Influence of thermal treatment on corrosion resistance of nickel with molybdenum coatings

Electrolytic Ni-Mo and Ni-P-Mo coatings were obtained from a citrate electrolyte in a current density range j = 2.5 - 300 mAcm-2. To determine the influence of phase composition change on corrosion resistance of the obtained coatings their annealing in temperature range from 300 to 1000 oC in argon atmosphere was carried out. Electrochemical corrosion investigations were conducted in 5 M KOH, using classical (voltammetry) and spectroscopic (electrochemical impedance spectroscopy) methods. On the basis of these studies it was found that corrosion resistance of Ni-Mo and Ni-P-Mo coatings depends on molybdenum contents. Ni-Mo alloy, which contains 30% of molybdenum and Ni-P-Mo alloy, which contains 2% of molybdenum, show greatest corrosion resistance. Thermal treatment of the obtained alloys did not improve their corrosion resistance. Moreover, it was confirmed that corrosion resistance of alloys with phosphorus is greater than corrosion resistance of alloys without phosphorus. It was shown that Ni-P-Mo alloy containing 2% of molybdenum is most resistible to the aggressive activity of the alkaline environment. This is due to the least value of the corrosion current, the double-layer capacitance and the factor of electrochemical active value surface and to the greatest value of polarization resistance and electric charge-transfer resistance. (author)