ABSTRACT

Depending on environmental conditions UNS(1) S32205 duplex stainless steel may suffer from Stress Corrosion Cracking (SCC) assisted by H2S initiated by local corrosion processes that involve the selective dissolution of the ferritic phase or the austenitic phase. The intent of this paper is first to study the evolution of the susceptibility of SCC assisted by H2S according temperature and pH. In a second phase, links are highlighted between the differences of cracking resistance and localized corrosion morphologies. Results show that the temperature of highest susceptibility to SCC assisted by H2S depends on the pH of the environment, moving from 80 °C at low pH (2.8 – 3.5) to temperature between 50 – 20 °C at higher pH (4.5 – 6.0). The maximum of cracking susceptibility seems to correlate with selective corrosion of ferrite coupled with transgranular cracks of the austenite.

INTRODUCTION

With their high mechanical properties and good corrosion resistance, duplex stainless steels (DSS) represent an attractive technical-economic choice for oil and gas applications such as risers, flowlines and topside equipment. However, with the presence of hydrogen sulfide (H2S) and chlorides (NaCl), it is known that these alloys can suffer from pitting corrosion, selective dissolution of one phase and stress corrosion cracking assisted by H2S (SCC). Consequently, a large number of studies have been published on the resistance of DSS in H2S containing environments over the last forty years.

A number of authors mentioned that a maximum SCC susceptibility for DSS was observed in a temperature range between 50 and 100 °C. In NACE solution A, A. El-Yazgi et al.1 observed a minimum ductility at 60 – 70 °C. This minimum is attributed to the combination between hydrogen diffusion process and localized pitting. The critical pitting temperature is 50 °C, above 80 °C the pits would become too numerous. The dissolution, which is thermally activated, would cause blunting of any cracks initiated and would also hinder the entry of hydrogen, by the corrosion product. Below 50 °C, the cracking would very similar to hydrogen embrittlement in hydrogen atmosphere. J. Sakai et al.2 also found that susceptibility of a 23 % Cr DSS to SCC is greatest at temperature from 50 to 100 °C. Likewise, P. R. Rhodes et al.3 found a minimum of SSC resistance close to 60 °C in pH 3 solutions at 0.2 and 1 bar H2S. Similarly, K. van Gelder et al.4 observed for their DSS that temperature exerts the largest influence on the SCC in sour environments. At 20 °C no crack were observed up to PH2S = 0.5 bar, whereas at 80 °C cracking occurred partial pressures of H2S as low as 0.1 bar, despite the fact that the solubility of H2S decreases with the increasing temperature5.

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