Corrosion failure investigation of seamless stainless steel tube used in die-casting machine of Mg alloy

• Sigma phase was related to the corrosion failure of the SS tube.
• Sigma phase was formed during the production process of the SS tube due to improper heat treatment.
• The corrosion of SS tube with sigma phase exhibited an intergranular corrosion.
• The corrosion of SS tube with sigma phase also exhibited pronounced oxide spallation.

The examination of a corrosion failure type 316 austenitic stainless steel tube, installed in one of the Mg alloy die-casting machines, is described. The metallographic examination manifested a large amount of sigma phase was related to the corrosion failure. A comparison is made with samples taken from the far end of corrosion pit of the failure tube, which operated at a same process. No precipitates were found in the contrastive samples. The results illustrated that the sigma phases were not formed during the operation process instead of the production process of the steel tube. The results are discussed in terms of the contribution of sigma phase to the corrosion performance. In addition, the corrosion mechanism of AISI 316 stainless steel with sigma phase was also discussed in details. In high temperature supercritical atmosphere environment, it exhibited an obvious intergranular corrosion and pronounced oxide spallation.