The use of nitrate for inhibiting corrosion fatigue crack growth of aircraft AA7050 aluminium alloy in 3.5% NaCl solution

The development of more environmentally friendly corrosion inhibitor such as nitrate (NO₃^-) as an alternative to chromium-based compounds is needed, especially for aircraft aluminium alloys. This paper presents a comparative study on nitrate (NO₃^-) and chromate (CrO₄^2-) inhibitions of corrosion fatigue crack growth of aircraft AA7050 aluminium alloy in 3.5% NaCl solution. Corrosion fatigue tests were conducted using centre-crack tension (CCT) specimens immersed in 3.5% NaCl solution with and without NO₃^- and CrO₄^2- inhibitors at the frequency, f of 10 Hz and stress ratio, R of 0.1. To gain better understanding to the corrosion inhibition mechanism of nitrate and chromate, corrosion tests were also conducted using potentiodynamic polarisation technique. Results showed that nitrate could behave as inhibitor and it slows down the fatigue crack growth rate similar to that of chromate owing to its capability of forming passive film despite its oxidising effect is lower compared to chromate inhibitor.