Effect of molybdenum additions on the microstructures and corrosion behaviours of 316L stainless steel-based alloys

Alloys were made by alloying 5, 10, 15, 17.5 and 20?wt-% Mo with Type 316L stainless steel. Sigma phases containing 21–29?wt-% Mo formed along the austenite grain boundaries with the addition of 5?wt-% Mo and increased with additions up to 15?wt-% Mo, but they decreased with further additions. Laves phases containing 33–40?wt-% Mo co-precipitated at additions of 10?wt-% Mo which increased with further Mo increases. The corrosion resistance, assessed by potentiodynamic polarisation in a 10?mM NaCl solution adjusted to pH 4, increased relative to Type 316L for alloys made with 5 and 10?wt-% added Mo, but decreased with further additions due to preferential corrosion of the Laves phase. The alloy made with 10?wt-% added Mo had the highest corrosion resistance due primarily to the high Mo content of the austenite.