The Corrosion Resistance of P/M Stainless Steels and Selected Alloys in Methanol-Based Fuels

The change from gasoline to alternate fuels such as those based on methanol, is expected to create material compatibility problems because of the enhanced reactivity of the powder metal (P/M) materials currently in use. These problems are most serious for the steels containing copper or those that have been copper infiltrated. P/M austenitic stainless steels offer the possibility of overcoming the inherent corrosion problems of the current P/M alloys. Test samples of 304L and 316L were processed on production equipment and sintered in pure hydrogen or a simulated dissociated ammonia mixture. Corrosion testing was performed in SAE-approved mixtures of “aggressive methanol” and gasoline (termed CM15A and CM85A). To accelerate the corrosion test and simulate an auto-oxidized fuel mixture, a small amount of t-Butyl Hydroperoxide was added. These preliminary tests confirm that Fe-0.8 %C and Fe-2% Cu-0.8%C steels will rust in these test fuels, within 24 hours. The austenitic stainless steels are very resistant to corrosion, if sintered in an atmosphere of pure hydrogen; less than 2% of the surface is covered in rust, even after 1500 hours of immersion. However, the corrosion resistance deteriorates if the samples are sintered in a synthetic dissociated ammonia atmosphere. Additions of small amounts of Sn and Cu are necessary to regain the superior corrosion resistance. Chemical analysis and metallographic examinations were performed on the test samples to determine the effect of nitrogen in the sintering atmosphere. The various test fuels for accelerated testing of metal components are identified and suggestions are made for further testing.