Iron corrosion in CO₂ /brine at low H₂ s concentrations: An electrochemical and surface science study

The effect of H₂ S 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 H₂ S concentrations (5 ppm) have an inhibiting effect on corrosion in the presence of CO₂. At higher H₂ S concentrations (up to 500 ppm), the corrosion rate is higher but below the H₂ S -free case. The characterization of the iron surfaces after corrosion uses photoelectron spectroscopy and electron microscopy. For Fe exposed to H ₂ S -containing solutions, a sulfur peak (S 2_p) appears at a binding energy of 162.1 eV, attributable to disulfide (S₂ ^2-) adsorption. The Fe 2 p_3/2 peak attributed to Fe(II) at the surface shifts by about 1 eV in the presence of 5 ppm H₂ S compared to the H₂ S -free case due to bonding of Fe to S and O. At higher H₂ S concentrations, the formation of a S-rich surface is indicated. Morphological changes are observed on the iron surface after exposure to H₂ S -containing solutions. A thin protective film forms at 5 ppm H₂ S, while at higher H₂ S concentrations a thicker, more porous surface phase forms. A mechanism for corrosion inhibition of iron in different concentrations of H₂ S in CO₂ /brine is proposed.