Inhibition of the Corrosion of Mild Steel and Aluminum in Acidic Media by Some Purines

Inhibition of the Corrosion of Mild Steel and Aluminum in Acidic Media by Some Purines

Chapter One

Aims of the Research

This research aims to investigate some selected purines as eco-friendly inhibitors for the corrosion of mild steel and aluminium in 0.1 M HCl, H₂SO₄ and H₃PO₄ (a low acid concentration) at 303 and 333 K respectively.

Objectives of the Research

The objectives of the research are as follows:

To carry out a comparative study of the effect of adenine (AD), guanine (GU),hypoxanthine (HYP) and xanthine (XN) on the corrosion of mild steel and aluminium in 0.1 M HCl, H₂SO₄ and H₃PO₄ using gravimetric technique at 303 and 333 K
To investigate the adsorptive properties, thermodynamics and kinetic parameters of the purines from weight loss
To establish the effect of each purine derivative at 303 K on the current density and corrosion potentials of mild steel and aluminium in HCl, H₂SO₄ and H₃PO₄ at 303 K, using potentio dynamic polarisation
To evaluate the interaction of each purine derivative with the mild steel and aluminium surfaces in HCl, H₂SO₄ and H₃PO₄ at 303 K, by electrochemical impedance spectroscopy.
To investigate the synergistic effects ofiodide ions ( using [KI]= 0.005 M) on the adsorptive behaviour of the selected purines on mild steel and aluminium in the different acid media at 303
To carry out quantum chemical calculations in order to get useful theoretical information about the selected Molecular dynamics simulations will be employed to understand the interactions of the inhibitors with the Fe (1 1 0) and Al (1 1 0) surfaces.

CHAPTER TWO

LITERATURE REVIEW

Corrosion Inhibitors

Corrosion inhibitors are compounds that can reduce the rate of corrosion of a metal through the mechanism of adsorption, which may be physical (i.e, involving the transfer of charges from charged inhibitor to a charged metal surface) or chemical adsorption (which involves the transfer of electrons from the inhibitor to the vacant d-orbital of the metal).

Most corrosion inhibitors have been observed to posses one or more of the following properties (Eddy et al, 2010):

Possessionof hetero atoms in aromatic rings or long carbon chain
Possession of polar functional groups such as –COOH, -NH₂, etc as well as π-

Based on the above criteria, several organic inhibitors have been investigated including triazoles, benzotriazoles, organic dyes, amino acids, schiff bases, imidazoles and purines.

Triazoles and benzotriazoles derivatives as corrosion inhibitors

Li et al. (2011) synthesized two triazole derivatives [1-phenyl-2-(5-(1,2,4) triazol-1- ylmethyl-(1,3,4) oxadizaol-2-ylsulphanyl)-ethanone (PTOE) and 2-(4-tert-butyl- benzylsulphanyl)-5-(1,2,4) triazol-1-ylmethyl-(1,3,4) oxadiazole (TBTO)] as new corrosion inhibitors for the corrosion of mild steel in 1 M hydrochloric acid. The inhibition efficiencies of the different inhibitors were evaluated using weight loss and electrochemical techniques such as electrochemical impedance spectroscopy (EIS) and polarization curves. The results obtained from electrochemical investigation revealed Nthat these compounds acted as mixed-type inhibitors retarding the anodic and cathodic corrosion reactions and did not change the mechanism of either hydrogen evolution reaction or mild steel dissolution. The adsorption of the inhibitors obeyed the Langmuir adsorption model.The effect of molecular structure on the inhibition efficiency of the inhibitors was investigated using ab initio calculations while electronic properties such as the energy of the highest occupied molecular orbital (HOMO), the energy of the lowest unoccupied molecular orbital (LUMO), dipole moment (µ) and molecular orbital densities were also calculated.

Khadom et al. (2009) investigated the adsorption of 4-amino-5-phenyl-4H-1, 2, 4- triazole-3-thiol (APTT) as a corrosion inhibitor for mild steel in hydrochloric acid (HCl) solution using the weight loss technique. The degree of surface coverage by APTT was used to calculate the free energy of adsorption, using Bockris-Swinkels isotherm. The dependence of free energy of adsorption, on the surface coverage, was ascribed to the surface heterogeneity of the adsorbent. The adsorption of APTT molecules on the surface occurred without modifying the kinetics of the corrosion process.

Gopi et al.(2007) studied the corrosion inhibition of mild steel by means of newly synthesized triazole phosphonates 3-vanilidene amino 1,2,4-triazole phosphonate (VATP), 3-anisalidene amino 1,2,4-triazole phosphonate (AATP) and 3-paranitro benzylidene amino 1,2,4-triazole phosphonate (PBATP) together with cetyl trimethyl ammonium bromide (CTAB) in natural aqueous environment using weight loss, potentiodynamic polarisation and AC impedance measurements. Addition of molybdate was noticed to increase the inhibition efficiency of triazole in a synergistic manner. Results from experimental observations indicated VATP as a better corrosion inhibitor for mild steel in aqueous solution. Additionally the formulation consisting of VATP, sodium molybdate and CTAB offered good corrosion inhibition efficiency.

Matheswaran and Ramasamy (2010) studied benzotriazole as an inhibitor for the corrosion inhibition of mild steel in 1 N citric acid using weight loss method. The results obtained indicated that the corrosion inhibition efficiency of the benzotriazole varied with the temperature and acid concentrations. Also, it was found that the corrosion inhibition behaviour of benzotriazole was better when the concentration of inhibitor was increased. Kinetic treatment of the results showed first order kinetics. Selvi et al. (2003) also synthesized some benzotriazole derivatives (namely, N-[1- (benzotriazolo-1-yl)alkyl] aryl amine (BTMA), N-[1-(benzotriazolo-1-yl)aryl] aryl amine (BTBA), and 1-hydroxy methyl benzotriazole (HBTA) and found that these compounds possess excellent inhibition properties for corrosion of mild steel in 0.5 M H₂SO₄ at room temperature. Potentiodynamic polarization and AC impedance studies were used to investigate the inhibition mechanism. Benzotriazole derivatives were found to act as mixed type inhibitors. Among the compounds studied, HBTA exhibited the best inhibiting performance giving more than 95% IE. They also found that R_ct values increased while C_dl values tend to decrease with increase in the concentration of benzotriazole. The observed trend was attributed to the adsorption of benzotriazole on the metal surface.

Dyes as corrosion inhibitors

The inhibitive properties of some organic dyes have been investigated by some researchers. For instance, Abdeli et al. (2009) studied the inhibiting behaviour of nile blue and indigo carmine organic dyes on corrosion of mild steel in 1 M HCl, using

weight loss, potentiodynamic polarization and electrochemical impedance spectroscopy techniques. Polarization curves indicated that both inhibitors are mixed anodic–cathodic in nature, and Langmuir isotherm was found to be the best isotherm that described the adsorption behaviour.

CHAPTER THREE

MATERIALS AND METHODS

Material Preparation

Mild steel sheet of composition {wt.%: Mn (0.6), P (0.36), C (0.15), Si (0.03) and Fe (98.86)} and aluminium sheet (AA 1060 type) of purity 98.5%, were mechanically press-cut to form coupons, each of dimensions, 5 cm x 4cm x 0.15 cm for gravimetric studies and 2 cm x 2 cm x 0.15 cm for electrochemical studies. The coupons were wet polished with different grades of SiC abrasive paper (#400 to #1200), washed with distilled water, rinsed with absolute ethanol, cleaned in acetone and allowed to dry in the air before being preserved in a desiccator prior to corrosion testing. The reagents HCl, H₂SO₄, H₃PO₄ and KI used in the study were analar grades. Distilled water was used for their preparations. Acid concentrations of 0.1 M HCl, H₂SO₄ and H₃PO₄ were used for gravimetric (weight loss) and electrochemical studies respectively. The concentration of the halide (KI) used for inhibitive synergistic studies was 0.005 M while the concentration range for the inhibitors was 2 x 10^-3 to 1 x 10^-2M respectively.

Gravimetric Method

Previously weighed mild steel and aluminium metal coupons were completely immersed in 150 cm³ of the test solutions (0.1 M HCl, H₂SO₄ and H₃PO₄) in an open beaker placed in a water bath maintained at 303 and 333 K respectively. After every 24 hours, the corrosion products were removed by washing each coupon (withdrawn from the test solutions) in a solution containing 40% NaOH and 100 g/L of zinc dust for mild steel coupons while aluminium coupons were dipped in 70% nitric acid for 2 mins and later washed with distilled water using bristle brush and rinsed with distilled water. The washed coupons were rinsed in acetone and dried in air before re-weighing. The difference in weights for a period of 120 hours was taken as the total weight loss. From the weight loss results, the inhibition efficiency (IE_exp) of the inhibitor, the degree of surface coverage (θ) and the corrosion rate (CR) of mild steel and aluminium were calculated using equations 3.1 to 3.3 respectively (Eddy and Ebenso, 2010).

CHAPTER FOUR

RESULTS

Gravimetric Measurements

Figures 4.1 – 4.16 show the variation of weight loss (g) with time (hrs) at 303 and 333 K for the corrosion of mild steel and aluminium in 0.1 M HCl, H₂SO₄ and H₃PO₄ acids in the presence of various concentrations of adenine (AD), guanine (GU), hypoxanthine (HYP) and xanthine (XN). The plots on Figures 4.1 – 4.16 were obtained from the data on Appendices I –VIII. From the plots (Figures 4.1 – 4.8), it is evident that weight loss of mild steel in the different acid media decreased in the presence of AD, GU, HYP and XN indicating that the purines retarded the corrosion of mild steel in HCl, H₂SO₄ and H₃PO₄. It can also be deduced from the plots that although the weight loss of mild steel increased over time, the corrosion rates of mild steel coupons in the presence of the inhibitors decreased with increasing inhibitor concentrations.

CHAPTER FIVE

DISCUSSION

Gravimetric Measurements

From the plots obtained (Figures 4.1 – 4.16) it is evident that the degradation of mild steel in the absence of adenine (AD), guanine (GU), hypoxanthine (HYP) and xanthine (XN) is highest in H₃PO₄ at both 303 and 333 K, while the rate of corrosion of aluminium appeared to be highest in HCl at both temperatures. The observed difference in the behaviours of mild steel and aluminium in 0.1 M HCl, H₂SO₄ and H₃PO₄ can be explained on the basis of the different nature of the metals and their relative positions on the galvanic series (Appendix IX). The Galvanic series provides a guide to the corrosion resistance of metals and alloys. It is clear from the Galvanic series on Appendix IX that aluminium with a more negative potential should corrode more readily than mild steel. This may be responsible for higher corrosion rates for aluminium compared to those of mild steel in HCl solutions (Tables 4.1 and 4.3). However, higher corrosion rates were observed for mild steel in H₂SO₄ and H₃PO₄ than those for aluminium in the same acid solutions (Tables 4.1 and 4.3).

The high resistance to corrosion of aluminium media may be due to the naturally formed oxide film on aluminium surface which protects it in air or neutral solutions. Even upon immersion in acidic and alkaline media, this film is preserved for long periods of time (Pourbaix, 1974). Since the potential of zero charge (pzc) for aluminium oxide (Al₂O₃) occurs from pH 9.0 to 9.1 (Cabot et al., 1991; Fouda et al., 2000), chloride or any aggressive anion is likely to be adsorbed at pH values <9.0. This leads to localization of the corrosion attack by preferential adsorption at weak sites on the passive film. The lower corrosion rates observed for aluminium in H₂SO₄ and H₃PO₄solutions are indications that these media do not readily dissolve the passive oxide film on the aluminium surface, so that the corrosion rates are lower.

Solubility of the corrosion products is another factor that may be responsible for the different behaviours of mild steel and aluminium in the different acid media. From the knowledge of chemical equilibria, a soluble corrosion product signifies more dissolution of the metal in the acid medium resulting in higher corrosion rates and lower inhibition efficiencies.

CHAPTER SIX

SUMMARY, CONCLUSIONS AND RECOMMENDATIONS

Summary and Conclusions

The results from the study show that some of the purines studied possess good inhibition properties for the corrosion of mild steel and aluminium in 0.1 M HCl, H₂SO₄ and H₃PO₄. The inhibition efficiencies increased with increasing concentrations of the inhibitors. The inhibition efficiencies of purines for the corrosion of mild steel in 0.1M HCl, H₂SO₄ and H₃PO₄ solutions decreased in the order: AD > HYP > GU > XN (in HCl); HYP > AD > GU > XN (in H₂SO₄); and GU > AD > HYP > XN (in H₃PO₄),

respectively. For aluminium in 0.1 M HCl, H₂SO₄ and H₃PO₄ solutions, the inhibition efficiencies decreased in the order: GU > AD > HYP > XN (in HCl). However, only HYP inhibited the corrosion of aluminium in H₂SO₄ solution. Their was no evidence of corrosion inhibition by AD, GU and XN in H₂SO₄ or any of the purines in H₃PO₄ solutions as they rather enhanced the corrosion of aluminium thereby functioning as corrosion catalysts. It was also observed that the inhibition efficiencies decreased with inmersion time for all the systems and for some of the systems at 333 K. Moreover, IE% of the selected purines was slightly enhanced in the presence of KI for some of the systems investigated.

The results obtained revealed that AD, GU, HYP and XN are good corrosion inhibitors for the corrosion of mild steel in HCl, H₂SO₄ and H₃PO₄. All the purines inhibit the corrosion of aluminium in HCl while only HYP inhibit the corrosion of aluminium in H₂SO₄.

Impedance measurements show that the inhibitors functioned by adsorption of the purines on the metal/corrodent interface while polarization measurements show that the adsorbed purines inhibited the corrosion process via mixed inhibition mechanism, affecting both the anodic metal dissolution reaction and the cathodic hydrogen evolution reaction.

Furthermore, the inhibition efficiencies of the purines obtained from gravmetric, polarization and impedance studies were observed to be similar and in good agreement.

From the activation energies (E_a), heats of adsorption (Q_ads) and standard free energies of adsorption (∆G_ads) obtained from the study, it is evident that the inhibitors are adsorbed on the steel and aluminium surfaces through both physical and chemical interactions suggesting that both molecular as well as protonated purine species were responsible for the observed inhibiting actions of the compounds in the different acid media. Adsorption characteristics of the purines fitted both the Langmuir and Temkin isotherms.

SEM micrographs, FTIR and impedance spectra showed the presence of protective layers over the mild steel and aluminium surfaces, providing evidence for the corrosion inhibitory effects of the inhibitors. While DFT-based quantum chemical computations of parameters associated with the electronic structures of the purine molecules confirmed their inhibiting potential, which was further corroborated by molecular dynamics modeling of the adsorption of the single molecules on the metal surface.

RECOMMENDATIONS

Further study is required to assess the following:

Use of purine derivatives as corrosion inhibitors in coatings on metallic implants in the field of medicine should be explored under in vitro and in vivo
Synergistic studies involving two or more of the studied purine derivatives should be Corrosion inhibition studies by purines should be carried out under field conditions to simulate operational conditions including the effects of flow, pressure, a wider temperature range and pH of the aqueous systems so as to explore the possibility of using them as inhibitor formulations in industries.
Quantum chemical calculations and molecular dynamics involving a rigorousmodeling of inhibitor-metal surface, water-metal surface, and inhibitor-water interactions should be This is because inhibition effectiveness depends on the interplay between them.

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