3.2    Potentiodynamic polarization measurement
 
The inhibition efficiency (IE %) was calculated for the aluminium alloy in 1 M H3PO4 containing 100 ppm, 500 ppm, 1000 ppm, 1500 ppm, 2000 ppm and 2500 ppm of Trigona sp. honey and  Apis sp. honey by using Equation 3.2. The percentage inhibition efficiency value was obtained from icorr data for both honey. From the result, it was obvious that current density of corrosion decreases as the concentration of the inhibitors increases. Better results and performances were observed when 1500 ppm of Trigona sp. and Apis sp. was used. The anodic and cathodic reactions were affected by the addition of Trigona sp. honey and also Apis sp. honey as seen in Figures 4.8 and 4.9. This situation may lead to the reducing of metal dissolution at anode and retarding the hydrogen evolution reaction at cathodic site after the addition of inhibitors. This resulted from the adsorption of inhibitor molecules on the aluminium surface (Farag et al., 2015). 
The value of corrosion potential Ecorr indicates two different polarizations which are anodic polarization and cathodic polarization. As the concentrations of inhibitors increase, the anodic and cathodic current densities will decrease. It was proven that the additive affected both anodic and cathodic processes. The value Ecorr shifts to positive (noble) direction when anodic polarization occurs and it will shift to negative (active) direction for cathodic polarization (Fouda et al., 2014). As seen in Table 4.5, compared to the Ecorr of  Trigona sp. honey concentration with the 1 M H3PO4 solution (blank), the values of Ecorr was less negative (anodically) and slightly more negative (cathodically) from 1500 ppm to 2500 ppm. This was consistent for  Apis sp. honey inhibitor. Therefore, the presence of inhibitors resulted in the shifting of the corrosion potential (Ecorr) towards the positive direction. According to literature report, when the displacement in Ecorr was bigger than 85 mV with respect to the corrosion potential of the blank, the inhibitor can be classified as a cathodic or anodic type (Shylesha et al., 2011). Otherwise, if the displacement is less than 85 mV, the inhibitor can be considered as mixed type (Abdullahi et al., 2014). In this study, the maximum displacement of both honeys was lower than 85 mV which can prove that the  Trigona sp. honey and  Apis sp. honey act as mixed type inhibitors with predominant anodically.
By extrapolating the anodic and cathodic Tafel lines to corrosion potential of the polarization curves, corrosion current density, icorr was determined. The icorr will decrease with increase in the concentration of the inhibitor. It was suggested that the adsorption of inhibitor molecules at the surface of aluminium occurred to form protective film on the aluminium surface (Yadav et al., 2015). Thus, the inhibitors act via adsorption on aluminium surface and blocking the active corrosion sites (Yurt et al., 2014). It was also due to the phenyl ring which has high electron density in the p-coumaric acid in the inhibitors. As a result, it will block the active site of the aluminium surface (Chakravarthy and Mohana, 2014). From the Tables 4.5 and 4.6, the value of anodic Tafel slope (βa) was observed to change with the addition of the inhibitors. It was suggested that the inhibitors were first adsorbed onto the metal surface and blocked the reaction sites of the metal surface resulting in the effecting of the anodic mechanism. However, the values of cathodic Tafel slope constant (βc) almost remain unchanged in the presence of the inhibitors. This situation suggested that the inhibitors did not interupt the mechanism of cathodic reaction with hydrogen evolution reaction and the inhibitors inhibit the corrosion by blocking the active sites on the aluminium surface (Caliskan and Akbas, 2012)
3.2    Electrochemical impedance spectroscopy (EIS)