Abstract Dense and well‐adherent enamel–nano‐Ni composite/enamel–nano‐nickel composite coatings were prepared on 304 steel. Their thermal shock (TS) resistance at 400°C as well as the chemical stability in sulfuric acid solution were investigated, and compared with pure enamel coatings. Results indicated that the nanocomposite coatings with 5 wt% of Ni particles exhibited the best resistance to TS. Almost no spalling of the coating occurs after 50 cycles of TS at 400°C. They also provided enough protective effect to the substrates from sulfuric acid corrosion. After 7 days of corrosion, the weight loss of Ni‐5 enamel coating was only 1.107 mg/cm 2 .
In this study, a vanadium (V) and tannic acid-based composite conversion coating (VTACC) was prepared on 6063 aluminum alloy (AA6063) to increase its corrosion resistance. The surface morphology and compositions of the VTACCs were characterized using scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), and X-ray photoelectron spectroscopy (XPS). The corrosion resistance of the coatings was investigated by linear polarization and electrochemical impedance spectra (EIS). The self-healing ability of the coating was detected by SEM, EDS, and scanning vibrating electrode technique (SVET) measurements. The coating mainly consisted of metal oxides, including Al 2 O 3 , VO 2 , V 2 O 3 , and V 2 O 5 , and metal organic complexes (Al and V-complexes). The electrochemical measurement results indicated that the best corrosion resistance of VTACC was acquired when the treatment time was 12 min. Furthermore, because a new coating with vanadium rich oxide was developed on the scratch area, artificial scratch VTACC surfaces were repaired after several days of immersion in 3.5-wt% NaCl solution.
Ceramic coatings on 6063 aluminum alloy were prepared by plasma electrolytic oxidation (PEO) using pulsed bipolar power supply under different anodic and cathodic current densities. The scanning electron microscope (SEM), x-ray diffraction (XRD), potentiodynamic polarization curves analysis and tribometer were used to investigate the microstructure, phase composition, corrosion and wear resistance properties of PEO coatings. It is found that with the anodic and cathodic current densities increasing, the whole layer thickness increased while the rate of inner compact layer increased and then decreased. The proportion of α-Al2O3 phase in PEO coatings increased when anodic current density increased. However, higher cathodic current density would result in less α-Al2O3 phase in the coating. All these phenomena can be explained by the effects of anodic and cathodic current on PEO process. Higher anodic current density would enhance the discharge reaction and generate more heat and higher temperature, which are the essential conditions for γ-Al2O3 phase transforming to α-Al2O3 phase. But it also damages the ceramic coatings, causing large voids and microcracks in PEO coatings. Certain level of cathodic current can drive Al3+ losing in the electrolyte back to the oxide-electrolyte interface and inhibit the large size sparks, which favors the growth of ceramic coatings. However, too high cathodic current would inhibit the anodic process and heat generation, so that the rate of compact layer and proportion of α-Al2O3 phase decrease. Furthermore, higher rate of compact layer and larger proportion of α-Al2O3 phase in the ceramic coatings significantly improve the corrosion and wear resistances. It is possible to obtain the desired structure, composition and properties of PEO coatings by controlling anodic and cathodic current densities.
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