galvanized steel may not be used for certain foodstuffsowing to the toxicity of zinc salts. In many chemicalprocesses, the selection of a particular metal may bedetermined by the need to avoid the contamination ofthe environment with traces of metallic impurities thatwould affect the color or taste of products or catalyzeundesirable reactions; therefore, copper and copperalloys cannot be used in soap manufacture, since tracesof copper ions result in the coloration and rancidifica-tion of the soap.}

6 In these circumstances, it is essential touse unreactive and relatively expensive metals, eventhough the environment would not result in the rapiddeterioration of cheaper metals such as mild further possibility is that the contamination of theenvironment by metal ions due to the Corrosion of onemetal can result in an enhanced Corrosion of anotherwhen the two are in contact with the same environ-ment. Thus, the slow uniform Corrosion of copper by acuprosolvent domestic water may not be particularlydeleterious to copper plumbing, but it can result inthe rapid pitting and consequent perforation of galva-nized steel and aluminum that subsequently comesinto contact with the copper-containing , it is necessary to point out that for a numberof applications, metals are selected in preference toother materials because of their visual appearance, andfor this reason.

7 It is essential that brightness and reflec-tivity are retained during exposure to the atmosphere;stainless steel is now widely used for architectural pur-poses, and for outdoor exposure, the surface mustremain bright and rust-free without periodic the other hand, the slow-weathering steels, whichreact with the constituents of the atmosphere to form anadherent uniform coating of rust, are now being used forcladding buildings, in spite of the fact that a rustysurface is usually regarded as aesthetically interaction of a metal or alloy (or a nonmetallicmaterial)

8 With its environment is clearly of vital impor-tance in the performance of materials of construction,and the fact that the present work is largely confined toa detailed consideration of such interactions couldcreate the impression that this was the sole factor ofimportance in material selection. This, of course, is notthe case, although it is probably true that this factor ismost neglected by the design Definitions of CorrosionIn the case of nonmetallic materials, the termcorrosioninvariably refers to their deterioration from chemicalcauses, but a similar concept is not necessarilyapplicable to metals.

9 Many authorities1considerthat the termmetallic corrosionembraces all interac-tions of a metal or alloy (solid or liquid) with itsenvironment irrespective of whether this is deliberateand beneficial or adventitious and deleterious. Thus,this definition of Corrosion , which, for convenience, isreferred to as thetransformationdefinition, includes,for example, the deliberate anodic dissolution of zincin cathodic protection and electroplating as well asthe spontaneous gradual wastage of zinc roofingsheet, resulting from atmospheric the other hand,corrosionhas been defined2as the undesirable deterioration of a metal or alloy, thatis.

10 An interaction of the metal with its environmentthat adversely affects those properties of the metalthat are to be preserved. This definition which isreferred to as thedeteriorationdefinition is alsoapplicable to nonmetallic materials such as glass,concrete, etc. and embodies the concept that corro-sion is always deleterious. However, the restriction ofthe definition to undesirable chemical reactions of ametal results in anomalies that become apparent froma consideration of the following , when exposed to an industrial atmosphere,reacts to form the reaction product rust of approximatecomposition Fe2O3 H2O, which, being loosely adher-ent, does not form a protective barrier that isolates themetal from the environment; the reaction thus proceedsat an approximately linear rate until the metal is com-pletely consumed.