Performance evaluation of phosphite NaCo(H₂PO₃)₃·H₂O as a corrosion inhibitor for aluminum in engine coolant solutions

Abstract

The inhibiting action of a novel inorganic compound (NaCo(H₂PO₃)₃·H₂O) (NaCoPh) synthesized in our laboratory on the corrosion of aluminum in engine coolant solutions has been studied. In this study, potentiodynamic polarization measurements with SEM/EDX investigations were employed. Polarization curves reveal that NaCoPh is a mixed type (cathodic/anodic) inhibitor for aluminum corrosion in the engine coolant solution. The inhibition efficiency increases with increasing concentration of NaCoPh but decreases with increasing temperature and flow rate. SEM/EDX investigations reveal the adsorption of NaCoPh on the aluminum surface. A mixed adsorption (physisorption and chemisorption) for NaCoPh was proposed.

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1. Introduction

In general, engine coolant solutions are used for cooling internal engines parts. Engine coolant is usually a water–ethylene glycol mixture.^1–3 Many additives such as corrosion and scale formation inhibitors are added in engine coolant solutions.^4–6

For many years, the application of corrosion inhibitors, such as inorganic inhibitors, has been the most common method for reducing the corrosion of radiators.^7–10 Corrosion protection by inorganic inhibitors is mostly based on the adsorption of these compounds on the metal surfaces to produce an impervious physical layer which prevents any further corrosion reactions from taking place. These compounds are often simultaneously affected by both anodic and cathodic corrosion reactions; therefore, they are sometimes referred to as mixed-type corrosion inhibitors.¹¹

Classically, brass or copper metal were used for radiators manufacture. Modern radiators were constructed using aluminum. This construction is more economically and less weight than traditional materials.

The objective of this study was to examine the inhibition performance of the mixed phosphate NaCo(H₂PO₃)₃·H₂O (NaCoPh) synthesized in crystalline in solution. Its influence on the corrosion behavior of aluminum in engine coolant solution was investigated. The effects of engine coolant solution temperature and flow rate on corrosion and inhibition processes were also assessed. Electrochemical techniques, such as potentiodynamic polarization with SEM/EDX observations were used to elucidate the inhibition mechanism of NaCoPh.

2. Experimental

2.1. Synthesis

NaCo(H₂PO₃)₃·H₂O (NaCoPh), examined in the present study as a corrosion inhibitor, was synthesized according to a procedure previously reported in the literature.¹² Solutions of CoCl₂·6H₂O (10 ml, 1 mmol), H₃PO₃ (10 ml, 3 mmol) and Na₂CO₃ (10 ml, 0.5 mmol) were mixed in a beaker. The mixture was stirred for six hours and then allowed to stand for two weeks at room temperature. At the end of this period, large prismatic pink crystals have deposited, which were filtered-off and washed with a water–ethanol solution (20 : 80). The sample was characterized by high precision powder X-ray diffraction (XRD), using a Panalytical diffractometer operating with CuKα radiations. The sample was scanned between 10 and 45 (2θ). Full pattern matching refinement was performed with the Jana 2006 program package.¹³ The background was estimated by a Legendre function, and the peak shapes were described by a pseudo-Voigt function. The refinement of peak asymmetry was performed using four Berar–Baldinozzi parameters (Fig. 1). The crystal structure of NaCo(H₂PO₃)₃·H₂O, from single crystal data, has been determined by Kratochvil et al.¹⁴

Fig. 1 XRD patterns of NaCoPh powder.

2.2. Specimen

Experiments were performed on aluminum sheets of the following percentage composition: Al (99.89%), Si (0.03%), Cu (0.02%), Mg (0.03%) and Zn (0.01%). Prior to each experiment, the aluminum electrodes with dimension 1.0 × 1.5 × 0.2 cm were first abraded by a series of emery papers (up to grade 1200) and washed thoroughly with distilled water and degreased with acetone.

2.3. Test solution

The engine coolant solution (test solution, pH 7.8) is a mixture of technical grade ethylene glycol (70%) and tap water (30%). Ethylene glycol was obtained from commercial source with grade 99.21%. The main impurities that present in ethylene glycol are 1.3 butanediol (0.3%) and diethylene glycol (0.49%). The tap water used in test solution contained 191 mg L⁻¹ of Na⁺, 9.8 mg L⁻¹ of K⁺, 330 mg L⁻¹ of Ca²⁺, 11.8 mg L⁻¹ of Mg²⁺, 94.8 mg L⁻¹ of Cl⁻ and 120 mg L⁻¹ of HCO₃⁻.

The temperature was adjusted to within ±0.2 °C in the range (298–358 K) using a water thermostat.

2.4. Electrochemical measurements

Potentiodynamic polarization measurements were carried out in the newly designed water-jacketed electrolytic cell as previously reported.¹⁵ The cell assembly consisted of aluminum sheet as working electrode, a platinum foil as counter electrode, and a saturated calomel electrode (SCE) as a reference electrode. All potentials quoted in this paper were referred to the SCE. Electrochemical experiments were performed using A potentiostat/galvanostat (EG&G model 273). Potentiodynamic polarization curves were recorded at constant sweep rate of 2.5 mV s⁻¹ in a potential range of ±250 mV vs. open circuit potential.

2.5. The surface morphology analysis

The surface morphology of aluminum sheets were investigated after 720 h exposure time in engine coolant solution in the presence and absence of NaCoPh by recording SEM images of the samples using JEOL-JEM 1200 EX II electron microscope.

Energy dispersive X-ray spectroscopy (EDX) investigations were carried out in order to identify the elemental composition of the species formed on the metal surface after its immersion in engine coolant solution in the presence and absence of NaCoPh. EDX examinations were carried out using a Traktor TN-2000 energy dispersive spectrometer.

3. Results and discussion

3.1. Effect of NaCoPh concentration in static condition

Anodic and cathodic polarization curves for aluminum in absence and presence of various concentrations of NaCoPh in engine coolant at 298 K in static conditions are shown in Fig. 2.

Fig. 2 Potentiodynamic polarization curves for aluminum in absence and presence of various concentrations of NaCoPh in engine coolant at 298 K in static conditions.

Electrochemical kinetic parameters, such as Tafel slops (anodic B_a and cathodic B_c), corrosion potential (E_corr) and corrosion current density (j_corr) were estimated by extrapolation of the Tafel lines and are presented in Table 1. The inhibition efficiency (η%) of NaCoPh is calculated according to eqn (1).¹⁶

η% = [(j⁰_corr − j_corr)/j⁰_corr] × 100 (1)

where j⁰_corr and j_corr are the corrosion current densities in the absence and presence of NaCoPh respectively. The estimated inhibition efficiencies are listed in Table 1. Data in Table 1 reveal that the current density j_corr decreases considerably in the presence of NaCoPh.

Table 1 Electrochemical kinetic parameters and the corresponding inhibition efficiency of aluminum in absence and presence of various concentrations of NaCoPh in engine coolant at 298 K in static conditions

NaCoPh conc. (M × 10^−4)	Ecorr mV (SCE)	Ba mV dec^−1	Bc mV dec^−1	jcorr μA cm^−2	η%
Blank	−572	142.4	−125.6	12.58	—
0.5	−562	141.2	−128.7	8.55	32.03
1.0	−538	145.5	−131.8	5.95	52.70
1.5	−527	139.5	−135.2	3.24	74.24
3.0	−506	135.8	−144.4	0.98	92.20

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Fig. 2 shows that both the anodic metal dissolution and cathodic reduction reactions are hindered when NaCoPh is added to the engine coolant. Further inspection, clearly shows that the effect of inhibiting anodic metal dissolution reaction is more prominent over retarding cathodic reduction reaction.¹⁷ It is also observed form Table 1 that addition of NaCoPh slightly shifts E_corr values in the anodic direction. The inhibitor can be classified as anodic or cathodic type inhibitor when the change in E_corr is larger than 85 mV.¹⁸ Such value is 66 mV NaCoPh (Table 1); consequently it might be classified as mixed-type inhibitor with predominant anodic impact.

It reveals form Table 1 that, the changes in values B_a and B_c were very small with the addition of NaCoPh. This suggests that the presence of NaCoPh in engine coolant solution does not alter the process mechanism and works as adsorptive inhibitor by blocking the active sites on the metal surface.¹⁹

The inhibition efficiency increases with increasing NaCoPh from 0.5 × 10⁻⁴ to 3.0 × 10⁻⁴ M. Increasing the concentration of NaCoPh above 3.0 × 10⁻⁴ M dose not cause significant change in the inhibition efficiency. The maximum inhibition efficiency (η% = 92.20) is achieved at 3.0 × 10⁻⁴ M, which indicates that NaCoPh is a good inhibitor for aluminum in engine coolant solution. This is likely because adsorptive and protective film of NaCoPh molecules is formed on the aluminum surface. The film acts as a physical barrier to restrict the diffusion of corrosive species to the metal surface.²⁰ The adsorption of NaCoPh on the aluminum surface can be attributed to the presence of oxygen atoms in NaCoPh molecule. The oxygen atoms are regarded as the reaction centre for the adsorption process. The adsorption of NaCoPh on a metal surface usually involves the replacement of one or more water molecules adsorbed at the metal surface.²¹

3.2. The effect of flow rate

The effect of increasing engine coolant flow rate (from 0.0 to 1.2 m s⁻¹) on the potentiodynamic anodic polarization behavior of aluminum in the absence and presence of 3.0 × 10⁻⁴ M NaCoPh at 298 K was examined and the data are represented in Fig. 3 and Table 2.

Fig. 3 Potentiodynamic polarization curves for aluminum in engine coolant containing 3.0 × 10⁻⁴ M NaCoPh at 298 K at different engine coolant flow rates.

Table 2 Electrochemical kinetic parameters and the corresponding inhibition efficiency of aluminum in engine coolant solution in the absence (blank) and presence of 3.0 × 10⁻⁴ M NaCoPh at 298 K at different flow rates

Solution	Flow rate (m s^−1)	Ecorr mV (SCE)	Ba mV dec^−1	Bc mV dec^−1	jcorr μA cm^−2	η%
Blank (without inhibitor)	0.0	−572	142.4	−125.6	12.58	—
	0.3	−579	141.0	−122.5	12.66	—
	0.6	−582	148.4	−128.5	13.25	—
	0.9	−581	141.2	−121.2	14.82	—
	1.2	−585	145.3	−127.0	15.02	—
NaCoPh 3.0 × 10^−4 M	0.0	−506	135.8	−144.4	0.98	92.20
	0.3	−509	136.2	−148.8	1.02	90.94
	0.6	−519	131.4	−145.5	1.35	89.81
	0.9	−530	129.4	−144.9	1.99	86.57
	1.2	−535	126.5	−139.8	2.23	85.15

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It can be noticed that the increase of corrosion current density is recognizable with increasing engine coolant flow rate in both blank and inhibited solutions.

Inspection of the data reveals that increasing flow rate (inhibited solution) increases the j_corr form 0.98 to 2.23 μA cm⁻². At the same time, E_corr values are slightly shifted in the cathodic direction. Worthy also to notice that the inhibition efficiency decreases with increasing coolant flow rate. This means that the extent of inhibition efficiency of NaCoPh in the dynamic condition is not as effective as that obtained in the static condition. This behavior is due to the fact that the impact effects by solution flow could degrade the effectiveness of corrosion inhibitor, and thus significantly enhance the corrosion rate and consequently decrease the inhibition efficiency of NaCoPh.²² This degradation of the effectiveness of the corrosion inhibitors by solution flow may have prevented the strong adsorption of the inhibitor on the metal surface.

3.3. Influence of temperature and thermodynamic parameters

Fig. 4 illustrates the influence of temperature (298–358 K) on the potentiodynamic polarization behavior of aluminum in engine coolant solution containing 3.0 × 10⁻⁴ M NaCoPh with solution flow rate of 1.2 m s⁻¹. The effect of solution temperature on E_corr, B_a, B_c and j_corr derived from Fig. 4 and inhibition efficiency are presented in Table 3. The results obtained indicate that the rates of aluminum corrosion in the presence of NaCoPh increases with temperature while the inhibition efficiency decreases. This may be attributed to the higher dissolution rates of aluminum at elevated temperature and a possible desorption of adsorbed inhibitor.²³

Fig. 4 Potentiodynamic polarization curves for aluminum in engine coolant containing 3.0 × 10⁻⁴ M NaCoPh with solution flow rate of 1.2 m s⁻¹ at different temperatures.

Table 3 Electrochemical kinetic parameters and the corresponding inhibition efficiency of aluminum in engine coolant solution in the absence (blank) and presence of 3.0 × 10⁻⁴ M NaCoPh with solution flow rate of 1.2 m s⁻¹ at different temperatures

Solution	Temperature K	Ecorr mV (SCE)	Ba mV dec^−1	Bc mV dec^−1	jcorr μA cm^−2	η%
Blank (without inhibitor)	298	−585	145.3	−127.0	15.02	—
	318	−580	144.1	−128.2	15.45	—
	338	−581	142.2	−125.5	16.22	—
	358	−574	139.7	−129.7	18.15	—
NaCoPh 3.0 × 10^−4 M	298	−535	126.5	−139.8	2.23	85.15
	318	−509	120.2	−142.5	2.45	84.14
	338	−512	123.4	−145.2	3.27	79.83
	358	−507	119.0	138.8	4.05	77.68

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Thermodynamic parameters are important to study the inhibitive mechanism. The thermodynamic functions for aluminum corrosion in engine coolant solution in the absence and presence of 3.0 × 10⁻⁴ M NaCoPh were obtained by applying Arrhenius and transition state equations:²⁴

where E_a is the activation energy of the metal dissolution reaction, ΔH_a is the enthalpy of activation, ΔS_a is the entropy of activation, R is the molar gas constant, T is the absolute temperature, A is the frequency factor, h is the Plank's constant and N is the Avogadro's number.

Arrhenius and transition state plots in absence and presence of NaCoPh are shown in Fig. 5 and 6, respectively. The calculated values of E_a, ΔH_a and ΔS_a are given in Table 4.

Fig. 5 Arrhenius plots for aluminum in engine coolant in the absence and presence of NaCoPh.

Fig. 6 Transition state plots for aluminum in engine coolant in the absence and presence of NaCoPh.

Table 4 Activation thermodynamic parameters of the corrosion process in the absence and presence of NaCoPh

Solution	Ea kJ mol^−1	ΔHa kJ mol^−1	ΔSa J K^−1 mol^−1
Blank	4.98	2.78	−175.04
3.0 × 10^−4 M NaCoPh	10.26	8.55	−177.20

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The value of activation energy in engine coolant solution containing NaCoPh is greater than that without NaCoPh. Generally, the energy barrier of corrosion reaction increases with the presence of NaCoPh and could be interpreted by physical adsorption.²⁵

The positive values of ΔH_a both in absence and presence of inhibitor reflect the endothermic nature of the aluminum dissolution process.²⁶

Large and negative values of ΔS_a imply that the activated complex in the rate determining step represents an association rather than a dissociation step, meaning that a decrease in disordering takes place on going from reactants to the activated complex.²⁷

3.4. Adsorption isotherm and free energy of adsorption

Adsorption behavior can be deduced from the adsorption isotherm, which shows the equilibrium relationship between concentrations of inhibitors on the surface and in the bulk of the solution.²⁸

The values of surface coverage (θ = η%/100) for the different concentrations of the studied inhibitor have been used to explain the best adsorption isotherm to determine the adsorption process. The data obtained from polarization measurements have been tested with several adsorption isotherms. Langmuir adsorption isotherm was found to fit well with our experimental data. The equation for the Langmuir adsorption isotherm can be written as follows:²⁹

where C_inh is the inhibitor concentration, K_ads is the adsorptive equilibrium constant for the adsorption/desorption process and θ is the degree of surface coverage.

It was found that the plot of C_inh/θ versus C_inh (Fig. 7) yield to straight lines with a linear correlation coefficient (R²) very close to unity, indicating that NaCoPh was adsorbed on the aluminum surface according to Langmuir adsorption isotherm.³⁰ The calculated adsorptive equilibrium constant K_ads value was 10 000 M⁻¹ and the standard adsorption free energy (ΔG^o_ads) was calculated by the following equations:³¹

ΔG^o_ads = −RT ln(55.5K_ads) (5)

where R is the molar gas constant, T is the absolute temperature and 55.5 is the concentration of water in solution (mol l⁻¹).

Fig. 7 Langmuir adsorption plot for aluminum in engine coolant containing different concentrations of NaCoPh at 298 K in static conditions.

The relatively high value of adsorption equilibrium constant reflects the high adsorption ability of NaCoPh on the aluminum surface.³²

The calculated ΔG^o_ads value is −32.71 kJ mol⁻¹. It is well known that the absolute values of ΔG^o_ads of the order of 20 kJ mol⁻¹ or lower indicate a physisorption; those of order of 40 kJ mol⁻¹ or higher involve charge sharing or transfer from the inhibitor molecules to the metal surface to form a coordinate type of bond (chemisorption).³³ NaCoPh presents then complex interactions (both physical and chemical adsorption) on the aluminum surface, as already reported in the literature.³⁴

3.5. The surface morphology investigations

Fig. 8a shows the SEM photo of the aluminum surface after immersion in the engine coolant solution at 298 K. A rough corroded surface with a considerable surface damage was noticed for the aluminum surface after immersion in blank solution. The corresponding EDX spectra presented in Fig. 8b confirms the existence of aluminum and oxygen signals. This indicates that the compound that may be found on the surface is mainly aluminum oxide. The presence of carbon, sodium and calcium signals comes from engine coolant solution.

Fig. 8 SEM image (a) and EDX analysis (b) for the aluminum surface after immersion in the engine coolant solution.

On other hand, in the presence of 3.0 × 10⁻⁴ M of NaCoPh in the engine coolant solution, the surface of aluminum was smoother and the corroded areas obviously diminished as seen in Fig. 9a. The corresponding EDX spectrum (Fig. 9b) shows additional signals characteristic of the existence of Co and P signals. These singles may be comes form NaCoPh adsorption on the metal surface. The presence of low content O and Na signals is due to the oxygen and sodium atoms presented in NaCoPh. The disappear of C and Ca signals is as a result of the decreasing the interaction between metal and the aggressive solution.

Fig. 9 SEM image (a) and EDX analysis (b) for the aluminum surface after immersion in the engine coolant solution containing 3.0 × 10⁻⁴ M of NaCoPh.

This data clearly proves that NaCoPh exhibited good inhibition effect and corrosion process was suppressed via adsorption on the aluminum surface, which results then in decreasing the interaction between aluminum and the engine coolant solution.

4. Conclusions

Several conclusions might arise from the present study as follow:

(1) NaCoPh acts as a good new inhibitor (more than 92% at 3.0 × 10⁻⁴ M) for the corrosion of aluminum metal in the engine coolant solution.

(2) The inhibition efficiency increases with the increasing NaCoPh concentration but decreases with increasing in the engine coolant solution temperature and flow rate.

(3) The corrosion process is inhibited by the adsorption of NaCoPh on the metal surface following Langmuir adsorption isotherm.

(4) A mixed inhibition mechanism was proposed for the inhibitive effects of NaCoPh based on the polarization results.

(5) Comprehensive adsorption (physisorption and chemisorption) for NaCoPh was proposed based on the value of ΔG^o_ads.

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