Pitting corrosion mechanism of Cl−- and S2−-induced by oxide inclusions in Fe-based amorphous metallic coatings
Introduction
Many key instruments used in the marine environment, such as drilling pipelines and ships, will be corroded to varying degrees under long-term service. Coating materials with high corrosion resistance and enhanced wear resistance are urgently needed [[1], [2], [3]]. Fe-based amorphous alloys have been widely used in ships, naval military components, and nuclear radiation because of their high strength, hardness, high thermal stability, and corrosion resistance. These alloys will become valuable corrosion- and wear-resistant surface engineering materials in the industry [4]. However, the high brittleness and poor plasticity of Fe-based amorphous alloys limit their application as structural materials. Such shortcomings are addressed by Fe-based amorphous metallic coatings (AMCs) prepared by thermal spraying.
High-velocity oxygen fuel (HVOF) is a new thermal spray technology developed in the 1980s for preparing Fe-based AMCs [5]. Coatings prepared by HVOF exhibit low porosity and high corrosion resistance [[6], [7], [8]]. Compared with 316 L stainless steel and Ni-based AMCs, HVOF-sprayed coatings show excellent corrosion resistance in harsh environments [4,9]. However, Fe-based AMCs after thermal spraying have lower corrosion resistance than the corresponding counterpart ribbon (amorphous phase content, 100%). This characteristic is considered to be due to the defects formed during thermal spraying, such as pores and oxides [[10], [11], [12], [13]]. In addition, the corrosion damage of Fe-based AMCs mainly originates from the existence of pores in the coatings [14].
Activated combustion-high velocity air fuel (AC-HVAF) technology is a new supersonic flame spray method. AC-HVAF and HVOF have similar spray process, but, instead of oxygen, air is used in the former to reduce the cost. In AC-HVAF, powder particles are heated to melt or semi-melt and accelerated to 700–850 m/s. The high-speed flight rapidly impacts the surface of the matrix to form a coating. The degree of oxidation of the spraying material is significantly reduced, and the bonding strength with the matrix is high [15,16]. Therefore, AC-HVAF can be used to prepare coatings with lower oxide content and higher density than traditional supersonic flame spraying (HVOF) [17].
Oxide inclusions are unavoidable during the preparation of coatings. This phenomenon increases the tendency of pitting corrosion. Pitting corrosion can be greatly concealed and may abruptly occur; it is a classical problem and causes difficulties in material science [18,19]. The pitting corrosion of a crystalline material is due to the local dissolution of manganese sulfide inclusions in stainless steel [20]. Structural homogeneity leads to better resistance of amorphous alloys compared with crystalline materials in most environments. However, the pitting mechanism of an amorphous material remains ambiguous. The intersplat region of AMCs is inferred to be the priority of pitting initiation. Otsubo et al. [21] optically observed the local corrosion of Fe-16Cr-30Mo-(C, B, P) AMCs in 1 M HCl and reported that pitting preferentially occurs in the pore of the sprayed powder joint. Other studies indicated that the corrosion of Fe-based AMCs in Cl−-containing solutions is due to the destruction of the homogeneity of the passive film in the pore [11,22,23] and inclusion (mainly oxide) areas, the breakdown of the passive film, and the initiation of pitting corrosion caused by Cl−. Lu et al. [24] observed similar phenomena in Fe-based AMCs. Nevertheless, the action mechanism of Cl−-induced pitting corrosion in amorphous materials remains unclear.
Zhang et al. [25,26] proposed that the pitting corrosion of Fe-based AMCs originates from the amorphous matrix region (narrow Cr-poor region) with a width of approximately 100 nm near the interface because of the action of oxides. To date, information on the microscale structure and composition of the pitting corrosion of AMCs remains insufficient. No direct evidence for the initiation site of the pitting corrosion is available. The internal mechanism between local dissolution and pitting of oxide inclusions in AMCs is ambiguous. In recent years, with the continuous improvement of computer hardware and related theories, many methods have been developed to help elucidate the mechanism of pitting corrosion. Some of these techniques are density functional theory, molecular dynamics (MD), and Monte Carlo simulation. Numerous related parameters, such as the corrosion reaction mechanism and electronic structure, have been determined. In the atomic level, the interaction between corrosion ions and the interface has been established [[27], [28], [29]]. Among these methods, MD simulation provides detailed information of the diffusion of corrosive particles and their adsorption on metal surfaces. MD has become an effective tool for studying complex systems [30,31]. In the present work, the combination of electrochemical experiments and MD simulation was used to analyze pitting corrosion of AMCs. The key problems of the initiation location of the pits and the underlying micro mechanism of the pitting corrosion of AMCs were discussed based on the macrotest and microelectronic structure to elucidate the pitting mechanism of amorphous materials. Results provide a theoretical basis for the local dissolution of oxide inclusions in Fe-based AMCs.
Section snippets
Materials
High-purity metals (Fe: 99.9%, Cr: 99.9%, Mo: 98.5%, Mn: 99.7%, W: 99.9%, FeB: 99% containing 20.06% B, C: 99.9%, Si: 99.9%) with nominal composition ratio (atomic percentage) were placed in a vacuum furnace under argon atmosphere to prepare master alloys. Amorphous powder was prepared by industrial gas atomization. The powders were then sieved at different meshes, and only powders with particle size of <45 μm were sprayed on 316L substrates. The Fe54.2Cr18.3Mo13.7Mn2.0W6.0B3.3C1.1Si1.4 (wt%)
Microstructure and structural characteristics of coatings
The surface and cross-sectional morphologies of AMCs are shown in Fig. 1. Fig. 1a shows that the surface of the as-sprayed coatings is melted uniformly. Few unmelted or semi-melted particles are observed locally, which easily form voids near and overlap with the particles. Fig. 1b shows a polished surface with numerous voids. A large amount of oxygen is involved in the spraying process to promote the formation of oxides, so some gray inclusions and primary oxide areas [36] can also be observed
Interaction between the stability of passive film and pitting corrosion properties
The passivation film can block the interaction between the coating surface and the corrosion medium. The corrosion resistance of the amorphous coating depends on the stability of the passivation film to a great extent. From the results of the polarization curve (Fig. 3), the passivation current density is sensitive to the S2− concentration, and the passivation current density increases as the S2− concentration increases. In the mixed solution, the passivation current density is more sensitive
Conclusion
- (1)
Pitting corrosion occurs in Fe-based AMCs in different concentrations of Na2S, NaCl, and mixed solutions. The passivation current density of AMCs increases with increasing concentration of Na2S. After adding 3.5 wt% NaCl, varying the Na2S concentration shows minimal effect on the corrosion system, and Cl− is dominant in the corrosion.
- (2)
There are oxides of Fe, Cr and Mo with different valences in the passive film. The Cr2O3 inclusion shows the greatest negative value for the adsorption energy, and
CRediT authorship contribution statement
Y. Wang:Conceptualization, Writing - review & editing, Funding acquisition.M.Y. Li:Writing - original draft, Software, Data curation.F. Zhu:Software, Formal analysis.W.T. Dong:Resources, Validation.X.Y. Zhang:Methodology, Investigation.L.L. Sun:Supervision, Visualization, Investigation.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (under grant No. 51974091) and Natural Science Foundation of Heilongjiang Province (under grant No. LH2019E021).
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