Electrodeposited Nİ-B coatings produced at different current densities: viability as an alternative to hard chromum

Hard chromium (Cr⁶-based) coatings are widely used in transportation, tooling, and general mechanical components due to their high hardness and abrasion resistance. However, occupational and environmental hazards associated with hexavalent chromium have intensified the search for technically equivalent yet safer alternatives. Among several candidates, nickel–boron (Ni–B) alloy coatings produced by current-assisted deposition (electrodeposition) have drawn increasing attention because they can combine high hardness, favorable tribological behavior, and process scalability while avoiding Cr⁶. Despite the growing body of work on Ni–B coatings, the interplay between current density, microstructural development, and tribological performance is still system-specific and requires careful optimization for a given substrate and bath chemistry. Objective This study systematically evaluates whether electrodeposited Ni–B coatings can serve as a viable substitute for hard chromium coatings on ST37-grade steel substrates. The primary objective is to clarify how current density (5, 10, and 15 A/dm²) governs microstructure, hardness, friction, and wear, and to benchmark these responses against a hard chromium reference. The overarching research question is: Under what deposition conditions do Ni–B coatings deliver a performance envelope that is competitive with hard chromium while providing clear health, safety, and environmental advantages? Materials and Methods Commercial ST37 steel coupons were prepared using standard surface treatments prior to deposition. Ni–B coatings were produced from an acidic electrolyte under three current-density conditions: 5, 10, and 15 A/dm². The study focused on isolating the effect of current density while keeping other variables constant and industrially realistic. Microstructural and phase characterization involved: • Scanning Electron Microscopy (SEM) for surface morphology and cross-sectional integrity, • X-ray Diffraction (XRD) for phase/atomic-order signatures typical of Ni–B systems (e.g., broadened peaks consistent with very fine grains or partial amorphous character). Mechanical and tribological properties were assessed via: • Vickers microhardness at low loads to capture near-surface properties relevant to wear, • Ball-on-disk tribometry (bilye–disk) to obtain friction vs. sliding distance curves and specific wear rates over a total sliding distance of 500 m. The tribometry protocol emphasized stability of friction signals and repeatability of wear tracks. A hard chromium coated counterpart, prepared according to accepted industrial practice, served as a reference to contextualize the performance of Ni–B coatings. All comparative assessments were conducted on identically prepped ST37 substrates to avoid confounding from substrate metallurgy. Results Morphology and cross-sectional integrity. At 5 A/dm², Ni–B coatings exhibited a nodular surface with localized porosity, indicating nucleation-dominated growth and less dense coalescence. Increasing to 10 A/dm² promoted a compact, homogeneous, and continuous morphology with improved layer integrity through the cross-section. At 15 A/dm², surface roughening and coarse nodule formation became more prominent, consistent with mass-transport limitations and localized current crowding that can lead to microdefects. In contrast, the hard chromium reference showed the well-documented microcracked morphology associated with internal stresses during deposition. Phase features. XRD patterns of Ni–B coatings were dominated by Ni-based peaks, with peak broadening consistent with very fine crystallites and/or partial amorphous character induced by boron incorporation. The evolution of peak widths across current densities suggested subtle changes in short-range order but remained within the expected envelope for electrodeposited Ni–B systems. Microhardness. Vickers microhardness trends supported the morphological observations: the 10 A/dm² coating consistently delivered the most balanced hardness response, aligning with its dense and uniform microstructure. The 5 A/dm² coating, while serviceable, did not reach the same hardness levels, and the 15 A/dm² condition showed greater dispersion attributable to surface defects and roughness. Friction and wear. Ball-on-disk tests over 500 m revealed that 10 A/dm² Ni–B coatings maintained more stable and generally lower friction profiles compared with the 5 and 15 A/dm² coatings. Correspondingly, specific wear rates were minimized near 10 A/dm², reflecting an advantageous combination of hardness and structural continuity. The 5 A/dm² coating exhibited higher friction transients and wear—consistent with its porosity—while the 15 A/dm² condition suffered from defect-assisted wear mechanisms. As expected, the hard chromium reference performed strongly in hardness-driven wear resistance; nevertheless, the 10 A/dm² Ni–B condition was competitive, especially when friction stability and overall integrity are taken together. Discussion The data demonstrate that current density is a first-order driver of Ni–B coating quality on ST37 steel. The 10 A/dm² deposition window emerges as a process optimum for this electrolyte/substrate system—balancing deposition rate, mass transport, and nucleation/growth dynamics to minimize voids and coarse nodules while preventing excessive roughness. This microstructural "sweet spot" translates directly to tribological advantages: lower and more stable friction, reduced wear, and consistent microhardness. Placing these findings in context, the hard chromium benchmark remains formidable in terms of absolute hardness; however, its microcracked morphology can compromise integrity in certain cyclic or corrosive environments. By comparison, a dense Ni–B layer at 10 A/dm² avoids Cr⁶ hazards and achieves a balanced performance envelope that is highly attractive for general-purpose wear scenarios where a combination of hardness, toughness, and friction stability is needed. Environmental and Regulatory Context A key differentiator is the elimination of hexavalent chromium from the coating process. This shift reduces regulatory burden, mitigates worker exposure, and eases end-of-life considerations. For manufacturers seeking REACH/RoHS-aligned solutions and improved ESG profiles, current-assisted Ni–B represents a technically grounded pathway that does not sacrifice functional performance when properly optimized. Limitations and Scope of Inference The present study confines its variables chiefly to current density, a single electrolyte family, and the ST37 substrate. Although this control clarifies causality, it may limit direct extrapolation to other steels, bath chemistries, or post-treatments (e.g., heat treatments that can modify Ni–B crystallinity and hardness). Future work should consider: (i) heat treatment schedules to enhance hardness and wear further, (ii) corrosion–wear synergy in relevant media, and (iii) counterface effects under varied loads/speeds to build comprehensive life predictions. Practical Implications From an implementation standpoint, targeting ß~10 A/dm² in similar acidic Ni–B baths on ST37 (S235JR-equivalent) substrates provides a pragmatic recipe for industrial transition away from hard chromium in many applications—particularly automotive/transport fixtures, molds/dies, and general tribological components subjected to moderate loads and sliding. The combination of Cr⁶-free processing, stable friction, and competitive wear resistance lowers barriers to adoption where compliance, worker safety, and sustainability are decisive. Conclusions Electrodeposited Ni–B coatings produced on ST37 steel exhibit a strong dependence on current density. Across the explored window, 10 A/dm² yielded the most favorable microstructure (dense and homogeneous) and, consequently, the best-integrated property set—balanced microhardness, lower and more stable friction, and reduced specific wear over 500 m sliding. While hard chromium maintains advantages in peak hardness, the 10 A/dm² Ni–B coating demonstrated a competitive and application-ready performance without the environmental and health liabilities of Cr⁶. These results support Ni–B at ß~10 A/dm² as a credible, safer alternative to hard chromium for a broad range of industrial wear scenarios. Original Contributions 1. A controlled, side-by-side comparison of current density effects (5/10/15 A/dm²) on Ni–B coatings over ST37 substrates, aligned to practical deposition windows. 2. An integrated property map linking SEM/XRD microstructure to microhardness, friction stability, and wear over 500 m sliding. 3. A process recommendation (ß~10 A/dm²) that balances microstructural integrity and tribological performance while removing Cr⁶ from the value chain.