ABSTRACT
Sensitization of 5xxx series aluminum alloys is a significant concern for the U.S. Navy as the alloys have found use in a variety of ship structures for improved strength-to-weight ratios and enhanced corrosion resistance. Sensitization occurs when precipitates of ß-phase magnesium aluminide form at grain boundaries when 5xxx alloys are exposed to elevated temperatures for prolonged periods. These ß-phase precipitates are anodic relative to the aluminum matrix and corrode rapidly in the presence of an aggressive electrolyte. This corrosion can result in large crack formation under applied stress. Galvanic primers have been found to be effective at reducing this crack formation by effectively polarizing the sensitized structure into a passive potential range. This paper will review primers that Luna has developed and explore a variety of commercially available ones. Performance test results will be presented including those obtained using a unique atmospheric corrosion test method that measures time to crack initiation on coated/bare notched and sensitized 5xxx alloy samples. The method allows for mechanical load and test environment variability and is low-cost to enable rapid sample throughput. The top coating candidates are 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 by reducing the time to crack initiation and crack growth rate in accelerated atmospheric testing. The protective coatings may find use in extending the service life of existing structures and protecting new ones from sensitization effects.
INTRODUCTION
The Navy uses 5xxx series aluminum alloys in ship structures for improved strength-to-weight ratios and enhanced corrosion resistance. Specifically, UNS A95456 (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 UNS A95083 (AA5083). Magnesium is a primary alloying element of these alloys (>3.5% Mg) which renders them susceptible to sensitization. Sensitization occurs when precipitates of ß-phase magnesium aluminide form at grain boundaries when 5xxx alloys are exposed to elevated temperatures for prolonged periods. These ß-phase precipitates are anodic relative to the aluminum matrix and corrode rapidly in the presence of an aggressive electrolyte. Figure 1 shows an example of an as received 5xxx alloy microstructure compared to a sensitized one. Table 1 shows typical Mg wt% and yield strength for five different 5xxx alloys. Sensitization of 5xxx alloys thus promotes pitting, intergranular corrosion (IGC) and intergranular stress corrosion cracking (IGSCC). In order to mitigate this corrosion risk, a series of galvanic coatings have been characterized, that when applied to sensitized 5xxx aluminum alloys, retard pitting, IGC, and IGSCC initiation. Custom aluminum-rich primers were developed and characterized along with several commercially available zinc and magnesium-rich primers.
