APPENDIX F: THE ATMOSPHERIC CORROSION CHEMISTRY OF IRON AND LOW ALLOY STEELS

Abstract

The chemical complexities of the interaction of a surface with its environment are nowhere better illustrated that in the case of iron and its alloys. The corrosion of these metals has been studied for at least two centuries. Nonetheless, the chemical processes by which iron is degraded in use are fully understood only in simplified laboratory simulations and in the simplest and cleanest of real environments. This is particularly true for steel products put into field use within the past half century and thus subjected to environments containing a wider variety and greater concentrations of reactive trace constituents than in earlier times. Although iron, the carbon steels (iron with ≤1% carbon and small and varying amounts of Mn, S, P, and Si), and the weathering steels (iron with <1% Cu, Mn, Si, Ni, and Cr) differ in composition, their interactions with corrosive atmospheres demonstrate more similarities than differences. In particular, corrosion rates and surface morphologies vary, but many of the reactive atmospheric species and many of the corrosion products are the same. Such a picture is less appropriate for the stainless steels, since their passive surfaces and ability to repassivate after attack are unique within the steels. As a consequence, we concentrate on the low alloy steels, and the discussion of the applicability of our analysis to stainless steels is deferred until near the end of this appendix. F.2 FORMaTiOn RaTeS FOR RUST LaYeRS The formation of rust on iron and steels is a chemically complex process. For classification purposes, it is possible to identify three stages in the development of common corrosion layers. First is the formation of a thin oxide/hydroxide film