Previous work by the present authors has described the influence of a range of variables on atmospheric chloride-induced stress corrosion initiation behavior of Type 304 stainless steels using polished C-rings1. This paper extends the previous studies in order to establish the effect of surface condition on the initiation of SCC. Three machining conditions were utilized in this study, each using different machining combinations in terms of material removal, either by lathe turning or turning followed by grinding. These conditions all produced significantly greater crack initiation than was seen under the same conditions on polished specimens, although the ground surface condition had less effect than the other two. The results highlight the importance of surface condition in the initiation and prevention of SCC in plant.
Atmospheric chloride-induced stress-corrosion cracking (SCC) of 300 series austenitic stainless steels has been observed in many plants across the world operating in maritime or marine environments. This phenomenon is most commonly observed at elevated temperatures (>60°C) but SCC has been observed on plants which have not operated above ambient temperature. It is well known that the SCC susceptibility can be significantly influenced by the manufacturing process through the action of surface irregularities as stress raisers and also due to the possible influence of surface cold-worked layers. This paper presents the results of a study of the effects of relative humidity, temperature, material sensitization, chloride deposition, applied stress and surface condition on SCC initiation and extends previously published work by the authors using polished C-ring specimens. A range of surface conditions have been produced by lathe turning the external surface of type 304H stainless steel pipe. The surface condition was characterised as a function of manufacturing variables: spindle speed, feed rate and depth of cut. The C-ring specimens were coated with known concentrations of magnesium chloride and exposed to humid air in an environmental test chamber. The SCC susceptibility as a function of surface condition was characterised by optical examination using a digitising microscope stage to measure the positions, lengths and numbers of cracks initiated after a set exposure (1000 hours). The susceptibility to SCC was determined as a function of stress, sensitization and surface salt concentration and was compared to previous data on polished specimens. Surface condition of material was found to be of great importance in the initiation or avoidance of cracking. Three surface-modified conditions were tested: high feed rate, deep cut; high feed rate, shallow cut; and low feed rate, shallow cut, then ground. (The amount of material removed was the same in each case. It proved impossible to generate a consistent finish with low feed rate without polishing or grinding.) Both high lathe feed rate and high cut depth were found to induce more extensive crack initiation, and the number and length of cracks was found to decrease with increasing sensitization for surface modified specimens in the same manner as with polished specimens. Increasing the stress applied to specimens by 30% was found to approximately double the number of cracks initiating.
