Effects of environmental factors such as H2S partial pressure, PH, C1 concentration and temperature on SSC property of modified 13Cr steels were systematically investigated. Critical H2S partial pressure for SSC is affected mainly by temperature and chloride concentration, while pH effect was quite small. In mild condition such as chloride free solution at 25ºC, the steels presented the passive state inhibiting hydrogen entry into steel so that no SSC susceptibility was observed. With increase in C1 concentration, corrosion morphology changed from passive state into either pitting or general corrosion allowing hydrogen entry into steel so that SSC susceptibility was observed. In higher temperature condition, passive film also became unstable to cause general corrosion, however, cracking susceptibility was decreased. Based on the test results, a principal SSC mechanism of modified 13Cr steels is considered to be hydrogen embrittlement in condition that some breakdown of passive film undergoes beforehand.
In recent years, several types of modified 13Cr steels[l-6] have been Spotlighted because of their improved resistance against weight loss corrosion and sulfide stress cracking(SSC).
So far, the weight loss corrosion properties of these steels have been extensively investigated in relation to various environmental factors and alloying element contents so that their application limits have been almost clarified[7]. With regard to SSC property, meanwhile, more investigation seems to be needed to determine the consistent application limits of them. Although some SSC data at ambient temperature have been reported [1-6,8], the complex effects of environmental factors have not been systematically made clear yet.
This paper focuses on clarifying the effects of environmental factors such as H2S partial pressure, pH, C1 concentration and temperature on SSC property of modified 13Cr steels in oil and gas environments.
Materials
Two types of modified-13Cr steels were used in this study. Their chemical compositions are listed in Table 1. These steels were hot rolled and heat treated in a seamless pipe mill. Mechanical properties of test materials are listed in Table 2.
SSC test
V-notched four point bent beam test was conducted to investigate the effect of such environmental factors as PH, temperature, H2S Partial Pressure and C1 concentration upon SSC susceptibility of test materials. The dimensions of specimen are shown in Figure1. The specimen was loaded to 100% actual yield strength. The condition oftest environments is listed in Table 3. The base of test solution was sodium chloride solution saturated with CO2 and H2S gases at the desired partial pressure and temperature. Some amount of sodium bicarbonate was added in order to adjust the in-situ pH to a desired value in which pH was estimated by the method proposed by A.Miyasaka[9]. The test duration was 336 hours. The corrosion morphology as well as the occurrence of pitting and cracking on a specimen just after testing were visual1y observed.
The SSC susceptibility in relation to H2S partial pressure, pH, temperature and C1 concentration are shown in Figure2,3,4,5,7 and 8.
