Salt spray corrosion behaviour of austenitic stainless steel matrix composites

This work deals with the possibility of using intermetallics as addition to P/M stainless steel in order to try to sinter these steels in nitrogen-base atmospheres. 316L, was chosen as stainless steel matrix, and two intermetallics (from Ti-Al and Cr-Al binary systems), with a sieve size of less than 80 mum, were added in the amount of 3% vol. to obtain MMCs. Powders were mixed, compacted at 700 MPa by uniaxial compacting, and then sintered at two temperatures (1120 and 1230 degreesC) in five different atmospheres (95N(2)/5H(2), 80N(2)/20H(2), 25N(2)/75H(2), hydrogen and vacuum). A complete microstructural study was carried out both by optical and scanning electron microscopy (SEM). Corrosion tests by salt spray fog were done in order to measure the possible improvements of intermetallic addition on the corrosion behaviour of these steels. SEM studies were also carried out on as corroded samples in order to understand the mechanisms of corrosion. Intermetallics absorb nitrogen from the nitrogen based atmospheres, and they develop a duplex (ferrite / austenite) microstructure when composite materials are sintered in hydrogen and vacuum. These microstructural features explain the results obtained in salt spray fog test.