Effect of Cold‐Work on Corrosion of Iron and Steel in Hydrochloric Acid

Zone‐refined iron corrodes at the same rate whether cold‐worked or annealed. Cold‐working of iron containing 0.007–0.15% carbon increases the corrosion rate in , the rate increasing still more after heat treating the iron at 77°–100°C for 2 hr. Cold‐working of iron containing 0.01–0.02% nitrogen increases the corrosion rate only after heat treatment at 77°–200°C. Two maxima in the rate appear for heat treated N alloys but only one for C alloys, corresponding to precipitation of two different nitrides but only one iron carbide. Heat treatment above 200 °C reduces the corrosion rate for both alloys, but pure iron is not affected by any heat treatment schedule. Polarization measurements show that corrosion in all instances is controlled cathodically.

These results are explained by lower H₂ overvoltage of imperfection sites introduced by cold‐work, and associated with C or N atoms (Cottrell atmospheres). In absence of C and N, imperfection sites have the same apparent H₂ overvoltage as iron, and their tendency to dissolve anodically is not pronounced. The C and N atoms segregated at imperfections come from interstitial sources as well as through dissociation of carbides and nitrides, the compounds being less stable than the corresponding Cottrell atmospheres. Increase in corrosion caused by cold‐work, especially when followed by heat treatment at 77°–200°C, is in part explained by annealing out some imperfections, such as lattice vacancies, causing re‐formation of carbides or nitrides in finely divided form. Increased peripheral area of the precipitate increases galvanic action. Carbides dissociate more rapidly at low temperatures than do nitrides, accounting for the observed difference in behavior of Fe‐N and Fe‐C alloys. Effect of cold‐work on corrosion of metals in general is greatest when a second phase precipitates to form active galvanic cells, whereas the increase in internal energy of a disarrayed metal lattice has little if any effect. Preferred grain orientation of surface metal sometimes resulting from cold‐work may either increase or decrease corrosion.