Lab Studying Stainless Steel Protective Coatings
Researchers from Sandia National Laboratories have reportedly tested a variety of nickel mixtures as protective coatings on stainless steel to protect it from corrosion caused by sea air. One such problem that could reportedly benefit from the research includes the use of stainless steel canisters that store spent nuclear fuel in coastal areas.
The results of the study were recently published in the scientific journal Frontiers in Metals and Alloys. The work was supported by the Department of Energy’s Office of Nuclear Enginering.
According to the lab’s release, the researchers found that the specific material applied, and the specific application process used, impacted the properties of the coating, including how protective it was against corrosion.
“Through our research, it became clear that it would not be easy to completely eliminate the possibility of a type of corrosion known as stress corrosion cracking,” said Charles Bryan, an expert on the storage of spent nuclear fuel and co-lead on the project.
“Stress corrosion cracking is likely to eventually occur at some interim storage sites. It might take hundreds of years, but it could happen, so people started thinking about mitigation and repair technologies. We started looking at cold spray, which is a technique industry is very interested in, and at corrosion-resistant polymer coatings.”
Nuclear power is the nation’s leading source of carbon-neutral electricity.— Sandia National Labs (@SandiaLabs) April 17, 2023
See how Sandia researchers are testing various coatings to protect spent nuclear fuel canisters against corrosion https://t.co/8cWhAxyfEv#EarthWeek pic.twitter.com/R5jcxK23DU
Nuclear fuel rods that no longer produce enough heat for a nuclear power plant are transferred to a pool of water at a reactor site and, after several years, the spent nuclear fuel is then placed inside a stainless-steel canister. These dry storage canisters are highly radioactive, with stress corrosion cracking posing a potential risk in aging canisters.
As a result, Sandia collaborators at the Electric Power Research Institute began working with nuclear power plant operators to analyze samples inside the canisters. Bryan said that they found chloride salts in every sample.
Sandia researchers also carried out a large experiment to ascertain if the welds used to manufacture the dry storage canisters produced enough stress to allow stress corrosion cracking to occur, Bryan said. They found that the welds do produce enough stress.
Sandia engineer Sam Durbin is studying the possibility of radioactive materials coming out of potential stress corrosion cracks, which is key for evaluating the possible radioactive exposure risk to the general public, Bryan said. Instead, the primary concern with canister stress corrosion cracking is degradation of the fuel rods, and possibly exposing workers to radioactive material if they repackage the spent fuel for permanent geologic disposal, he added.
Three years ago, researchers began studying crack mitigation and repair technologies, including a variety of cold spray coatings to see if they could protect 1/2-inch-thick pieces of stainless steel from chloride corrosion.
“We found that you have to be very cognizant of the kind of material you are spraying onto what other kind of material,” said engineer Erin Karasz, who is the lead author of the paper.
“I was surprised at how much the porosity determined the behavior when corrosion got going in between the cold spray coating and the steel. There seems to be a specific level of porosity, below which the cold spray has enhanced corrosion resistance.”
The cold spray process takes small metal particles, about as wide as half of a human hair, and sprays them onto a surface using gas hotter than a commercial pizza oven, but much cooler than the temperature needed to melt metal, Karasz said. The inert gas “splats” the small metal particles onto the stainless steel using pressures 10 to 20 times higher than the pressure of a car tire.
The team reportedly tested three different nickel-based metal mixtures: two with known anti-corrosive properties and pure nickel as a comparison. Then, they tried two different gases, nitrogen and helium, testing the effect of tapering off the coating on the metal or leaving a sharper edge between the coated area and the uncoated area.
Sandia reports that it was found that the gas used to spray on the metal particles had a strong impact on how porous, or spongy, the coating was. The porosity of the coating greatly impacted the corrosion behavior of the coating, Karasz said.
Afterwards, Karasz soaked the small pieces of cold-spray-coated stainless steel in a very corrosive ferric chloride bath for three days to test the corrosion protection properties. Corrosion was found on all the samples, but the location and shape of the corrosion differed, suggesting further refinement of the coatings is needed.
The next step, said Rebecca Schaller, a materials scientist and co-lead on the project, is to test stainless steel with cold spray as well as other polymer coatings under more relevant atmospheric conditions. Additionally, coatings will be tested on welded stainless steel to test them with stress and corrosion.
“When we put anything on these canisters, first and foremost we’re trying to make sure it’s not doing any harm,” Schaller said. “What Erin was trying to establish is where we need to look at in these studies to optimize the coatings and ensure that they’re not going to create more problems for us in the future.
“Cold spray is a newer technique and very few people have looked at it under atmospheric corrosive conditions, let alone its corrosion performance over hundreds of years.”