Effect of plating time on surface morphology and corrosion behavior of electroless Ni-P coating in simulated concrete pore solution

As a protective coating, electroless Ni-P (ENP) coating with various deposit times was fabricated on the carbon steel. The morphology, microstructure, and composition of the ENP coating were investigated via SEM, AFM and XRD. The anti-corrosion capability and protective mechanism of the coating exposed to chloride-contaminated simulated concrete pore solution (CL-SCPS) were evaluated by OCP, potentiodynamic polarization, EIS, and Mott-Schottky analysis. As deposit duration increased, the Ni and P content percentage and compactness of ENP coating increased, but "trenches " appeared. The corrosion behavior of ENP coating was closely related to the thickness and surface morphology. That is to say, the coating with the thin layer and defects could represent low corrosion resistance. Moreover, passivation would occur on the ENP coating in an alkaline environment. The performance of the passive film was also related to the thickness and morphology of the coating. The oxidation products could partially repair the surface "trenches ". The ENP coating with an 80-min deposit duration had the highest corrosion resistance in the short immersion stage. Coating with a 60-min deposit duration represented optimal corrosion resistance and desirable electrochemical stability in the long-term service in aggressive media.