¹ Department of Chemistry, Sri Meenakshi Government College, For Women, Madurai, India

² Corrosion Research Centre, PG and Research Department of Chemistry, GTN Arts College, Dindigul - 624 005, India

³ Corrosion Research Centre, RVS School of Eng. and Technology, Dindigul-624 005, Tamilnadu, India

⁴ Department of Chemistry, VSB Engineering College, Karur-639 111, Tamilnadu, India

⁵ Department of Chemistry, Arupadavedu Institute of Technology, Chennai

Abstract

The inhibition efficiency (IE) of Amino Trimethylene phosphonic acid (ATMP) in controlling corrosion of carbon steel immersed in ground water in the absence and presence of Zn²⁺ has been evaluated by weight loss method. It is observed that the synergistic formulation consisting of 250 ppm ATMP and 10 ppm of Zn²⁺ has 98% IE. Polarization study reveals that ATMP-Zn²⁺ system functions as a cathodic inhibitor system. AC impedance study reveals that a protective film is formed on the metal surface.

Keywords: carbon steel, corrosion inhibition, synergistic effect, cathodic inhibitor, phosphonic acid.

Introduction

The usage of ground water is common in all industries. The role of dissolved oxygen in ground water is the predominant factor to cause mild steel corrosion [1-2]. Depending upon the metal/environment combinations, different types of inhibitors are used in suitable concentrations. Speller [3-5] reported that "Compound films" formed by phosphate - chromate mixture are more effective than those of either alone. The synergistic effect of halides and organic compound inhibitors are reported often in the literature [6-9]. Several inhibitors such as phosphonic acid [10-11], thiourea [12], carboxy methyl cellulose [13], and sodium dodecyl sulphate [14] have been used to control corrosion of carbon steel. Inhibitors for carbon steel in near neutral aqueous solutions are soluble chromates, dichromates nitrates, borates, benzoates and salts of carboxylic acids. Corrosion inhibition due to the formation of oxide layer on Cu metal surface in concentrated propionic acid and dilute citric acid [15] has been reported. The corrosion inhibition of carbon steel in ground water by adipic acid and Zn²⁺ system has been reported [16]. Existence of synergism between succinic acid and Zn²⁺ in controlling corrosion of carbon steel in well water has been investigated [17]. Inhibitors such as benzoate, phthalate and other carboxylates [18-20] stabilize the oxide film on iron surface; presumably, their inhibitive action results from the bonding of the O -ion. Carboxylates are anodic inhibitors. The corrosion inhibition of steel by salicylic acid in acidic media has been investigated [21]. In the present study, synergistic effect of AminoTrimethylene phosphonic acid ATMPand Zn²⁺ in corrosion inhibition of carbon steel in ground water has been investigated in detail. Amino Trimethylene phosphonic acid is an environment friendly organic compound, C₃H₁₂P₃O₉N, which is mainly used in open water circulation cooling system, petroleum pipelines and boilers. While the inhibition efficiencies have been evaluated by weight loss method, the mechanistic aspects are based upon the results of potentiostatic polarization and AC impedance studies.

Experimental

Preparation of the specimen

Weight loss method

Carbon steel specimens in triplicate were immersed in 100 mL of ground water containing various concentrations of the inhibitor in the absence and presence of Zn²⁺ for five days. The weights of the specimens before and after immersion were determined using a balance Shimadzu AY62 model. The corrosion products were cleaned with Clarke's solution [22]. The inhibition efficiency (IE) was then calculated using the equation

where w1 and w1 are the corrosion rates (mdd) in absence and presence of inhibitor, respectively.

Potentiostatic polarization study

Tangents were drawn on the cathodic and anodic polarization curves. From the point of intersection of the two tangents, I_corr and E_corr were calculated.

AC impedance measurements

The CHI 66A Electrochemical Workstation model was used for AC impedance measurements. The cell set up was the same as that used for polarization measurements. The real part (Z') and imaginary part (Z'') of the cell impedance were measured in ohms for various frequencies. The R_t (charge transfer resistance) and C_dl (double layer capacitance) values were calculated.

Results and discussion

Parameters of ground water

ATMP alone has 80% IE, whereas Zn²⁺ has 5% IE. In the absence of ATMP, the rate of transport of Zn²⁺ from the bulk of the solution towards the metal surface is slow. Similar observations have already been reported by Rajendran et al. [23-24]. When ATMP is combined with Zn²⁺ ions it is found that the IE increases. For example, 250 ppm ATMP has only 80% IE and 10 ppm of Zn²⁺ has only 5%. Interestingly their combination shows 98% IE. This suggests a synergistic effect between ATMP and Zn²⁺ ions; ATMP is able to transport Zn²⁺ towards the metal surface.

Analysis of the results of potentiostatic polarization study for the ATMP-Zn²⁺ system

Polarization study has been used to investigate the protective film formed on the metal surface [25-31].

When carbon steel is immersed in ground water, R_t value is 386 Ω cm² and C_dl value is 1.32 × 10^-8 F cm^-2. When ATMP and Zn²⁺ are added to ground water, Rt value increases from 386 Ω cm² to 509 Ω cm². The C_dl decreases from 1.32 × 10^-8 F cm^-2 to 1.00 × 10^-8 μF cm^-2. This suggests that a protective film is formed on the surface of the metal. This accounts for the very high IE of ATMP-Zn²⁺ system. Equivalent circuit diagram for such a system is shown in scheme 1.

Mechanism of corrosion inhibition

The results of the weight loss study show that the formulation consisting of 250 ppm of ATMP and 50 ppm of Zn²⁺ has 98% IE in controlling corrosion of carbon steel, in ground water. A synergistic effect exists between Zn²⁺ and ATMP. Polarization study reveals that this formulation functions as a cathodic inhibitor.

AC impedance spectra reveal that a protective film is formed on the metal surface.

a) When the solution containing ground water, 50 ppm of Zn²⁺ and 100 ppm of ATMP is prepared, there is formation of Zn²⁺-ATMP complex in solution.

c) On the metal surface, Zn²⁺-ATMP complex is converted into Fe²⁺-ATMP complex on the anodic sites. Zn²⁺ is released.

d) Zn²⁺-ATMP + Fe²⁺ → Fe²⁺-ATMP + Zn²⁺

e) The released Zn²⁺ combines with OH^- forming Zn(OH)₂ on the cathodic sites.

f) Zn²⁺ + 2 OH^- → Zn(OH)₂ ↓

g) Thus the protective film consists of Fe²⁺-ATMP complex and Zn(OH)₂.

Conclusions

The present study leads to the following conclusions:

• a synergistic effect exists between Amino Trimethylene Phosphonic acid (ATMP) and Zn²⁺ in controlling the corrosion of carbon steel immersed in ground water;

• polarization study reveals that ATMP-Zn²⁺ system functions as a cathodic inhibitor system;

• AC impedance spectra reveal that a protective film is formed on the metal surface.

References