Corrosion inhibition of 6061-SiC by rare earth chlorides

Purpose - The research work in this paper aims to focus on understanding the corrosion inhibition of 6061-8 (vol.%) SiC in 3.5 per cent NaCl solution using different concentrations (250, 500, 750 and 1,000 ppm) of cerium and lanthanum chloride. Design/methodology/approach - The corrosion inhibition of 6061-SiC in 3.5 per cent NaCl solution using the rare earth chloride inhibitors was analyzed by different electrochemical techniques. The techniques employed were linear polarization, Tafel extrapolation and electrochemical impedance spectroscopy (EIS). Further, surface characterization, before and after inhibitor addition, was studied using scanning electron microscopy (SEM) and energy dispersive analysis using X-ray. Findings - It was observed that the polarization resistance increased after addition of LaCl3 and CeCl3, with maximum increase noticed for 250 ppm LaCl3 and 1,000 ppm CeCl3. CeCl3 addition showed better improvement in polarization resistance value compared with LaCl3 addition. Pitting nucleation resistance also increased with addition of LaCl3 and CeCl3, with maximum obtained for 250 ppm LaCl3 and 500 ppm CeCl3. EIS studies showed that there was a significant increase in resistance of areas not covered by the surface film after addition of LaCl3 and CeCl3, when compared with the case without inhibitor, with a maximum increase observed with 1,000 ppm CeCl3. Rare earth chloride addition resulted in an increase in resistance on both cathodic intermetallic sites as well as the pitted regions by formation of precipitates of their oxide/hydroxide on those locations. This gave the high pitting nucleation resistance as well as improved corrosion resistance. Research limitations/implications - it was observed that optimum concentrations of CeCl3 and LaCl3 resulted in good corrosion resistance properties on 6061-SiC in 3.5 per cent NaCl solutions. Even small quantities of these inhibitors resulted in high corrosion resistance. However, it should be noted that both LaCl3 and CeCl3 did not follow a simple increase in corrosion resistance with composition, despite both being rare earth chloride inhibitors, and this issue merits further research. Practical implications - Metal matrix composites (MMC) are of great use in the aerospace, military and automotive industries due to their good mechanical strength/density and stiffness/density ratios. A typical example might be the reinforcement of Al alloys with SiC particulates, which leads to a new generation of engineering materials. However, the addition of a reinforcing phase can cause discontinuities in any protective surface film, increasing the number of sites where corrosion can be initiated and rendering the composite liable to severe attack. Thus, this research work was performed to investigate if a suitable concentration of lanthanide salts (LaCl3 and CeCl3) could be identified that could improve both uniform and pitting corrosion resistance. Originality/value - Earlier studies on the corrosion inhibition of 6061-SiC used cerium conversion coatings. More recently (i.e. during the last 1-2 years) work has started on lanthanum conversion coating on Al alloys. However, little work has been carried out on use of these lanthanide salts (CeCl3 and LaCl3) as corrosion inhibitors for 6061-SiC. The present research work was performed in order to better understand the effectiveness of these inhibitors to reduce corrosion attack on 6061-8(vol.%) SiC.