Figure 1a and b show OM images of ZS and C-ZS before and after treatment in the humidity chamber (95% RH at 40℃ for 72 h). The inset in each image displays a surface SEM image and the surface roughness could be attributed to the skin-pass process.15,16 The surface morphology of C-ZS was similar to that of ZS, indicating that the very thin (~ 100 nm) CC coating did not substantially alter the surface morphology. After exposure to high humidity, only negligible morphological changes were observed on the ZS sample; however, dark spots appeared on the C-ZS sample, indicating that the dark spots were associated with the CC coating process.
Figure 2a shows the time-lapse OM images of the same location on C-ZS during the humidity treatment. A dark spot with a diameter of ~ 20 µm formed after 6 h of exposure. After 72 h, this spot expanded to ~ 100 µm. This suggests that the dark spot resulted from localized corrosion under high humidity conditions. Figure 2b shows the top-view SEM image of the dark spot after humidity treatment for 72 h. The dark spot consisted of particles ~ 15 µm in size. A cross-sectional SEM image of the dark spot is shown in the upper panel of Fig. 2c. Note that the red A and B in Figs. 2b and 2c correspond to the same location. Particles were observed both on the surface of the zinc alloy layer and the steel substrate beneath, demonstrating the depth of corrosion. The enlarged SEM image of the region enclosed by the white dotted box, along with the associated EDS mappings are shown in the lower panels of c. The observed particles were primarily composed of oxygen and zinc, indicating that the dark spot is composed of zinc-containing oxides and hydroxides, corrosion products of zinc in a humid environment.
If a uniform and dense CC coating is applied to ZS, it should effectively protect the underlying zinc layer from corrosion. Therefore, the formation of dark spots on C-ZS is believed to be caused by surface damage during the CC coating under acidic conditions. To test this hypothesis, a hydrophilic PEO-SH layer was applied to the surface of ZS before the CC coating was applied. After coating with PEO-SH, the water contact angles of ZS changed from 79° to 43°, indicating the formation of a hydrophilic PEO layer on P-ZS (Figure S1 in the Supplementary Information). Figure 3a-c show the surface morphologies of ZS, C-ZS, and CP-ZS, respectively, before high humidity treatment. ZS exhibits a smooth plateau–valley surface, resulting from the skin-pass process, with no visibly damaged areas. In contrast, the surface of C-ZS appears irregular and is characterized by the presence of numerous protrusions measuring 1–2 µm in size. Cross-sectional SEM and EDS analysis revealed that these protrusions were zinc oxides or hydroxides, indicating localized corrosion of the zinc alloy layer during acidic CC coating treatment (Figure S2 in the Supplementary Information). The magnified image in the inset of Fig. 3b shows that the CC coating layer on the protrusion was broken. Therefore, C-ZS was not uniformly coated, and the protrusions are gaps in the coating that make C-ZS susceptible to further corrosion when exposed to high humidity conditions. The surface morphology of CP-ZS was similar to that of ZS, with no visibly damaged areas, indicating that the self-assembled PEO layer effectively protects the surface from damage by aggressive acid and water molecules.10
The effectiveness of the self-assembled PEO layer in preventing the formation of dark spots on CP-ZS was evaluated by subjecting CP-ZS to high humidity conditions. Figure 4a and b show time-lapse OM images of dark spots formed on C-ZS and CP-ZS when exposed to 95% RH at 40°C. Approximately 50 dark spots appeared on C-ZS after 24 h of exposure, and this number increased to approximately 58 after 72 h. In contrast, no dark spots were observed for 48 h on CP-ZS, and only 4 dark spots appeared after 72 h, confirming that the self-assembled PEO layer was effective in protecting the surface from localized corrosion.