Laboratory autoclave tests were carried out to investigate the corrosion and cracking behaviors of steels in brine saturated liquid hydrogen sulfide (H2S). Traditional low-alloy steel and 13%Cr steel underwent severe corrosion .and sulfide stress cracking (SSc). Based on the test results, anti-SSC steel and high chromium and molybdenum stainless steel were selected as candidate materials for the environments. The corrosion mechanism of steel in the solution was attributed to the promotion of cathodic reaction of H2S reduction due to the catalytic effect of water in liquid H2S.
In oil and gas wells with extremely high partial pressure of H2S, the liquefaction of H2S during operation in tubings or flowlines is very probable because the temperature of the produced fluid falls as the fluid rises from the bottom to the top of the well. Carbon and low-alloy steel do not corrode in pure liquid H2S. This can be easily understood from the case of H2S gas cylinders, for which neither corrosion nor cracking have been reported. In oil and gas wells, however, condensed water and chlorides usually coexist with Iiquified H2S, Although corrosion 1) and cracking2) data have been reported for steel in aqueous solution saturated with high pressure sour gas, corrosion or cracking data have not been reported for steel in liquid H2S solution containing such impurities as water and chlorides. This report details the corrosion and cracking behaviors of low-alloy and stainless steels in the above-mentioned environments. Selection of materials to be applied in the environments can be determined by the experimental results.
It is appropriate to review the phases of H2S in order to understand the variety of corrosion environments in sour has wells. Figure 1 shows the Pressure-Temperature phase diagram3) for the H2S-H20 binary system. The coexisting? phases for each region in Figure 1 are listed in Table 1. The environments of importance in this report are those containing liquid H2S and water. They are region A and the line a-b (Table 1). In region A two phases of H2S-rich solution*1 (Ls) and H20-rich solution *2 (La) coexist. The region on the line a-b is in three phase equilibrium of LS/ La and gas. It is not known to what extent steel corrodes or cracks in these regions. In region B (the region under the line a-bod and at the right hand of point a), H2S gas coexists with water, in which H2S is dissolved. Corrosion and cracking behaviors of steel in this region have been reported1)2). In regions C, D and the line e-b-d, steel does not corrode4) because solid hydrate H2S-6H20 forms and covers the steel surface which prevents the steel from corrosion. Furthermore, in region D, because all water in the system is consumed to produce hydrate, the liquid phase consists only of pure liquid H2S.