Effect of CO2 and H2S on the composition and stability of passive film on iron alloys in geothermal water

Potential of Martensitic Stainless Steel X5CrNiCuNb 16-4 as Pipe Steel in Corrosive CCS Environment

“…This may be due to increasing carbide precipitation depleting the metal matrix of chromium and therefore prohibiting further surface passivation. Due to passivation X5CrNiCuNb16-4 shows very low corrosion rates in water saturated supercritical CO 2 depending either on the lack of electrolytes [42] or more possible on the cathodic reaction described in equation (1) leading to a higher H 2 CO 3 concentration and therefore more acidic and reactive environment as in the CO 2 saturated liquid phase [7], [26]. In CO 2 -containing water carbonic acid is formed due to the low pH and because the solubility of iron carbonate FeCO 3 is low [29] siderite forms.…”

“…The flowing corrosive media removes the remaining film causing the pit to grow wider, because the same steps will occur from the beginning on the newly exposed surface (f). Simultaneously crack propagation will take place due to the dynamic load and forces at the pit bottom, because the stress concentrations and plastic deformation lead to the production of slip bands [26] highly susceptible to the corrosion explained. The crack flanks are immediately wetted repeating the corrosion steps within the crack itself.…”

“…Alloying elements [22] as well as heat treatment (temperature and time of austenitisizing, cooling rate as well as temperature and time of annealing [23]- [25]) shows significant influence upon the corrosion resistance of steels, e.g. high Ni-and Cr contents [26], [27] improve general corrosion resistance as well as retained austenite improves pitting corrosion resistance [26]. Higher austenitizing temperature of martensitic steels [28]- [30] as well as higher annealing temperature of lean duplex stainless steels [22], [23], [28] decrease the pitting potential.…”

Investigation of the deterioration of passive films in H2S-containing solutions

“…Generally, the passive film consists mainly of Fe 2 O 3 , Fe(OH) 3 , and Cr 2 O 3 [13]. Banas et al [14] found that CO 2 could stimulate the dehydroxylation of the outer part of the oxide film. Liang et al [15] found that the surface film was composed of Cr 2 O 3 , MoO 3 , FeS, FeS 2 , Fe(OH) 2 , and FeO after immersing the sample in a sulfate-reducing bacteria medium.…”

“…However, the inhibition effect of H 2 S on stainless steels has seldom been reported. To date, several investigations have been conducted with regard to the corrosion behavior of stainless steels in the presence of H 2 S. Banas et al [14] found that the presence of H 2 S in thermal water could lead to the decrease in the stability of the passive film on high alloyed stainless steels by inhibiting the formation of oxides. Ge et al [17] found that the addition of sodium sulfide to cooling water could affect the Cr oxide in the inner passive film, thereby resulting in an increased acceptor density.…”