Effect of Nitrogen and Phosphorus on the Stress Corrosion Cracking of Austenitic Steels in MgCl 2 Solutions

Abstract

The influence of phosphorus and nitrogen on the stress corrosion cracking behaviour of austenitic stainless steels similar to Type 316 (but with variations in nickel and molybdenum contents) has been investigated. Nitrogen additions decreased the critical stress for stress corrosion crack (SCC) nucleation but did not increase the maximum crack growth rate in Type 316 tested in MgCl₂ at 154°c. In a molybdenum-free steel, phosphorus additions were far more deleterious than nitrogen additions and gave increased cracking rates and nucleation frequencies.

The stacking fault energy was only decreased a little by the phosphorus addition and nitrogen tended to increase the stacking fault energy, so that changes in SCC cannot be attributed simply to stacking fault energy changes. An ordered carbide type structure was observed in a slow-cooled specimen containing a very high nitrogen content. Only slight freckling contrast and no precipitate diffractions were observed in the range of nitrogen contents usedfor the SCC tests. The strengthening by nitrogen and the slip planarity observed were similar from 13 wt% nickel to 30 wt% nickel. However no cracking was observed in the alloy with 30% nickel. Whilst the change in dislocation behaviour may account for some increase in SCC susceptibility at 13 and 20 wt% nickel, slip planarity is not sufficient to produce crack growth at high nickel contents.

The phosphorus addition gave little hardening and little increase in slip planarity. It does, however, have a marked effect on the slip step oxide growth characteristics, producing tunnels rather than slots. It seems, therefore, that the effect of phosphorus is mainly a chemical one, that is, on the rate of dissolution or on the nature of the oxide film, rather than on the dislocation behaviour.