Corrosion resistance, mechanical and tribological properties of electroless NiP multilayered coatings

摘要

Electroless plating is an autocatalytic method of metal deposition in which the reduction of metallic cations is assisted by the electrons supplied by the reducer, which is placed in the same solution, without the use of an external potential. Since the discovery of electroless nickel plating by Brenner and Riddell in 1946, this system has become one of the preferred solutions for engineering applications in which high corrosion resistance and superior mechanical properties are demanded. On the other hand, the properties of electroless nickel layers are defined by the phosphorous content of the alloy.In this sense, low phosphorous deposits (3-5%P) present high corrosion resistance and medium mechanical properties, medium phosphorous content coatings (6-9%P) show moderate corrosion resistance and good mechanical properties, whilst high phosphorous content layers (10-12%P) exhibit superior corrosion resistance at the expense of limited mechanical properties. However, the well-defined mechanical properties and corrosion resistance of the electroless nickel layers are sometimes limited when high performing materials, exhibiting superior mechanical properties and high corrosion resistance, are demanded. Thus, in the recent years the use of the multilayer approach, sequentially stacked layers of similar and/or dissimilar nature, is becoming very popular in the surface engineering field in order to overcome the limitations of current surface treatments based on metal plating commonly used in surface engineering. In this work, the multilayer approach was used in order to enhance the corrosion resistance, mechanical and tribological properties of a low phosphorous electroless nickel deposit (3-5%P) maintaining or even improving the mechanical properties. The electrochemical corrosion characterization of the coatings was done by means of potentiodynamic polarization curves and electrochemical impedance spectroscopy techniques. Furthermore, composition (EDX), morphology (FE-SEM) and crystalline structure (XRD) were studied along with the mechanical and tribological properties of the coatings and compared with the one-step deposited coatings. It is worth mentioning that the use of the same electroless nickel bath employed in the multilayer approach helps preventing the cross-contamination commonly observed in this kind of processes when dissimilar layers are plated on the same substrate.