Effect of Cathodic Kinetics on Intergranular Corrosion of AA5083 Under Atmospheric Conditions

Tuesday, 3 October 2017: 09:20
Camellia 2 (Gaylord National Resort and Convention Center)
P. Khullar and R. G. Kelly (University of Virginia)
Al-Mg 5XXX series alloys are widely used for marine applications due to their corrosion resistance. However, alloys containing more than 3 wt% Mg, when exposed to standard service temperatures for extended periods of time are known to become sensitized and susceptible to localized corrosion; particularly intergranular corrosion (IGC). Many studies have investigated this IGC phenomenon in AA5XXX and attribute it to precipitation of a more anodic β-phase (Al3Mg2) along grain boundaries after exposure to environments during service in which the surface temperatures can exceed 60 C repeatedly. The majority of the electrochemical studies to understand IGC for AA5083 have been conducted in full immersion under potentiostatic or galvanostatic conditions, but much of the marine infrastructure is exposed to atmospheric conditions/thin films.

This study aims to develop a quantitative understanding of the effect of cathodic kinetics on IGC propagation at open circuit conditions. A new experimental setup using a sintered Ag/AgCl electrode as a combined reference and counter electrode was used to measure cathodic kinetics in thin films. This setup avoided the counter electrode products usually associated with a platinum electrode (H+ or OH-) and maintained uniform current density by placing the electrode right above and parallel to the sample.

Preliminary results indicate that under open circuit conditions (as experienced during service), the cathodic kinetics govern the IGC behavior under droplets/thin films. Different oxidizers like ozone (introduced in the gas phase) and potassium persulfate (introduced in solution) were used to increase cathodic kinetics. AA5083 samples were also coupled to carbon steel, which acted effectively as a galvanostat by suppling the cathodic current required to sustain the anodic activity on AA5083.

Initial thin film electrochemical results agree well with the existing corrosion framework for IGC in full immersion. An understanding of IGC in thin films/atmospheric exposures will provide a basis for developing a framework for IGC in service conditions.