**3.2 Localled corrosion of ALMMC's**

If is generally accepted that MMC are in general more prone to corrosion than their monolithic counterparts (Berkely et al., 1998; Turnbull and Corros, 1992; Trzskoma, 1991). Conflicting views have been presented on the causes of the localised corrosion. The results of the studies showed that galvanic corrosion between the matrix and the reinforcement occurs. However, this is related to the machining conditions. Three different machining process; Wielding Electrical Discharge Machine (WEDM), Cemented Carbide Turning and Single Point Diamond Turning were employed for investigation. The test results for different process are shown in Table 5 (Yue et al., 2002).


Table 5. Electrochemical parameters for different machining conditions (Yue et al., 2002)

Corrosion Behavior of Aluminium Metal Matrix Composite 391

**ENERGY (KEV) ENERGY (KEV)**

Fig. 4. EDS spectrums of (a) Al2Cu and (b) (CuFeMn) Al6 inclusions (Feng et al., 1998)

Fig. 5. Scanning electron micrographs of pits on interfaces of (a) SiCp-2024 Al matrix, and

Fig. 6. Corroded surface of the as-rolled specimen after the polarization test (a), (b) showing

**Cu**

**Mn**

**0.0 2.0 4.0 6.0 8.0 10.0**

**(a) (b)**

**Fe**

100FS 100FS

**0.0 2.0 4.0 6.0 8.0 10.0**

(b) inclusions-2024 Al matrix (Feng et al., 1998)

pit morphology (Yue et al., 2000)

**Al Al Cu**

The electrical discharge machining showed the highest value of pitting potential. The resolidified layer did not show any extensive pitting. The results show that surface conditions have a major effect on pitting potential and the resistance to pitting may be shown by Epit - ECorrosion. The difference above is not sufficient to predict corrosion susceptibility. It may be observed that silicon carbide is an insulator and there is hardly any possibility of cathodic reaction occurring on the surface of particles. The theory that Al/SiC is sensitive to corrosion because of micro galvanic coupling applies to some intermetallic compounds, cathodic to the matrix such as CuAl2 which is formed. So far there is no general agreement on the role of SiC particulates on the mechanism of localized corrosion. The electrochemical behaviour of Al2024/AlSiC has been also investigated by scanning micro reference electrode imaging system (Feng et al., 1981; Isacs & Vyas, 1981). The results of investigations on Al2024/Al SiC (A) are given in Table 6.


Table 6. Summary of electrochemical data (Feng et al., 1981; Isacs & Vyas, 1981)

It was observe that pitting potential Ep decreased as the volume fraction of SiC particulate reinforcement increased. The relation between the volume fraction and EProtection It was clearly observed that the pitting attack occurred at SiC/Al interface which contained intermetallic Cu and Al precipitates. The presence of Mg, Cu, and Fe compounds in Al6013/20% Vol. of SiC has been confirmed also in another work in recent years (Zaki et al., 2000). The interfacial regions may act as active centers for localized corrosion on immersion in sodium chloride solution. The EDS spectrum of Al2Cu is shown in Figure 4. The pits on Al 2024/SiC interface are shown in Figure 5. In Al 2024/SiC MMC, Mg may segregate in addition to the precipitates of Al2Cu Mg and Al2Cu. The segregated magnesium may form active galvanic couple with Al matrix (Jamaludin et al., 2008). There is also the possibility of the intermetallic precipitates to act as local anodes or cathodes because of the difference between the open circuit potentials of these intermetallic with Al matrix. As seen above the role of the precipitates and inclusion is not clearly understood. However, the evidence of localized corrosion of Al MMC suggests, that the Al/SiC interface in active and responsible for localized corrosion. This is also confirmed by studies on (Al 2009/SiC W) (W = whisker). In the rolled material extensive pitting occurred, and on removing the corrosion products it was observed that the pits contained particles CuAl2 (Rohatgi, 2003). On heat treatment the amount of CuAl2 particles was significantly reduced (Rohatgi, 2003) and the rate of corrosion also diminished which suggested that the heat treatment diminished Mg, Fe and CuAl2 precipitates Figure 6 shows the effect of heat treatment on the corrosion behaviour of T6 and as rolled Al 2009/Sic (w) composite. The corroded surface of as rolled specimens is shown in Figure 6.

The electrical discharge machining showed the highest value of pitting potential. The resolidified layer did not show any extensive pitting. The results show that surface conditions have a major effect on pitting potential and the resistance to pitting may be shown by Epit - ECorrosion. The difference above is not sufficient to predict corrosion susceptibility. It may be observed that silicon carbide is an insulator and there is hardly any possibility of cathodic reaction occurring on the surface of particles. The theory that Al/SiC is sensitive to corrosion because of micro galvanic coupling applies to some intermetallic compounds, cathodic to the matrix such as CuAl2 which is formed. So far there is no general agreement on the role of SiC particulates on the mechanism of localized corrosion. The electrochemical behaviour of Al2024/AlSiC has been also investigated by scanning micro reference electrode imaging system (Feng et al., 1981; Isacs & Vyas, 1981). The results of

Fraction Epitting Eprotection Ecorrosion

0.01 m NaCl

0 – 430 – 497 – 565 – 653 – 620 – 612 – 612 – 574 5 – 460 – 528 – 597 – 750 – 700 – 670 – 670 – 610 10 – 485 – 555 – 625 – 740 – 765115(T) – 720 – 725 – 688 15 – 538 – 632 – 662 – 700 – 720 – 720 – 750 – 671 20 – 550 – 650 – 692 – 670 – 670 – 775 – 775 – 671

It was observe that pitting potential Ep decreased as the volume fraction of SiC particulate reinforcement increased. The relation between the volume fraction and EProtection It was clearly observed that the pitting attack occurred at SiC/Al interface which contained intermetallic Cu and Al precipitates. The presence of Mg, Cu, and Fe compounds in Al6013/20% Vol. of SiC has been confirmed also in another work in recent years (Zaki et al., 2000). The interfacial regions may act as active centers for localized corrosion on immersion in sodium chloride solution. The EDS spectrum of Al2Cu is shown in Figure 4. The pits on Al 2024/SiC interface are shown in Figure 5. In Al 2024/SiC MMC, Mg may segregate in addition to the precipitates of Al2Cu Mg and Al2Cu. The segregated magnesium may form active galvanic couple with Al matrix (Jamaludin et al., 2008). There is also the possibility of the intermetallic precipitates to act as local anodes or cathodes because of the difference between the open circuit potentials of these intermetallic with Al matrix. As seen above the role of the precipitates and inclusion is not clearly understood. However, the evidence of localized corrosion of Al MMC suggests, that the Al/SiC interface in active and responsible for localized corrosion. This is also confirmed by studies on (Al 2009/SiC W) (W = whisker). In the rolled material extensive pitting occurred, and on removing the corrosion products it was observed that the pits contained particles CuAl2 (Rohatgi, 2003). On heat treatment the amount of CuAl2 particles was significantly reduced (Rohatgi, 2003) and the rate of corrosion also diminished which suggested that the heat treatment diminished Mg, Fe and CuAl2 precipitates Figure 6 shows the effect of heat treatment on the corrosion behaviour of T6 and as rolled Al 2009/Sic (w) composite. The corroded surface of as rolled specimens is

0.1 m NaCl 0.5 m NaCl

0.5 NaCl

0.1 NaCl

0.5 m NaCl

Table 6. Summary of electrochemical data (Feng et al., 1981; Isacs & Vyas, 1981)

investigations on Al2024/Al SiC (A) are given in Table 6.

6.1 m NaCl

Volume

shown in Figure 6.

0.01 m NaCl

Fig. 4. EDS spectrums of (a) Al2Cu and (b) (CuFeMn) Al6 inclusions (Feng et al., 1998)

Fig. 5. Scanning electron micrographs of pits on interfaces of (a) SiCp-2024 Al matrix, and (b) inclusions-2024 Al matrix (Feng et al., 1998)

Fig. 6. Corroded surface of the as-rolled specimen after the polarization test (a), (b) showing pit morphology (Yue et al., 2000)

Corrosion Behavior of Aluminium Metal Matrix Composite 393

Velocity Temper(0) Temper (F) Temper T4 1.0 11.8 9.9 9.6 2.7 12.6 10.8 10.1 3.8 12.9 11.3 11.4

Temperature (oC) Velocity Temper(O) Temper(F) Temper(T4)

2.7 172 17.2 148 75

2.7 17.8 17.8 163 90

Table 8. The effect of temperature on the erosion – corrosion behavior of Al 6013 – 2051 C (p)

The erosion-corrosion rate increased, linearly with velocity in the presence of SiC particles. It was also found that Temper (T4) of the alloy showed the best resistance to corrosion. The rate of erosion corrosion varied also with temperature. The best resistance offered by T4 may be attributed to the homogenization of the surface structure, less clustering of SiC particles, a uniform distribution of secondary intermetallic phases such as CuAl2 and minimization of micro-crevices (Zaki, 2000). The localized attack was confined to Al 6013/20 SiC (p) interface. A large number of secondary phase particles were observed. After studies showed the presence of Cu 3.55 %, Fe 1.77 %, Mg 1.71 %, and some Cl (0.32%) a high dislocation density was observed at the interface Figure 8 (Zaki, 2000). The formation of coherent films was made more difficult by the protrusion of the particles. This factor adds significantly the erosion –corrosion caused by polystyrene particles. The surface is subjected to a cycle of destruction and reformation of a protective film as a result of impact of polystyrene particles. The corrosion product which accumulates at the interface may act as cathode and increase the cathode / anode area ratio causing an overall increase in the rate of corrosion. Alloy Al 6013 / 20 SiC (p) in temper T4 offered of temper T4 offered a good resistance to erosion–corrosion. It can be used in water containing Silica or other particulate matter without undertaking any major risk. Al 6013 reinforced with 20 Vol. % SiC (p) was designed to have improved mechanical properties over those of AAl1l 6061/SiC (p). The corrosion resistance of al 6013 /20 Sic (p) was determined in fog testing cabinet (Zaki, 2000). A

Table 7. Variation of Erosion-Corrosion Rate with Velocity in 3.5wt%% NaCl + 2%Vol

Polystyrene (Zaki, 2007)

50

in 3.5 wt % NaCl + 2%Vol Polystyrene (Zaki, 2007)

Corrosion Rate(In 3 weight% NaCl + Vol% Polystyrene(mpy)

1.0 12.1 10.3 9.9 1.9 3.6 11.2 10.1 2.7 14.2 12.1 11.4

3.8 14.9 13.6 10.3 1.0 11.9 11.9 117 1.9 15.5 15.5 161

3.8 19.6 19.6 159 1.0 13.3 13.3 12.3 1.9 13.1 15.1 13.6

3.8 19.7 19.7 17.6

Erosion Corrosion Rate(mpy)
