The influence of thermal treatment on the corrosion resistance of electrolytical nickel coatings containing molybdenum, titanium or vanadium
Joanna Panek, Antoni Budniok Uniwersytet Śląski, Instytut Nauki o Materiałach, ul. Bankowa 12, 40-007 Katowice
Quarterly No. 3, 2005 pages 62-69
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abstract Electrolytic composite layers Ni+Me (Me = Mo, Ti, V) were obtained by electrolytic deposition of nickel from an electrolyte containing Mo, Ti or V powder suspension. The layers were plated on steel base (St3S) under galvanostatic conditions. Corrosion tests were conducted on the layers containing comparable amount of incorporated metallic powder: about 20 at.%. The results of structural investigation of the obtained layers by the X-ray diffraction method show, that they consist in crystalline phase of nickel and incorporated Mo, Ti or V (Fig. 1a-c). Surface morphology of obtained layers as well as the cross-sectional images of obtained layers, before and after thermal treatment, were investigated by scanning microscope (Figs 2, 3). It was stated, that incorporation of metallic powder into the electrolytic nickel matrix causes the obtaining of layers characterized by great, developed surface area. The presence of electrolytical nickel nano-agglomerates plated on metallic powder particles confirms the adsorption mechanism of layers' deposition. Thermal treatment of Ni+Me composite layers was conducted in vacuum, at a temperature of 800÷950oC, depending on the kind of layer. Thermal treatment changes the surface morphology and phase composition of the layers. As a result of solid-state reaction in Ni+Ti layer Ni3Ti intermetallic compound is arising. Thermal treatment of Ni+Mo layer leads to obtaining Ni3Mo phase and Mo(Ni) solid solution, whereas in Ni+V layer only V(Ni) solid solution is formed (Fig. 1d-f)). Corrosion tests of Ni+Me composite layers were conducted in alkaline environment (5M KOH). As a measure of corrosion resistance served the corrosion potential and current values - Ekor and jkor, determined by Stern method, as well as the polarization resistance - Rp and the corrosion rate in mm/year (Tab. 1). The values of these parameters were compared to the values determined for nickel matrix. It was stated, that the changes of surface morphology and phase composition arising as a result of thermal treatment of the layers improved their corrosion resistance, what is confirmed by their lower corrosion current density and the corrosion rate, and by the higher values of corrosion potential and polarization resistance (Tab. 1). The highest corrosion resistance exhibits Ni+Ti layer. Key words: nickel, titanium, molybdenum, vanadium, electrolytic composite layers, intermetallic compounds, corrosion resistance