**4.3.1 Inhibition mechanism of NH**

The inhibition performances of honey could be explained as follows: Fourier transform infrared (FTIR) spectrum in Figure 3 demonstrates that honey is a mixture of various compounds containing carbon (C), oxygen (polyphenols), nitrogen and sulphur (glucosinolates) which all can be adsorbed on the corroded metal (Radojcic *et al*., 2008). The bands at about 1055.3 and 1418.1 cm−1 are consists of C, O, H and N atoms, meanwhile the peak at 1255.6 cm−1 is due to sulphur (S). A band appearing near 2935.7 cm−1 proves the existence of C, O and H atoms in NH. A band located at 3355.1 cm−1 corresponds to O, N and H atoms.

The adsorption of NH onto the surface of Al-Mg-Si alloy may take place through all these functional groups. The simultaneous adsorption of the four functional groups forces the natural honey molecule to be horizontally oriented on the surface of Al-Mg-Si alloy (Gao *et al*., 2008). As the corrosion resistant concentration increases, the area of the metal surface covered by the corrosion resistant molecule also increases, leading to an increase in the IE.

Fig. 3. FTIR spectrum of NH.

IE (%)

CR *i*corr *R*p *R*ct

200 64.66 63.43 66.34 64.48 400 67.16 66.04 68.48 67.39 600 73.21 73.24 78.73 79.42 800 84.63 84.12 85.22 84.61 1000 91.85 91.58 89.61 90.19

200 66.84 67.62 66.81 69.88 400 78.70 78.68 77.69 77.23 600 79.66 80.02 79.50 80.49 800 87.81 88.01 87.13 88.01 1000 92.50 92.67 91.93 92.46

200 70.27 71.84 71.64 71.13 400 85.42 86.19 85.07 83.14 600 87.75 88.40 85.66 85.16 800 89.34 89.90 88.62 89.06 1000 93.98 93.98 93.38 92.92

Table 5. Values of IE (%) for Al-Mg-Si alloy at various concentrations of NH, VL and TS

The inhibition performances of honey could be explained as follows: Fourier transform infrared (FTIR) spectrum in Figure 3 demonstrates that honey is a mixture of various compounds containing carbon (C), oxygen (polyphenols), nitrogen and sulphur (glucosinolates) which all can be adsorbed on the corroded metal (Radojcic *et al*., 2008). The bands at about 1055.3 and 1418.1 cm−1 are consists of C, O, H and N atoms, meanwhile the peak at 1255.6 cm−1 is due to sulphur (S). A band appearing near 2935.7 cm−1 proves the existence of C, O and H atoms in NH. A band located at 3355.1 cm−1 corresponds to O, N

The adsorption of NH onto the surface of Al-Mg-Si alloy may take place through all these functional groups. The simultaneous adsorption of the four functional groups forces the natural honey molecule to be horizontally oriented on the surface of Al-Mg-Si alloy (Gao *et al*., 2008). As the corrosion resistant concentration increases, the area of the metal surface covered by the corrosion resistant molecule also increases, leading to an increase in the IE.

PP LPR EIS

Inhibitor *c* (ppm)

Honey

Vanillin

Tapioca

and H atoms.

**4.3.1 Inhibition mechanism of NH** 

#### **4.3.2 Inhibition mechanism of VL**

The inhibition process of vanillin could be explained as follows: FTIR spectrum illustrate that vanillin is an aromatic aldehyde containing carbonyl, methoxy, and hydroxyl groups arranged around the aromatic ring (Figure 4). The bands at about 1153.7 to 1199.9 cm−1 and 2362.3 cm−<sup>1</sup> in the spectrum are assigned to carbonyl group, meanwhile the bands located between 1429.8 to 1664.9 cm−1 are refers to hydroxyl group and aromatic compound (benzene ring).

Fig. 4. FTIR spectrum of VL.

Improvement of Corrosion Resistance of Aluminium Alloy by Natural Products 389

Uninhibited and inhibited samples were analyzed by SEM and EDS in order to identify the morphology and composition of the corrosion products before and after immersion in

The SEM micrograph of the unexposed Al-Mg-Si alloy is shown in Figure 6. It shows that the surface of the metal is absolutely free from any pits and cracks. Polishing scratches are

Figure 7 corresponds to the SEM of the specimen surface after 60 days of immersion in seawater. Flakes showing corrosion products like metal hydroxides and its oxides can be observed, however no pits or cracks were noticed (Gao *et al*., 2008). The figures also show the presence of micro organisms (plankton) on the surface of the specimen which contributes to the corrosion process. The corrosion process in deep seawaters occurs under very specific conditions and is characterized mainly by high chloride contents, the presence

Fig. 6. SEM analysis of the unexposed Al-Mg-Si alloy surface.

of CO2 and H2S and micro organisms (Yagan *et al*., 2006).

Fig. 7. SEM of Al-Mg-Si alloy surface after 60 days of immersion

**4.4 Surface morphology studies** 

**4.4.1 Unexposed specimen** 

**4.4.2 Unexposed specimen** 

seawater at 25°C.

also visible.

The adsorption of vanillin onto the surface of the aluminium alloy may take place through all these functional groups. The simultaneous adsorption of the three functional groups forces the vanillin molecule to be horizontally oriented at the surface of the aluminium alloy (Li *et al*., 2008). As the inhibitor concentration increases, the area of the metal surface covered by the inhibitor molecule also increases, leading to an increase in the IE.

Similar to the findings reported previously (El-Etre, 2001; Emregul and Hayvali, 2002; Li *et al*., 2008) the adsorption of vanillin mechanism is related to the presence of carbonyl, methoxy, and hydroxyl groups arranged around the aromatic ring in their molecular structures.
