A highly anticorrosive chromium-free conversion coating prepared on electroless Ni-P coating

Two kinds of chromium-free conversion coatings, including manganate conversion coating (MCC) and manganate/silicate composite conversion coating (MSCC), were prepared on electroless Ni-P coatings (ENPC) to improve the anticorrosive property of ENPC instead of chromate conversion coating (CCC). The EDX and XPS analyses illustrated the chemical compositions of MCC and MSCC mainly contained Ni(OH)2, NiO and MnO2, which acted as a physical barrier to inhibit the transfer of the charges and the movement of the ions in corrosion process. Compared with the MCC, an additional layer of polymeric silicate was formed on the surface of MSCC, which leaded to better corrosion resistance. In potentiodynamic polarization test, the MSCC showed the lowest corrosion current density (icorr) with value of 0.050 μA/cm2, which decreased by around two orders of magnitude compared with untreated ENPC. Moreover, the polarization resistance (Rp) of MSCC was about 7 times higher than that of CCC. The fitting parameters of EIS explained the reason why the MSCC exhibited better anticorrosive performance than the MCC. The result of neutral salt spray test (NSS) intuitively proved that the MSCC can replace the CCC to improve the anticorrosive property of ENPC.