The theory of metallic corrosion in the light of quantitative measurements. Part VI.— The distribution of corrosion

In previous parts of this research it has been shown that the rate of oxygen supply to the whole metal surface is the factor which generally controls the rates of corrosion of zinc and mild steel in the conditions studied, though other factors may affect them appreciably in special conditions. Such factors are the evolution of hydrogen and the formation of protective films or masses of corrosion products which may result from increased alkalinity, or other causes. Neither the basic potential of the metal nor the conductivity of the electrolyte exercised control over a wide range of salt concentration. Corrosion distribution was at first sporadic but was soon concentrated at a few large irregularly shaped areas. For many purposes distribution is more important than the total amount of corrosion, especially when it is so localized as to be called pitting. The irregular distribution of corrosion was formerly believed to be caused by chemical and physical heterogeneity of the metal, but differential aeration is now usually believed to be a more frequent cause of local corrosion and this view was accepted in Parts I to III of this research.