NACE Resource Center
|Corrosion - Design|
Corrosion Performance of Metals
The following numbers refer to the corrosion rates, expressed in terms of depth of metal removed in unit time, for mild steel and various stainless steels in different acids.
Penetration rates (mm/year) for various steels in different acids
Clearly any of the stainless steels would provide better life than mild steel in hot concentrated nitric acid, but in hydrochloric acid only the 12 per cent Chromium 12 per cent Nickel steel is significantly better than the mild steel, which in the sulfuric acid is better than any of the stainless steels, which in the sulfuric acid is better than any of the stainless steels, costing about an order of magnitude more per unit weight.
These differences are largely related to the differing oxidizing characteristics of the acids, the corrosion resistance of stainless steels mainly depending upon the formation of a protective chromium oxide film upon their surfaces, so that in the absence of an adequate supply of oxygen, as with the hydrochloric acids, the chromium addition is of little benefit. Essentially the same point can be seen in relation to the atmospheric corrosion of metals, that good corrosion resistance in a particular atmosphere is no guarantee of good resistance in other circumstances, as the following figures indicate.
('Cor-Ten' is a proprietary material developed by United States Steel Company, and is one of a series of low alloy steels known as 'weathering steels'.)
The weathering steel, 'Cor-Ten', owes its improved corrosion resistance over mild steel in industrial and rural atmospheres to the development of an adherent rust film due to the incorporation of low-concentrations of certain alloying elements, but the relative improvement in those atmospheres is not nearly so marked in a marine atmosphere. When immersed in sea water 'Cor-Ten' is not significantly better than mild steel and its use is not recommended in such conditions, although there are examples of it having been so used, no doubt because of its corrosion resistance in other situations and the assumption that this will be good irrespective of the environment.
The same point an be made in relation to other forms of corrosion; thus molybdenum additions to ferritic steels are beneficial with respect to stress corrosion cracking resistance in carbonate environments, but are detrimental in hydroxide solutions, and so on. The general point to be grasped is that in specifying the operating conditions for a component or structure it is vital that the environmental conditions be defined with as much precision as any other parameters, such as stress, temperature etc. Only by so doing can the selection of a material from the corrosion resistance viewpoint be made satisfactorily.