SCC and Atom Probe Studies of 304 Stainless Steel as a Function of Palladium and Ruthenium Alloying Additions

Stress-Corrosion Cracking (SCC) represents significant challenges to the long-term reliability and availability of structural and pressure bearing components in many industries.  The primary research objectives of this programme is to provide a fundamental understanding of the SCC mechanisms of 304 stainless steel modified with platinum group metal (PGM) alloying additions. Although limited work has been conducted on stainless steels, PGMs are known to profoundly influence the corrosion behaviour of titanium, and ferritic steels in acid environments.  Type 304L austenitic stainless steel has regulatory approval for many applications and minor changes to the alloy while remaining within the overall specification avoid costly regulatory re-approval.

SCC experiments were performed at ambient temperature in acidified potassium tetrathionate solutions to replicate features of failures which occur in plant (e.g., intergranular nature of SCC in sensitised material). These tests were used as a screening method to rank the SCC resistance of the PGM-modified alloys subjected to a ‘sensitising’ heat treatment of 650C for 24 hrs.  PGM alloys based on Pd additions showed no improvement over the SCC failure times of the industrial control while results for the PGM alloys based on Ru additions indicate an improved resistance to SCC.

Microstructural investigations using high resolution analytical TEM indicate that chromium depletion levels are present at grain boundaries in both the 304 industrial standard and PGM modified alloys and it is reasonable to conclude that levels of chromium depletion do not appear to be the controlling factor for the improved performance of Ru alloys.  Scanning atom probe analysis was used to perform 3D compositional analysis along a grain boundary in a palladium alloyed specimen. Palladium alloying additions prove to have no observable influence on the SCC resistance of as Pd is depleted locally at grain boundaries due to formation of MnPd intermetallic precipitates.