**3.4 Effect of Inhibitors**

It has been shown in earlier sections that Al/SiC metal matrix composites such as Al 6013-20 SiC (p) exhibit improved mechanical and physical properties compared to wrought alloys. However, they are more susceptible to pitting than their monolithic counterparts (Beccario et al., 1994; Trazaskama, 1990). They also exhibit a higher corrosion rate at velocities greater than the 2.3 ms-1 (Zaki, 2000). 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).

Studies on to investigate the effect of inhibitors on Al 6013 – 20 SiC (p) included weight loss, Electrochemical and re-circulation loop studies (Zaki, 2009).

Following inhibits solutions were used


The results of inhibitive action of K2Cr2O7 + 1000 pm NaHCO3 are summarized in Table 10.


Note; All experiments were conducted in 3.5 wt% NaCl

Table 10. The results of inhibitor action of k2Cr2O7+1000 ppm NaHCO3 (Zaki, 2009)

The corrosion behavior of Al6013–20SiC (p) is a very strong function of Al (OH)3 and once the film formation is completed it becomes independent of oxygen (Beccario et al., 1994). The crystals of boehmite have been observed on the surface of the alloy. The data generated in highly aggressive environment shows promising applications potential of this alloy in salt

It has been shown in earlier sections that Al/SiC metal matrix composites such as Al 6013-20 SiC (p) exhibit improved mechanical and physical properties compared to wrought alloys. However, they are more susceptible to pitting than their monolithic counterparts (Beccario et al., 1994; Trazaskama, 1990). They also exhibit a higher corrosion rate at velocities greater than the 2.3 ms-1 (Zaki, 2000). 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

Studies on to investigate the effect of inhibitors on Al 6013 – 20 SiC (p) included weight loss,

The results of inhibitive action of K2Cr2O7 + 1000 pm NaHCO3 are summarized in Table 10.

Corrosion rate in mpy(MDD) with no inhibitor

1.0 11.8(22.1) 3.07(5.76) 1.9 11.6(21.7) 7.63(14.32) 2.7 12.9(24.1) 8.4(15.77)

3.8 13.6(25.5) 9.63(18.08)

1.0 9.9(18.5) 3.61(5.68) 1.9 10.4(19.5) 4.31(8.09) 2.7 10.8(20.2) 5.53(10.38)

3.8 11.3(21.2) 6.60(12.39) 1.0 9.6(18.5) 2.01(3.77) 1.9 10.1(18.2) 2.70(5.07) 2.7 10.8(20.2) 3.40(6.38)

3.8 11.4(21.4) 3.80(7.13)

Table 10. The results of inhibitor action of k2Cr2O7+1000 ppm NaHCO3 (Zaki, 2009)

Corrosion rate in mpy(MDD)with inhibitors

water and humid environment typical of sea coastal environment in the Gulf Region.

attention in the last decade (Hinton & Arnold, 1986; Davenport et al., 1991).

Electrochemical and re-circulation loop studies (Zaki, 2009).

a. 1000 ppm K2Cr2O7 + 1000 pm NaHCO3 + 3.5 wt % NaCl

Following inhibits solutions were used

b. 1000 ppm Cerium chloride + 3.5 wt% NaCl c. 1000 ppm sodium molybdate + 3.5 wt % NaCl

> Velocity (ms-1)

Note; All experiments were conducted in 3.5 wt% NaCl

**3.4 Effect of Inhibitors** 

Alloy Designation

Al 6013-20 SiC(p)-O

Al 6013-20 SiC(p)-F

Al 6013-20 SiC(p)-T4

The reduction in the corrosion rate with K2Cr2O7 +NaHCO3 has been attributed to the formation of protective layer of boehmite Al (OH)3, 3H2O and bayrite Al2O3, H2O. The breakdown of the oxide layer leads to pitting. The reduction in the corrosion resistance at increased velocities is caused by continuous removal of protective layer by erodent particles. The protrusion of particulates also makes it difficult to achieve a passivating layer; hence the resistance to the impact of velocity is lowered.

The preferred site for localized corrosion is Al/SiC interface as this site is abundant in intermetallic compound (Zaki, 1998). The existence of thermal stresses and dislocation density at interface affects the kinetics of erosion corrosion and increases the sensitivity if Al/SiC interfaces to erosion-corrosion. Because of the encouraging results of inhibition treatment of Al7057, and Al1000, with cerium chloride and sodium molybdate, studies were further conducted on Al6013 –20 Vol. % SiC(p) MMC. The effect of inhibition treatment is shown in Table 11 below.


Table 11. Effect of Inhabition Treatment

As shown by table 11 cerium chloride is a more effective inhibitor than sodium molybdate as shown by a larger reduction in corrosion rate brought about by addition of cerium chloride compared to sodium molybdate. The corrosion rate of temper of the MMC is reduced from 19.13 mpy to 3.96 with Cerium Chloride at 100°C which is very significant. Electrochemical studies were also conducted at 50, 70 and 100°C to observe the effect of temperature on inhibition. The electrochemical data obtained by above studies is shown Table 12.

The results of studies summarized in Table 12 clearly established that cerium chloride is a more affective inhibitor than sodium molybdate. The large difference between the corrosion potential (Ecorr) and the pitting potential (Ep) shows that the cerium chloride is a more affective inhibitor in 3.5 wt % NaCl. The corrosion potential (Ecorr) shifts closer to Ep which shows the sensitivity of the MMCS to localized pitting in Sodium Chloride without inhibition. The cathodic polarization curve of temper T4 of the alloy in 3.5 wt% NaCl +1000 ppm CeCl3 in dearated condition is shown Figure 11. The curves are overlaid on the main curve. A maximum reduction in current density (from 234 to 25.1uA/cm2) is exhibited by Temper T4 in cerium chloride (Zaki, 2009). The current densities recorded are summarized in the Table 13.

Corrosion Behavior of Aluminium Metal Matrix Composite 399

Cerium chloride acts as a strong cathodic inhibitor for the alloy. Sodium molybdate on the other hand acts as an anodic inhibitor which acts by raising the pitting potential (Up) in the positive direction while maintaining Ecorr negative to Ep. A typical cyclic polarization curve of the temper T4 of the alloy in 3.5 wt% NaCl + 1000 ppm NaMoO4 is shown in Figure12.

Fig. 12. A typical cyclic polarization curve of Al 6013-20 SiC (p)-T4 temper of the alloy in 3.5

Fig. 13. Surface morphology of Al 6013-SiC (p) in 3.5 wt. % NaCl containing 1000 ppm CeCl3

wt.% NaCl + 1000 ppm sodium molybdate in deaerated conditions

(Zaki, 2009)

The corrosion potential tends to shifts to more positive values.


Table 12.

Fig. 11. A cathodic polarization curve of temper T4 of the alloy in 3.5 wt% NaCl + 1000 ppm CeCl3 in deaerated condition (Zaki, 2009)


Table 13. Current Densities of MMCS after Inhibition (Zaki, 2009)

50 F 3.301 -0.783 0.57 2.80(5.28) Cerium Chloride 50 T.4 2.43 -0.78 1.1 0.47(0.88)

70 F 9.14 -0.915 3.993 1.68(3.14) Cerium Chloride 70 T.4 10.07 -0.993 2.155 0.92(1.71)

100 F 100 -0.868 100 6.50(12.16) Sodium Molybdate 100 T.4 58 -0.947 74 3.96(7.41)

> **Legend Title NaCL CeCl3 Na2Mo**

**-0.60**

**-0.65**

**-0.70**

**-0.75**

**-0.80**

**E vs. SCE(V)**

**-0.85**

**-0.90**

**-0.95**

**-1.00**

Table 13. Current Densities of MMCS after Inhibition (Zaki, 2009)

CeCl3 in deaerated condition (Zaki, 2009)

50 O 9.004 -0.8 3.727 1.60(2.99)

70 O 1.281 -0.909 4.757 2.04(3.81)

50 O 44.5 -0.8 150 1.24(3.22) 50 F 11.91 -0.716 216.5 1.92(3.60) 50 T.4 41.3 -0.791 72.8 0.47(0.88) 70 O 58.77 -0.909 272.6 2.74(5.11) 70 F 23.76 -0.916 137.0 2.12(4.06)

70 T.4 34.87 -0.867 111.5 0.75(1.46) 100 O 100 -0.87 100 8.60(16.00)

> **-8.00 -7.50 -7.00 -6.50 -6.00 -5.50 -5.00 -4.50 -4.00 -3.50 -3.00 log (I/area)**

Fig. 11. A cathodic polarization curve of temper T4 of the alloy in 3.5 wt% NaCl + 1000 ppm

Sr. No. Media Icorr(µA│cm2) 1 3.5 wt % NaCl 234 2 3.5 wt% NaCl +1000 ppm Cecl3 25.1 3 3.5 wt% NaCl + 1000 ppm NaMoo4 178

Temper R(K.ohms) Ecorr(mv) Icorr(µA/cm2 Corrosion

rate mpy(mdd)

Solution

Sodium Molybdate

Table 12.

Temperature °C

Cerium chloride acts as a strong cathodic inhibitor for the alloy. Sodium molybdate on the other hand acts as an anodic inhibitor which acts by raising the pitting potential (Up) in the positive direction while maintaining Ecorr negative to Ep. A typical cyclic polarization curve of the temper T4 of the alloy in 3.5 wt% NaCl + 1000 ppm NaMoO4 is shown in Figure12. The corrosion potential tends to shifts to more positive values.

Fig. 12. A typical cyclic polarization curve of Al 6013-20 SiC (p)-T4 temper of the alloy in 3.5 wt.% NaCl + 1000 ppm sodium molybdate in deaerated conditions

Fig. 13. Surface morphology of Al 6013-SiC (p) in 3.5 wt. % NaCl containing 1000 ppm CeCl3 (Zaki, 2009)

Corrosion Behavior of Aluminium Metal Matrix Composite 401

the form of mothballs can be observed in Figure 15. It has been reported that cathodic reaction proceeds at the sites of intermetallic precipitates of copper and its solution becomes alkaline. The film of cerium oxide replaces the film of aluminum hydroxide with increased exposure time (Muhammad & Edwin, 2004; Misra et al., 2007). Whereas the studies on the inhibition of AlMMC are still lacking, there is sufficient evidence to show that cerium chloride is an effective inhibitor for corrosion protection of AlMMC Sodium molybdate is not as effective as cerium chloride shown by the studies reported above composite in

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

 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 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

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

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,

copper and secondary phase particles of Mg and Fe were observed.

be due to reduction in the concentration of intermetallic precipitates.

chloride containing environment.

particles has been reported in literature.

**3.5 Corrosion mechanism** 

interface.

pits developed were deeper.

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)

the form of mothballs can be observed in Figure 15. It has been reported that cathodic reaction proceeds at the sites of intermetallic precipitates of copper and its solution becomes alkaline. The film of cerium oxide replaces the film of aluminum hydroxide with increased exposure time (Muhammad & Edwin, 2004; Misra et al., 2007). Whereas the studies on the inhibition of AlMMC are still lacking, there is sufficient evidence to show that cerium chloride is an effective inhibitor for corrosion protection of AlMMC Sodium molybdate is not as effective as cerium chloride shown by the studies reported above composite in chloride containing environment.
