Corrosion and Hydrogen Permeation in H2S Environments with O2 Contamination, Part 3: The Impact of Acetate-Buffered Test Solution Chemistry

Abstract

This paper highlights the importance of considering the magnitude of acetate (ethanoate) species concentration on corrosion and hydrogen permeation rates, important factors associated with cracking initiation in steels for sour service qualification. Materials selection relies on standards, such as NACE TM0177 and NACE TM0284, which stipulate that oxygen pollution should be avoided during testing in H2S-containing media. The 5% NaCl test solutions in current standards are buffered using acetic acid (CH3COOH)/sodium acetate (CH3COONa) to fix the solution pH over long periods. In this third paper, as part of a series of articles that evaluate how oxygen entry modifies the corrosion of (and hydrogen permeation across) low alloy steel membranes in H2S-containing solutions, the effect that changing the solution chemistry has through testing X65 steel in different concentrations of acetic acid and sodium acetate in H2S-saturated 5% NaCl solutions, i.e., Solutions A and B (NACE TM0177-2016) and the high-strength line pipe (HLP) solution of NACE TM 0284-2016, is investigated. Increasing the total acetic acid + acetate concentration encourages a higher average X65 corrosion rate and longer-sustained hydrogen charging flux, assigned to the cathodic reaction rate enhancement by acetic acid and the iron solubilizing effects of acetates. Introducing 300 ppb of dissolved oxygen does not push the solution pH outside of the permitted error range but increases average X65 corrosion rates and, again, helps sustain hydrogen permeation flux for longer. Through an evaluation of the surface structure and electrochemical data, this appears to be due to an increase in the permeability and protectiveness of the troilite FeStroilite dominant scale and a possible acetic acid buffering of local pH increase at surface cathodic sites during corrosion. The HLP solution (at pH 3.5), with the highest acetic acid and acetate concentration, is the most aggressive. In this electrolyte, an iron sulfide overlayer structure is attained with an oxygen-rich inner layer between the metal substrate and a thick iron sulfide film.