Effect of Mineral Acids on Corrosive Propensity of Metals

Transcription

1 Effect of Mineral Acids on Corrosive Propensity of Metals ka Sharma, Arpita Sharma, Guddi Choudhary, Swati Yadav Department of Chemistry, University of Rajasthan, Jaipur (Rajasthan) INDIA Abstract The corrosive propensity of mild steel, aluminium and copper in various concentrations of mineral acids viz. HCl and H 2 SO 4 has been studied by employing weight loss method. The effect of acid strength on the metal corrosion has been investigated at room temperature at different immersion periods. In H 2 SO 4 media, the corrosion rate of mild steel has been observed to be higher than that of aluminium and copper; while in HCl, aluminium has been observed to get corroded fast. The thermodynamic as well as kinetic parameters have also been evaluated. The surface morphological analysis has been carried out by Optical micrographs. Keywords: metal corrosion, mineral acids, corrosion parameters, thermodynamic and kinetic parameters, optical micrograph I. INTRODUCTION Regardless of all the recent advances in polymer technology and the development of high strength plastics, industry and civilization would grind to a halt without the metals. Unfortunately all metals are subject to corrosion, a continuous process that consumes increasingly scarce raw materials. Corrosion is the deterioration of materials by chemical interaction with their environment, resulting in to a less desirable material from the original metal [1]. Metals are usually exposed to the action of bases or acids in the industries. Acid being corrosive causes severe damage to properties of metal resulting in sudden failure of material in service [2-4]. The corrosion costs in most of the countries are in the range of 2-4 % of the gross national product [5]. Consequently, it is needed to investigate the trend of the metal-dissolution on their exposure to various mineral acids, as this is an important factor in material selection that determines the service life of the material [6-7]. The present work has been focused to study the corrosive tendency of passive and active metals viz aluminium, copper and mild steel in various concentrations of hydrochloric acid and sulphuric acid by employing weight loss method. II. EXPERIMENTATION A. Specimen (coupon) preparation Rectangular specimen (coupons) of industrially used metals viz aluminium, copper and mild steel having the compositions as: uminium: () 97.6%, (Mn)1.3%, (Fe).87%, ().11%, (Zn).11%; : (Mn).51%, 68 (Fe).19%, ()93.6%, (Zn)5.7%, and : (Mn).3%,.1% (Mo),.39% (Cr), Fe These specimens have been mechanically pressed cut to form different coupons, each of dimension exactly (3 cm x 2.4 cm x.16 cm) with a hole about.12 mm diameter drilled at one end for free suspension. The specimens have been mechanically polished, degreased with acetone, washed with distilled water and well polished with emery paper, cleaned, rinsed and dried then stored in desiccators [8]. B. Test Solutions Mineral acids viz. hydrochloric acid and sulphuric acid each of 2 N and.5 N strengths have been prepared using standard procedures. For the preparation of all solutions double distilled water was used and all reagents used were of analytical grade. C. Chemical (Weight loss) Measurements The pre-treated (raw) coupons were introduced in the test solutions. The weight of coupons was measured before and after each immersion. The mass loss was taken as the difference in weight of the coupons before and after immersion. Since weight loss is not uniform with time, thus average loss is reported [9-1]. From the experiment data, various corrosion parameters such as corrosion rate (ρ corr ) (mmy -1 ), percentage inhibition efficiency (IE %), apparent activation energy (E a ) were evaluated. D. Surface morphological analysis To scan morphology of metal surface, the optical micrograph of the coupons were taken before and after immersion in different concentrations of aggressive media, viz. HCl and H 2 SO 4 solutions [7]. III. RESULT AND DISCUSSIONS Corroding process of all three metals in acidic media, viz. HCl and H 2 SO 4 was investigated. As an established fact, aluminium is more corrosion resistant than other materials due to a compact, strongly adherent and continuous microscopically thin film of aluminium oxide, 2 O 3 ; but this film is resistive to corrosion in the ph range from 4-9 [11-12]. Figures 1-4 reveal that weight loss of uminium is found to be more in the HCl media as compared to H 2 SO 4. This is because of the anion effect

2 i.e. presence of aggressive anions like Cl - accelerates the process of corrosion and form a soluble aluminium - complexes while SO 4 ions are found to show inhibitive effect [13-15]. It has been observed that with the lapse of the time period, the weight loss also increases. It is also evidenced by hydrogen evolution that significant weight loss is observed in HCl media compare to H 2 SO 4 where practically no hydrogen evolution take place in initial hours. Metal Coupons Table 1: Various corrosion parameters for uminium, and Mild steel in various concentration of HCl Immersion time (h) Weight Loss (g) HCl.5 N 2 N corr Activation Weight corr (mmy -1 ) Energy (Ea) Loss (mmy -1 ) x1-2 KJ/mol (g) x Activation Energy (Ea) KJ/mol

3 Corrosion Rate (mmpy) International Journal of Science and Advanced Technology (ISSN ) Volume 2 No 12 December 212 Table 2: Various corrosion parameters of uminium, and Mild steel in various concentration of H 2SO 4. Metal Coupons Immersion time (h) Weight Loss (g) H 2 SO 4.5 N 2 N corr Activation Weight corr (mmy -1 ) Energy Loss (mmy -1 ) (Ea) (g) KJ/mol Activation Energy (Ea) KJ/mol Mild steel was found to get more corroded in H 2 SO 4 as compared to copper and aluminium, because iron is more reactive than the as per the standard potential series. uminium being passive is not effected much in initial hours as sulphate ion are inert, but later with the passage of time, the oxide film dissolves and considerable weight loss was observed for aluminium. Compared to copper, aluminium is active, hence the metal dissolution trend can be placed in the order as: > > Increasing the acid concentration (strength), the corrosion rate was also found to increase Fig 1: Corrosion Rate Vs time (h) for,, in.5 N HCl 7

4 Corrosion Rate (mmpy) Corrosion Rate (mmpy) Corrosion Rate (mmpy) International Journal of Science and Advanced Technology (ISSN ) Volume 2 No 12 December Surface morphology of the coupons (before immersion) Fig 2: Corrosion Rate Vs time (h) for,, in 2. N HCl.25 uminium Fig 3: Corrosion Rate Vs time (h) for,, in.5 N H 2SO Fig 4: Corrosion Rate Vs time (h) for,, in 2. N H 2SO 4 Surface morphological analysis The coupon surfaces were analyzed by carrying out optical micrographs with the help of LABOMED microscope. Optical micrographs of samples of,, and before and after immersion in mineral acids (HCl / H 2 SO 4 ) have been reported as: Surface morphology of coupons (after immersion in aggressive media) 71

5 In.5 N H 2 SO 4 In 2. N H 2 SO 4 uminium uminium 72

6 In.5 N HCl In 2. N HCl uminium uminium 73

7 IV. CONCLUSIONS Dissolution of uminium was found to be fast in HCl; while in H 2 SO 4 mild steel has high corrosion rate. With the increase in acid concentration (strength), corrosion rate also increases. On comparing the optical micrographs of the coupons in (2 N and.5 N) HCl and H 2 SO 4, corrosion was more pronounced at higher level of acid concentration. [13] A. Buyuksagis, A. A. Aksut, Effects of cohols on the corrosion of uminium loys in 1 N HCl Solution, Protection of Metals, vol. 44 (5) pp , 28. [14] M. Yasuda, F. Weinberg, D. Tromans, Pitting Corrosion of and - Single-. Crystals J. Electrochem. Soc., vol. 137, pp , 199. [15] T. I. Wu, J. K. Wu, Effect of Sulfate Ions on Corrosion Inhibition of AA 775 uminum loy in Sodium Chloride Solutions, Corrosion, vol. 51 (3) pp , ACKNOWLEDGMENT Authors thank head, Department of Chemistry for providing necessary research facilities. Arpita and Guddi thanks UGC for financial support. Authors also express gratitude towards CDPE, University of Rajasthan, Jaipur for facilitating to carry out optical micrographs. REFERENCES [1] J. J. Moore, Corrosion of Metals. A text book of Chemical Metallurgy, Butterworth-Heinemann Ltd.: Boston, New Jersey, 1994, pp [2] Mars G. Fontana, Corrosion Engineering, 2 nd ed., Tata McGraw Hill: New Delhi, 25. [3] Barbara A. Shaw, Robert G. Kelly, Electrochem. Soc. Interface, pp , 26. [4] R. R. Pierre, Corrosion Inspection and Monitoring, John Wiley & Sons: New York, 27. [5] Vedula S. Sastri, Corrosion Inhibitors: Principles and Applications, John Wiley & Sons: England, [6] E. Osarolube, I. O. Owate, N. C. Oforka, Corrosion behaviour of mild and high carbon steels in various acidic media, Scientific Research and Essay, vol. 3 (6) pp , 28. [7] E. A. Noor, A. H. -Moubaraki, Corrosion Behavior of in Hydrochloric Acid Solutions Int. J. Electrochem. Sci., vol. 3, pp , 28. [8] Rekha N. Nair, N. Kharia, I. K. Sharma, P. S. Verma, ka Sharma, Corrosion Inhibition Study of uminium in Acid Media by Mango (Mangifera indica) Leaves as Eco-Friendly Inhibitor, J. Electrochem. Soc. India, vol. 56 (1/2) pp , 27. [9] Rekha N. Nair, Shashi Sharma, I. K. Sharma; P. S. Verma, ka Sharma, Inhibitory efficacy of Piper nigrum linn. Extract on corrosion of AA11 in HCl, RASAYAN J. Chem., vol. 3 (4) pp , 21. [1] ka Sharma, P. S. Verma, I. K. Sharma, Rekha N. Nair, Energy Conservation through Combating Metal-Corrosion by Means of Natural Resources, Proc. World Acad. Sci. Eng. Technol. (PWASET), vol. 39 (ISSN: ) pp , 29. [11] C. Kammer, uminum Handbook, 1 st edn., uminum Verlag, Dusseldorf, 1, 1999, pp [12] Christian Vargel, Corrosion of uminium, Oxford, Elsevier: UK,