Effect of Chitosan on the Corrosion Inhibition for Aluminium Alloy in H₂SO₄ Medium

Abstract

Aluminum AA5052 is an inexpensive and lightweight metal that has been used in a variety of applications, including the Bipolar Plate in Proton Exchange Membrane Fuel Cells (BPs PEMFC). The alloy has good electrical conductivity and low corrosion. Corrosion rate (CR) increases during PEMFC operation with increasing temperature. Inorganic dyes, such as chromate, are commonly used to reduce metal corrosion. Unfortunately, they are toxic and have a negative impact on the environment. Chitosan, which is a green, cheap, non-toxic, and environmentally friendly organic solvent, can be used to solve this problem. Electrophoretic Deposition (EPD) technique was used to coated the surface of AA5052 with chitosan. CR was measured using electrochemical and weight loss methods (in 0.5 M H₂SO₄). The best research results are as follows. The lowest CR values were obtained at the EPD time of 20 min at a chitosan concentration of 0.5% wt. The results of the study using the weight loss method indicated that after soaking for 72 h, the chitosan inhibitor can reduce the corrosion rate (CR) with an inhibitor efficiency of 87.89%, while the electrochemical method obtained a higher efficiency of 95.12%. An increase in temperature will result in a decrease in the efficiency of the inhibitor. Testing with SEM-EDX, after being coated with chitosan inhibitor, the metal surface looks smoother and the Al composition is reduced and it is detected that there is adsorption of O, N and S elements that coated the metal to form insoluble complex compounds, so the corrosion rate decreases.

1. Introduction

Aluminum and its alloys have many advantages over other materials because of their high electrical conductivity, low cost, and light weight [1]. The resistance of aluminum in a corrosive environment under certain conditions is due to the presence of an oxide layer on its surface [2]. Alloy AA5052 contains 97.2–97.8% aluminum (Al) and 2.2–2.8% magnesium (Mg). The addition of Mg to AA5052 metal can increase its strength without reducing its ductility [1]. AA5052 can be applied to aircraft, automotive, construction, and Bipolar Plate components in Proton Exchange Membrane Fuel Cell (BPs PEMFC) systems. The selection of AA5052 as BPs is generally based on its properties, namely high conductivity, low cost, and lightness compared with other metals [3,4]. Generally, PEMFC are operated under acidic conditions at temperatures of 40 °C to 80 °C (313–353 k) [5,6]. The acidic environment conditions were simulated by Yang [5] using 0.5 M H₂SO₄ solution. Acidic environmental conditions and relatively high operating temperatures can cause the corrosion of AA5052 [7]. In general, corrosion is defined as damage to metal surfaces caused by chemical reactions between the surface and the environment. Due to the corrosion on this BPs, it is very important to handle it, so that the service life of this fuel cell device can be longer.

Inorganic inhibitors such as chromate can be used to reduce corrosion. Unfortunately, this inhibitor has a negative impact on the environment due to its toxicity [8,9,10,11,12,13,14]. Based on these facts, green organic inhibitors are the best alternatives that can prevent and reduce corrosion [15,16]. Chitosan is an organic compound derived from chitin; it has the potential to serve as a corrosion inhibitor, because it fulfills all the requirements. Chitosan can be produced from chitin through hydrolysis and deacetylation processes that involve amino (-NH2) and hydroxyl (-OH) groups, both of which act as absorbents to reduce the corrosion rate through the absorption process [11,12,13,14]. Chitosan is not soluble in water but soluble in acid acetate and easily degraded, it can be utilized as a basic material in industries, health applications and as a corrosion inhibitor [11,12,13,14,15,16].

Temperature plays an important role in the corrosion process, especially in PEMFC operation. Therefore, the effect of temperature differences on the corrosion rate of AA5052 uncoated and coated with chitosan in 0.5 M H₂SO₄ at different times and temperatures was determined.

In order to calculate the magnitude of the corrosion rate from the mass loss, the Equation (1) can be used

CR = (m₁ − m₂)/(A.t),

(1)

where CR is Corrosion Rate (mg/cm² h, m₁: Initial Mass (mg), m₂ is Final Mass (mg), A is Specimen Surface Area (cm²), and t is Immersion (h).

Meanwhile, to calculate the value of the efficiency of the inhibitor, Equation (2) can be used:

%IE: (CRblank − CRinh)/CRblank × 100%,

(2)

where Crblank is Corrosion rate without inhibitor (mg/cm² h), Crinh is Corrosion rate with inhibitor (mg/cm² h), %IE is Inhibitor efficiency [17].

The measurement of the corrosion rate of the Tafel extra polarization method can be calculated with the followin Equation (3):

CR = K1 × Icor/ρ × EW,

(3)

where CR is Corrosion rate (mpy), K1 = 0.1288 (mpy.g/A.cm², Icor is Corrosion current (A/cm² = Density (g/cmEW = Equivalent weight (g/mol).

Similar to the weight loss method, to determine the efficiency of the inhibitor, the following equation can be used (Equation (4)):

IE: (Iblank − Iinh)/Iblank × 100%,

(4)

where Iblank is Current without inhibitor (A/cm²), Iinh is Current with inhibitor (A/cm²) IE = Inhibitor efficiency [18].

Activation energy can be defined as the minimum amount of energy required to activate atoms or molecules under a condition in which they can undergo chemical reaction or physical conversion. Theoretically, the corrosion rate can also be analyzed using the activation energy (E_a) equation, as formulated by the following Arrhenius Equation (5):

$$\ln \left(\mathrm{CR}\right)=\left(-\frac{E_a}{R}\right).\left(\frac{1}{T}\right)+\ln A),$$

(5)

where CR is the corrosion rate (mpy), E_a is the activation energy (kJ/mol), R is the molar gas constant (8.3145 J/K.mol), T is the temperature (°K), and A is the exponential factor Arrhenius; this is a simple linear regression equation with the equation y = bx + a, where y = ln (CR), b = slope = −E_a/R, x = (1/T), and a = ln (A).

The purpose of this study was to determine the rate of corrosion and to what extent the inhibitory effect of chitosan can reduce the corrosion, while also analyzing the activation energy and surface morphology of the metal using SEM-EDX.

2. Materials and Methods

2.1. Equipment and Materials

AA5052 alloy with size of 20 mm × 10 mm × 2.5 mm was selected during this experiment. Chitosan (Merck, with standard analysis) was used as a green organic inhibitor. The corrosion rates of AA5052 coated and uncoated chitosan were evaluated by using the weight loss and electrochemical methods (potentiostate CorrTest CS350) to analyze the potentiodynamic polarization. The morphology and element compositions on the surface of AA5052 with and without chitosan were analyzed by using a scanning electron microscope (SEM, Hitachi TM3000) equipped with energy dispersive X-ray spectroscopy (EDX, Carl Zeiss EVO MA 10). Firstly, AA5052 alloy was washed with acetone and polished by sand paper with grids of 200, 400, 600, 1000, and 2000. After that, AA5052 alloy was washed with acid and base pickling solution and then rinsed with deionized water. Cleaned AA5052 alloy was perforated using a drill with a diameter of 2.5 mm. To hold the AA5052 alloy, a polyvinyl chloride (PVC) cable was attached. The AA5052 alloy surface was smeared with epoxy, except for a 1.0 cm⁻² of its surface. Chitosan solution (0.5% wt) was prepared using a total 0.15 g of chitosan that was diluted in 30 mL acetic acid (CH₃COOH). The solution mixture was stirred until homogeneous using a magnetic stirrer for 10 min [14]. AA5052 alloy was placed into the electrophoresis deposition (EPD) cell that contained 0.5% wt chitosan solution. The EPD process was performed at room temperature with AA5052 as the cathode. The EPD process was conducted for 10, 20, and 30 min with an applied voltage of 20 V. Corrosion rate tests included weight loss analysis of AA5052 coated and uncoated chitosan; these were immersed in 0.5 M H₂SO₄ for 72, 120, and 168 h at room temperature [19,20,21,22,23].

2.2. Electrochemical Method with Potentiodynamic Polarization

The corrosion rates of AA5052 alloys with and without chitosan were verified and analyzed using the potentiodynamic polarization method. This test was performed using the potentiostat CorrTest, model CS350 at temperatures of 298, 313, 333, and 353 K. Potentiodynamic polarization cell was arranged as three electrode configurations in which theAA5052 acted as the working electrode (WE), platinum (Pt) rod acted as the counter electrode (CE), and Ag/AgCl acted as the reference electrode (RE). The applied potential was in the range of −0.9 V to −1.0 V with a scan rate of 2 mVs ⁻¹.

3. Results and Discussion

3.1. Results and Calculating the Corrosion Rate Weight Loss Method

During the testing of the corrosion rate using the weight loss method, the sample was immersed in a solution of 0.5 M H₂SO₄ with an EPD time of 10, 20, and 30 min. The immersion time variation was 72, 120, and 168 h [17,18]. The results of the corrosion rate testing process using the weight loss method are presented in

Table 1. Results of Analysis of time effect on EPD process and calculation of the corrosion rate of AA5052 uncoated and coated with Chitosan inhibitor.

EPD Time	Immersion Time	CR	EF%
(min)	(h)	(mg cm−2 h−1)
	72	0.491
Blank	120	0.1660
	168	0.1702
	72	0.0245	8.54
0.1	120	0.0667	59.87
	168	0.0625	63.29
	72	0.0181	87.89
0.2	120	0.0617	62.88
	168	0.0552	67.60
	72	0.0398	73.29
0.3	120	0.0878	47.16
	168	0.0819	51.86

, including the calculation data of the corrosion rate and the efficiency of the inhibitor. At 72 h immersion, the corrosion rate of uncoated AA5052 metal sample was 0.1491 mg cm⁻² h⁻¹, whereas the corrosion rates of AA5052 metal samples coated with chitosan inhibitor for 10, 20, and 30 min were 0.0245, 0.0181, and 0.0398 mg cm⁻² h⁻¹, respectively. At 120 h of immersion, the corrosion rate of AA5052 metal samples uncoated with inhibitor was 0.1661 mg cm⁻² h⁻¹, whereas the corrosion rates of AA5052 metal samples coated with chitosan inhibitor for 10, 20, and 30 min were 0.0667, 0.617, and 0.0878 mg cm⁻² h⁻¹, respectively. At 168 h of immersion, the corrosion rate of uncoated AA5052 metal samples was 0.1702 mg cm⁻² h⁻¹, whereas the corrosion rates of AA5052 metal samples coated with chitosan inhibitor for 10, 20, and 30 min were 0.0625, 0.0552, and 0.0819 mg cm⁻² h⁻¹, respectively. The data of the best efficiency value showed that the smallest corrosion rate occurred in the 20 min EPD time process (87.89%) at 72 h of immersion.

The chitosan inhibitors adsorbed on the metal surface containing polar atoms, such as O, N, and S, can form a protective layer, and under certain conditions, the metal can undergo a passivation process or form an oxide layer [14,15]. Al₂O₃ on the metal surface leads to a decrease in the corrosion rate [16,17].

3.2. Effect of Temperature on Corrosion Rate and Inhibition Efficiency

The corrosion rate of AA5052 with and without chitosan increased with increasing temperature. The corrosion rate of AA5052 with an EPD time of 20 min was 0.0181 mg cm⁻² h⁻¹ at 298 K, which was then increased to 0.43 mg cm⁻² h⁻¹ (313 K), 0.62 mg cm⁻² h⁻¹ (333 K), and 1.38 mg cm⁻² h⁻¹ (353 K). Increasing temperature accelerates anodic and cathodic reactions, reduces dissolved oxygen in solution, and also accelerates cathodic diffusion, consequently increasing the corrosion rate [18]. The highest efficiency of chitosan inhibitor is at a temperature of 298 °K of 87.89%, and the influence of temperature will reduce the efficiency of the inhibitor as shown in

Table 2. Effect of temperature on corrosion rate and inhibitor efficiency on 20 min EPD time technique in 0.5 M H₂SO₄ media.

AA5052		Temperature (°K)
	298	313	333	353
Uncoated
CR (mg cm−2 h−1)	0.1491	0.70	0.87	1.73
Coated
CR (mg cm−2 h−1)	0.0181	0.43	0.62	1.38
EF%	87.89	42.31	38.10	20.19

.

3.3. Activation Energy Analysis

The activation energy value of metal AA5052 uncoated and coated with chitosan inhibitor is determined in

Table 3. Values of activation energy (E_a) for AA5052 alloys with and without chithosan for 20 min EPD time.

Sample AA 5052	Linear Regression	−Ea/R	Ea (kJ mol−1)
Uncoated	y = −2471x + 7.457	−2471	20.55
Coated	y = −3105x + 9.020	−3105	25.82

. The activation energy value (E_a) of AA5052 without chitosan inhibitor coating was 20.55 kJ mol⁻¹, whereas the activation energy value (E_a) of AA5052 coated with chitosan inhibitor was 25.82 kJ mol⁻¹. The higher activation energy indicated that AA5052 coated with chitosan was more difficult to corrode compared with uncoated AA5052. Figure 1 presents a plot graph of Ln CR vs. (1/T) metal AA5052 uncoated and coated with chitosan inhibitor.

3.4. Potentiodynamic Polarization Test Using Electrochemical Method for 20 min EPD Time

The Ecorr values for uncoated AA5052 were lower than those of coated with chitosan at each temperature tested (see

Table 4. Tafel of potentiodynamic polarization for AA5052 coated Chitosan (EPD Time 20 min and uncoated in 0.5 M H₂SO₄ at 298, 313, 333, and 353 K.

AA 5052	T (K)	Ecorr (V vs. Ag/AgCl)	Icorr (mA cm−2)	CR (mpy)	IE (%)
Uncoated	298	−0.4829	144.8	62.96	-
	313	−0.5880	490.0	213.12	-
	333	−0.6558	959.5	417.34	-
	353	−0.5925	2794.2	1215.31	-
Coated	298	−0.5519	7.07	3.07	95.12
	313	−0.6277	88.29	38.40	81.98
	333	−0.6844	501.93	218.31	47.69
	353	−0.6345	2011.90	875.06	28.00

). For example, −0.4829 V vs. Ag/AgCl Ecorr for uncoated AA5052 (Figure 2a), was lower than 0.5519 V vs. Ag/AgCl Ecorr for AA5052 coated with chitosan (Figure 2b) at the test temperature of 298 K. This fact revealed that chitosan predominantly protected the cathodic region. The corrosion rate can be determined using the Icorr value.

The higher the Icorr value, the higher the corrosion rate. Overall, the corrosion rate gradually increased with increasing temperature. The activation energy (E_a) of the particles increased with increasing temperature; consequently, the corrosion reaction became faster [16,17]. Furthermore, the highest efficiency of chitosan was observed at 95.12% at 298 K and then gradually decreased with increasing temperature. This finding was due to the physisorption properties of chitosan; the dominant adsorption reaction occurred on the cathodic side.

Based on Table 4, the corrosion potential values (Ecorr) of metal AA5052 coated with the inhibitor layer were always below the corrosion potential value (Ecorr) of metal AA5052 uncoated with an inhibitor layer; chitosan is a mixed inhibitor with dominant protection in the cathodic area [19,20,21]. A decrease in the value of corrosion potential (Ecorr) at a temperature range of 298 K to 333 K indicated an increase in the anodic reaction, in which the oxidation process took place faster. At a temperature of 353 K, the corrosion potential (Ecorr) increased, indicating the formation of a passive layer in the anodic region [22].

The corrosion rate can be determined based on the Icorr value. The higher the Icorr value, the higher the increase in the corrosion rate. At temperatures in the range of 298 K to 353 K, the corrosion rate of metal AA5052 without and with chitosan inhibitor coatings constantly increased. The increase in temperature caused the increase in kinetic energy of the reacting particles; it exceeded the magnitude of the activation energy price. As a result, the reaction speed rate (corrosion) also increased. The efficiency of chitosan inhibitors was highest at 298 K, but the efficiency decreased with increasing temperature, thereby indicating that chitosan adsorption worked physically [17].

3.5. SEM-EDX Analysis

Based on Figure 3, the EPD process of AA5052 using chitosan for 20 min EPD time will produce layers with various thicknesses used SEM ranging from 56.7 μm to 85.5 μm.

The surface morphology of AA5052 metal can be seen through the SEM-EDX. Variations in the thickness of the inhibitor layer can be caused by the length of time the potential EPD is used, the concentration of the suspension (coating) used and the electrical conductivity of the solvent used. The EPD process will usually produce a layer thickness ranging from 0.1 μm to approximately 100 μm [18,19,20].

Elements Analysis of Chitosan Inhibitor Composition SEM-EDX testing will produce data in the form of mass percentage, atomic percentage, and so on. Comparison of element compositions contained in metal AA5052 without inhibitor coating and metal AA5052 coated with chitosan inhibitor is presented in Figure 4.

Sample AA5052 shown in Figure 4 indicates that the elements Al and Mg are very dominant., the mass percentage of Al elements reaches 96.38% while the percentage of Mg elements reaches 3.62%. There are other elements, but the amounts are so small that it cannot be assessed for the large spectrum because of the large purity of the element Al. Elemental Mg in AA5052 is usually 2–2.5%. The addition of elemental magnesium (Mg) will increase strength and hardness on aluminum without degrading too much tenacity.

In sample AA 5052 coated with chitosan inhibitor (Figure 5), there was a the composition of additional elements C, N, and O and a small proportion of P and S elements was caused by the adsorption of chitosan on the surface of AA 5052 through the EPD process. The presence of these elements which are adsorbed on metal surfaces, results in corrosion being inhibited, for more details can be seen in

Table 5. Elements composition of metallic surface AA5052 uncoated and coated with chitosan inhibitor coating.

Elements	Weight (%)
	Uncoated	Coated
Al	96.38	42.45
Mg	3.62	2.06
C	-	33.44
N	-	4.77
O	-	16
P	-	0.66
S	-	0.62

. This study is in accordance with previous studies [11,12,13,14,15,16,17].

4. Conclusions

The results of the research that used the method of weight loss showed the corrosion rate at immersion for 72 h; the chitosan inhibitor efficiently reduced the corrosion rate by 87.89%. However, by using the electrochemical method, an efficiency of 95.12% was observed. The change in temperature resulted in a decrease in the efficiency of this inhibitor, but it still inhibited corrosion when compared with the uncoated metal.

The activation energy (E_a) of AA5052 uncoated inhibitor coating was 20.55 kJ/mol, whereas the value of activation energy (E_a) of AA5052 coated with chitosan inhibitor (EPD time of 20 min) was 25.82 kJ/mol. The higher the value of E_a, the more difficult the corrosion process, because the process required higher energy.

The morphological result from SEM-EDX shows that with coated chitosan inhibitor for EPD time of 20 min, the metal surface looks smoother and the Al composition is reduced, while the adsorption of O, N, and S elements was detected on the metal to form insoluble complex compounds in which the corrosion rate decreases.