Cathodic Control of Intergranular Corrosion in Sensitized AA5083 -H131

Al-Mg 5XXX alloys are widely used for marine applications due to their low cost, high strength-to-weight ratio and good weldability. However, alloys containing more than 3 wt% Mg, when exposed to standard service temperatures associated with marine service for extended periods of time, can 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 service environments.

Conventional laboratory accelerated corrosion test methods like ASTM B117 are often used to simulate and accelerate the atmospheric corrosion of materials. Previous work has shown that a modified accelerated test with strong oxidizers and relevant humidity range correlates much better with field data for corrosion of silver. To develop a modified accelerated corrosion test to predict and evaluate IGC in AA5XXX, a better understanding of IGC initiation and propagation under natural conditions is required. Previous studies have investigated the role of metallurgical and electrochemical factors on IGC and indicate that degree of sensitization (DoS), orientation and time of exposure have a strong influence on grain boundary precipitation of β-phase and hence IGC in AA5XXX. However, the majority of IGC studies have been conducted in full immersion (usually with anodic polarization) while much of the marine infrastructure is exposed to atmospheric conditions (at open circuit). Irrespective of the exposure conditions, the anodic dissolution at the fissures has to be balanced by the cathodic current. In a setup with potentiostat, virtually all of the cathodic current is supplied by the counter electrode, irrespective of the working electrode condition. Under open circuit conditions, the cathodic current can only be supplied by the reduction reactions happening on the same surface, with most of that cathodic current occurring outside the fissure. Hence, the cathodic kinetics on the alloy surface can play an important role in IGC propagation under open circuit conditions.

Based on initial results, cathodic kinetics was hypothesized to be the primary factor controlling whether or not IGC occurred under open circuit conditions for a highly sensitized material. The hypothesis was tested by using strong oxidizers (to replace oxygen) and better cathodes (via galvanic coupling) as proxies for faster cathodic kinetics (and increased cathodic current availability). The results showed that a threshold for cathodic kinetics (and cathodic current) was the key difference in open circuit exposures where consistent IGC was observed vs no/minimal IGC (only minor (< 25 µm) pitting observed). Mixed potential theory was used to explain the role of cathodic kinetics in controlling the IGC behavior observed. The framework described here for IGC in Al-Mg alloys is applicable to any type of localized corrosion which has the important characteristic feature of discrete cathodic and anodic sites. This new understanding of cathodically limited localized corrosion was then applied to some previous work to explain the corrosion morphology obtained by others.

The output of this research is of both scientific and technological importance. An understanding of IGC in thin films/atmospheric exposures will provide a basis for developing a framework for IGC in thin films and comparing it to existing corrosion framework for IGC in full immersion. This research provides the foundation for the development of a modified accelerated corrosion test that is based on the fundamental understanding of the role of cathodic kinetics in localized corrosion. This cathodic limit can manifest itself as the need for a minimum exposed sample area or the need for oxidizers stronger than oxygen to provide faster cathodic kinetics. A modified test that correlates well with the field exposures will be more applicable for service life prediction in natural environments.