Key Takeaways & Executive Findings
- •• LPBF 420 + 5wt% WC/W2C MMCs exhibit pH-dependent pitting mechanisms: pits form near carbides in acidic/neutral media, while particle dissolution drives nucleation in alkaline conditions. • In-situ reaction layers show superior corrosion resistance compared to both matrix and reinforced particles across all pH levels, offering a design pathway for enhanced durability. • Critical pitting potential ranking (neutral > alkaline > acidic) inversely correlates with pit growth kinetics (alkaline > acidic > neutral), highlighting distinct thermodynamic and kinetic controls. • The study provides critical insights for optimizing LPBF MMC compositions and service environments to mitigate localized corrosion in chloride-containing solutions.
Abstract
An effective approach to enhance the surface degradation characteristics of laser powder bed fusion (LPBF) type 420 stainless steel involves the incorporation of spherical cast WC/W2C to create LPBF metal matrix composites (MMCs). However, the corrosion behavior of stainless steel and cast WC/W2C varies inversely across different pH levels, and the phenomenon of pitting corrosion in LPBF MMCs under varying pH conditions remains insufficiently explored. In LPBF 420 + 5wt% WC/W2C MMCs, pits form adjacent to cast WC/W2C in acidic and neutral environments, attributed to the presence of chromium-rich carbides and galvanic coupling effects. The dissolution of the reinforced particles facilitates pit nucleation in alkaline conditions. Notably, in-situ reaction layers exhibit superior corrosion resistance to the matrix or the reinforced particles across all pH levels. The distinct corrosion mechanisms influence the pitting corrosion behavior, with the corrosion ranking based on critical pitting potential being neutral > alkaline > acidic, contrasting the observed kinetics of pit growth (alkaline > acidic > neutral).
1. Introduction
Type 420 martensitic stainless steel offers high mechanical performance and reasonable corrosion resistance [1–3]. Laser powder bed fusion (LPBF) is a well-known additive manufacturing technique that selectively melts regions on the powder bed using a high-energy laser in a layer-by-layer fashion [4–7]. The primary advantages of the LPBF method include rapid component fabrication without the need for specialized tooling and a reduction in material waste [8–9]. Type 420 stainless steel has already been manufactured using LPBF, demonstrating excellent mechanical properties and corrosion resistance [10–11].
Two main approaches are applied to enhance the mechanical characteristics of type 420 stainless steel. The first approach involves post-heat treatments, which refine the grains and increase the carbon content in the substrate, improving tensile strength and hardness. In addition, the transformation of retained austenite and the formation of Cr-rich carbides further enhance the mechanical and corrosion resistance [12–14]. The second approach involves adding reinforced particles, such as TiC, WC, and SiC, to create LPBF stainless steel-based metal matrix composites (MMCs) [15–18]. In our previous work, spherical cast WC/W2C particles, ranging in size from 15 to 45 μm, were selected as the reinforced particles. The lower thermal stability of W2C releases W and C during laser scanning, which react with the matrix to form a solid in-situ reaction layer. LPBF 420 + 5wt% WC/W2C MMCs exhibit excellent compressive strength and wear performance [19–21]. The corrosion resistance, improved by WO3 in the passive film and the austenite phase from C in the matrix, overcomes the degradation caused by galvanic coupling between reinforced particles and matrix, resulting in a higher critical pitting potential (Epit) than conventional and LPBF 420 stainless steel [20].
The pitting resistance in LPBF 420 + 5wt% WC/W2C MMCs is controlled by the most accessible pit nucleation sites in the reinforced particle, in-situ reaction layer, or stainless steel matrix. However, the sensitivity to pitting corrosion in stainless steel and spherical cast WC/W2C behaves oppositely with increasing pH values. The pitting corrosion resistance of stainless steel improves as the pH of the solution increases [22–23]. In addition, the stability of the passive film in type 304L and type 308L stainless steel increases with higher pH values, ranging from 10.5 to 13.5 [24]. However, with increasing pH value, the corrosion resistance of cast WC/W2C shows a trend opposite to that of stainless steel. For example, WC is stable in the acidic and base solutions in the WC–Co hard materials but becomes less stable in alkaline environments [25–26]. The stability evolution of the in-situ reaction layer formed between the reinforced particles and the matrix under varying pH conditions remains unclear. Therefore, the pitting corrosion mechanisms for LPBF 420 + 5wt% WC/W2C MMCs in acidic, neutral, and alkaline solutions must be elucidated.
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Yiqi Zhou, Peihu Yuan, Decheng Kong, Xiaochang Xu, Shuoyang Wang, Lili Li, Tingting Liu, Xiaogang Li, Xuanhui Qu, Yu Yan, Chaofang Dong (2025). Pitting corrosion behavior of additively manufactured spherical WC/W2C-reinforced stainless steels in chloride-containing solution. Int. Journal of Minerals, Metallurgy and Materials (矿物冶金与材料学报). https://doi.org/10.1007/s12613-024-3075-5
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Frequently Asked Questions
What is the main focus of this study?
The study investigates the pitting corrosion behavior of additively manufactured stainless steel reinforced with spherical WC/W2C particles in chloride-containing solutions across different pH levels.
How does pH affect the corrosion behavior of LPBF 420 + 5wt% WC/W2C MMCs?
In acidic and neutral environments, pits form near chromium-rich carbides due to galvanic coupling, while in alkaline conditions, dissolution of reinforced particles facilitates pit nucleation. The critical pitting potential ranking is neutral > alkaline > acidic, but pit growth kinetics follow alkaline > acidic > neutral.
What role do in-situ reaction layers play in corrosion resistance?
In-situ reaction layers formed between the reinforced particles and the matrix exhibit superior corrosion resistance compared to both the matrix and the reinforced particles across all pH levels, contributing to overall improved pitting resistance.
What are the practical implications of this research?
The findings provide insights for optimizing LPBF metal matrix composites for applications in chloride-containing environments, guiding material design and service condition selection to mitigate localized corrosion.
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