**3.3 Flow induced corrosion and Erosion corrosion**

The resistance of metallic equipment and structures to the impact of flow induced corrosion is extremely important as it affects their operational life and integrity of equipment. Whereas the effect of velocity on the erosion/corrosion of steel copper, and aluminium alloys are widely reported in literature the information on the metal matrix composite is scanty (Rohatgi, 2004; Griffen &Turnbull, 1994; Lin et al., 1992; Mansfield & Jeanjagnet, 1984; Chen & Mansfeild, 1997; Hihara, 2010; Colman et al., 2011). Studies on Al 6013–20 SiC were conducted in a customized recirculation loop as shown in Figure 7. It consisted of entry valves, a manometer, a centrifugal water pumps, a flow meter and several specimen holders to accommodate flat specimens. Each specimen holders contained four specimens which were housed in an outside container. The velocity was varied by varying the chamber of the specimen holders. Three tempers of Al6013-20 SiC (p) were investigated in the loop. In which a solution of 3.5wt%% NaCl was flowing at velocities ranges from 1-4 ms-1.

Fig. 7. Schematic diagram of PVC recirculating loop (Zaki, 2001)

After exposure of 100 hours it was shown that temper (0) annealed, and temper F, as fabricated, showed a lower resistance to corrosion in 3.5 wt% NaCl with and without polystyrene suspended particles. Upon increasing the temperature form 30 to 50 and 90 C, the erosion corrosion rate increased as shown in Table 7 and 8 (Zaki, 2007).

The resistance of metallic equipment and structures to the impact of flow induced corrosion is extremely important as it affects their operational life and integrity of equipment. Whereas the effect of velocity on the erosion/corrosion of steel copper, and aluminium alloys are widely reported in literature the information on the metal matrix composite is scanty (Rohatgi, 2004; Griffen &Turnbull, 1994; Lin et al., 1992; Mansfield & Jeanjagnet, 1984; Chen & Mansfeild, 1997; Hihara, 2010; Colman et al., 2011). Studies on Al 6013–20 SiC were conducted in a customized recirculation loop as shown in Figure 7. It consisted of entry valves, a manometer, a centrifugal water pumps, a flow meter and several specimen holders to accommodate flat specimens. Each specimen holders contained four specimens which were housed in an outside container. The velocity was varied by varying the chamber of the specimen holders. Three tempers of Al6013-20 SiC (p) were investigated in the loop. In which a solution of 3.5wt%% NaCl was flowing at

**3.3 Flow induced corrosion and Erosion corrosion** 

Fig. 7. Schematic diagram of PVC recirculating loop (Zaki, 2001)

the erosion corrosion rate increased as shown in Table 7 and 8 (Zaki, 2007).

After exposure of 100 hours it was shown that temper (0) annealed, and temper F, as fabricated, showed a lower resistance to corrosion in 3.5 wt% NaCl with and without polystyrene suspended particles. Upon increasing the temperature form 30 to 50 and 90 C,

velocities ranges from 1-4 ms-1.


Table 7. Variation of Erosion-Corrosion Rate with Velocity in 3.5wt%% NaCl + 2%Vol Polystyrene (Zaki, 2007)


Table 8. The effect of temperature on the erosion – corrosion behavior of Al 6013 – 2051 C (p) in 3.5 wt % NaCl + 2%Vol Polystyrene (Zaki, 2007)

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

Corrosion Behavior of Aluminium Metal Matrix Composite 395

Time Temper-0 Temper-F Temper-T4 200 10.23 V (19.8) 8.42 (15.78) 7.12 (13.35) 400 9.11 (17.8) 7.78 (14.58) 6.18 (11.09) 600 6.38 (11.96) 6.06 (9.49) 4.38 (8.21) 800 21.92 (9.23) 3.98 (7.46) 2.82 (5.28) 1000 4.66 (8.74) 3.76 (7.05) 2.63 (4.53) 1200 4.27 (8.01) 3.68 (6.90) 2.50 (4.83)

A decrease in corrosion rate with increased exposure period was observed for all three tempers. The MMC temper T4 showed the highest resistance to pitting. The surface of the composite was often covered with a gelatinous product of aluminum hydroxide Al(OH)3. The pit environment was acidic and bubbles of hydrogen rose from the surface forming corrosion chimneys. The hydrogen bubbles pump up AlCl3(OH)3 to the outside which reacts with water to form Al(OH)3 (Burleigh et al., 1995). The pitting depth in temper T4 were lower than pitting depths in F and O tempers. It was reported that a high concentration of intermetallic compounds was observed at Al/SiC inter-phases which lead to localized corrosion (A). The corrosion rate of Al6013-20SiC (p) decreased for all tempers on increasing the temperature form 50 to 75°C and increased again on raising the temperature to 100°C. This change may be attributed to the changes brought about by the composition of the protective films from being bayerite (AlO(OH)) to boehmite (Al2O3, H2) as shown by FTIR

Fig. 10. Cross section of singleton salt fog corrosion test cabinet

Table 9. Corrosion rates of Al6013/20SiC(p) in Salt Spray Chamber

(Fourier transformation infra-red) spectroscopy).

schematic of salt spray chamber is given in Figure 9. The cabinet comprised of a basic chamber level matic test reservoir (1.0 gal salt solution), reservoir (3.0 gal), bubble tank, twin optic fog assembly, and accessories such as a lower assembly bubble tank heater, control valves, and cabinet heaters. The cross section of the assembly is shown in Figure 10. The results obtained for O, F, and T4 Tempers of the alloy composite in the fog cabinet are shown in Table 9.

Fig. 8. TEM micrograph of Al 6013/SiC interface showing dislocation generations

Fig. 9. A schematic of salt spray chamber

schematic of salt spray chamber is given in Figure 9. The cabinet comprised of a basic chamber level matic test reservoir (1.0 gal salt solution), reservoir (3.0 gal), bubble tank, twin optic fog assembly, and accessories such as a lower assembly bubble tank heater, control valves, and cabinet heaters. The cross section of the assembly is shown in Figure 10. The results obtained for O, F, and T4 Tempers of the alloy composite in the fog cabinet are

Fig. 8. TEM micrograph of Al 6013/SiC interface showing dislocation generations

Fig. 9. A schematic of salt spray chamber

shown in Table 9.

Fig. 10. Cross section of singleton salt fog corrosion test cabinet


Table 9. Corrosion rates of Al6013/20SiC(p) in Salt Spray Chamber

A decrease in corrosion rate with increased exposure period was observed for all three tempers. The MMC temper T4 showed the highest resistance to pitting. The surface of the composite was often covered with a gelatinous product of aluminum hydroxide Al(OH)3. The pit environment was acidic and bubbles of hydrogen rose from the surface forming corrosion chimneys. The hydrogen bubbles pump up AlCl3(OH)3 to the outside which reacts with water to form Al(OH)3 (Burleigh et al., 1995). The pitting depth in temper T4 were lower than pitting depths in F and O tempers. It was reported that a high concentration of intermetallic compounds was observed at Al/SiC inter-phases which lead to localized corrosion (A). The corrosion rate of Al6013-20SiC (p) decreased for all tempers on increasing the temperature form 50 to 75°C and increased again on raising the temperature to 100°C. This change may be attributed to the changes brought about by the composition of the protective films from being bayerite (AlO(OH)) to boehmite (Al2O3, H2) as shown by FTIR (Fourier transformation infra-red) spectroscopy).

Corrosion Behavior of Aluminium Metal Matrix Composite 397

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

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

> Corrosion rate in 3.5% NaCl+1000 ppm

0 4.72(8.86) 3.8(7.13)

T4 1.71(3.21) 0.9(1.69) 0 8.3(15.5) 5.06(9.5)

T4 2.54(4.77) 2.01(3.72) 0 12.90(24.2) 8.05(15.11)

T4 10.19(19.13) 5.41(10.15)

Sr. No Temperature °C Temper Mpy(mdd) Mpy(mdd)

1 50 F 2.24(4.13) 1.8(3.38)

2 70 F 6.53(12.26) 4.01(7.5)

3 100 F 11.60(21.7) 8.26(15.41)

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

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

Corrosion rate in 3.5% NaCl+1000 ppm Namoo4, CeCl3

resistance to the impact of velocity is lowered.

shown in Table 11 below.

Table 11. Effect of Inhabition Treatment

Table 12.

in the Table 13.

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