Effect of nickel and nitrogen contents on stress corrosion cracking (SCC) of 22%Cr-3%Mo-base duplex stainless steel was investigated in simulated sour environments with respect to both the base metal and the heat-affected zone (HAZ) of welding. The threshold stress and the critica) chloride concentration for SCC ware evaluated as a function of the ferrite content (a-content) in the alloy. The threshold stress is highest at the a-content of 40 to 45%, and is lowered with decreasing and increasing the a-content from its value. The alloy whose a-content exceeds 80% at the HAZ has also high susceptibilities to pitting corrosion and intergranular corrosion (IGC). The critica) chloride concentration for cracking increases with the decrease in the a-content. Moreover, the contents of chromium, nickel and molybdenum in the a-phase are considered to be an important factor for determining the critica) chloride concentration.KEY WORDS: duplex stainless steel, sour environment, stress corrosion cracking. ABSTRACTThe effect of Cr, Ni, and Mo contents of Ni-base alloy on the corrosion behavior in H2S-0O2-C1 environment with elemental sulfur at elevated temperature was investigated. The environmental with elemental sulfur was more corrosive than that without elemental sulfur. The stress corrosion cracking (SSC) resistance of Ni-base alloys increases with increasing Ni and especially Mo contents. The main effect of the elemental sulfur is considered that the generation of H2S due to the cathodic reaction of elemental sulfur at a location underneath sulfur deposftion, which is related to the viscosity of elemental sulfur. Recommendable chemical composition of Ni-base alloys in H2S-CO2-C1--S environment was proposed trom the view points of phase stability, stress corrosion cracking, and hydrogen embrittlement (HE). Suitable high-Ni alloys had more than 12%Mo and 15%Cr and the phase stability parameter (Md) value below 0.911.

The effects of C, Zr, Ti, and Nb on the oxidation resistance of 17% Cr steels have been investigated by means of isothermal heating and cyclic heating at temperatures up to 1000~ It has been found that C has a detrimental effect on the oxidation resistance of 17% Cr steels. The oxidation behavior of steels containing Zr is different from that of steels containing Ti and Nb. Zr improves the oxidation resistance of 17% Cr steels to a great extent. Particularly, alloys containing Zr at a concentration which exceeds the stoichiometric equivalent of the total amount of C and N show excellent oxidation resistance up to 1000~ Electron probe microanalysis shows the presence of a protective oxide layer consisting of Si-rich oxide at the metal-oxide interface, and no Zr is detected in the scales. With steels containing Ti and Nb, on the other hand, there is no protective Si-rich oxide, but Ti and Nb are detected in the scales. These scales are not protective. As the effective factors of excellent oxidation resistance of Cr steels containing Zr, the behavior of carbide, grain growth, and phase transformation have been investigated.

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The crevice corrosion behavior of 22% Cr duplex stainless steel (SS), which has become widely used because of its high strength and stress corrosion cracking (SCC) resistance, was investigated in simulated sour gas conditions. The crevice corrosion is accelerated with increasing partial pressure of H2S and temperature, decreasing pH of the test solution, and the addition of sulfur. The duplex SS was more resistant to the crevice corrosion than AIS1^1) 316 SS in the H2S-Cl-environment.A simple model of the crevice corrosion, in which the initiation process and the mode of crevice corrosion were discussed, was proposed. The crevice corrosion incubation time based on this model was calculated, and the effect of the crevice gap and the partial pressure of H2S was clarified. The mode of the crevice corrosion changed with the partial pressure of H2S, which was explained by the stability of the NiS film and the depassivation pH in the crevice.

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