How Zinc Can Protect Steel

Steel is so versatile that it is used in all industries, but it is particularly vulnerable to corrosion. The wide range of uses has greatly extended, at an economical cost, the use of zinc to prevent corrosion. 

Fundamentally, corrosion is an electrochemical process. That is, it is accompanied and accelerated by the passage of very small electric currents between the corroding metal and any other metal with which it is in electrical contact or between different areas on the surface of the corroding metal. For these currents to flow, a potential difference must exist, either between the two pieces of metal or between the different parts of the same piece, and moisture or other electrolyte must be present on the surface to act as a conductor for the current. Potential differences sufficient to cause current flow can arise from very small local variations in the chemical constitution of the surface phase differences across a grain boundary (e.g., between ferrite and pearlite or cementite) are quite sufficient to constitute an electrolytic cell. 

Electrolytic cells set up in this way comprise a series of positive anodes and negative cathodes between which currents flow and at which electrochemical reactions take place. The reactions at the cathodes may be regarded as electron-consuming and those at the anodes as electron-producing. The actual disintegration and corrosion are associated with loss of electrons and so occur at the positive anodic areas. 

The simplest way of hindering the corrosion processes at a metal surface is to seal it off by an impermeable and adherent barrier coating, which denies access to moisture and oxygen. The barrier coatings used on iron and steel need to be strongly resistant to the corrosive influence of their environ- 

Zinc can fulfill these requirements and can be applied to an iron or steel surface in a variety of ways and also at different stages in the manufacture of products. Zinc is, however, more than a barrier coating—as discussed later. 

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Rain or high relative humidity (which can cause condensation) increases corrosion of zinc that is roughly proportional to the time of wetness in a particular environment. Factors that determine time of wetness have been discussed by Grossman (1978). Temperature generally has only an indirect effect in air the relative humidity usually drops as temperature increases. At humidities exceeding 75%, corrosion rates rise perceptibly. A drop in temperature has the opposite effect. In solutions, reaction rates increase with temperature but—more important—in hot tap water there is a potential reversal between zinc and steel typically between 60 and 90°C, which then limits the benefits of a zinc coating to its barrier effect. In addition, localized pitting may occur, as noted in Section IV (How Zinc Can Protect Steel). 

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