Corrosion behaviour of AA 6351 in glycol/water solutions degraded at elevated temperature

Common heat transfer fluids such as glycol/water solutions show a low aggressiveness, unless pollution or high temperature exposure (degradation) occur. This paper deal with the study of the corrosion behaviour of AA 6351 aluminium alloy in ethylene or propylene glycol (EG or PG)/water solutions (1:1 in volume) degraded at 108 °C in contact with AA 6351, as glycol oxidative degradation catalyzer. The tests in degraded solutions, performed at 80 °C over a period of 60 days, showed that degradation causes an increase in the uniform corrosion rates and a remarkable pitting attack. Pitting corrosion has been mainly attributed to the action of copper ions dissolved from the aluminium alloy and detected in the solutions by atomic adsorption analysis. In conjunction with the degradation studies, tests were also performed at 80 °C in buffered PG/water solutions (pH 4) containing the acids reported to be produced during the process of glycol oxidative degradation (oxalic, glycolic, acetic, formic acids), at the concentrations 1 or 10 mM. In these solutions the corrosion behaviour of AA 6351 was investigated over a period of 30 days by measuring the gravimetric corrosion rates and the polarization resistance values and by recording the polarization curves after 2 h or 30 days of immersion. Only oxalic acid and 10 mM glycolic acid significantly increased AA 6351 corrosion rates, but only in the first period of immersion. On the contrary, when the surface films were formed in the solutions of these acids, they afforded the highest protectiveness. The analogous behaviour of AA 6351 in oxalic and glycolic solutions has not been attributed to the formation of an insoluble aluminium salt film with the organic anions, but rather to the growth of an oxide film with peculiar characteristics of thickness and protectiveness