Effect of Reinforcement on the Pitting Behavior of Aluminum‐Base Metal Matrix Composites

Abstract

This paper examines the effect of graphite and silicon carbide reinforcements on the pitting behavior of graphite/aluminum (Gr/Al) and silicon carbide/aluminum (SiC/A1) metal matrix composites. Electrochemical corrosion tests were performed on both Gr/A1 and SiC/A1 composite specimens. Identical tests were completed on powder metallurgy processed aluminum and wrought aluminum of the same composition. The electrochemical behavior of the SiC/A1 composites was essentially identical to that of the powder processed and wrought aluminum alloys; however, the pitting attack on the SiC/Al composites was distributed more uniformly across the surface, and the pits penetrated to significantly less depths. The presence of graphite in the Gr/A1 composites did not cause an electropositive shift in corrosion potential as anticipated, but caused a substantial decrease in resistance to passive film breakdown. This effect is the predominant reason for the poor performance of Gr/At composites in marine environments.Interest in implementation of metal matrix composites (MMC) in marine applications is rapidly expanding due to the materials' higher strength and modulus than that which can be attained by conventional alloying. Utilization of MMC in marine environments requires adequate corrosion resistance. To date, composites considered for marine application are typically aluminum-based, specifically 5000 series or 6061 Al, with reinforcements of graphite (Gr) and silicon carbide (SIC). It has been reported tl~at aluminum alloys suffer localized attack by pitting in chloride environments (1, 2). A reinforcement of Gr or SiC added to the aluminum matrix could potentially decrease the corrosion resistance as compared to an unreinforced alloy due to the structure of the composite. Continuous graphite/aluminum (Gr/A]) composites typically consist of Gr/Al wires (graphite multifilament tows infiltrated with aluminum) diffusion bonded between aluminum foils. This composite configuration could promote wicking of the electrolyte (thereby increasing the corrosion rate) at the foil-to-wire and wire-to-wire interfaces once pitting corrosion progresses through the aluminum foils. The structure of the discontinuous silicon carbide/aluminum (SiC/A1) composites, that of silicon carbide whiskers or particles blended into a powder aluminum matrix could increase the probability of pitting/crevice corrosion at exposed SiC-A1 interfaces in the composite. Dull et al. (3,4) studied the Gr/A1 system in 3.5% NaC1.They concluded that the corrosion rate was slightly higher for Gr/AI relative to 6061 aluminum alloy controls. The explanation for this increased corrosion rate was galvanic corrosion at the graphite fiber-aluminum interface.Trzaskoma et al. (5) studied the SiC/A1 system in 0.1N NaC1. They concluded that the SiC whiskers did not influence the susceptibility of 6061 aluminum to pit initiation, but promoted the development of a smaller size pit morphology relative to 6061 aluminum controls.In this research, both SiC/A1 and Gr/A1 composites were evalua...

Effect of fly ash reinforcement on the corrosion behaviour of cast Al-Mg alloy A535 in 3.5 wt% NaC1 solution

“…Similarly, Aylor and Moran [17] observed that SiC did not alter the corrosion potential of AA6061 in aerated seawater.…”

“…The corrosion behaviour of Al-based MMCs reinforced with particles such as Al 2 O 3 , garnet, TiC, AlN and SiC particles have been studied by several workers [12][13][14][15][16][17][18][19][20][21]. A close look at the results obtained from these studies shows that three types of corrosion can occur in particulate Al MMCs at room temperature.…”

Corrosion Behavior of Aluminium Metal Matrix Composite

“…They also exhibit a higher corrosion rate at velocities greater than the 2.3 ms -1 . A variety of surface modification techniques such as anodizing, chromate conversion coatings and organic finishing have been suggested for the protection of aluminum metal matrix composite from localized corrosion (Aylor & Moran, 1986;Lin et al, 1989;Mansfield et al, 1990). Cerium coatings have been the focus of attention in the last decade (Hinton & Arnold, 1986;Davenport et al, 1991).…”

Aluminum Alloys for Al/SiC Composites

“…al., 2000;Aylor & Moran, 1985;Candan & Bilgic, 2004;Hihara & Latanison 1992;Pardo et. al., 2003;Pardo et.…”

“…Pitting corrosion is believed to be one of the main mechanisms for damage of high-strength aluminum alloys, and usually it initiates by the breakdown of the passive film of the metal surface. Accordingly, the effect of reinforcement on the pitting behavior of aluminum-base metal matrix composites was studied earlier (Aylor & Moran, 1985). Aylor & Moran studied the effect of processing type on the corrosion behavior, utilizing different 6061 Al processing forms (wrought vs. powder metallurgy) and SiC/Al composite heat-treating (as-fabricated vs T-6 temper).…”