Atmospheric pitting corrosion of AA7075-T6 under evaporating droplets with and without inhibitors

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Pitting corrosion of polished and etched AA7075-T6 in the presence of 3.5 wt% NaCl electrolyte was investigated both with standard full immersion testing, and under evaporating electrolyte droplets in a fixed humidity environment. The effects of chromate-, vanadate-, and cerous-based corrosion inhibiting salts were also examined. A scanning Kelvin probe (SKP) was used to investigate the corrosion potential behavior in the electrolyte droplets. During immersion testing, widespread pitting at intermetallic particles was observed in uninhibited solution. Similar but less severe attack was observed with additions of 0.03 mM vanadate and cerous ions. At higher concentration of vanadate, small numbers of very large pits were found, while higher concentrations of cerous ions provided very good inhibition. Chromate provided nearly complete inhibition of corrosion in all concentrations of immersion testing. In droplet testing, corrosion potentials measured by SKP were associated with metastable pitting, inhibition/passivation, and a form of corrosion attack in which pitting at the edge of the test droplet resulted in the formation of an adjacent, secondary droplet. A model is described for the formation of the secondary droplet and associated attack, by deliquescence of water vapor due to an excess of ions from the anode diffusing to the edge of the droplet.