ARTIGOS

Influence of Zinc Sulphate on the Corrosion Resistance of L80 Alloy Immersed in Sea Water in the Absence and Presence of Sodium Potassium Tartrate and Trisodium Citrate

 

Grace Baby^a, Susai Rajendran^b,*, V. Johnsirani^a and Abdulhameed Al-Hashem^c

^aPG Department of Chemistry, Sakthi College of Arts and Science for Women, Oddanchatram-624 619, India

^bCorrosion Research Centre, Department of Chemistry, St Antony`s College of Arts and Science for Women, Dindigul-624005, India

^cSenior Research Scientist, Petroleum Research Centre, Kuwait Institute for Scientific Research, Kuwait

 

ABSTRACT

In cooling water systems, seawater can be used. L 80 can be used as a pipeline for carrying sea water. However, this alloy will undergo corrosion. Corrosion can be prevented by the addition of inhibitors such as sodium potassium tartrate (SPT), trisodium citrate (TSC) and zinc sulphate. Corrosion resistance of L 80 alloy in sea water, in the absence and presence of the above inhibitors, has been evaluated by polarization study and AC impedance spectra. It was observed that SPT and TSC show better inhibition efficiency in the presence of Zn ²⁺. Further, it was found that the SPT-Zn system is better than the TSC-Zn system.

Keywords: corrosion inhibition, seawater, sodium potassium tartrate, trisodium citrate, polarization study, AC impedance spectra, L80 alloy.

 

Introduction

Seawater makes up the oceans and seas, covering more than 70 percent of Earth surface. Seawater is a complex mixture of 96.5 percent of water, 2.5 percent of salts, and smaller amounts of other substances, including dissolved inorganic and organic materials, particulates and a few atmospheric gases [1]. Almost anything can be found in seawater. This includes dissolved materials from Earth crust, as well as materials released from organisms. The most important components of seawater that influence life forms are salinity, temperature, dissolved gases (mostly oxygen and carbon dioxide) and nutrients [2].

Seawater can be used in cooling water systems, especially in ships and in marine environments. In these systems, L 80 alloy can be used to carry out seawater. Hence, knowledge of corrosion resistance of L80 alloy in seawater, in the presence of inhibitors, will be useful. Corrosion resistance of aluminium and its alloys [3, 4], zinc and its alloys [5, 6], copper and its alloys [7, 8], mild steel [9, 10], stainless steel [11, 12], nickel and its alloys [13, 14] in seawater has been evaluated. Corrosion inhibitors such as inorganic compounds [15, 16] and natural products [17, 18] have been used to prevent the corrosion of metals and alloys. In the present study, corrosion resistance of L 80 alloy in seawater, in the presence of sodium potassium tartrate (SPT) and trisodium citrate (TSC), has been evaluated by electrochemical studies, such as polarization study and AC impedance spectra. Influence of Zn²⁺ on the corrosion resistance of the above systems has also been evaluated.

 

Experimental

Electrochemical studies

In the present work, corrosion resistance of L 80 alloy immersed in various test solutions was measured by Polarization study and AC impedance spectra.

Polarization study

In the present study, polarization studies were carried out in a CHI Electrochemical work station/ analyzer, 660A model. It was provided with an automatic IR compensation facility. A three electrode cell assembly was used, as shown in Fig. 1.

 

 

The working electrode was L 80 alloy. SCE was the reference electrode. Platinum was the counter electrode. A time interval of 5 to 10 min was given for the system to attain a steady state open circuit potential. The working electrode and the platinum electrode were immersed in sea water, in the absence and presence of the inhibitor. Saturated calomel electrode was connected with the test solution through a salt bridge. From the polarization study, corrosion parameters, such as corrosion potential (E_corr), corrosion current (I_corr), Tafel slopes anodic = b_a and cathodic = b_c and LPR (linear polarization resistance) values, were considered. The scan rate (V/S) was 0.01. Hold time at (Efcs) was zero and quiet time (s) was two.

AC impedance spectra

In the present investigation, the same instrument and set-up used for the polarization study was used to record AC impedance spectra. A time interval of 5 to 10 min was given for the system to attain a steady state open circuit potential. The real part (Z’) and imaginary part (Z’’) of the cell impedance were measured in ohms at various frequencies. AC impedance spectra were recorded with initial E(v) = 0, high frequency (Hz = 1 x 105), low frequency (Hz = 1), amplitude (V) = 0.005 and quiet time (s) = 2. From Nyquist plot, the values of charge transfer resistance (R_t) and double layer capacitance (C_dl) were calculated.

where Rs = solution resistance.

C_dl values were calculated using the relationship:

where f_max = frequency at maximum imaginary impedance.

Sea water

The composition of sea water used in this study is given in table 1. Sea water was collected in the Bay of Bengal, located at Kanampadi, East Coast Road, Chennai, India.

 

 

L80 alloy

L80 alloy was manufactured according to 5CT API specification. This is a controlled yield strength material with a hardness testing requirement.

L80 is usually used in wells with sour (hydrogen sulfide) environments. Chemical composition of L80 alloy is given in table 2.

 

 

The remaining Wt % was iron.

Used inhibitors

Pure samples of Sodium Potassium Tartrate (SPT), Tri Sodium Citrate (TSC) and Zinc Sulphate were used as corrosion inhibitors.

 

Results and discussion

Electrochemical studies

In the present work, corrosion resistance of L80 alloy immersed in various test solutions was measured by polarization study and AC impedance spectra.

L80 alloy –sodiumpotassium tartrate (SPT) system

Polarization study

It is seen from table 3 that when SPT is added to sea water, the corrosion resistance of L80 alloy increases. This is revealed by the fact that when SPT is added to sea water, LPR values increases and I_corr value decreases, as shown in Figs. 2 to 5.

 

 

 

 

 

 

Influence of Zinc sulphate (ZnSO₄.7H₂O)

When ZnSO₄.7H₂O was added to the above system, LPR value increased to a great extent (Fig. 5) and I_corr decreased considerably. This indicates that, in the presence of SPT - ZnSO₄.7H₂O system, the corrosion resistance of L80 alloy in sea water increases to a great extent. A protective film is formed on the metal surface. This prevents transfer of electrons from anodic site to cathodic site. Hence, LPR value increases, which results in a decrease in the corrosion current value.

AC impedance spectra

The AC impedance spectra (Nyquist plots, Bode plots) of L80 alloy immersed in various test solutions are shown in Figs. 6 to 11. The corrosion parameters are given in table 4. It is observed from table 4 that, when SPT is added to sea water, the corrosion resistance of L80 alloy increases. This is revealed by the fact that there is an increase in R_t value, impedance value and decrease in C_dl value.

 

 

 

 

 

 

 

 

Influence of Zinc sulphate (ZnSO₄. 7H₂O)

When ZnSO₄.7H₂O is added to the above system, the corrosion resistance further increases (Fig. 12). This is revealed by the fact that there is an increase in R_t values and impedance value. There is a decrease in C_dl. Thus, electrochemical studies reveal that the corrosion resistance of L80 alloy in sea water decreases in the following order:

 

 

Sea water + SPT + ZnSO₄ > Sea water + SPT > Sea water.

L80 alloy – TSC system

Polarization study

It is seen from table 5 that, when TSC is added to sea water, the corrosion resistance of L80 alloy increases. This is revealed by the fact that when TSC is added to sea water, LPR values increases and I_corr value decreases (Figs. 13 and 14). There is the formation of a protective film on the metal surface. So, the transfer of electrons from anodic site to cathodic site is restricted. Hence, LPR value increases and, correspondingly, corrosion current value decreases.

 

 

 

 

Influence of Zinc sulphate (ZnSO₄.7H₂O)

When ZnSO₄.7H₂O is added to the above system, LPR value increases tremendously (Fig. 15) and I_corr decreases very much. This indicates that, in the presence of SPT - ZnSO₄.7H₂O system, the corrosion resistance of L80 alloy in sea water increases to a great extent.

 

 

AC impedance spectra

The AC impedance spectra (Nyquist plots, Bode plots) of L80 alloy immersed in various test solutions are shown in Figs. 16 to 19. The corrosion parameters are given in table 6. It is observed from table 6 that, when TSC is added to sea water, the corrosion resistance of L80 alloy increases. This is revealed by the fact that there is an increase in R_t value and impedance value and a decrease in C_dl value.

 

 

 

 

 

 

Influence of Zinc sulphate (ZnSO₄ .7H₂O)

When ZnSO₄.7H₂O was added to the above system, the corrosion resistance further increased. This is revealed by the fact that there is an increase in R_t values (Fig. 20) and impedance value. There is a decrease in C_dl. Thus, electrochemical studies [19-28] reveal that the corrosion resistance of L80 alloy in sea water decreases in the following order:

 

 

Sea water + TSC + ZnSO₄ > Sea water + TSC > Sea water

 

Conclusion

The corrosion resistance of L80 alloy in sea water, in the absence and presence of sodium potassium tartate (SPT), trisodium citrate (TSC) and zinc sulphate (ZnSO₄ . 7H₂O), has been evaluated by electrochemical studies, such as polarization study and AC impedance spectra. The study led to the following conclusions:

L80 alloy – SPT system

Electrochemical studies revealed that the corrosion resistance of L80 alloy in sea water decreases in the following order:

Sea water + SPT + ZnSO₄ > Sea water + SPT > Sea water

L80 alloy – TSC system

Electrochemical studies revealed that the corrosion resistance of L80 alloy in sea water decreases in the following order:

Sea water + TSC + ZnSO₄ > Sea water + TSC > Sea water

Addition of Zinc sulphate improves the corrosion resistance of the TSC and SPT systems.

This study also led to the conclusion that the SPT-Zinc system offers better corrosion resistance than the TSC-Zinc system.

 

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Received June 15, 2018; accepted August 20, 2020

 

^* Corresponding author. E-mail address: susairajendran@gamil.com

 

Acknowledgements

The authors are thankful to their respective managements for their help and encouragements. Special thanks are due to the Congregation of The Immaculate Conception (CIC), Rev. Sr. Gnana Sundari, Provincial, Madurai Province of CIC, Rev. Sr. M Margaret Inbaseeli, the Secretary and Rev. Sr. Mary Pramila Shanthi, the Principal, St Antony`s College of Arts and Science for Women, Dindigul-624005, India.