A Two-Stage Conversion Process For Az31B Corrosion
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Permanganate phosphate coating is considered as one of the most promising alternative coatings to chromate conversion coating because of the similarity between permanganate and chromate. However, the permanganate phosphate coating always has net-work cracks. In this paper, a thick and crack-free permanganate phosphate coating was developed by a two-stage conversion process. Results of scanning electron microscope (SEM) with X-ray diffraction (XRD) analysis showed that some more corrosion resistant compounds appeared in the coating after a two-stage conversion treatment, which were not found in the single conversion coating. Moreover, the two-stage conversion coating has a higher thickness with few cracks. Furthermore, electrochemical tests and salt spray test further exhibited that the two-stage conversion coating had a better corrosion performance than either of the two single conversion coatings.