Effect of pH of the Electroless Bath on Microstructure and Corrosion Behavior of Ni-Co-La2O3-CeO2 Coating

In this study, Ni-Co-La₂O₃-CeO₂ composite coatings were deposited on an AISI 430 steel substrate using the electroless method. The microstructure and corrosion behavior of coatings obtained at different bath pHs (8, 8.5, 9, 9.5, and 10) were investigated. Coating characterization was performed using a scanning electron microscope (SEM). To assess corrosion resistance, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) tests were conducted in a 3.5% NaCl aqueous solution. Microstructural examination showed that the coating formed at pH 9 has greater uniformity than the other coatings. Additionally, under this condition, the highest weight percentage of reactive elements (La and Ce) was present in the coating. The Tafel polarization test results demonstrated that applying the composite coating significantly reduces the corrosion current density of the uncoated sample. The positive effect of the coating on increasing the corrosion resistance of the steel is particularly significant for the coating formed at pH 9. In this case, the corrosion current density was reduced by more than 20 times (from 17.35 µA·cm⁻² to 0.8 µA·cm⁻²) compared to the uncoated sample. The results obtained from electrochemical impedance spectroscopy (EIS) further support these findings. According to the EIS data, the charge transfer resistance for the uncoated sample was 4089 Ω·cm², while applying coatings at pH levels of 8, 8.5, 9, 9.5, and 10 increased the charge transfer resistance to 13,214, 19,840, 28,318, 17,060, and 9446 Ω·cm², respectively.