**4. Results and discussion**

380 Corrosion Resistance

The study test solution was tropical seawater collected approximately 100 m from the shoreline of Pantai Teluk Kalong, Kemaman, Terengganu. Pantai Teluk Kalong is located near Teluk Kalong Industrial Estate (oil and gas industrial area) and Kemaman Supply Base

Pantai Teluk Kalong was selected in this research due to widely application of Al-Mg-Si alloy in the shipping, marine, oil and gas industrials surrounding the area. The values of physicochemical properties of seawater such as salinity, dissolved oxygen, pH and temperature were monitored during the immersion test. Average selected physicochemical

pH Temperature (°C) Salinity (g/L) Dissolved oxygen (mg/L)

The corrosion inhibitors tested in this study were natural honey, vanillin and tapioca starch. The choice of natural honey (NH), vanillin (VL) and tapioca starch (TS) as corrosion

The Nicolet 380 Fourier transform infrared (FTIR) spectrometer was used to determine the

All electrochemical measurements (PP, LPR and EIS) were accomplished with Autolab frequency response analyzer (FRA) and general purpose electrochemical system (GPES) for

i. the selected natural products contained the possible adsorption functional groups

7.63 28.3 34.8 7.66

Alloys % Weight Silicon Si 0.40–0.80 Iron Fe 0.7 Copper Cu 0.15–0.40 Manganese Mn 0.15 Magnesium Mg 0.80–1.20 Chromium Cr 0.04–0.35 Zinc Zn 0.25 Titanium Ti 0.15 Others (each) 0.05 Others (total) 0.15 Aluminium Al Remainder

Table 1. The chemical-composition of Al-Mg-Si alloy

data of the seawater used are reported in Table 2.

Table 2. Physicochemical properties of seawater

inhibitors for the study were based on the following:

iii. good solubility in water and non-toxic

function group for each inhibitor.

**3.4 Electrochemical measurements** 

ii. these natural products are commercially available at low cost

**3.3 Corrosion inhibitors** 

port which is about 6.78 km from Chukai, Terengganu, Malaysia.

**3.2 Test solution** 

Many corrosion phenomena can be explained in terms of electrochemical reactions. Therefore, electrochemical techniques can be used to study these phenomena. Measurements of current-potential relations under carefully controlled conditions can yield information on corrosion rates, coatings and films, pitting tendencies and other important data.

#### **4.1 Electrochemical measurements**

The corresponding corrosion potential (*E*corr), corrosion current density (*i*corr), anodic Tafel slope (*b*a), cathodic Tafel slope (*b*c) and *CR* for uninhibited and inhibited systems from PP measurement are listed in Table 3. The data demonstrates that the *E*corr values shift to more positive values as the concentration of added studied inhibitors are increased. On the other hand, the corrosion current densities are markedly declined upon addition of the studied corrosion inhibitors. The extent of its decline increases with increasing of the corrosion inhibitor concentration. Moreover, the numerical values of both anodic and cathodic Tafel slopes decreased as the concentration of inhibitors were increased. This means that the three natural products have significant effects on retarding the anodic dissolution of aluminium alloy and inhibiting the cathodic hydrogen evolution reaction.

Anodic and cathodic processes of aluminium corrosion in seawater are dissolution of aluminium and reduction of dissolved oxygen, respectively, as

$$4\text{Al} \rightarrow 4\text{Al}^{\text{++}} + 12\text{e}^{\cdot} \tag{1}$$

$$\text{2O}\_2 + 6\text{H}\_2\text{O} + 12\text{e}^- \to 12\text{OH}^-\tag{2}$$

Hence, Al3+ reacts with OH- to form aluminum hydroxide near the aluminium surface as below

$$4\text{Al} + 3\text{O}\_2 + 6\text{H}\_2\text{O} \to 4\text{Al(OH)}\_3\tag{3}$$

Improvement of Corrosion Resistance of Aluminium Alloy by Natural Products 383

1000 ppm of NH, VL and TS i.e. 112.72, 145.05 and 177.00 kΩ cm2, respectively. A higher of *R*p indicates the lower of the corrosion rate. The corrosion resistance obtained by LPR measurement was compared between the studied inhibitors. The inhibitor concentration was plotted against the values of *R*p for each studied inhibitor (Figure 1). The results show that the values of *R*p after addition of inhibitor increase with the following sequence: NH <

The values of *R*ct and double layer capacitance, *C*dl for Al-Mg-Si alloy at various concentrations of NH, VL and TS are presented in Table 4. The results show that the *R*ct values increase with the addition of corrosion inhibitors when compared with those without corrosion inhibitor. Furthermore, the values of *R*ct are observed to increase with the increasing corrosion inhibitor concentration, which can be attributed to the formation of a protective over-layer at the metal surface. It becomes a barrier for the charge transfers.

The values of *R*ct for the alloy in inhibited solution with NH were enhanced up to 10 times higher as compared to that of the value of *R*ct in uninhibited solution. Meanwhile, VL and TS shown better performance in improving the value of *R*ct for Al-Mg-Si alloy in studied aggressive solution, where they were increased the values up to 13 and 14 times higher,

> 200 400 600 800 1000 *c* (ppm)

It should be noted that while *R*ct values increase with the addition of corrosion inhibitor, the capacitance, *C*dl values decrease indicating the formation of a surface film. Thus, effective corrosion resistance is associated with high *R*ct and low *C*dl values (Yagan *et al*., 2006). Increase in *R*ct values and decrease in *C*dl values by NH, VL and TS indicated that the studied inhibitors inhibit the corrosion of Al-Mg-Si alloy in seawater by adsorption mechanism (Noor, 2009) and the thickness of the adsorbed layer increases with the increase

VL < TS.

respectively.

*R*p (kcm2

)

NH VL TS

Fig. 1. Rp versus concentration of the studied inhibitors

of inhibitor concentration.


Table 3. The electrochemical parameters of Al-Mg-Si alloy in absence and presence of different concentrations of NH, VL and TS

and the hydroxide precipitates on the surface due to its low solubility product. Aluminium hydroxide changes gradually to aluminium oxide, resulting in the formation of passive film (Aramaki, 2001):

$$2\text{Al(OH)}\_{3} \rightarrow \text{(Al}\_{2}\text{O)} + 3\text{FeO} \tag{4}$$

However, this nature oxide film does not offer sufficient protection against aggressive anions and dissolution of aluminium substrate occurs when exposed to corrosive solution.

Seawater predominantly consists of about 3.5% of sodium chloride (NaCl) and many other ions. Chloride ions are very strong and could easily penetrate the passive film. Thus, dissolution of the aluminium substrate occurs and results in corrosion. The adsorption of the corrosion inhibitor competes with anions such as chloride. By assuming that the corrosion inhibitor molecules preferentially react with Al3+ to form a precipitate of salt or complex on the surface of the aluminum substrate, the anodic and cathodic processes subsequently suppressed by inhibitor molecules. Thus, this result suggests that the protective film that was formed comprise aluminium hydroxide, oxide and salts or complexes of the corrosion inhibitor anions.

Polarization resistance, *R*p values for Al-Mg-Si alloy in seawater in the presence and absence of corrosion inhibitor were determined using linear polarization method. The values of *R*<sup>p</sup> are tabulated in Table 3. Generally, the value of *R*p increased with increasing inhibitor concentration for all studied inhibitor. The highest *R*p values for Al-Mg-Si alloy obtained at

*b*a (mV dec-1)

Blank -796 1.622 101 274 1.078 11.71

200 -554 0.593 65 93 0.381 34.78 400 -546 0.551 69 85 0.354 37.15 600 -553 0.434 75 89 0.289 55.06 800 -550 0.258 67 92 0.166 79.23 1000 -556 0.137 48 59 0.088 112.72

200 -693 0.538 56 66 0.349 35.28 400 -642 0.346 83 61 0.230 52.48 600 -590 0.330 32 67 0.215 57.13 800 -545 0.198 80 76 0.129 90.98 1000 -530 0.122 40 43 0.079 145.05

200 -578 0.483 65 133 0.315 41.29 400 -565 0.237 82 86 0.155 78.43 600 -577 0.199 65 128 0.130 81.67 800 -567 0.173 44 57 0.113 102.89 1000 -514 0.103 43 49 0.064 177.00

Table 3. The electrochemical parameters of Al-Mg-Si alloy in absence and presence of

and the hydroxide precipitates on the surface due to its low solubility product. Aluminium hydroxide changes gradually to aluminium oxide, resulting in the formation of passive film

 2Al(OH)3 (Al2O3) + 3H2O (4) However, this nature oxide film does not offer sufficient protection against aggressive anions and dissolution of aluminium substrate occurs when exposed to corrosive solution. Seawater predominantly consists of about 3.5% of sodium chloride (NaCl) and many other ions. Chloride ions are very strong and could easily penetrate the passive film. Thus, dissolution of the aluminium substrate occurs and results in corrosion. The adsorption of the corrosion inhibitor competes with anions such as chloride. By assuming that the corrosion inhibitor molecules preferentially react with Al3+ to form a precipitate of salt or complex on the surface of the aluminum substrate, the anodic and cathodic processes subsequently suppressed by inhibitor molecules. Thus, this result suggests that the protective film that was formed comprise aluminium hydroxide, oxide and salts or

Polarization resistance, *R*p values for Al-Mg-Si alloy in seawater in the presence and absence of corrosion inhibitor were determined using linear polarization method. The values of *R*<sup>p</sup> are tabulated in Table 3. Generally, the value of *R*p increased with increasing inhibitor concentration for all studied inhibitor. The highest *R*p values for Al-Mg-Si alloy obtained at

PP LPR

CR (10-2mmyr-1)

*b*c (mV dec-1)

*R*<sup>p</sup> (kΩ cm2)

Inhibitor *c*inh

NH

VL

TS

(Aramaki, 2001):

(ppm)

*E*corr (mV)

different concentrations of NH, VL and TS

complexes of the corrosion inhibitor anions.

*i*corr A cm-2) 1000 ppm of NH, VL and TS i.e. 112.72, 145.05 and 177.00 kΩ cm2, respectively. A higher of *R*p indicates the lower of the corrosion rate. The corrosion resistance obtained by LPR measurement was compared between the studied inhibitors. The inhibitor concentration was plotted against the values of *R*p for each studied inhibitor (Figure 1). The results show that the values of *R*p after addition of inhibitor increase with the following sequence: NH < VL < TS.

The values of *R*ct and double layer capacitance, *C*dl for Al-Mg-Si alloy at various concentrations of NH, VL and TS are presented in Table 4. The results show that the *R*ct values increase with the addition of corrosion inhibitors when compared with those without corrosion inhibitor. Furthermore, the values of *R*ct are observed to increase with the increasing corrosion inhibitor concentration, which can be attributed to the formation of a protective over-layer at the metal surface. It becomes a barrier for the charge transfers.

The values of *R*ct for the alloy in inhibited solution with NH were enhanced up to 10 times higher as compared to that of the value of *R*ct in uninhibited solution. Meanwhile, VL and TS shown better performance in improving the value of *R*ct for Al-Mg-Si alloy in studied aggressive solution, where they were increased the values up to 13 and 14 times higher, respectively.

Fig. 1. Rp versus concentration of the studied inhibitors

It should be noted that while *R*ct values increase with the addition of corrosion inhibitor, the capacitance, *C*dl values decrease indicating the formation of a surface film. Thus, effective corrosion resistance is associated with high *R*ct and low *C*dl values (Yagan *et al*., 2006). Increase in *R*ct values and decrease in *C*dl values by NH, VL and TS indicated that the studied inhibitors inhibit the corrosion of Al-Mg-Si alloy in seawater by adsorption mechanism (Noor, 2009) and the thickness of the adsorbed layer increases with the increase of inhibitor concentration.

Improvement of Corrosion Resistance of Aluminium Alloy by Natural Products 385

Owing to the adsorption of the corrosion inhibitor molecules onto the surface of Al-Mg-Si alloy, a thin film is formed on the aluminium alloy to retard the corrosion. Thus, in this case, all studied inhibitors worked as the filming corrosion inhibitor to control the corrosion rate. Instead of reacting with or removing an active corrosive species, the filming corrosion inhibitor function by strong adsorption and decrease the attack by creating a barrier

The values of IE (%) from the PP, LPR and EIS measurements obtained by using following

'

' *CR CR CR*  (5)

(6)

(7)

(8)

*corr corr corr i i i* 

'

'

The values of the result are presented in Table 5. All these parameters showed a similar trend. In all cases, increasing the inhibitors concentration is accompanied by an increase in the IE (%) and maximum for 1000 ppm. The IE (%) for all the measurements obtained from three different methods; PP, LPR and EIS are in good agreement. The inhibitive properties

The protection action of inhibitor substances during metal corrosion is based on the adsorption ability of their molecule where the resulting adsorption film isolates the metal surface from the corrosive medium. Consequently, in inhibited solutions, the corrosion rate is indicative of the number of the free corrosion sites remaining after some sites have been

The adsorption of NH, VL and TS compounds on the aluminium alloy surface reduces the surface area available for corrosion. Increases in inhibitor concentration results in amplify the degree of metal protection due to higher degree of surface coverage, *θ* (*θ* = IE%/100). This is resulting from enhanced inhibitor adsorption. The higher *θ* were acquired at 1000 ppm of NH, VL and TS i.e. 0. 9036, 0.9185 and 0.9587, respectively. Further investigation using surface analytical technique i.e. FTIR and SEM-EDS enable to characterize the active materials in the adsorbed layer and identify the most active molecule of the studied

of the studied natural products can be given by the following order: NH < VL < TS.

*p p p R R R* 

*ct ct ct R R R* 

between the metal and their environment (Al-Juhni and Newby, 2006).

IEPP (%) = 100

IEPP (%) = 100

ERP (%) = 100

IEEIS (%) = 100

**4.3 Inhibition mechanism** 

blocked by inhibitor adsorption.

inhibitors.

**4.2 Inhibition Efficiency (IE)** 

equation:

The equivalent circuit fitting for these experimental data is a Randles circuit. The Randles equivalent circuit is one of the simplest and most common circuit models of electrochemical impedance. It includes a solution resistance, *R*s in series to a parallel combination of resistor, *R*ct, representing the charge transfer (corrosion) resistance and a double layer capacitor, *C*dl, representing the electrode capacitance (Badawy *et al*., 1999). In this case, the value of *R*s can be neglected because the value is too small as compared to that of the value of *R*ct. The equivalent circuit for the Randles cell is shown in Figure 2.


Table 4. *R*ct and *C*dl of Al-Mg-Si alloy in seawater obtained using impedance method

Fig. 2. The equivalent circuit for the Randles cell

#### **4.2 Inhibition Efficiency (IE)**

384 Corrosion Resistance

The equivalent circuit fitting for these experimental data is a Randles circuit. The Randles equivalent circuit is one of the simplest and most common circuit models of electrochemical impedance. It includes a solution resistance, *R*s in series to a parallel combination of resistor, *R*ct, representing the charge transfer (corrosion) resistance and a double layer capacitor, *C*dl, representing the electrode capacitance (Badawy *et al*., 1999). In this case, the value of *R*s can be neglected because the value is too small as compared to that of the value of *R*ct. The

Inhibitor *c* (ppm) *R*ct (k cm2) *C*dl (F cm-2)

Blank 11.76 23.98

Table 4. *R*ct and *C*dl of Al-Mg-Si alloy in seawater obtained using impedance method

*R*s

200 33.10 8.11 400 36.06 7.79 600 57.12 5.16 800 76.39 3.81 1000 119.84 2.31

200 39.04 7.06 400 51.63 5.43 600 60.27 4.39 800 98.05 3.16 1000 155.84 1.95

200 40.73 7.30 400 69.72 3.86 600 79.23 3.37 800 107.45 2.79 1000 166.09 0.99

*C*dl

*R*ct

equivalent circuit for the Randles cell is shown in Figure 2.

NH

VL

TS

Fig. 2. The equivalent circuit for the Randles cell

Owing to the adsorption of the corrosion inhibitor molecules onto the surface of Al-Mg-Si alloy, a thin film is formed on the aluminium alloy to retard the corrosion. Thus, in this case, all studied inhibitors worked as the filming corrosion inhibitor to control the corrosion rate. Instead of reacting with or removing an active corrosive species, the filming corrosion inhibitor function by strong adsorption and decrease the attack by creating a barrier between the metal and their environment (Al-Juhni and Newby, 2006).

The values of IE (%) from the PP, LPR and EIS measurements obtained by using following equation:

$$\text{IEF}\_{\text{FP}}\left(\%\right) = \text{100}\left(\frac{i\_{corr} - i\_{corr}'}{i\_{corr}}\right) \tag{5}$$

$$\text{IE}\_{\text{FP}}\left(\%\right) = 100 \left(\frac{\text{CR} - \text{CR}^{\prime}}{\text{CR}}\right) \tag{6}$$

$$\mathcal{E}\_{\rm RF} \left( \% \right) = 100 \left( \frac{R\_p^{\prime} - R\_p}{R\_p} \right) \tag{7}$$

$$\text{IE}\_{\text{EIS}}\left(\%\right) = 100 \left(\frac{R\_{ct}^{'} - R\_{ct}}{R\_{ct}}\right) \tag{8}$$

The values of the result are presented in Table 5. All these parameters showed a similar trend. In all cases, increasing the inhibitors concentration is accompanied by an increase in the IE (%) and maximum for 1000 ppm. The IE (%) for all the measurements obtained from three different methods; PP, LPR and EIS are in good agreement. The inhibitive properties of the studied natural products can be given by the following order: NH < VL < TS.

#### **4.3 Inhibition mechanism**

The protection action of inhibitor substances during metal corrosion is based on the adsorption ability of their molecule where the resulting adsorption film isolates the metal surface from the corrosive medium. Consequently, in inhibited solutions, the corrosion rate is indicative of the number of the free corrosion sites remaining after some sites have been blocked by inhibitor adsorption.

The adsorption of NH, VL and TS compounds on the aluminium alloy surface reduces the surface area available for corrosion. Increases in inhibitor concentration results in amplify the degree of metal protection due to higher degree of surface coverage, *θ* (*θ* = IE%/100). This is resulting from enhanced inhibitor adsorption. The higher *θ* were acquired at 1000 ppm of NH, VL and TS i.e. 0. 9036, 0.9185 and 0.9587, respectively. Further investigation using surface analytical technique i.e. FTIR and SEM-EDS enable to characterize the active materials in the adsorbed layer and identify the most active molecule of the studied inhibitors.

Improvement of Corrosion Resistance of Aluminium Alloy by Natural Products 387

[C=C] 1418.1 [C, O, H, N]

1255.6 [S]

1055.3 [C, O, H, N] [C, O, H] 1646.7

500 1000 1500 2000 2500 3000 3500 4000 Wavenumbers (cm-1)

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

> 500 1000 1500 2000 2500 3000 3500 4000 Wavenumbers (cm-1)

2362.3 [C=O]

to 1664.9 cm−1 are refers to hydroxyl group and aromatic compound (benzene ring).

1429.8 - 1664.9 [C, H , O, Benzene ring ]

1153.7 - 1199.9 [C=O]

3355.1 [O, N, H]

2935.7

70

Fig. 4. FTIR spectrum of VL.

%Transmittance

Fig. 3. FTIR spectrum of NH.

**4.3.2 Inhibition mechanism of VL** 

75

80

85

% Transmittance

90

95

100


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