The corrosion behavior of electroless Ni-P coatings in concentrated KOH electrolyte

Abstract

Abstract Ni-P has been widely used as a protective coating for many substrates. The corrosion resistance of Ni-P in neutral solutions such as NaCl, or acidic electrolytes such as HCl and H2SO4, has been extensively studied. However, the corrosion behavior of Ni-P coatings in caustic media, such as KOH, has received much less attention. Typically, corrosion behavior is studied through the use of electrochemical methods with corrosion rates determined from corrosion currents and potentials measured from Tafel curves. In this work, the corrosion rates of Ni-P coatings, with P concentrations varying from 2 to 11 wt%, in highly alkaline KOH (11 M) are obtained directly through electron microscopy measurements of cross sections and subsequent correlation with electrochemical data. Phosphorus concentration affects the corrosion rate; corrosion rate increases with increasing P content, peaks out at about 6–8 wt% P, and then decreases with any further increase in P content. This behavior is correlated to internal stress levels developed in the coatings.