Department of Chemistry, Faculty of Science; Benha University, Benha, Egypt.

Abstract

The effect of 2-amino-4,6-dihydroxy pyrimidine (ADHP) on the corrosion of carbon steel in 1 M HCl has been studied using weight loss and galvanostatic polarization measurements. The percentage inhibition efficiency was found to increase with increasing concentration of the inhibitor, and with decreasing temperature. The inhibitive effect of these compounds was explained on the basis of the formation of an insoluble complex adsorbed on the metal surface. The adsorption process follows Langmuir adsorption isotherm. The effect of temperature on the rate of the corrosion in the absence and presence of these compounds was also, studied. The activated thermodynamic parameters were calculated and explained.

Keywords: corrosion test, C-steel, polarization.

Introduction

Acid solutions are generally used for the removal of undesirable scale and rust in several industrial processes. Hydrochloric acid is widely used in the pickling process of steel. The use of inhibitors is one of the most practical methods for protection against corrosion especially in acid solutions to prevent metal dissolution and acid consumption [1]. Most well known acid inhibitors for steel corrosion are organic compounds containing [2-10]. It has been observed that most of the organic inhibitors act by adsorption on the metal surface [11].

The adsorption of corrosion inhibitor depends mainly on physical chemical properties of the molecules such as functional group, steric factor, molecule size, molecular weight, molecular structure, aromaticity, electron density at the donor atoms and π-orbital character of donating electrons [12].

The present investigations aim to study the effect of 2-amino-4,6-dihydroxy pyrimidine (ADHP) as corrosion inhibitors on the corrosion of 1018 carbon steel in 1 M HCl using weight loss and galvanostatic polarization measurements. The effect of temperature on the dissolution of carbon steel in 1 M HCl containing the inhibitors used was also studied and some thermodynamic parameters were computed.

Experimental method

Carbon steel of type 1018 used in this study has the chemical composition C 0.2%, Mn 0.6%, P 0.04 %, Si 0.003% and the remainder iron. Weight loss measurements were performed using compounds of the dimensions (3 × 3 × 0.1) cm³. For galvanostatic polarization, a cylindrical rod embedded in araldite with exposed surface area of 0.2 cm² was used. The electrodes were polished with different emery papers, degreased with acetone and rinsed by distilled water, before being inserted in the test solution. For weight loss measurements, the steel coupons were left hanged in the test solution (1 M HCl) for 150 minutes at 25 ± 1 °C before recording the loss of their weight.

The galvanostatic cathodic and anodic polarization measurements were carried out using a three-compartment glass cell and an EG&G model 363 potentiostate/galvanostate corrosion measurement system. Platinum electrode was used as a counter electrode (separated from the cell solution by a sintered glass frit) and a saturated calomel electrode (inside a luggin's probe) as a reference electrode.

The inhibition efficiency IE was calculated using the following equation:

Results and discussion

Weight loss measurements

To study the adsorption behavior of ADHP on 1018 carbon steel in 1 M HCl, the adsorption isotherm must be defined. Attempts were made to fit θ values to various isotherms including Frumkin, Langmuir, Temkin and Freundlich isotherms. By far the results were best fitted by Langmuir adsorption isotherm.

Langmuir adsorption isotherm could be represented using the following equation:

where K and C are the equilibrium constants of adsorption process and additive concentration, respectively.

The results show that all the linear correlation coefficients and all the slopes are close to unit, which confirms that the adsorption of ADHP in 1 M HCl follows Langmuir adsorption isotherm. The considerable deviation of the slopes from unity shows that the isotherm cannot be strictly applied. This deviation is attributable to the interaction between adsorbed species on the metal surface [15].

Straight line relationships were obtained. This means that the adsorption of ADHP on the steel surface obeys the Langmuir adsorption isotherm. The standard free energy of adsorption, (ΔG⁰_ads), was calculated from the equilibrium constant of adsorption using the following equation

Then the standard adsorption entropy (ΔS⁰_ads) can be obtained using the thermodynamic fundamental equation [17]:

The negative values of ΔG⁰_ads suggested that the adsorption of ADHP onto the steel surface was a spontaneous process and its values also indicate that the inhibition process becomes less favorable as the temperature is increased from 20 °C to 50 °C. The positive values of adsorptive equilibrium constant (k) reveal a better adsorption, which leads to an increase in the inhibition efficiency [18]. The negative values of ΔG⁰_ads also suggest a strong interaction of the inhibitor molecule on the carbon steel surface [19]. These negative values indicate that adsorption of ADHP on the steel surface is an exothermic process [20].

Generally, an exothermic process signifies either physi-or chemisorption. The heat of physical adsorption is relatively low, on the order of 4-41 kJ.mol^-1 , whereas that of chemisorption is much higher, of a magnitude of 100-500 kJ.mol^-1 [21]. These concepts can be made valid for the adsorption of ADHP onto the steel surface, whereby it can be affirmed that this is due to a physical phenomenon.

The increase of the activation energy in the presence of the inhibitor is attributed to an appreciable decrease in the adsorption process of the inhibitors on the metal surface with increase of temperature and a corresponding increase in the reaction rate because of the greater area of the metal that is exposed to the acid. A decrease in inhibition efficiency with rise in temperature, with analogous increase in corrosion activation energy in the presence of inhibitor compared to its absence, is a good evidence of the physical adsorption of ADHP on the steel surface.

Galvanostatic polarization studies

Inspection of the data given in this table reveals that, as the concentration of the inhibitor increases, the values of b_a and b_c increase. This indicates that the inhibitor affects both anodic and cathodic reactions i.e mixed inhibitor [24]. The values of Ecorr are shifted to more negative potentials, the values of I_corr decrease and the values of IE increases indicating the inhibiting effect of these compounds.

Mechanism of inhibition

The formed complex is adsorbed on the metal surface and thereby isolating the metal from corroding attack.

Conclusions

1-The corrosion of 1018 carbon steel in 1 M HCl is inhibited by 2-amino-4,6dihydroxy pyrimidine(ADHP).

2-The inhibition efficiency was found to increase by increasing the inhibitor concentrations, and/or decreasing the temperature.

3-The inhibitive effect of these compounds takes place through the adsorption of their compounds on the metal surface.

4-The adsorption process follows Langmuir adsorption isotherm.

5-The inhibition efficiency obtained from weight loss measurements showed good agreement with those obtained from galvanostatic polarization measurements.

References