Development of Pitting Resistant Fe-Cr-Mo Alloys

Abstract

Abstract Because of the resistance of iron-chromium stainless steels to chloride stress corrosion, this alloy system was used as a base for developing superior resistance to various forms of corrosion by means of alloying with molybdenum, nickel, and the six metals of the platinum group. The effects of these alloying elements were evaluated by accelerated laboratory tests for pitting, intergranular, general, and stress corrosion. The optimum ductility and resistance to pitting, intergranular, and stress corrosion were found for an alloy of Fe-28% Cr-4% Mo with carbon not exceeding 0.010% and nitrogen below 0.020% (C+N <0.025%). This alloy resists pitting and crevice corrosion in 10% FeCl3-6H2O at 50 C (122 F) with six crevices on the specimen surfaces, and it resists all intergranular attack on a welded specimen in the boiling ferric sulfate-50% H2SO4 test. Addition of 2% Ni to this alloy extends its general corrosion resistance in oxidizing and organic acids to boiling 10% H2SO4 and 1% HCI, in which it is also self-repassivating. The nickel addition makes the alloy subject to stress corrosion cracking (SCC) in the boiling 45% MgCl2 test, but not in the NaCl wick test, which more nearly simulates plant exposures and cracks 18Cr-8Ni stainless steel. Additions of small -amounts of any of the six platinum metals, e.g., 0.020% Ru, also make the 28Cr-4Mo alloy passive in boiling 10% H2SO4. But only ruthenium, iridium, and osmium do this without impairing pitting resistance in halide solutions. For self-repassivation, 0.50% Ru is required, and this amount makes the alloy subject to stress corrosion in the boiling 45% MgCl2 test, but not in the wick test.