Galvanic Protection Coatings for 5xxx Series Aluminum Alloys
Adam Goff, Fritz Friedersdorf, Charles Sprinkle, and Nate Brown
The Navy uses 5xxx series aluminum alloys in ship structures for improved strength-to-weight ratios and enhanced corrosion resistance. Specifically, AA5456 is the primary material of construction of the Ticonderoga (CG-47) class cruiser superstructures. The superstructure of the LCS Freedom class and the hull and superstructure of the LCS Independence class are constructed from AA5083. Magnesium is a primary alloying element of these alloys (>3.5% Mg) which renders them susceptible to sensitization. Sensitization occurs when precipitates of β-phase magnesium aluminide form at grain boundaries when 5xxx alloys are exposed to elevated temperatures for prolonged periods. These β-phase precipitates are anodic relative to the aluminum matrix and corrode rapidly in the presence of an aggressive electrolyte. Sensitization of 5xxx alloys thus promotes pitting, intergranular corrosion (IGC) and intergranular stress corrosion cracking (IGSCC). In order to mitigate this corrosion risk, Luna is developing and demonstrating lightweight galvanic coatings intended to extend the service life of existing structures and protect new ones from sensitization effects. Luna has demonstrated custom aluminum-rich primers and commercially available zinc and magnesium-rich primers that, when applied to sensitized 5xxx aluminum alloys, reduce the local mixed potential to below that of the β-breakdown (βbr) potential, thereby preventing pitting, IGC, and IGSCC initiation.
The presentation will review a variety of coatings that Luna has developed and explore a variety of commercially available ones. A variety of performance test results will be presented including those obtained using an atmospheric corrosion test uniquely developed by Luna that measures time to crack initiation on coated/bare notched and sensitized 5xxx alloy samples. The method allows for load and test environment variability and is low-cost to enable rapid sample throughput. The top coating candidates will be presented, based on a variety of performance testing, including the aforementioned atmospheric corrosion tests. The galvanic coatings have been found to drastically increase the lifetime of coupons and require higher stress intensity for coupon fracture in accelerated atmospheric SCC testing.
The presentation will conclude with a summary of future work that will be carried out under the project, including next-steps in coating development and how these new types of coatings may be used to mitigate the ongoing problem of 5xxx sensitization and resultant corrosion across the Navy. With this array of functional materials for use in coating systems, the Navy will be able to effectively manage 5xxx aluminum sensitization and corrosion on existing and future Navy ships.
This work is being funded by the Office of Naval Research under Contract No. N00014-15-C-0043.