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NASA Launches Study of Corrosion Exposure Testing

Corrosion Technology Laboratory Investigates Method to Correlate Marine Atmospheric Exposure Tests with Accelerated Corrosion Tests

The Beachside Corrosion Test Site at Kennedy Space Center (KSC) includes around 600 feet of test racks for long-term atmospheric corrosion test specimens, which are exposed to the aggressive marine environment of the Atlantic Ocean. Photo courtesy of NASA.

Corrosion protection is a high priority at NASA. The NASA launch facilities at John F. Kennedy Space Center (KSC) are positioned within 1,000 feet of the Atlantic Ocean on Merritt Island, a location that exposes them to salty ocean air, high ambient air temperatures, and extensive ultraviolet (UV) light combined with acidic rocket exhaust from launch vehicles. To provide corrosion control-related technical innovations and engineering services for NASA's launch and ground support structures at KSC, as well as DoD and other customers, KSC maintains the Corrosion Technology Laboratory, which combines people, equipment, and facilities to support state-of-the-art corrosion research and testing.

In June 2010, Corrosion Technology Laboratory researchers began a study to correlate results of exposure tests to determine if accelerated corrosion testing techniques could be reliably correlated to NASA's long-term atmospheric exposure. Specifically, the study used a timescale to measure the time it takes a sample placed under corrosive conditions in each test to reach a similar, if not the same, corrosion rate. Lab officials set out to determine how many hours of exposure in an accelerated test environment such as a neutral salt spray fog chamber would correlate to one year of atmospheric corrosion at KSC's Beachside Atmospheric Corrosion Test Site.

"We began this project in order to better understand our corrosion environment and determine the feasibility of using accelerated techniques in our coatings qualification process," said Eliza Montgomery, postdoctoral research fellow at the KSC Corrosion Technology Laboratory. With successful correlation, more reliable accelerated tests could be developed for faster material qualification times, she explained. Alternatively, accelerated tests might also be ruled out in many cases if correlation is unsuccessful.

The corrosion effect of solid rocket booster exhaust on KSC launch pad structures is shown just after a launch and before refurbishment. Photo courtesy of NASA.

"One of the challenges of corrosion is that there are many parameters or factors involved. Realistically you cannot duplicate all those factors in an accelerated corrosion test, but there may be some factors that have a more significant impact than others," said Luz Marina Calle, senior materials research scientist at the NASA Corrosion Technology Laboratory. The research project will enable Montgomery, Calle, and other lab researchers to better understand why the tests do or do not correlate, and possibly assist them in developing accelerated exposure tests that make up for what the current accelerated corrosion testing techniques lack.

The study's corrosion exposure tests used sample panels made of 1010 carbon steel, the type of steel used to construct the launch structures at KSC. The researchers compared results of long-term atmospheric exposure testing with results from two accelerated corrosion testing techniques—NASA's alternating seawater spray test and the ASTM B117 neutral salt spray fog method. For the long-term atmospheric exposure test, carbon steel panels were placed on the KSC Beachside Atmospheric Corrosion Test Site racks at 30-degree angles. Additionally, data were recorded on atmospheric conditions at the test site, including temperature, relative humidity, total precipitation, chloride deposition, sulfur concentration, and wave height. Data for exposure duration times were collected over three sets of timeframes: one month, successive months, and one year.

During the alternating salt spray test, carbon steel panels were placed at 30-degree angles on test racks at KSC's Beachside Atmospheric Corrosion Test Site close to the atmospheric exposure test racks. Here they were continuously exposed to the same atmospheric conditions as the atmospheric exposure test site, and also exposed to seawater spray for 10 minutes every hour, 24 hours a day, for periods lasting 30, 60, 120, and 180 days. The neutral salt spray fog test exposed the carbon steel panels in a 5 percent sodium chloride neutral salt spray fog chamber for 100; 250; 470; 500; 750; 1,000; 1,500; and 1,900 hours using the ASTM B117 method.

The researchers compared the corrosion rates and corrosion behaviors of the panels in the atmospheric exposure and accelerated exposure environments to identify possible timescale correlations. They measured corrosion rates for all panels using the ASTM G1 weight loss method, and used visual and x-ray photoelectron spectroscopy methods to identify the initial corrosion products. The initial corrosion rates were compared to longer-term data to measure the consistency and sustainability of the corrosion rates as a function of exposure time.

Over the years, thousands of samples have been tested on the atmospheric exposure test racks at the KSC Beachside Corrosion Test Site. Photo courtesy of NASA.

When trying to correlate the results of the exposure tests, however, they faced several challenges. One problem was trying to relate data collected on the atmospheric conditions, which synergistically and simultaneously influenced the corrosion rates. Montgomery noted that many atmospheric variables affected the atmospheric exposure corrosion rate, which didn't remain constant over time but fluctuated depending on varying atmospheric conditions at different times of the year.

Additionally, the accelerated corrosion test data showed the corrosion rates to be the same after short exposure times and long exposure times, and these rates were much higher than any corrosion rate recorded over a one-year period in the atmospheric corrosion test.

Another challenge in correlating results of the three exposure tests involved factoring in differences in the corrosion products formed on the sample panels, which also affect the corrosion rates. The salt fog chamber induced a carbon steel corrosion product that became localized where moisture droplets formed and then created a stream of water across the surface. The resulting corrosion products had a low density, were flaky, and formed as vertical ridges.

The alternating salt spray test produced an aggressive corrosion product along the path of the seawater stream, as well as general atmospheric corrosion that formed across the surface when the salt spray was not operating. The corrosion products formed as blisters and were much denser than in the case of the neutral salt spray fog chamber. The atmospheric exposure caused corrosion to initially occur at local anodic sites and then form as general corrosion across the metal surface. There were no preferentially corroding areas, and the corrosion products were more diverse and dense than those formed during the accelerated tests.

Samples on the alternating salt spray test rack were exposed to seawater spray for 10 minutes every hour, 24 hours a day, during various exposure time frames. Photo courtesy of NASA.

The researchers determined that corrosion rates for long-term atmospheric corrosion and accelerated corrosion testing were poor values to use for data correlation, because the conditions of the accelerated corrosion tests were far too destructive to make a reliable comparison to atmospheric exposure conditions. "When we looked at just the corrosion rates, we could make the time scales correlate, but the data weren't even remotely realistic. That was the problem," said Montgomery. "What we really measured was the aggressiveness of the accelerated corrosion tests."

Although the researchers concluded that it is impractical to correlate the neutral salt spray fog test data with atmospheric exposure test data, they believe the alternating salt spray test is the best candidate for correlating data with the atmospheric exposure tests because it shares the same atmospheric conditions and has the capability to accelerate corrosion with the seawater spray pumped from the ocean. Now they are focusing on developing the alternating salt spray test as a more reliable accelerated corrosion test.

The next steps for correlating the testing techniques, Montgomery noted, include shortening the test exposure times to determine if any timescales can be correlated, measuring surface oxides to determine the main differences between the corrosion products formed during the tests, and understanding which factors may limit the correlation of alternating salt spray test results to atmospheric exposure results.

"The project has been challenging but has provided very useful information," said Calle. "We're hoping that we will be able to develop a more accurate accelerated corrosion test than what is available now."

Editor's note: A version of this article originally appeared in the September 2012 issue of Materials Performance magazine.

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