The nitrate cracking of high purity iron.

The mechanism of intergranular stress-corrosion cracking of a high purity iron in nitrate solutions (nitrate cracking) has been investigated. The work considers the role of the nitrate ion in promoting this attack in an inherently susceptible material, and incorporates a study of corrosion in unstressed specimens, and the influence on stress-corrosion behaviour of various environmental parameters including cation, temperature, and concentration, as well as the effects of applied potential and additional ions. Metallurgical aspects are considered by comparing the stress-corrosion behaviour of high purity iron with two pure iron-based alloys - Fe/1 wt% Mn, which can be made resistant or susceptible according to heat treatment, and Fe/1 wt% Cr, which is totally immune. It is shown that, for cracking to occur, a critical combination of environmental and metallurgical factors must be present. The specimen life is divided into three stages; (i) incubation period, (ii) crack initiation period, (iii) crack propagation period. Stage (i) is connected with the formation of a partially protective oxide film on the specimen surface, and its brittle fracture coincident with a sensitive grain boundary owing to the yielding of the specimen under the conditions of the test. Stage (ii) concerns the formation of an "embryo" crack which propagates in stage (iii) by a cyclic sequence of electrochemical (yield-assisted dissolution) and mechanical (brittle fracture) stages. Tests under applied anodic potential correlate well with isolated tests for the various materials and suggest that a grain boundary must be inherently subject to electrochemical attack in order to provide the conditions for stress-corrosion crack propagation. Furthermore, tests under applied potential might provide a suitable alternative test for nitrate cracking using unstressed specimens. Finally, suggestions for future work, and the prevention of nitrate cracking are made