The effect of methyldiethanolamine (MDEA) aqueous solution ageing on the corrosion mechanisms of A414(F) steel under acidic conditions

• The corrosion morphology of A414(F) steel depends on the MDEA solution composition • The corrosion behavior of A414(F) in Fresh vs Aged MDEA solution was tested • The extreme corrosion rate is driven by the local H 2 SO 4 impact and Cl-ions presence • A protective FeS 2 film grows in the presence of H 2 S, which prevents corrosion The comprehensive analysis of the A414(F) structural steel corrosion mechanism in an aqueous methyldiethanolamine (MDEA) solution degraded under real industrial conditions (Aged) vs Fresh one was conducted. The accelerated corrosion test was conducted at 150°C to evaluate the corrosion activity of the solutions. The relationship between the composition of the aged solution and the steel corrosion morphology was examined. The results showed that A414(F) steel corroded at a low rate of about 0.16 mm/year in the Fresh MDEA solution. However, the corrosion rate increased extremely up to 9.2 mm/year (∼ 60 times higher) when the coupons were tested in the Aged MDEA solution. This was attributed to a complex corrosion mechanism involving the destruction of the protective oxide film in the presence of chloride ions, as well as to the local H 2 SO 4 -acid corrosion in the presence of elemental sulfur due to mercaptans (precursors to elemental sulfur). Saturation of the Aged MDEA solution with hydrogen sulfide has a strong inhibitory effect (∼93% of the inhibition efficiency), reducing the corrosion rate of steel from 9.2 mm/year to 0.62 mm/year, a reduction of more than 15 times. This finding underscores protective properties of a dense FeS 2 -pyrite film, even in the presence of other aggressive agents, which has practical implications for industrial corrosion control. Based on the data obtained, direct recommendations were provided for improving corrosion management in industry.