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 H 2 S (∼ 0.5 μM total dissolved H 2 S) on the electrocrystallization of siderite on plain carbon steel and its various micro-alloyed counterparts are explored. This small concentration of H 2 S in a CO 2 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 H 2 S. The results contrast with literature results for similar H 2 S concentrations at room temperature or for higher concentrations at elevated temperature, where passivating films of iron sulfide have been reported. The effect of trace H 2 S 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.

Discussion paper: “In situ SR-XRD study of FeCO3 precipitation kinetics onto carbon steel in CO2-containing environments: The influence of brine pH” D. Burkle et al.. Electrochimica Acta 255 (2017) 127–144

“…In earlier work we have presented detailed studies of anodic electrocrystallisation of iron carbonate layers on carbon steel at elevated temperature in CO2 containing solution [2][3][4][5][6][7][8][9][10][11][12]. We used both insitu synchrotron X-ray diffraction, and ex-situ measurement using a rotating disc electrode to control hydrodynamics.…”

Contrast Inversion of Ferrite and Pearlite in a Sweet Corrosion Environment

“…The microstructure of steel plays a key role in determining its mechanical, structural, as well as chemical and electrochemical degradation properties, e.g., corrosion resistance [1][2][3][4][5][6][7][8][9]. Precisely the same materials chemistry with different microstructure provides a very different set of local properties [2,8,9].…”

Effect of thiosulphate/H2S on crevice corrosion behaviour of P110 carbon steel in CO2-saturated solution

“…The brine solution, containing CO 2 and H 2 S, is a corrosive medium, which accelerated localised corrosion such as pitting corrosion and sulphide stress corrosion cracking [12][13][14][15][16][17][18][19][20][21]. To date, a couple of studies have been conducted with a focus on the role of trace H 2 S in corrosion of carbon steel in the CO 2 -saturated solution [20][21][22][23][24][25][26][27][28][29]. Some of these investigations showed that addition of trace H 2 S into the CO 2 -saturated solution reduced the corrosion rate of carbon steel at room temperature due to the formation of thin sulphide film [21][22][23], whereas other studies concluded oppositely [20,[24][25][26][27][28][29].…”

“…To date, a couple of studies have been conducted with a focus on the role of trace H 2 S in corrosion of carbon steel in the CO 2 -saturated solution [20][21][22][23][24][25][26][27][28][29]. Some of these investigations showed that addition of trace H 2 S into the CO 2 -saturated solution reduced the corrosion rate of carbon steel at room temperature due to the formation of thin sulphide film [21][22][23], whereas other studies concluded oppositely [20,[24][25][26][27][28][29]. These studies found that the addition of an amount of H 2 S in the CO 2 -saturated solution dramatically increased the corrosion rate of carbon steel at higher temperature with a partial pressure of CO 2 at 10-12 MPa [24,25].…”