Indian Journal of Chemical Technology Vol. 15, March 2008, pp. 174-179 Corrosion characteristics of mild steel in aqueous solution of formic acid containing some acetic acid S K Singh, Ashim K Mukherjee* & M M Singh Department of Applied Chemistry, Institute of Technology, Banaras Hindu University, Varanasi 221 005, India Email: ashimmukherjee@gmail.com Received 8 June 2007; revised 15 November 2007 Weight loss and potentiostatic techniques were used at 25, 35 and 45 C to investigate the corrosion behaviour of mild steel in aqueous solution of formic acid containing different amounts of acetic acid, maintaining the total concentration of the acids at 20%. It was found that the corrosion rate of mild steel in the mixture of the two acids is a function of acid composition and temperature. The maximum corrosion rate was observed in 20:0 % (formic:acetic) acid solutions by both techniques. The corrosion rate of mild steel in the aqueous mixture of formic and acetic acid having different compositions (%) followed the order 20:0> 19:1>18:2> 15:5> 10:10. Active corrosion behaviour of mild steel was observed over the whole range of potentials at each composition and temperature. The cathodic polarisation curves were almost identical irrespective of the composition of the two acids in the mixture. Keywords: Mild steel, Acetic acid, Formic acid, Corrosion, Passivity In general corrosion of most metals follows the same mechanism in both formic and acetic acids, with some notable exceptions. For example, aluminium is corroded at a fairly rapid rate by formic acid even at room temperature 1. The copper base alloys, i.e. brasses, silicon bronzes, aluminium bronzes and phosphor bronzes are generally corroded at rates too high to permit their use as materials of construction. Teeple 1 discussed the corrosion behaviour of various metals and alloys in contact with different organic acids. Field corrosion test for 27 days in a 2% formic acid, 1.5% formaldehyde solution at 300 F indicated corrosion rate of mild steel >0.5 IPY. Thus, it cannot be used as constructional material without precaution. Corrosion behaviour of 430 stainless steel 2,3, SS 41 steel 4 and AISI 316 stainless steel 5 in formic and acetic acids have been studied by Sekine and his coworkers. Corrosion of copper in aqueous solutions of formic and acetic acids taken individually has been studied by Singh and Singh 6. They reported that formic acid was more corrosive than acetic acid. Having studied the corrosion behaviour of steels, oxygen free Cu and Ti in formic acid of different concentrations, Pletka et al. 7 concluded that the corrosion of oxygen free Cu in formic acid was strongly dependent on the presence or absence of oxygen. The corrosion behaviour of duplex SUS 329JI stainless steel in HCOOH solution was investigated by Sekine and Momai 8, while mild steel in formic acid 9 and acetic acid 10 separately has been investigated by Singh and Gupta. Comparison of corrosion behaviour of pure Fe, Ni, Cr and type 304 stainless steel in formic acid solution has been worked out by Sekine and Chinda 11. The influence of water content on the corrosion of metals in monocarboxylic acids has been investigated by Constantinescu and Heitz 12. A review of the literature pertaining to corrosion by organic compounds indicates that the information available is not comprehensive. Most of the investigations pertain to pure acids, but the study of formic acid contaminated with acetic acid as a significant theme has remained marginalized in the field of corrosion science till now. The corrosion of a zinc surface exposed to formic acid at various relative humidity has been investigated by Johnson and Leygraf 13. Maeng 14 has studied the effect of formic and acetic acids on the corrosion fatigue of some stainless steel samples. Corrosion and inhibition behaviour of mild steel in 20% acetic and formic acid solutions has been investigated by Quraishi and Sharma 15,16. Actual corrosion incidents of structural
SINGH et al.: CORROSION OF MILD STEEL IN AQUEOUS FORMIC ACID 175 components of chemical plants made of stainless steel in acetic and formic acids at sea shore regions induced by airborne sea salt have been reported by Katsumi 17. This prompted us to investigate the corrosion of mild steel in formic and acetic acid mixtures. The aim of the present work is to study the role of acetic acid in promoting or retarding the corrosion of mild steel in aqueous solution of formic acid. Experimental Procedure AR grade glacial acetic acid and formic acid were used for weight loss studies, while for polarisation studies sodium acetate, sodium formate and sodium sulphate were used as well. Mild steel samples having percentage composition: C, 0.23; Mn, 0.11; Si, 0.02; P, 0.02; S, 0.02; Ni, 0.02; Cu, 0.01; Cr, 0.01; Fe, remainder, were used as test specimens. For weight loss experiments the specimens were cut into pieces of 3 3 0.1 cm. For polarisation measurements, the working electrodes were cut into rectangular pieces of 3 1 0.1 cm, inserted into the electrochemical cell through a copper rod and fixed with a screw. The upper part of the samples adjacent to the copper rod was coated with wax in order to avoid electrolytic contact. The remaining exposed surface was chosen approximately equal to 1 1 cm in such a way that the effective exposed area of both sides (inclusive of edges) turned out to be 2 cm 2. The polarisation experiments were carried out in a three-necked glass assembly. The electrode system consisted of the working electrode of the specimen, a counter electrode of platinised platinum (1 1 cm) and a reference electrode in the form of a saturated calomel electrode (SCE) with a Luggin probe containing saturated solution of KNO 3. The experiments were performed in static and undeaerated solutions at 25, 35 and 45 C (± 0.2 C) and the potentials were quoted with respect to the SCE. Potentiostatic polarisation was performed using a Wenking POS 73 potentiostat by manually changing the potential stepwise at a rate of 10 mv/min. The surface morphology of the samples was characterised with Scanning Electron Microscopy (SEM). The mild steel samples were mounted on the specimen stub and inserted into the evacuated chamber of the JEOL JSM Scanning Electron Microscope. The images were obtained at an operating voltage of 10 KV. The experiments were performed by varying the composition of the two acids in the solution keeping the value of total acid concentration constant at 20%. Results and Discussion Corrosion behaviour by weight loss method The corrosion rate of mild steel in 20:0, 19:1, 18:2, 15:5 and 10:10, mixture by percentage (v/v) of formic and acetic acid has been determined for an immersion period of 24 h at 25, 35 and 45 C. Further, for the 20:0 and 10:10 formic and acetic acid mixtures, it was determined for varying immersion periods (6, 12, 24, 48, 72, 96, 120, 144 and 168 h) to evaluate the effect of exposure time on the corrosion rate. The variation in corrosion rate with the time of exposure at each temperature has been illustrated in Fig. 1. The nature of variation of corrosion rate with exposure time is found to be identical at all the temperatures for both the investigated mixture compositions. There is an initial sharp increase leading to a maximum at 24 h, and then a small gradual decrease till 168 h of exposure time. This is in conformity with the results reported by Singh and Gupta 9, who had investigated their system up to 72 h exposure time only. It has been reported elsewhere 18 that the corrosion rate in 5% solution of formic acid at 40 C was found to be relatively high in the initial stages, but decreases and stabilizes after 400-600 h. The conductance of formic and acetic acid in the mixture, as well as corrosion rate of mild steel at 25, 35 and 45 C have been plotted as a function of composition in Fig. 2. It can be observed from the curves that at all the three temperatures there is a Fig. 1 Variation of corrosion rate with exposure time of mild steel in 20% formic and 10% formic + 10% acetic acid mixture at different temperatures 25 C; o 35 C 20% formic acid + 0% acetic acid; 45 C; Δ 25 C; 35 C 10% formic acid + 10% acetic acid 45 C
176 INDIAN J. CHEM. TECHNOL., MARCH 2008 continuous decrease in both the corrosion rate as well as conductance with increase in acetic acid content. The corrosion rate however, increases with increasing temperature. This is believed to be due to the enhanced conductivity of the solution resulting from increase in dissociation of the electrolyte. Sekine et al. have observed that corrosion rate was markedly dependent on concentration and temperature for AISI 316 SS 5 and 430 SS 2,3 in formic and acetic acid solutions. Increase in corrosion rate of mild steel with increase in temperature has earlier been reported by Singh and Gupta in formic 9 as well as acetic acid 10 when taken separately. The gradual decrease in corrosion rate with successive changes in the composition of the mixture of the two acids seems to be obvious in view of the difference in their corrosivity. The corrosion rate of mild steel at each concentration of formic acid has been found to be greater than that in acetic acid. However, the observed rate represents neither the algebraic sum of the rates in individual acids nor their mean. The two acids seem to enjoy a synergistic relationship. It appears that acetic acid acts as an inhibitor for the corrosion of mild steel in formic acid. This is further evident from the SEM micrographs (Fig. 3), which show uniform corrosion in formic acid but localized and more intense corrosion in 10:10 % mixture. The corrosion in the mixture of acids occurs on those spots which are not covered by the inhibitor and hence is more intense. Since the dissociation constant of formic acid at 25 C is almost ten times greater than the corresponding value for acetic acid, the presence of formic acid will suppress the dissociation of acetic acid, thereby reducing its contribution towards the total conductance of the mixture. Moreover, the increased viscosity of the medium due to undissociated acetic acid molecules decreases the conductance of the mixture even below the conductance of 10% formic acid. This explains the observed corrosion rate in 10:10 mixture, which is even lower than that of 10% formic acid. The same logic could be extended for other mixture compositions as well. The increase in corrosion rate with rise in temperature likewise, can be explained in terms of increase in conductance of the mixture as a result of simultaneous increase in the degree of dissociation of both the acids. Electrochemical behaviour of mild steel in formic and acetic Fig. 2 Variation of corrosion rate and conductance with concentration at different temperatures Corrosion rate: ( ) at 25 C; ( ) at 35 C and ( ) at 45 C Conductance: (o) at 25 C; ( ) at 35 C and (Δ) at 45 C Fig. 3 (a) SEM micrograph of mild steel in 20% formic acid at 35 C and 900 X; (b) SEM micrograph of mild steel in 10% formic + 10% acetic acid at 35 C and 900 X. acid mixtures Besides weight loss measurements, the potentiostatic polarisation behaviour of mild steel in mixtures of formic and acetic acid in the ratio 20:0, 19:1, 18:2, 15:5 and 10:10 by percentage (v/v) was investigated at 25, 35 and 45 C. The polarisation curves for these compositions at 35 C are illustrated
SINGH et al.: CORROSION OF MILD STEEL IN AQUEOUS FORMIC ACID 177 in Fig. 4. Since the nature of the polarizaton curves at different temperatures is similar, it was decided to choose 35 C as the representative temperature in all further discussions. The nature of these curves in the mixture of the two acids is exactly the same as that obtained for either of the acids taken in isolation 9,10. The shift in polarisation curves with increase in acetic acid content in formic acid is gradual and is in accordance with expectations. Acetic acid being less corrosive than formic acid, any addition of acetic acid to formic acid is bound to decrease the corrosion rate of mild steel. The values of various corrosion parameters - corrosion current density (I corr ), corrosion potential (E corr ), limiting current density (I L ), the Tafel slope of anodic curve (β a ) and the corrosion rate calculated both from the polarisation curves as well as weight loss data at different compositions of formic and acetic acids have been recorded in Table 1. It is clear from the data that as the composition of acetic acid is increased from 0 to 10% in the mixture the I corr and I L values gradually decrease indicating a decrease in corrosion rate. The variation of corrosion rate with increasing concentration of acetic acid in the mixture obtained by both the techniques is similar in nature. The temperature of the system has a marked influence on the corrosion parameters, as illustrated in Table 1. The values of I corr and I L increase enormously for a ten degree rise in temperature at all compositions of the electrolyte mixture, and consequently a similar increase in the corrosion rate is also observed. An analysis of the cathodic polarisation behaviour in Fig. 4 shows that the nature of the curves is independent of the composition of the mixture. The curves gradually shift towards the region of higher Table 1 Corrosion parameters of mild steel in different compositions of formic and acetic acid Composition of I corr E corr I L Anodic Tafel Corrosion rate (mpy) formic and (μa.cm -2 ) (mv) (ma.cm -2 ) slope (β a ) Polarisation Weight loss acetic acid (%) (mv.decade -1 ) At 25 C 20:0 912.01-565 66.06 176 417.33 460.29 19:1 870.96-585 52.48 200 398.55 435.14 18:2 794.32-590 45.71 203 363.48 406.72 15:5 602.56-600 28.84 200 275.73 348.76 10:10 467.73-605 19.95 222 214.03 263.11 At 35 C 20:0 1513.56-550 59.18 182 692.60 735.75 19:1 1479.11-590 57.56 214 676.84 701.34 18:2 1380.38-600 52.48 220 631.66 674.25 15:5 1122.02-592 34.67 250 513.44 619.77 10:10 912.01-585 27.54 227 417.33 494.33 At 45 C Fig. 4 Polarisation curves of mild steel in different compositions of formic and acetic acid mixture at 35 C. 20% formic acid + 0% acetic acid; o 19% formic acid + 1% acetic acid; 18% formic acid + 2% acetic acid; 15% formic acid + 5% acetic acid; 10% formic acid + 10% acetic acid 20:0 1949.84-575 100 182 892.04 998.03 19:1 1927.52-580 72.44 187 882.03 956.82 18:2 1862.09-597 63.09 204 852.09 911.26 15:5 1659.59-595 41.68 208 759.43 825.41 10:10 1380.38-585 33.11 192 631.66 694.13
178 INDIAN J. CHEM. TECHNOL., MARCH 2008 Table 2 Activation energy calculated from the plot of log I corr versus 1/T(K 10-3 ) Composition of Slope m Activation energy formic:acetic acid E a (J.mole -1 ) (%) 20:0-1.6077 30.7819 19:1-1.6812 32.1904 18:2-1.8020 34.5022 15:5-2.1426 41.0247 10:10-2.2892 43.8322 Fig. 5 Plot of log I corr vs 1/T for various compositions of formic and acetic acid mixture. 20% formic acid + 0% acetic acid; o 19% formic acid + 1% acetic acid; 18% formic acid + 2% acetic acid; 15% formic acid + 5% acetic acid; 10% formic acid + 10% acetic acid current density with successive increase in formic acid content in the mixture. The shift becomes more pronounced at higher cathodic potentials. The corrosion of Fe in de-oxygenated non-aqueous formic acid was studied by Sekine and others 19. It was confirmed that the cathodic reaction is a hydrogen evolution reaction, without the formation of formaldehyde. The increase in the rate of cathodic reaction as indicated by the shift of cathodic polarisation curves towards a higher current density region with increasing formic acid content in the mixture can be easily explained in terms of the relative rates of cathodic reaction in formic and acetic acids. The behaviour of the cathodic polarisation curves in these experiments are in accordance with the observations made elsewhere that hydrogen evolution reaction occurs faster in formic acid than in acetic acid at the mild steel cathode 3. 20:0 and 10:10% formic and acetic acids occupy extreme positions on the right and left respectively and all other curves have positions according to the relative concentration of formic and acetic acids in the mixture. This indicates that even in their mixtures, the two acids retain their independent contributions towards the kinetics of cathodic reaction. The effect of temperature is also on expected lines. The rate of cathodic reaction in the mixture increases in the same manner as it happens in the pure acids. In order to calculate activation energies of the associated electrochemical reactions, log I corr derived from the anodic polarisation curves have been plotted against 1/T in Fig. 5. The plots are all linear, but the slopes for different compositions are not identical. The slopes and activation energies calculated from the graph have been depicted in Table 2. The magnitude of slope and corresponding activation energy increases continuously with increasing composition of acetic acid in the mixture of the two acids. This is in accordance with the observed corrosion rate. Conclusions From the study the following conclusions can be drawn: (i) The dissolution of mild steel in formic acid was inhibited in the presence of acetic acid. (ii) Mild steel exhibited active behaviour in all the investigated formic-acetic acid mixtures. (iii) The similarity in the nature of the anodic curves in mixtures with those in individual acids indicates a common mechanism of corrosion. References 1 Teeple H O, Corrosion, 8 (1952) 14. 2 Sekine I, Hatakeyama S & Nakazawa Y, Electrochim Acta, 32 (1987) 915. 3 Sekine I, Hatakeyama S & Nakazawa Y, Corros Sci, 27 (1987) 275. 4 Sekine I & Senoo K, Corros Sci, 24 (1984) 439. 5 Sekine I, Masuko A & Senoo K, Corrosion, 43 (1987) 553. 6 Singh V B & Singh R N, Corros Sci, 37 (1995) 1399. 7 Plekta H D & Schcuetze K G, Korros, 38 (1987) 317. 8 Sekine I & Momai K, Corrosion, 44 (1988) 136. 9 Singh M M & Gupta Archana, Mater Chem Phys, 46 (1996) 15. 10 Singh M M & Gupta Archana, Corrosion, 56 (2000) 371. 11 Sekine Isao & Chinda Akira, Corrosion, 40 (1984) 95 12 Constantinescu E & Heitz E, Corros Sci, 16 (1976) 857.
SINGH et al.: CORROSION OF MILD STEEL IN AQUEOUS FORMIC ACID 179 13 Johnson C M & Leygraf C, J Electrochem Soc, 153 (2006) B547. 14 Maeng W Y, Eng Mater, 345-346 (2007) 999. 15 Quraishi M A & Sharma H K, Indian J Chem Technol, 11 (2004) 321. 16 Quraishi M A & Sharma H K, J Appl Electrochem, 35 (2005) 33. 17 Katsumi Y, Advances in Welding Technology of Stainless Steels, 27 (2001) 32. 18 Zaritskii V D, Fiz-Khim-Mekh Mater, 26 (1990) 108. 19 Sekine Isao, Ohkawa Hideki & Handa Takashi, Corros Sci, 22 (1982) 1113.