The Role of H2S in the Corrosion and Hydrogen Embrittlement of Steel

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also suggested that the H2S acts to poison the recombination reaction, thus facilitating h y d r o g e n e n t r y . Because of the complexity, there is still much u n k n o w n about the role of H2S in hydrogen embrittlement. This paper describes the results of experiments carried out to u n d e r s t a n d the factors controlling the e n trance of hydrogen into steel. I n particular, the effect of surface oxides and the presence of HfS on the v a r i ous corrosion reactions and hydrogen p e r m e a t i o n were investigated. These results will serve as the foundation for future studies which combine corrosion and m e chanical testing to u n d e r s t a n d the mechanisms of the e m b r i t t l e m e n t of steels i n the presence of HfS.
Hydrogen sulfide is k n o w n to accelerate the rate o f cracking of high strength steels in acidic, aqueous e n v i r o n m e n t s (1-5). The steels, carbon and low alloy, which are most affected are those h e a t - t r e a t e d to strengths above 80-90,000 psi and the failures are typified b y i n t e r g r a n u l a r cracking which occur well below the yield strength (6). This type of failure is most troublesome in oil and gas production where high strength steels are required for drill pipe and production t u b u l a r (7). I n this case, the e n v i r o n m e n t is aqueous b r i n e with pH in the range of 2-5. The t e m peratures range from a m b i e n t at the well head to as much as 450~ for the v e r y deep gas wells. HfS can be present in large quantities of the gas phase. F a i l ures of alloy steels have also been documented in petroleum refining where HfS is present (8-10). F a i l ures are less f r e q u e n t in refining, however, due to the need and use of lower strength steels. Those that have occurred have been a t t r i b u t e d to h a r d welds a n d / o r improper h e a t - t r e a t m e n t s . The e m b r i t t l e m e n t p h e n o m e n a observed in these e n v i r o n m e n t s have been n a m e d sulfide stress cracking (SSC). Materials are screened for susceptibility b y a n N.A.C.E. sponsored test (11) which subjects tensile or C-ring specimens to a static load in an aqueous solution containing 5% NaC1, and 0.5% acetic acid saturated with HfS gas. Using this and other testing schemes, much has been l e a r n e d about the p h e n o m e nology of sulfide stress cracking. It is k n o w n that sulfide stress cracking is influenced by three general classes of variables: e n v i r o n m e n t , metallurgy, a n d state of stress. E n v i r o n m e n t a l factors include pH, acid and salt composition, concentration of HfS, a n d t e m perature. Metallurgical variables include hardness, strength, ductility, microstructure, and composition of the alloy. The state of stress variables are i m p o r t a n t to measure and control. They include the stress i n t e n s i t y at the crack tip and the m a n n e r i n which the stress is applied (tensile, biaxial, plane strain, etc.). There a r e m a n y h u n d r e d s of experiments and field experiences which have been reported, b u t due to the large n u m ber of variables which must be considered and the difficulty in v a r y i n g only one parameter, the mechanisms of e m b r i t t l e m e n t are still uncertain. It has been suggested that sulfide stress cracking is a form of hydrogen e m b r i t t l e m e n t (1, 12). Hydrogen is present due to the corrosion of the steel i n the acidic e n v i r o n m e n t . At the corrosion potential, the dissolution of iron, Fe ~ Fe ++ -}- 2 e - (anodic reaction), is accompanied b y the cathodic reaction, 2H + + 2 e 2H(ads). The h y d r o g e n atoms which are produced at the steel surface m a y recombine to form H2 or may be absorbed b y the steel as atomic hydrogen. I t is * Electrochemical Society Active Member. Key words: metals, absorption, passivity, polarization.
The Role of H2Sin the Corrosion and Hydrogen Embrittlement of Steel
B. J. Berkowitz and H. H. Horowitz* Exxon Research and Engineering Company, Corporate Research Science Laboratories, Linden, New Jersey 07036
ABSTRACT
Hydrogen permeation in a high strength ferritic steel was shown to depend on the passivity of the surface and the presence of HfS. In the case where HfS is not present, an oxide on the surface of the steel reduced the kinetics of the proton discharge rate and, even more so, the kinetics of the molecular hydrogen evolution rate. Thus, a high percentage of hydrogen permeated the steel. After the oxide was removed, there was an increased proton discharge rate and an equally increased hydrogen evolution rate. thus resulting in very low percentage of hydrogen permeation. When H~S was present, the hydrogen discharge reaction remained high but the recombination reaction was suppressed so the percent of permeation was very high. Thus, HfS plays a multiple role in increasing hydrogen permeation of passivated steel in slightly acidic environments. It increases the rates of iron corrosion and proton discharge and poisons the hydrogen evolution react'ion on the depassivated surface, thereby permitting a large fraction of hydrogen atoms to enter the metal.