The interrelationship of corrosion rate of steel, dissolved oxygen concentration, agitation, electrode potential, and cathodic protection current density were studied under controlled laboratory conditions. While the corrosion rate of steel is essentially linear with amount of dissolved oxygen, the rate appears to fall almost to zero near 1.0 ppm dissolved oxygen rather than at zero ppm dissolved oxygen as commonly assumed. In the region below 2 ppm dissolved oxygen, there is almost no effect of agitation on corrosion rate; above 2 ppm there is a large effect. By measuring the corrosion rate of an unprotected specimen, it should be possible to compute the minimum current density needed for cathodic protection in the field.
In acidic environments, less current is needed for protection than is predicted by analysis of the mechanism of cathodic protection. This fact suggests that the anodic reaction in low pH corrosion proceeds by the removal of “chunks” of metal which contain several atoms. The “chunk effect” also appears to operate in the neutral range, but only when the corrosion rate is very high.
The effect of differential aeration on corrosion rates is discussed in detail. 5.2.4
