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据悉，劳伦斯利弗莫尔国家实验室(LLNL)的科学家们报道了增材制造（3D打印）不锈钢316L在海水中点腐蚀的研究成果。 研究以“Critical role of slags in pitting corrosion of additively manufactured stainless steel in simulated seawater”为题发表在《Nature Communications》上。
Pitting in seawater is one of the most difficult forms of corrosion to identify and control. As we all know, 316L stainless steel (316L SS) is the main material in marine applications, with good mechanical properties and pitting resistance. The advent of additive manufacturing (AM) technology, especially laser powder bed fusion (LPBF), has led to extensive microstructure and mechanical studies of LPBF 316L SS, however, the origin of surface pitting is not well understood, although pitting is critical for the certification of LPBF 316L SS prior to commissioning. Pitting corrosion acts like a hidden enemy, attacking metal surfaces, making them difficult to detect and control. This corrosion is mainly caused by long-term contact with seawater in nature and is especially tricky for naval vessels.
Here, the scientists demonstrated that manganese-rich silicate slag is responsible for the pitting corrosion of LPBF materials in sodium chloride. This is because they introduce harmful defects such as cracks or surface oxide heterogeneity. In addition, high-fidelity melt pool simulations were used to explain how slag forms in liquid metal and is deposited on the surface of the blank. The researchers' work has revealed how LPBF alters surface oxides through rapid solidification and high-temperature oxidation, resulting in a completely different pitting mechanism.
Figure 1: Surface characteristics of LPBF 316L SS.
The LLNL team found that the key role in pitting corrosion is tiny particles known as "slag", which are produced by deoxidizers such as manganese and silicon. In the traditional 316L stainless steel manufacturing process, these elements are usually added before casting, combined with oxygen, to form a solid phase in molten liquid metal, which is easily removed after manufacturing.
Figure 2: Pitting on the surface of 316L SS.
The researchers found that these dross also forms during laser powder bed fusion (LPBF) 3D printing, but remains on the metal surface and causes pitting corrosion. Pitting corrosion is extremely difficult to understand because of its randomness, but scientists have identified the properties of the materials that cause or cause this corrosion. Scientists have confirmed this with LLNL's superior material characterization and modeling capabilities, which have left no doubt that dross is the cause of pitting corrosion. While dross can also be generated in the traditional stainless steel manufacturing process, it is usually removed with a breaker, grinder, or other tool. The researchers added that prior to their study, there was hardly any information on how slag was formed and deposited during the additive manufacturing (AM) process.
Figure 3: Three-dimensional high-fidelity simulation of liquid metal flow in a stainless steel 316L molten pool by a powder bed laser grating.
To help solve these unanswered questions, the team used a combination of advanced techniques on AM stainless steel components, including plasma focused ion beam milling, transmission electron microscopy, and X-ray photoelectron spectroscopy.
Characterization techniques help uncover the complex interplay of factors that contribute to pitting corrosion and enable the team to analyze slag in a way that has never been done before in additive manufacturing.
This proves once again that scientists can use laser powder bed fusion additive manufacturing technology to improve material properties far beyond other technologies.
Now that the causes behind pitting corrosion are understood, the next step to improve the performance and longevity of 3D-printed stainless steel 316L will be to change the formulation of the powder feedstock to remove manganese and silicon to limit or eliminate slag formation, the team said. Detailed simulation of the laser's melting trajectory and melting behavior can also be performed to optimize the processing parameters of the laser and potentially prevent the slag from reaching the surface.
This research marks an important step forward in the ongoing fight against corrosion, not only deepening the scientific understanding of the corrosion process, but also paving the way for the development of improved materials and manufacturing technologies. By revealing the mechanisms behind slag and how it relates to pitting corrosion, engineers and manufacturers can work to create stainless steel parts that are not only robust but also highly resistant to seawater corrosion, extending beyond marine applications to other industries and harsh environments.
Paper Links:
Shohini Sen-Britain et al, Critical role of slags in pitting corrosion of additively manufactured stainless steel in simulated seawater, Nature Communications (2024). DOI: 10.1038/s41467-024-45120-6
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