Influence of Acetic Acid on the Integrity and Protectiveness by an Iron Carbonate (FeCO₃) Corrosion Product Layer

The effect of undissociated acetic acid on the integrity of an iron carbonate (FeCO₃) layer formed on platinum and X65 steel substrates is investigated. Experiments were conducted using buffered solutions with 0.8 mM to 5 mM undissociated acetic acid. For the platinum substrates, changes in the FeCO₃ layer were monitored via in situ electrochemical quartz crystal microbalance (EQCM) measurements. This investigation was supported by performing electrochemical experiments, under comparable conditions, on the X65 steel specimens, linking the integrity of FeCO₃ layer to its corrosion protection effect. The EQCM measurements revealed that the addition of acetic acid, at all concentrations tested here, caused partial dissolution of the FeCO₃ layer. The dissolution occurred selectively, with the outer layer dissolving first, leaving behind an inner FeCO₃ layer at the end of the immersion experiment. The electrochemical results revealed that introducing the acetic acid caused a sudden increase in the corrosion rate, which over time returned to a lower value similar in magnitude to that seen before addition of the acetic acid. This was attributed to the protection provided by the inner FeCO₃ layer, which was only mildly affected by the addition of acetic acid and was able to repair over time. It was proven that the dissolution of the protective FeCO₃ layer in the presence of acetic acid was not due to a small pH change but rather due to the formation of ferrous acetate complexes and changes in ionic strength, which decreases the FeCO₃ saturation values.