The sulfide stress corrosion cracking resistance of low Cr steels was investigated to clarify the application limit in sour environment. The SSC threshold stress of the 0.5%C?r pipe was 95% SMYS which is the same as the Cr-free one. The SSC critical hardness of the 0.5%Cr pipe was about Hv 250 in the 3.000ppm HM solution and increased with the decrease in H~S content. The KISSC value of the 0.5(ACI pipe was newly equal to the Cr-free one. The SSC resistance to the 0.5% Cr pipe was same as the sour resistant Cr-free pipe. The hydrogen permeation rate of the Cr bearing steel was slightly lower than that of the Cr-free steel. This first is attributed to the difference in corrosion film created on the corrosion surface. The linepipe steel with the Cr content of 0.5 % proved to have sulfide stress corrosion cracking resistance :as large as the sour resistant linepipe steel.
Plain carbon and low alloyed steels have high corrosion susceptibility in the wet carbon dioxide Environment. In particular, preferential corrosion like mesa attack is a serious problem since the Corrosion rate of steel induced by preferential corrosion is very high. Mesa attack is characterized by fairly large areas which corrode at a very high rate and film covered area which virtually do not suffer. Mesa attack develops when the protective corrosion film is destroyed or does not form locally. When the mesa attack has been initiated a galvanic cell is established where the film covers surface is cathodic and the mesa attacked area is anodic. The corrosion rate of mesa attack is more than 10mm/yr.
The protection of steel against C02 corrosion has been achieved by the injection of inhibitor and with use of a corrosion resistance alloy. The inhibitor has been mostly used to prevent corrosion in the pipeline. If the inhibitor injection equipment failed down, however, severe corrosion occurs in the pipeline. A low Cr linepipe steel having superior C02 corrosion resistance as well as good mechanical properties and weldability has been developed. This kind of steel has resistance to mesa attack as well as general corrosion On the other hand, the hydrogen sulfide content increases during the production because of souring. For example, sea water is injected to maintain reservoir pressure and also to sweep oil from the reservoir. Water injection always raises a concern that this may stimulate the growth of sulphfite-redacting bacteria in the reservoir. This can result in the generation of so much hydrogen sulfide that a corrosion problem develops in (he pipeline. However, the effect of Cr addition on SSC has not to be thoroughly studied.
This paper describes the result of an extensive study on the effect of Cr on the SSC resistance of the low alloy steel.
