Effect of Trace H2S on the Scale Formation Behavior in a Predominant CO2 Environment under Hydrodynamic Control: Role of Cr/Mo Micro-Alloying in Plain Carbon Steel

Abstract

The effects of the presence of trace H2S (∼ 0.5 μM total dissolved H2S) on the electrocrystallization of siderite on plain carbon steel and its various micro-alloyed counterparts are explored. This small concentration of H2S in a CO2 saturated (sweet) brine (0.5 M NaCl), at 80°C, in a slightly acidic environment (pH 6.6) at the elevated temperature was found to increase the anodic dissolution rate of the plain carbon steel and inhibit the electrocrystallization of carbonate scale. Highly porous surface scales resulted that contained sulfide uniformly distributed through the thickness through which anodic dissolution of the steel proceeded. There was a strong dependence on both electrode rotation rate and on micro-alloying (Cr, Mo) of the scale formation kinetics, morphology and eventual scale protectiveness. For the 1Cr steel, the electrocrystallization of carbonate was inhibited but was still observable electrochemically. The resultant scale was highly porous but contained little sulfur. With sufficient Mo (0.7 wt%) in the 1Cr steel, a two-layer scale was formed, with the inner layer coherent but not containing appreciable sulfur: it was probably a carbonate. With sufficient Cr (3.5 wt%) a coherent and protective layer, presumed to be a chromium oxide or oxyhydroxide that also contained Fe, 200–600 nm thick, protected the steel from the anodic activation by H2S. The results contrast with literature results for similar H2S concentrations at room temperature or for higher concentrations at elevated temperature, where passivating films of iron sulfide have been reported. The effect of trace H2S on the formation of protective, crystalline siderite scales at elevated temperature is a possible mechanism for initiation of corrosion of pipeline steels in operational conditions in nominally ‘sweet’ fields.