CORROSION CONTROL OF Al6063/SiC COMPOSITE USING NOVEL HYBRID CONVERSION COATINGS

ANYIM, IKECHUKWU KINGSLEY (2022) CORROSION CONTROL OF Al6063/SiC COMPOSITE USING NOVEL HYBRID CONVERSION COATINGS. Masters thesis, Landmark University, Omu Aran, Kwara State.

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Abstract

Several organizations have limited their usage of chromate due to its high risks of carcinogenicity and environmental issues connected with chromium in +6 oxidation state, which has given rise to the numerous researches for chromate replacements in conversion coatings. This research focuses on developing a substitute using ferric sulphate and hybrid, ferric sulphate/chromate salts as relatively environmentally acceptable alternatives to chromate conversion coating. Aluminum composite 6063 - 10wt.% silicon carbide specimens were coated for 180s in ferric sulphate (Fe2(SO4)3) and hybrid ferric sulphate/chromate coating baths. The coated specimens were subjected to corrosion by natural atmospheric exposure and immersion in 3.5% sodium chloride (NaCl) solution combined with electrochemical analyses, electrochemical impedance spectroscopic (EIS) and potentiodynamic polarization (PDP) measurements. Adhesion tests using Japanese industrial test method was performed on exposed specimens. Similarly, conversion coated samples were inspected in scanning electron microscope with attached EDS facility (SEM/EDS). Results obtained from specimens subjected to immersion in 3.5% NaCl and paint adhesion tests revealed that hybrid conversion coating (HCC) and ferric conversion coating (FCC) were superior to chromate conversion coating (CCC). Also, EIS measurements showed superior corrosion resistance for both HCC and FCC over CCC. The charge transfer resistance, Rct, determined from the Nyquist plots were 9.89 × 104 Ω for HCC, 6.35 × 103 Ω for FCC and 3.75 × 103 Ω for CCC. Thus, the order of corrosion resistance is CCC < FCC < HCC from EIS measurements. However, CCC in EIS measurements performed relatively better under PDP measurements with a lower corrosion rate of 0.012 mm/year in comparism to 0.058 mm/year for FCC whereas the corrosion rate for HCC was 0.002 mm/year. Although chromate performed better than ferric under polarization measurements. However, hybrid conversion coating displayed superior corrosion resistance and showed less delamination in paint adhesion tests. Thus, it was established that the hybrid Ferric sulphate/chromate conversion coating on Al 6063/10%SiC composites performed better than chromate and ferric sulphate with respect the resistance to corrosion as well as its paint adhesion capabilities.

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