Iron Corrosion in CO2/Brine at Low H2S Concentrations: An Electrochemical and Surface Science Study

The effect of H2S at the parts per million level on the early stage of iron corrosion in 3 wt % NaCl solutions saturated with CO2 is investigated using electrochemical and surface science techniques. Small H2S concentrations (5 ppm) have an inhibiting effect on corrosion in the presence of CO2. At higher H2S concentrations (up to 500 ppm), the corrosion rate is higher but below the H2S-free case. The characterization of the iron surfaces after corrosion uses photoelectron spectroscopy and electron microscopy. For Fe exposed to H2S-containing solutions, a sulfur peak (S 2p) appears at a binding energy of 162.1 eV, attributable to disulfide (S-2(2-)) adsorption. The Fe 2p(3/2) peak attributed to Fe(II) at the surface shifts by about 1 eV in the presence of 5 ppm H2S compared to the H2S-free case due to bonding of Fe to S and O. At higher H2S concentrations, the formation of a S-rich surface is indicated. Morphological changes are observed on the iron surface after exposure to H2S-containing solutions. A thin protective film forms at 5 ppm H2S, while at higher H2S concentrations a thicker, more porous surface phase forms. A mechanism for corrosion inhibition of iron in different concentrations of H2S in CO2/brine is proposed.