8/8/2019 Spr06_p24-26 What is Corrosion

1/3

24 The Electrochemical Society Interface Spring 2006

What is

Corrosion?by Barbara A. Shaw and Robert G. Kelly

Corrosion is degradation of materials properties due to interactions

with their environments, and corrosion of most metals (and manymaterials for that matter) is inevitable. While primarily associatedwith metallic materials, all material types are susceptible todegradation. Degradation of polymeric insulating coatings onwiring has been a concern in aging aircraft. Even ceramics canundergo degradation by selective dissolution. Like death andtaxes, corrosion is something we hope to avoid; but ultimately it issomething we must learn to deal with. The fundamental cause ordriving force for all corrosion is the lowering of a systems Gibbsenergy. As Fig. 1 illustrates, the production of almost all metals(and engineering components made of metals) involves adding

energy to the system. As a result of this uphill thermodynamicstruggle, the metal has a strong driving force to return to itsnative, low energy oxide state. This return to the native oxidestate is what we call corrosion and even though it is inevitable,substantial barriers (corrosion control methods) can be used toslow its progress toward the equilibrium state. Thus it is the rate ofthe approach to equilibrium that is often of interest. This rate iscontrolled not only by the nature of the metal surface, but also bythe nature of the environment as well as the evolution of both.

In light of the thermodynamic basis

for corrosion it is not surprising that costsassociated with corrosion are high. Severalstudies over the past 30 years have shown

that the annual direct cost of corrosion toan industrial economy is approximately3.1% of the countrys Gross NationalProduct (GNP). In the United States, thisamounts to over $276 B per year.1 AsFig. 2 reveals, the highest segments ofthe cost of corrosion are associated withutilities, transportation, and infrastructure.The Department of Defense alone hascorrosion costs of $20 B. Because of thesignificant economic, safety, and historical

impact of corrosion on society and because

corrosion of metals is an electrochemical

process, it is also not surprising that the

Corrosion Division is one of the oldestdivisions within ECS. The Division wasestablished in 1942, but corrosion has

been an important topic in the Societysince 1903. Reviews of the early literatureand history of the Division were preparedby Uhlig for the 50th and 75th anniversariesof the Society2,3 and a centennial reviewby Isaacs was published more recently.4Readers looking for a good overall sourceof corrosion information are encouragedto consult Uhligs Corrosion Handbook.5

Most corrosion processes involve at leasttwo electrochemical reactions (one anodicand one cathodic). A corroding surface

can be thought of as a short-circuited

battery; the dissolution reaction at theanode supplies electrons for the reduction

reaction at the cathode. A short circuitis the electrical connection made by aconductor between the two physical sites,which are often separated by very smalldistances. Thus, the study of corrosionprocesses involves the use of many of thesame tools that electrochemists studyingbatteries, fuel cells, and physical and

analytical electrochemistry use. Theapplication of mixed potential theoryto corrosion was originally presented

by Wagner and Traud6 and discussedlater in theJournal of The ElectrochemicalSocietyby Petrocelli.7 In 1957, Stern andGeary theoretically analyzed the shapeof polarization curves providing the basisfor the primary experimental technique(electrochemical polarization) used inelectrochemical studies of corrosion.8 Theformation of surface oxide films is criticalin mitigating the rate of metal dissolution,

so corrosionists have much in commonwith those studying dielectrics for otherpurposes. It is these thin (< 10 nm) nativeoxide films that make the technologicaluse of metallic materials possible byserving as barriers to dissolution.

Traditionally, corrosion is classifiedinto eight categories based on themorphology of the attack, as well asthe type of environment to which thematerial is exposed.9 Uniform or generalcorrosion is the most prevalent type ofcorrosion and examples of this type of

corrosion abound; including rusting ofsteel bridges, rusting of undergroundpipelines, tarnishing of silver, and patina

formation on copper roofs and bronzestatues. Anyone who has left a pieceof unprotected steel outside is familiarwith uniform corrosion. Fortunately,uniform corrosion is predictable and canbe controlled by various methods such aspainting the surface or applying a layer ofa sacrificial metal like zinc to steel. This

sacrificial corrosion of the zinc surfacelayer to protect the underlying steel isactually a form of galvanic or bimetalliccorrosion. In this case, like in a battery, weare using corrosion to our advantage. Thesurfaces of some metals (like aluminum,stainless steel, and titanium) are protectedfrom uniform corrosion by an extremelythin oxide film that forms naturally. Manypractical applications of materials dependon the presence of this protective oxide.

We would not be able to use planes (or justabout any other structures for that matter)made from aluminum if it were not forthis thin protective film. Unfortunately,this film can breakdown locally, resultingin forms of corrosion like pitting ofaluminum plates, crevice corrosion ofstainless steel fasteners, or stress corrosioncracking of pipes in nuclear reactors.Protection of structures and equipment

(continued on next page)

8/8/2019 Spr06_p24-26 What is Corrosion

2/3

The Electrochemical Society Interface Spring 2006 25

chemical, electrical, information), as wellas economists. Accurate models requireaccurate representations of processesover ten orders of magnitude in size (