Development of Pitting Resistant Fe-Cr-Mo Alloys

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.

Laser Surface Alloying of 316L Stainless Steel with Ru and Ni Mixtures

“…Higher nickel content enhanced the influence of Ru. This phenomenon has been observed by Steicher [3] and Higginson [11], although at lower concentrations of less than 0.5 wt% Ru.…”

“…However, owing to the high-cost associated with ruthenium, bulk alloying is currently not a feasible means, although opportunities to explore the method exist. For instance, Streicher [3] observed a synergistic benefit when ruthenium and nickel were added together to steels. This observation offers an opportunity to reduce the amount of ruthenium per bulk volume added in the alloying process, yet presents significant improved corrosion resistance.…”

“…It has been established that small additions of Ru in the steel shifts the corrosion potential to more noble values [1]. Streicher [3] pointed out that addition of Ni also has potential benefits. In this study, laser surface alloying of AISI 316 with Ru and with (Ru + Ni) mixed powders was investigated.…”

Application limits of stainless steels in the petroleum industry

“…This has been confirmed by exposure of specimens to FeCl 3 for five years at a temperature just below the critical pitting temperature determined by the 24 hour FeCl 3 -test (ASTM Practice G 48-92) without occurrence of pitting [11]. In another study, almost all alloys that proved to be susceptible to crevice corrosion in FeCl 3 and NaCl þ KMnO 4 started to corrode within 24 hours [12].…”

Caracterização por XPS de filmes passivos formados sobre aços de baixa liga em meio de bicarbonato

“…O filme passivo crescido sobre o aço inoxidável possui, em determinados locais, uma estrutura cristalina 13 . No entanto, o efeito do Mo no grau de amorfia do filme passivo sobre o aço inoxidável ainda não se encontra estabelecido.O efeito benéfico da presença de Mo na composição química dos aços inoxidáveis, no sentido de aumentar sua resistência à corrosão por pites, em soluções contendo cloreto já é conhecido há algumas décadas, e inúmeros trabalhos abordando esse assunto podem ser citados [14][15][16][17][18][19][20][21][22][23][24][25][26][27][28] . Muitas teorias têm sido propostas acerca do mecanismo de melhoria da resistência à corrosão por pites do aço inoxidável, através da adição de Mo.…”