**4.3.3 Inhibition mechanism of TS**

The inhibition process of tapioca starch can be explained as follows: FTIR spectrum shows that TS is composed of mixture of two molecular entities (polysaccharides), a linear fraction, amylase and highly branched fraction, amylopectin (Figure 5). Both of them are polymers of glucose.

Fig. 5. FTIR spectrum of TS.

Amylose is constituted by glucose monomer units joined to one another head to tail forming alpha–1, 4 linkages; these are linked with the ring oxygen atoms all on the same side. Amylopectin differs from amylase in that branching occurs, with an alpha–1, 6 linkages every 24–30 glucose monomer units. Amylopectin has phosphate groups attached to some hydroxyl group (Wu *et al*., 2009).

The peak at 923.4, 2931.3 and 3322.3 cm−1 in the FTIR spectrum are characteristic of hydroxyl group (OH). The adsorption of TS on Al-Mg-Si alloy surface would take place through all these functional groups. As the concentration of corrosion inhibitor increases, the part of the metal surface covered by the corrosion inhibitor molecule also increases, leading to an increase in the IE.
