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Magnesium-Rich Primer Challenges Chromate Systems

Wednesday, June 1, 2016

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A team of materials science researchers at the University of Virginia (UVA) has identified a magnesium-rich primer—used as part of a normal paint system—as a promising candidate to replace chromate-based paint systems that protect aluminum alloys.

“Several long-term studies, and some conducted in severe, natural environments, have demonstrated the primer’s excellent corrosion resistance,” said John Scully, professor of materials science and engineering at UVA.

Scully leads a team of graduate-level researchers investigating the primer’s effectiveness. The team’s research has benefitted from funding provided by the U.S. Department of Defense Corrosion Policy and Oversight Office under the aegis of the Technical Corrosion Collaboration (TCC).

The Primer’s Promise

“In spite of the promise that the magnesium-rich primer has shown over the past several years, the U.S. military has been slow to adopt a magnesium-rich primer for use on aircraft in the DoD fleet,” said Bill Abbott, a former program manager at Battelle and an independent consultant for the DoD Corrosion Office. 

C-130
Photo by Osakabe Yasuo, U.S. Air Force

An Air Force C-130 Hercules assigned to the 36th Airlift Squadron is ready to perform a low-cast, low-altitude airdrop at Yokota Air Base, Japan, in January 2016. The C-130 and other aircraft models have undergone ground and in-flight exposure testing since 2007 as the DoD evaluated the effectiveness of a magnesium-rich primer.

“We believe this is largely due to questions surrounding the exact reasons that magnesium provides protection, the effects of the chemical conversion of magnesium as it is ‘used up’ in service, how long the protection may last in service, and the performance of a magnesium-rich primer with various pretreatments and topcoats,” Abbott adds.

With support from the TCC program, three UVA materials science and engineering students have studied the magnesium-rich primer for several years, beginning when Andrew King (who earned his doctorate in 2014) kicked off the project in 2008. Balaji Kannan, a Ph.D. candidate, helped further the research starting in 2011, followed by R.J. Santucci, a Ph.D. candidate who joined the team in 2014.

The UVA researchers conducted laboratory studies of the electrochemical properties of paint systems using seven commercial pretreatments and two military-grade topcoats.

Active Aircraft Testing

The team also benefitted from the results of independent studies Abbott performed on similar paint systems, which were exposed in natural environments and flown on the exterior of military aircraft for periods of up to about five years.

“For these supporting analyses, we exposed several aircraft models to the severe environment of Daytona Beach, FL, while they were on the ground, from 2007 to the present,” Abbott said. “We also exposed these same models during flight from 2009 to 2014.”

The specific aircraft models that underwent ground and flight exposure testing included the Air Force C-130, C-5, and UH 60; the Navy P-3; and the Coast Guard HC-144.

For these tests, Battelle received a majority of funding from the Air Force through Hill Air Force Base, UT, while the DoD Corrosion Office and the Air Force Research Laboratory contributed funding for additional testing phases.

Long-Term Corrosion Protection

“All of the studies conducted by UVA and Battelle appear to present a consistent result, showing that the elemental magnesium present in the starting primer is in fact converted over time to magnesium hydroxide [Mg(OH)2] or magnesium carbonate (MgCO3),” said Scully. 

“Moreover, the results also explain the findings from independent natural environment studies, which suggest that the magnesium-rich primer system, which includes a pretreatment, primer, and topcoat, continues to provide excellent long-term corrosion protection, even after the magnesium is believed to be effectively ‘used up’ or converted to a form that might not continue to provide sacrificial-anode-based corrosion protection.”

P-3
Photo by Victor Pitts, U.S. Navy

During a program to test the performance of a magnesium-rich primer, the Navy P-3C Orion was subjected to severe outdoor exposure testing on the ground around Daytona Beach, FL, from 2007 to the present. The P-3 was also subjected to flight exposure testing from 2009 to 2014.

Recently, Santucci examined how the coating system performed on samples painted with different topcoats during a two-and-a-half-year exposure period. He concluded that the primer’s performance when used in conjunction with diverse topcoats is similar.

UVA researchers also confirm independent natural environment studies that demonstrate that the magnesium-rich primer provides excellent corrosion protection with a wide range of pretreatments, and also evinces little adverse effect on corrosion protection after two and a half years of exposure, even when the primer is used in conjunction with different types of military-grade topcoats.

“These conclusions will be of considerable importance to policy considerations that may impact the practical application of new chromate-free paint systems,” said Rich Hays, deputy director of the DoD Corrosion Office.

“These results should diminish concerns about whether systems of this type, based on an initial cathodic protection by magnesium, should lose their effectiveness over time,” Scully said. 

“This work is important in that it provides a technical basis for concluding that the corrosion protection will not be lost,” he added.

“A simple explanation is that the magnesium does provide cathodic protection for some period of time. In addition, over the long term, a barrier film of magnesium hydroxide will rapidly form at defects such as scratches and continue to provide excellent corrosion protection.”

Consistent, Positive Results

“The results from UVA’s research are consistent with other natural environment exposure results that have shown no evidence of corrosion degradation on any full paint stack-ups on aluminum alloys using the magnesium-rich primer,” Scully explained. 

This conclusion has been extended to such systems that have been exposed in a severe natural environment for years, and then scribed, followed by extended exposure.

“At the DoD Corrosion Office, we believe the positive results of these UVA studies represent the successful way in which the TCC program was designed to function,” noted Hays. 

“Under the auspices of the TCC, we fund and support research that addresses problems of immediate concern to the Department of Defense, which may lead to a rapid process of implementation on DoD’s fleet of aircraft, ships, ground vehicles, and infrastructure.”

 

A version of this article, written by Cynthia Greenwood, DoD Corrosion Policy and Oversight Office, originally appeared in Armed with Science, the official U.S. Defense Department science blog.

   

Tagged categories: Aerospace; Coating Materials; Colleges and Universities; Corrosion protection; Corrosion resistance; Department of Defense (DOD); Military; North America; Primers; Research and development

Comment from edwin call, (6/1/2016, 12:10 PM)

Has there been any testing with this primer and pre treatment on aluminum hull vessels in a marine saltwater environment? Are they commercially available? If so what paint manufacturers are producing them? Any feedback would be appreciated. Edwin.call@vigor.net Thanks,


Comment from Cynthia Greenwood, (6/1/2016, 5:54 PM)

Edwin - here are responses from our experts (or please email me at cynthiagreenwood@flaviusink.com if you have further questions) a) The pretreatments and primer/topcoat that UVA tested are commercially available. MgRP (Aerodur 2100) and Polyurethane based topcoat (Aerodur 5000) are produced by Akzo Nobel. We have tested seven different pretreatments. Prekote(Pantheon), Alodine 1200s and Alodine 5200 from Henkel, Surtec650 from Surtec are all commercial products too. I am not sure if there are any commercial players for anodization. Most of our current work and other previous work for MgRP are limited to 2024 and 7075 aluminum alloy which are primarily for aerospace application. The alloys used for hull vessels are 5086, 5052 and 6061(http://youboat.net/diy/metalBoats/aluminumAlloys.aspx). b) Another UVA researcher writes: While my project is currently focusing on AA5456 to assess primer protection capabilities for Naval hulls and superstructures (ie. the Littoral Combat Ship and the CG-47 Class Cruisers), the primary focus is Zn-rich primers with one Mg-rich primer also being tested. At this time, we have not done enough testing on the Mg-rich primer to have conclusive results. c) Bill Abbott writes: To my knowledge there has been no such testing but my guess is that the results would be negative at least for the thicknesses I have evaluated in the field. My basis for this comes from exposures essentially in the surf. This coating system fails rapidly but so does a full chromate system. Maybe a much thicker coating would perform well but no supporting data is available.


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