Researchers reveal the reason behind corrosion in 3D-printed stainless steel

Like an invisible enemy, pitting corrosion attacks metal surfaces, making it difficult to detect and control. This type of corrosion, which occurs mainly due to prolonged contact with seawater in nature, is a particular problem for marine vessels.

at recent days paper Published in Nature CommunicationsLawrence Livermore National Laboratory (LLNL) scientists investigated the mysterious world of corrosion of additively manufactured (3D printed) 316L stainless steel in seawater.

316L stainless steel is a popular choice for marine applications due to its excellent combination of mechanical strength and corrosion resistance. This is even more true after 3D printing, but even this flexible material is not immune to the scourge of wear.

The LLNL team discovered that the main players in this corrosion drama are small particles called “slag,” which are produced by deoxidizers such as manganese and silicon. In conventional manufacturing of 316L stainless steel, these elements are typically added before casting to bind with oxygen and form a solid phase in the molten liquid metal that can be easily removed after manufacturing.

The researchers found that these slags are also formed during laser powder fusion (LPBF) 3D printing but remain on the metal surface and begin to corrode.

“Corrosion is very difficult to understand due to its random nature, but we have identified the properties of materials that cause or initiate this type of corrosion,” said lead author and LLNL team scientist Shohini Sen-Britain.

“Although our slag appeared different from what is observed in conventionally manufactured materials, we hypothesized that it could be a cause of corrosion in 316L. We confirmed this by leveraging the impressive material characterization suite and modeling capabilities we have at LLNL, where we were able to demonstrate without “There's no doubt that malice was the cause. This was very helpful.”

While slag can also form during conventional stainless steel manufacturing, it is typically removed using chipping hammers, grinders, or other tools. These post-processing options would defeat the purpose of additive manufacturing (AM) of the metal, said the researchers, who added that before their study, there was almost no information about how slag is formed and deposited during AM.

To help answer these unanswered questions, the team used a range of advanced techniques, including plasma-focused ion beam milling, transmission electron microscopy, and X-ray photoelectron spectroscopy on AM stainless steel components.

They were able to zoom in on the slag and reveal its role in the corrosion process in a simulated ocean environment, finding that it created discontinuities and allowed chloride-rich water to penetrate the steel and wreak havoc. In addition, slag contains mineral impurities that dissolve when exposed to a seawater-like environment, further contributing to the corrosion process.

“We wanted to perform an in-depth microscopic study to find out what could be responsible for corrosion when it occurs in these materials, and if so, there might be additional ways to improve it by avoiding this particular factor,” said lead investigator Brandon Wood.

“There's a secondary phase that forms that contains manganese — these slags — that seem to be more responsible for that. Our team did some additional detailed microscopy looking at the vicinity of that slag, and sure enough, we were able to show that in this neighborhood, you have enhanced “It is a secondary indicator that this is likely the dominant factor.”

Using a transmission electron microscope, the researchers selectively lifted small samples of 3D-printed stainless steel from the surface — about a few microns — to visualize the slag through a microscope and analyze its chemistry and structure at atomic resolution, according to lead researcher Thomas Voisin.

Characterization techniques helped shed light on the complex interplay of factors that lead to corrosion, and enabled the team to analyze slag in ways that had never been done before in feedstock manufacturing.

“During the process, you melt the material locally with a laser, and then it hardens very quickly,” Voisin said. “Rapid cooling freezes the material in a non-equilibrium state; you're essentially keeping the atoms in a configuration they're not supposed to be, and you're also changing the mechanical and wear properties of the material.”

“Corrosion is very important for stainless steel because it is used a lot in marine applications. You can get the best materials with the best mechanical properties, but if they can't be in contact with seawater, that will greatly limit applications.”

The study represents an important step forward in the ongoing battle against corrosion, not only to deepen scientific understanding of corrosion processes but also to pave the way for the development of improved materials and manufacturing techniques, the researchers said.

By revealing the mechanisms behind slag and their relationship to corrosion, engineers and manufacturers can strive to create stainless steel components that are not only strong and durable, but also highly resistant to the corrosive forces of seawater, with implications extending far beyond the marine world. Applications and in other industries and types of harsh environments.

“When we 3D print the material, it is better for mechanical properties, and from our research, we also understand that it is better for corrosion as well,” Voisin said.

“The surface oxide that forms during the process develops at high temperatures, and this also gives it many different properties. What is exciting is understanding why the material corrodes, why it is better than other techniques, and the science behind it. Confirming, again and again, that we can Using AM laser powder fusion technology to improve the properties of our materials in a way that goes beyond anything we can do using other technologies.

Now that the team understands the reasons behind the etching, Sin-Bretagne and Voisin said the next steps to enhance the performance and longevity of the 3D-printed 316L stainless steel will be to change the composition of the powder feedstock to remove manganese and silicon to reduce or eliminate slag formation.

Researchers can also analyze detailed simulations of the laser melting path and melting behavior to optimize laser processing parameters and potentially prevent slag from reaching the surface, Voisin added.

“I think there's a real way to be involved in the design of these alloy compositions and the way they process them to make them more corrosion resistant,” Wood said.

“The long-term vision is to go back to the feedback cycle of prediction and validation. We have this idea that slag is a problem; can we then leverage our formulation models and process models to figure out how to change our basic formulations, like 'What we're getting is essentially a problem Reversible design. “We know what we want, now we just have to figure out how to get there.”