Investigation of Localized Corrosion Resistance for Surface Treated Martensitic Stainless Steels

Seawater corrosion remains a constant challenge for any metal-based components expected to be used in or near marine environments. Corrosion in less visible components, such as bearings, can be troublesome, as their degradation is not easily noticed until engine system performance is reduced or parts begin to fail. This work was conducted to investigate the effect of surface and heat treatments on the corrosion resistant properties of 13% Cr carburizable martensitic stainless steel (UNS 42670). Processing treatments considered include carburizing, carbonitriding and final annealing temperature. Samples of other aerospace bearing steels were tested for comparison. AC and DC electrochemical testing were conducted in a 350 mL cylindrical cell at ambient temperature and oxygen partial pressure. Samples were submerged in 0.1 M NaCl solution for polarization tests and in ASTM D 1141 synthetic seawater for electrochemical impedance spectroscopy (EIS) tests. SEM was then carried out with a Hitachi S-3400N Scanning electron microscope to observe corrosion damage morphology and elemental composition. Results of the electrochemical tests show the samples with lower final annealing temperatures had better performance. These samples had the lowest average corrosion rate in both NaCl solution and synthetic seawater. SEM images of these samples exhibited relatively less damage and build-up of corrosion products compared to the other surface treatments considered