No abstract

Conventional electroless nickel–boron deposits are produced using solutions that contain lead or thallium, which must be eliminated due to their toxicity. In this research, electroless nickel–boron deposits were produced in a stabilizer-free bath that does not include any toxic heavy metal. During processing, the plating rate increased from 10 to 14.5 µm/h by decreasing the concentration of the reducing agent, leading to increased bath stability. The thickness, composition, roughness, morphology, hardness, wear, and corrosion resistance of the deposits were characterized. The new deposit presents an excellent hardness of 933 ± 56 hv50, 866 ± 30 hk50, and 12 GPa from the instrumented indentation test (IIT), respectively, which are similar to that of hexavalent hard chromium coating. Moreover, by using both potentiodynamic polarization and salt spray tests it was shown that the coating presents higher corrosion resistance as compared to standard nickel-boron coatings. The new deposit exhibits properties close to those of the conventional electroless nickel–boron deposits. Therefore, it could replace them in any industrial applications.

Electroless nickel-boron coatings were deposited from a novel deposition bath that was stabilizer-free. Those coatings were analyzed by DSC to determine the crystallization temperature of nickel-boride phases, and then the best heat treatment conditions for the new coatings were selected using the Knoop hardness test and XRD. The results of DSC analysis and XRD were coherent with the findings of previous studies, which shows that the new coating has a boron content lower than 4% wt. The maximum hardness was obtained after heat treatment at 300 °C for 4 h and reached 1196 ± 120 hk50, which is much higher than the as-deposited coating. The heat-treated coatings were then fully characterized using optical and scanning electron microscopy, as well tribological and corrosion tests. Various sliding tests (ball-on-disk and ball-on-flat configurations) were conducted to analyze the coefficient of friction (COF) and wear behavior of the coatings. The maximum von Mises stress was calculated, and values of 624 MPa and 728 MPa were obtained for the ball-on-disk and ball-on flat, respectively, at a depth close to 14 µm from the surface, confirming the suitability of the applied load. Abrasive, adhesive, and fatigue wear mechanisms were observed on the worn sample morphology using SEM. It has been determined that during the corrosion test the OCP and corrosion potential values for the heat-treated coating increase as compared with the as-deposited one, whereas its corrosion resistance decreases.

Electroless nickel-boron (ENB) coatings from borohydride-reduced bath have received wide acceptance thanks to their excellent hardness and superior wear resistance. Therefore, they have been considered as an alternative to hard chrome. However, the presence of a small amount of toxic heavy metals such as Pb or Tl in the plating bath restricts their application. These metallic salts are used to stabilize the bath, which is necessary to avoid abrupt decomposition, and are harmful towards the environment. A new bath, that is exempt from stabilizer, was designed for ENB plating. In this bath, the bath stability is achieved through the optimization of the concentration of complexing agent, pH adjuster, and reducing agent. An ENB coating from the new bath presents promising properties including modified surface morphology, excellent hardness, low friction coefficient, etc. Salt spray test (ASTM B117-07) and potentiodynamic polarization test results showed that the new ENB coating has a significantly better corrosion resistance than the conventional ENB coatings stabilized by Pb or Tl salts. In conclusion, the present ENB coating is produced in an environmentally friendly bath, and the deposit presents properties close to those of the conventional ENB deposit.