In this study, a scalable surface modification strategy for M2 high-speed steel was applied using multiple-pass friction stir processing (FSP) with overlapping ratios of 25%, 50%, and 75%. A comprehensive investigation of the microstructure, surface hardness, wear, and corrosion resistance was conducted to elucidate the properties of FSPed M2 as a function of the overlapping ratio. In the single-pass FSPed M2, the major phase was martensite and the minor phases included retained austenite where refined carbides (M₆C, M₂₃C₆, and MC) were detected. However, back-tempering occurred near the overlapped zone (OZ) between consecutive tracks for the multiple-pass FSPed M2. The martensite formed in the first pass was turned into tempered martensite by the thermal cycle from the subsequent pass. This back-tempering resulted in a localized decline in hardness from 900 to 650 HV_0.2. Further wear tests revealed that the wear rates of the tempered zone (TZ) of the multiple-pass FSPed M2 (FSP25%: 1.40 × 10⁻⁵ mm³/N·m, FSP50%: 1.20 × 10⁻⁵ mm³/N·m and FSP75%: 1.00 × 10⁻⁵ mm³/N·m) are all higher than that of SZ of the single-pass FSPed M2 (0.75 × 10⁻⁵ mm³/N·m), indicating lower wear resistance of the TZ. Moreover, increased carbide content in the TZ led to the depletion of passivating elements near proximity of the tempered martensite, acting as the active sites for selective corrosion attack. The corrosion potential (E_corr) and corrosion current density (I_corr) increased significantly, with values of −397.6 ± 5.6 mV and 9.5 ± 0.8 μA·cm⁻² for FSP25%, −424.4 ± 6.0 mV and 14.7 ± 1.7 μA·cm⁻² for FSP50%, and −440.9 ± 2.8 mV and 17.1 ± 1.9 μA·cm⁻² for FSP75%.

In recent years, CoCrFeMnNi high-entropy alloy (HEA) has attracted considerable attention owing to its exceptional mechanical properties, thermal stability and corrosion resistance. Friction surfacing (FS), as a solid-state additive manufacturing technology that can avoid point defects such as pores, has great application potential in the fabrication of HEA coatings. However, relevant research has been extremely limited so far. In this article, a CoCrFeMnNi HEA coating was successfully fabricated on 304 stainless steel substrates via FS. The microstructure and corrosion behavior of as-cast HEA, solution-treated HEA and HEA coating prepared by FS were systematically analyzed. The results showed that the as-cast alloy exhibited severe dendritic segregation of Mn and Ni elements, with large-sized MnCr₂O₄-MnS composite inclusions widely distributed. Solution treatment could alleviate this segregation and partially suppress the corrosion rate. In contrast, the thermo-mechanical coupling imposed by FS eliminated the dendritic segregation and promoted compositional homogeneity. Moreover, FS effectively fragments the inclusions in the HEA, leaving only small-sized MnCr₂O₄ observable. These effects substantially reduced the defect density in the passive film on the FSed-coating surface, leading to a marked enhancement in corrosion resistance. The corrosion current density (i_corr) of the coating decreased from 0.55 μA/cm² in the as-cast state to 0.11 μA/cm², and the pitting potential increased from −120.3 mV to 125.7 mV.