Impedance spectroscopy studies of electroless Ni-P matrix, Ni-W-P, Ni-P-ZrO2, and Ni-W-P-ZrO2 coatings exposed to 3.5% NaCl solution.

Ni-P matrix, ternary Ni-W-P and Ni-P-ZrO₂ coatings, and quaternary Ni-W-P-ZrO₂ coatings were deposited using electroless method from a glycine bath. Their corrosion resistance was evaluated by electrochemical impedance spectroscopy (EIS) for various immersion times in a 3.5% NaCl solution. From among the investigated coatings, the ternary Ni-W-P coatings show the highest resistance to corrosion in the first hour of exposure to the 3.5% NaCl medium. An addition of ZrO₂ adversely affects the performance of both the Ni-P coatings and the Ni-W-P coatings. For all the coatings, including the ones containing tungsten, a marked decrease in pore resistance (R_por) over time is observed. This means that their corrosion resistance and capacity to protect the substrate decline. On the other hand, after 24 h immersion in the 3.5% NaCl solution the Ni-W-P coating shows the highest low-frequency impedance modulus (| Z|_{f = 0.01 Hz}). As regards corrosion resistance, the Ni-P coatings and the Ni-W-P coatings perform best. [ABSTRACT FROM AUTHOR]