Characterization of chromate conversion coatings on zinc using XPS and SKPFM

Chromate conversion coatings (CCCs) on zinc have been studied using SEM, AES, XPS and scanning Kelvin probe force microscopy (SKPFM), aiming at understanding the protective mechanism of the CCCs. The SEM images show that the size of microcracks increases with the dipping times in chromate bath, and no crack has been observed when the dipping time is shorter than 5 s. The AES depth profiles show that the concentration of chromium decreases with the depth in the chromate layer, and zinc oxide exists mainly at the interface between the chromate layer and the zinc metal substrate for a coating with dipping time 60 s. The photoreduction of Cr(VI) caused by the X-ray radiation in a vacuum chamber has been investigated by sequentially acquiring the XPS spectra for a chromate coating. By limiting the acquisition time within 45 min, the Cr(VI) reduction by the X-ray radiation is less than 17%. For as-chromated samples, the percentage of Cr(VI) varies from 32% to 42% of the total Cr content in the outer layer of chromate coatings with dipping time from 1 to 60 s. After immersion in the 0.01 M NaCl solution for 24 h, the relative amount of chromium decreased due to mainly loss of Cr(VI) species and zinc oxide/hydroxide emerged on the surface. A lower Volta potential of a chromate layer than the bare zinc has been observed using SKPFM in air, which is in agreement with the open circuit potential measured in 0.01 M NaCl solution using a normal reference electrode. This suggests that the chromate in the coating has a cathodic inhibitive effect in the corrosion of zinc.