On the role of phosphorus in the caustic stress corrosion cracking of low alloy steels

• Published in: Corrosion science 1989, Vol.29 (1), p.53,65-63,68
• Main Author: Briant, C.L.
• Format: Article
• Language: English
• Subjects: alloys; Applied sciences; Corrosion; Corrosion mechanisms; crack propagation; Cross-disciplinary physics: materials science; rheology; Exact sciences and technology; Materials science; Metals, semimetals and alloys; Metals. Metallurgy; Physics; Specific materials; steel; stress corrosion cracking; tensile strength
• ISSN: 0010-938X; 1879-0496
• DOI: 10.1016/0010-938X(89)90080-2

This paper reports on a study of the role of phosphorus in the caustic stress corrosion cracking of NiCrMoV and NiCrV steels. The study was performed by carrying out constant load stress corrosion tests on smooth bar tensile samples for fixed lengths of time. The samples were tested in an aqueous solution of 9 M NaOH at a potential of −400 mV(Hg/HgO). After the tests were completed, the samples were sectioned metallographically and the cracks were examined. The results showed that phosphorus, when segregated to the grain boundaries, accelerates anodic dissolution at the crack tip. In samples in which this segregation had occurred, the cracks tended to be narrower. Both factors would lead to accelerated times to failure in stress corrosion tests. When the overall cracking process was examined, it was found that after the test begins there is a long initiation period before any cracking is observed. This period is shorter in steels that have phosphorus segregated to their grain boundaries than in those that do not. During this initiation time an oxide grows on the surface of the sample. Metallography showed that the oxide contains cracks and that often these cracks are associated with a small amount of penetration into the metal. When the crack in the oxide happens to occur over a grain boundary that contains segregated phosphorus, the penetration achieved after each rupture event is especially great. Eventually cracks will form along grain boundaries in the metal and the stress intensification associated with them causes the velocity of crack growth to increase. This velocity continues to increase as the cracks become longer. When failure does occur, only a few cracks have progressed to a depth that is sufficient to produce failure.