Accelerated Corrosion Degradation of Coated Aluminum Alloy Systems through Galvanic Interactions
Authors: Boerstler Joshua T. , Frankel Gerald S. 
Published: February 2020

The use of structural aluminum alloys for aircraft is common due to its low density and relatively high strength. While exhibiting significant general corrosion resistance, these alloys are susceptible to various forms of localized corrosion, such as pitting, intergranular corrosion, etc. When in galvanic contact with fasteners noble to the aluminum alloy panel, these phenomena can be much more aggressive. In an effort to mitigate these problems, multi-layer coating systems are utilized to protect the aluminum alloy from aggressive environments and to prevent galvanic contact with dissimilar metals. However, defects in these coatings may lead to galvanic interaction with the fastener and localized attack of the underlying panel. In addition, delamination of coating at these sites can result in accelerated attack as the environment becomes more aggressive in these shielded areas.

Previous work for this project has focused on the galvanic interaction of aluminum alloy 7075-T6 panels with stainless steel 316 fasteners when coated with either a chromated (chromate conversion coating, chromate-rich primer) or non-chromated (adhesion promoting pretreatment, praseodymium-rich primer) system in two environments: immersion in five weight percent sodium chloride and a continuous fog salt spray chamber. A test panel configuration using a scribed Al alloy panel and isolated fasteners was used. Galvanic current data can be acquired during exposure to determine total charge passed due to galvanic interaction between the fastener and panel for the purpose of approximating the extent of aluminum dissolution and comparison of the two coating systems. Post-exposure optical profilometry has been conducted on samples to gain insight on morphology of the attack, as well as to quantify the corroded volume of material. Analyses of this galvanic current data and morphology have been coupled to survey what coating strategy could be more beneficial.

Current work focuses on identifying the susceptibility of certain areas of the test panel to localized attack and development of a damage function for galvanic corrosion degradation in coated aluminum alloy systems. Coating performance can be compromised at the fastener hole due to mechanical damage from the fastener and from inadequate coating coverage. Electrochemical impedance spectroscopy (EIS) has been utilized to determine the extent of coating protection at the fastener hole and assess the effects of defects that may serve as initiation sites for accelerated attack. Various fastener hole conditions have been compared to the protection afforded by the coating in areas away from the fastener hole. Impressed current has also been used to determine the feasibility of attack initiating when the unintentional coating defects within the fastener hole are not a factor. This process involves current being passed from a counter electrode to a panel without fasteners in solution, allowing control of the amount of charge passed, as well as the rate at which it is passed. With the absence of fasteners, current may be passed to any defect, regardless of proximity to the fastener hole.

This technique of supplying current via a potentiostat can also be used in conjunction with a controlled, artificial defect (scribe) as a means to compare various coating systems. Contrary to the aforementioned technique, during this galvanostatic stressing, a fastener is used as the counter electrode and the exposure is carried out in a salt spray chamber, not in bulk solution. This allows for analysis of the coating systems under wetting and drying cycles that more closely resemble in-service conditions, while utilizing impressed current to better control and accelerate the charge passed from fastener to panel compared to simple exposure of galvanic panels. Post-exposure, optical profilometry analysis will be conducted to ascertain volume, depth, and area of attack, which may then be utilized in the development of damage functions for galvanic corrosion degradation based on a variety of variables.

Acknowledgement: This work was supported by the Office of Naval Research through award number N00014-13-1-0738.