Effect of cathodic polarisation on pitting corrosion of AA5083-H321 aluminium–magnesium alloy in stagnant 3·5% NaCl solution

Abstract

Aluminium–magnesium alloy AA5083-H321 is a suitable material for construction of high speed boats, submarines, desalination systems etc. Cathodic protection is the proposed method for protection against its pitting corrosion. In this research, cathodic polarisation of this alloy has been studied in stationary 3·5% sodium chloride solution at –0·9, –1, –1·1 and –1·2 V(SCE). The electrochemical behaviour of the surface was investigated with EIS and DC methods. Morphological changes on the surface and cross-section of the samples were studied using scanning electron microscope (SEM) and EDAX analyser. The results showed that the fraction of the surface covered by pits, increases with applying more negative potentials. More alkaline electrolyte inside the pits on the surface tends to show non-ideal behaviour for capacitance inside the oxide layer and for oxide layer/solution interface. This behaviour is intensified with increasing of negative potential by deepening of pits during cathodic polarisation of the alloy. The final result is that cathodic protection of this alloy immersed in stationary conditions is impossible.