AN SKP and EIS study of microporous nickel-chromium coatings in copper containing electrolytes

Abstract

The corrosion behaviour of microporous nickel-chromium systems was studied by means of conventional electrochemical techniques and localized ones using two chloride based electrolytes with and without cupric ions in their composition. The conscious combination of different methodologies using these techniques has provided valuable information about the corrosion process. Open Circuit Potential (OCP) and Electrochemical Impedance Spectroscopy (EIS) measurements were performed in bulk solution, whilst Scanning Kelvin Probe (SKP) measurement were carried out using two methodologies: i) electrolyte droplets monitoring (measuring simultaneously potential and droplet height with time), and ii) potential maps of dried surfaces that previously were exposed to droplets. Further characterization was done based on the morphology of the attack and composition on the surface by Optical Microscopy (OM), Field Emission- Scanning Electron Microscope (FE-SEM) and X-ray Photoelectron Spectroscopy (XPS) together with the analysis of electrolyte composition with time by Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES). Results have shown the harmful effect of Cu2+ cations in the corrosion resistance and a different morphological impact on the surface. Such negative effect has revealed a new time constant at high frequency in the impedance diagrams as well as an abrupt potential decrease (due to a change in the cathodic reaction involved: Cu2+ + e− ↔ Cu+ takes place) using SKP droplet test. Apparently, Cu+ species were stabilized in bulk solution by the formation of chloride complexes, as was confirmed by the precipitation of a white CuCl compound during droplet evaporation, pointing out the key role of Cu+ in the corrosion process.