**3.5 Corrosion mechanism**

400 Recent Trends in Processing and Degradation of Aluminium Alloys

It is interesting to relate the surface morphology to localized corrosion. Typical features of surface morphology after inhibitor treatment are shown in figure 13. Deposition of two types of the particles in concentric rings is seen. These are particles of Ce2O3 and Al2O3. The square shaped particles of cerium oxide are shown in Figure 14. The oxide layer comprising of Ce2O3 and Al2O3 are very stable and protect the MMC from corrosion in 3.5 wt% NaCl. However, once the layer reaches a certain thickness, it flakes off. The broken oxide layer in

Fig. 14. Square-shaped particles containing predominantly cerium chloride formed on

cathodic polarization (Zaki, 2009)

Fig. 15. Broken oxide layer forming blisters (mothballs)

Despite decades of research no conclusive mechanism on the localized corrosion of Al/SiC(p) composites has been described – The role of intermetallic and dislocation generation at Al/Sic (p) interface has not been conclusively established. No attack a SiC particles has been reported in literature.

 From several reliable studies it may be concluded that the pitting potential of monolithic alloys depends on the alloy composition and Ep which is more positive than that of reinforced material (Monticelli et al., 1997; Trazaskoma et al., 1990). The pitting resistance of several MMC investigated followed the order, Al2024 = Al6013 – 20Sic (p) > AL 6061>, Al 6013-20SiC (p) T4=Al5456 (Zaki 2000). In the studies conducted an abundant distribution of copper and secondary phase particles of Mg and Fe were observed.

Copper particles were also present in pit cavities. Analysis of corroded regions at the interface showed a greater concentration of copper compared to the surface away from the interface. The presence of AlCl3 in the oxide film has been indicated by EDS studies (Trazaskoma et al., 1990). Results show a high concentration of copper (3.5%) and Fe (1.77%). There is therefore, a sufficient evidence to show that the increased reactivity at the interface is responsible for localized corrosion of composites. The intermetallic precipitates act as anodic or cathodic sites for initiation of localized corrosion. It is also observed that homogenization of the surface minimizes corrosion the reactivity at the interface is further minimized as shown by temper T4. The SiC particles do not provide any sites for initiation of pits. A higher concentration of copper in pit cavities may be attributed to higher velocities which transport copper ions. Dislocation generation at the interface further activates the interface.

Two more factors are reported to influence, the mechanism of corrosion; Na:YAG laser treatment and machining. Electrochemical studies undertaken showed that the corrosion potential (Ecorr) increased by 79mv and the corrosion current density decreased by an order of magnitude for the laser treated specimens whereas the untreated surface showed extensive corrosion accompanied by abundant pits. The decrease of corrosion is reported to be due to reduction in the concentration of intermetallic precipitates.

The effect different machining conditions, WEDM, Carbide Turning and Diamond Turning on the electrochemical corrosion behaviour are shown in Figure 16. No significant difference in pitting corrosion potential between the three machining condition was observed. The magnitude of corrosion current for the three machining conditions however differed. Diamond turned specimens showed shallow pits at isolated sites accompanied by a high corrosion rate, whereas Carbide Turned specimens showed extensive pitting because of the hindrance of repassivation of pits due to micro and large crevices present on the surface, pits developed were deeper.

Corrosion Behavior of Aluminium Metal Matrix Composite 403

Surface treatment has a significant effect are localized corrosion as shown by the effect of laser treatment and effect of machining on the surface. The scanning micro effloresce

Although no conclusion mechanism of localized corrosion of Al MMC exists, there is sufficient evidence to show that Al/SiC interface acts as a centre for localized corrosion and a reduction in the concentration of intermetallic compounds is accompanied by a reduction in localized corrosion as shown by the effect of tempers T4 and T6 on localized

Based on the studies conducted in the last they decades, the following are the major

1. The mechanical properties of fiber, particulate, or whisker reinforced composites are strongly dependent upon the micro-structural parameters, size, shape, orientation and

2. The tensile strength and Vickers micro-hardness increases significantly with increasing volume fraction of reinforcement as exemplified by Al6013, 6061, 2024 reinforced with particulate and whiskers. The strain to failure also decrease with increased volume fractions of reinforcement - Sliding velocity leads to lower wear rates and lower

3. Increasing cutting speed increased tool wear. The highest wear rate was shown by 15

4. Because of accumulation of stress concentration and high dislocation density Al MMC's are sensitive to stress corrosion cracking is 3.5 wt% NaCl – Al6061/20vol% SiC(p)-T6 shows a good resistance to stress corrosion cracking. The polarization curves shifted to

5. The corrosion rates of MMC's decreased with exposure time in long term immersion tests. Heat treatment lowered corrosion rates because of the homogeneous distribution of the precipitates an reduction in their concentration on the Al/SiC interphase electrochemical studies on MMC's showed that the pitting potential decreased with increasing volume fraction of SiC(p) in Al6013 and 6061 reinforce with SiC particulate.

6. The effect of machining conditions on corrosion showed that electrical discharge machining provided higher resistance to pitting than carbide turning or diamond

7. The lowest rate of corrosion was shown by temper T4 if A6013 – 20 SiC(p) is 3.5 wt%

8. Studies in salt spray chamber showed a good resistance of MMC's to Corrosion and

9. MMC's exhibited a beneficial effect of inhibitor treatment with cerium chloride and sodium molybdate. Cerium Chloride has paved away more effective inhibition than

10. The mechanism of corrosion of MMC's is not conclusively understood. It has been, however, shown that the Al/SiC interface us highly reactive due to the presence of

Heat treatment enhances corrosion resistance in corrosive environment.

conclusion on the mechanical and corrosion behaviour of Al MMCs

frication coefficient as shown by SiC and B4C reinforcements.

volume fraction of the reinforcement.

wt% SiC (p) and the lowest by 5 wt% SiC.

Shallow pits contain intermetallic CuAl2.

NaCl containing silica and other particulate matter.

higher current densities.

turning machining.

sodium molybdate.

intermetallic compounds.

electrode studies have shown that Sic/Al interface is the centre f localized corrosion.

corrosion.

**4. Conclusion** 

Fig. 16. Potentiodynamic polarization curves of the composite machined to different conditions (Yue et al., 2002)

In the eclectically discharged machined specimen, a resolidified layer of aluminium provided a blanketing effect on the substrate. A reasonable range of passivity was produces on the surface. The high resistance was provided by a layer of oxide on the resolidified aluminium layer created by machining. From the above evidence it is to be understood that surface morphology plays an important role at the Al /SiC interface. Scanning micro reference studies has been employed for direct mapping the active centers on surface of electrode with a low dimensional resolution in micrometers. Evidence of micro pitting has been observed at the open-circuit potentials which were more negative than the pitting potential. In their studies, the same conclusion was reached; i. e the SiC/Al interface is the active center for localized corrosion due to the precipitation of intermetallic compounds (Zaki, 2000).

The observation that heat treatment increases corrosion resistance is shown by temper T4 of Al6013 /20 SiC (p), it is further supported by studies on temper T6 of Al2009/SiC (w), which showed a higher resistance to pitting compared to as rolled specimens It has been already stated above their heat treatment induces homogenization which causes a reduction in the concentration of intermetallic compounds and hence, reduces localized Corrosion.

From the above discussion it may be concluded that the composites are more sensitive to pitting than their monolithic counterparts unless they are subjected to T4 or T6 heat treatment. These is sufficient evidence to show AL/SiC interface is the main target of localized corrosion due to the presence of intermetallic particulates and inclusion which may form micro-galvanic cells and induce localized corrosion. It is also observed that SiC particles are not attacked.

Surface treatment has a significant effect are localized corrosion as shown by the effect of laser treatment and effect of machining on the surface. The scanning micro effloresce electrode studies have shown that Sic/Al interface is the centre f localized corrosion.

Although no conclusion mechanism of localized corrosion of Al MMC exists, there is sufficient evidence to show that Al/SiC interface acts as a centre for localized corrosion and a reduction in the concentration of intermetallic compounds is accompanied by a reduction in localized corrosion as shown by the effect of tempers T4 and T6 on localized corrosion.
