Indian Journal of Chemical Technology Vol. I. July 1994. pp. 225-229 of mild steel in hypochlorite solution - An electrochemical and weight-loss study BGaur Protection Division, National Metallurgical Laboratory, Jamshedpur 831 007, India A K Singh & N J Rao Institute of Paper Technology, Saharanpur 247 00 I, India Received 5 October 1993; accepted 8 February 1994 Present paper reports studies on corrosion behaviour of mild steel in calcium hypochlorite solution. Mechanism of corrosion reaction has been discussed and kinetic electrochemical parameters have been derived from polarisation curves. rates, obtained from linear polarisation and weight-loss data, are observed to be independent of chloride but increase with hypochlorite concentration. These variations have been discussed Or! the basis of reaction mechanism. Hypochlorites are alkaline oxidising agerts and are among the most corrosive chemicals I. Calcium hypochlorite solutions are encountered during production and in other chemical processes for bleaching, sanitizing, etc. One such area is bleach plant in paper mill where the process machinery is attacked by pitting/crevice corrosion because of which their life reduces considerably!". In order to minimise corrosion problems, it is important to identify the mechanism of corrosion reactions and the factors which influence these reactions. Further, protection of steel equipments from corrosive media by electrochemical corrosion control is emerging as a useful alternative-". Consequently, a study is required on electrochemistry of corrosion behaviour and kinetic electrochemical parameters of steel in hypochlorite solution. The literature cites some work on this aspecr'-". This paper reports the work performed on electrochemical and weightloss behaviour of mild-steel in calcium hypochlorite solution. Experimental Procedure The experiments were performed on commercial grade mild-steel. Its composition was determined by chemical methodsl'l!'. Amount of the main constituents (wt%) in the tested steel are C(0.18), Mn(0.66), Si(0.04) and Fe(Balance). For electrochemical tests, the samples were cut in cylindrical form of the size 1.0 em length and 1.5 ern diameter. For weight loss measurements, rectangular coupons of size 2.5 ern x 3.75 ern were prepared. Heat treatment was given to these samples at 750 C for 2 h. The heat treated coupons were wet ground upto 600 grit and polished upto 4/0 grade. These polished samples were degreased in benzene and washed by 50% acetone solution. The samples were tested in sodium chloride solutions with chloride ion concentration ranging from 200-3000 ppm (ph Z 7) and in calcium hypochlorite solutions having free available chlorine (FACl 2 ) 150-600 ppm (ph Z 9) and chloride 1000-3000 ppm. The selected concentration ranges are such as normally observed in liquors of hypo washers in bleach plants. For weight-loss measurements, the coupons were kept immersed for one day in test solution. Corroded coupons were then cleaned in a solution of hydrochloric acid, 50 gpl stannous chloride and 20 gpl antimony(iii) chloride 12 and we.;~hed. Experimental set-up for polarisation measurements consisted of model K 47 corrosion cell, 173 potentiostat, 376 logarithmic current converter, 175 universal programmer and RE-0074 x-v recorder all of Princeton Applied Research, USA. Saturated calomel electrode was used as, reference electrode. The samples were polarised upto ± 250 mv with respect to corrosion potential at a scan rate of 2 my/so For linear polarisation measurements, the samples were scanned ± 25 mv with respect to corrosion potential at a scan rate of 1 m'v/s. Results rate of mild steel in test solutions was calculated from decrease in weight observed
, -~ 226 INDIAN J. CHEM. TECHNOL.,JULY 1994 Table 1 - rate in chloride solution Chloride loss rate ppm mg mpy 200 3.50 3.2 400 3.06 2.R 600 3.72 3.4 800 3.50 3.2 1000 3.61 3.3 1500 3.72 3.4 2000 4.37 4.0 2500 4.26 3.9 3000 4.37 4.0 Table 2 - rate in hypochlorite solutions Cl- Parameter Free available chlorine, ppm ppm 150 300 450 600 1000 2000 3000 loss, mg 30.60 51.59 76.72 76.61 rate, mpy 28.00 47.20 70.20 70.10 loss, mg 27.87 48.42 76.72 85.79 rate, mpy 25.50 44.30 70.20 78.50 loss, mg 29.95 57.27 74.97 99.45 rate, mpy 27.40 52.40 68.60 91.00 by. coupons ill weightloss tests using following formula I 3 534x W rate (in mpy) = ---- DxAxT where W= weight loss (mg) D= density (7.87 g/cm") A= exposed area of coupon (3.15 inch") T= time of exposure (24 h) ('mpy' is 0.001 inch per year) Tables 1 and 2 show corrosion rates of mild steel in chloride and hypochlorite solutions, respectively. The data have been depicted graphically in Fig. 1. rates in hypochlorite solutions (1000 ppm Cl ') were also calculated from linear polarisation data. Following formula was used for this purpose 13.. 0.129 x icorr x EW rate (m mpy) = D Xa 250 200 150,.. Q. E 100 1000 1000 C( (ppm) 3000 1.000 ~ 100'1'--------:---- -,---...-1 0: c o.. :::80 o u OLO--~20~0------1.0~0--~60~0----~S~00 Fru Avail (12 [pprn I - -1000 ppmci- Polo risalian Dolo --1000 ppm(1-, + ------ 2000pp",Ci} Wp;ghl.loss 0010...--3000ppmci Fig. 1 - Variation of corrosion rate with chloride and hypochlorite concentration (weight loss and linear poiarisation data) where icorr = corrosion current (pa) E W= equivalent weight (27.925 g) a = Exposed area of cylinderical sample (8.213 em") i.: was obtained by knowing the slopes of linear polarisation curves. rates thus calculated alongwith icorr are shown in Table 3. The data on corrosion rate are also depicted in Fig. 1. The cathodic part of polarisation curves of mild steel in hypochlorite and chloride (1000 ppm CI- ) solutions are shown in Fig. 2. Discussion reaction in calcium hypochlorite solution is influenced mainly by ph, free available chlorine and chloride ion concentration. Since ph of these solutions is neutral or slightly alkaline (~7.5-9), the dominating form of free available chlorine will be hypochlorite ion ( OCI- )14. As.r
GAUR et al.: CORROSION OF MILD STEEL 227 Table 3 - rate in hypochlorite solutions (Cl- 1000 ppm) (linear polarisation data) Free available chlorine ppm Reciprocal of ico", polarisation f.1a resistance 1/ Ror di/ se 150 23.96 2.084 X 10' 116 300 32.44 2.820 X 10 3 157 450 600 29.54 47.52 rate mpy 2.569 X!OJ 143 4.132 x 10 ' 230 such corrosion will be governed by chloride and hypochlorite ions. In view of the above, corrosion rates of mildsteel were measured in sodium chloride solutions. These are observed almost unaftected by variation in chloride concentration (Fig. 1, Table 1). Similar trend is also shown by weight-loss measurements (Fig. 1, Table 2) in calcium hypochlorite solutions, on comparing corrosion rate at varying chloride concentration. It was not possible to compare present results with earlier studiesl+" as the latter correspond to higher chloride concentrations. Since corrosion in neutral sodium chloride is controlled by concentration polarisation of oxygen reduction, as depicted by Fig. 2, corrosion current (hence rate) for iron dissolution will remain unaffected even if exchange current for Fe/ Fe 2 + increases with CI- concentration 16. rate of mild-steel is observed to increase with free available chlorine (Fig. 1, Table 2). To understand this and electrochemical mechanism of corrosion in the present metal-environment system, polarisation curves of mild steel were measured in calcium hypochlorite solution having chloride ion concentration fixed at 1000 ppm. Following redox systems are relevant for understanding corrosion mechanism in present metalenvironment system: Fe/Fel. OHh - Equilibrium potential for this system varies according to following equation: EFe/Fe(OH)2 = - 0.293-0.059 ph Thus this potential (with respect to SCE) ranges from -794 to - 853 mv for present hypochlorite solutions (ph"" 8.5-9.5). Exchange current density, 10' for this system can be considered as around 3 x 10-5 Azcm? (ref. 17). f3a has been reported between 200-320 my/decade in alkaline solutions!", its value was considered 200 mv/decade in present studies.. -560-610 -555-710 -655-810 -755 > -910-855 E @). W u - 1010-955 '" 0 C.. 10-5 10-4 10 3 Hl0-3 10-5 a. '0-553 -653-753 -853-525 -625-725 -825 @ 10 5 10 4 1()3 4Xl0-3 10-6 Curr~ntJA Fig. 2 - Cathodic polarisation curves in chloride and hypochlorite solutions [(--) Experimental, (--------) Theoretical filled curve J 1000crIT1 o ::r)o-c---:i-:-'c--:t-:------------------. 750 500 E 250 w 0 u '" -;: -250 2 C -500 " ~ -750-1000 -1250 ocr-s cr 10-7 10-6 10-5 Curre-nt Deonsity, A/cm2 Fig. 3 - Redox systems operative in metal- environment system Cathodic part of the curves can be considered, in present solutions due to OzlOH- and OCl- / Cl-. 02/0H- - E due to reduction of dissolved oxygen varies with ph as per following equation EO,IOW = 1.23-0.059 ph 1 0 "" 10- IO Azcm", f3a = 40 my/decade and cathodic Tafel slope f3c = 110 mv/decade'". Limiting current density, IL = 80 JiNcm 2 considering saturated concentration of oxygen in water at 25 C as it varies only slightly with dissolved salts?", 10 5
228 INDIAN J. CHEM. TECHNOL., JULY 1994 Solution (Cl- 1000 ppm) Table 4 - Electrochemical parameters Fe/Fe(OH)2 redox system id(i.jo, + (i,joc1-l E o. 4, fja fjc - A my A my/decade my/decade Sodium Chloride -773 0.2 x 10-4 200 127 Calcium Fca.Cly l Su ppm -857 0.7x 10-4 200 200 Hypo-F.A.CI 2 300 ppm - 857 0.9 x 10-4 200 200 Chlorite EA.CI 2 450 ppm - 835 1.5 x 10 4 200 200 With respect to saturated calomel electrode (SCE) potential 0.11 X 10-3 -625 Theoretical 1.16xlO-3 1.30 X 10-3 1.70 x 10-3 -617-627 -600 Ecorr*, my Experimental -634-610 -667-563 OC/- /C/- - Hypochlorite ions present in calcium hypochlorite solution reduce as below EO for this system varies in the following manner E OCI -/ Cl - = 1.69 - O.059pH + O.0295Iog(OCl- /Cl ") f3c=103 my/decade and 1 0 ""'10-12 A/cm2(ref. 8,9). IL"'" 10-4 A/cm 2 for the concentration of OCl- in present solutions", These three systems are shown together in Fig. 3, according to which corrosion of iron in present solutions will be governed by the concentration polarisation of OCl- and O 2, While in case of sodium chloride solutions, the same will be governed by the presence of O 2 only. From the estimated values of electrochemical parameters, for the three redox systems, cathodic part of polarisation curves were fitted. The fitting is acceptable (Fig. 2) in all cases except that for mild steel in 600 ppm FACl 2 hypochlorite solution considering reversible potential for iron around - 800 mv (with respect to SCE). Table 4 shows parameters which gave reasonable fit with experimental curves. Thus, for Fe/Fe(OHlz, the exchange current density increases with addition of OCI-. The anodic and cathodic Tafel slopes are 200 my/decade except for sodium chloride solution. As expected the limiting current density increases with addition of OCI-. Ecorr derived theoretically, after fitting, matches well with the experimental value. Difference in EO and 10 of Fe/Fe(OHlz and OCl-.ci- are so large (Fig. 3, Table 4) that L; is expected to be very high ("'" 10-3 A'cm"). So mass transfer effects are likely to be present. To avoid them, solution was stirred while measuring Tafel plots. However, stirring appears to change the metal surface characteristics so rapidly that no meaningful curve could be drawn. Also presence of oxygen, in the test solutions, is not expected to influence the corrosion reaction significantly. This was checked by recording polarisation curve after purging N2 gas. For calculated corrosion rates from linear polarisation curves, their slope was obtained. Since corrosion of iron in present solutions is governed by concentration polarisation, the corrosion current may be obtained by following equation". Pa ~i I =-xcorr 2.3 ~E rates (Table 3) were obtained from polarisation data and f3a (Table 4). The rates are greater than those derived from weight-loss measurements (Fig. 1). This is expected since corrosion rate is maximum in the beginning and it decreases, due to formation of corrosion products, with time to reach an equilibrium value. rates are ohserved to increase with free available chlorine concentration (Fig. 1). From the mechanism of corrosion it is observed that exchange current density for Fe/Fe(OH)z and limiting current density for OCl- /Cl : both increase with free available chlorine concentration. These changes result in increase of corrosion rate with free available chlorine. Conclusion The experiments performed to study corrosion on mild steel in hypochlorite solutions indicate that corrosion rates increase with free available chlorine concentration and are independent of chloride ions. Polarisation curves indicate that reactions can be understood in terms of three redox systems namely Fe/Fe(OHh, O/OH - and OCl- /Ct. The rate of corrosion is controlled by concentration polarisation of the latter two systems, though effect of dissolved oxygen is not significant. Electrochemical parameters have been derived by fitting the polarisation curves. It has been observed that exchange current density for Fe/Fe(OH)2 and limiting current density for OCl- /Cl : increase with free available chlorine concentration.
GAUR et al.: CORROSION OF MILD STEEL 229 Acknowledgement The authors wish to acknowledge the financial support of Department of Science and Technology, Government of India, New Delhi. One of us (AKS) is thankful to Prof. Mohd Ajmal, Aligarh Muslim University for extending the facilities of his laboratory and to Prof Desmond Tromans, University of British Columbia, Vancouver, Canada for some useful suggestions. References 1 Nelson J K, in Process industries corrosion, edited by B J Moniz & W I Pollack (NACE, Houston), 1986,297-310. 2 Sharp W B A, Pulp Pap Can, 84 (1983) 1'90-1'92. 3 Garner A, Pulp Pap Can, 86 (1985) T419-T426. 4 Henrikson S & Kucera V, Pulp paper industry corrosion problems, Vol. 3 (NACE, Houston), 1982, 137-147. 5 Garner A, Pulp Pap Can, 82 (1981) T414-T425. 6 Gaur B, Singh A K, Kumar S, Rao N J & Tewari V K TappiJ, 73 (1990)67-71. ' 7 LaliberteLH&GarnerA, TappiJ,64(1981)47-51. 8 Hine F & Yasuda M, J Electrochem Soc, 118 (1971) 170-173. 9 Wu Jiann-Kuo, J Electrochem Soc, 135 (1987) 1462-1467. 10 Vogel A I, Quantitative inorganic analysis (Longmans, Green and Company Ltd, London), 1964,639-643. 11 Jain S K, An introduction to metallurgical analysis (Vikas Publishing House Pvt Ltd, Sahibabad), 1986, 153, 165, 168. 12 Thompson D H, in Handbook of testing and evaluation, edited by W H Ailor (John Wiley, New York), 1984, 136. 13 Dean Jr S W, France Jr W D & Katchan S J, in Handbook of testing and evaluation, edited by W H Ailor (John Wiley, New York), 1984, 174. 14 Smook G A. Handbook for pulp & paper technologists (TAPPI,Atlanta). 1982, 161. 15 Foley R T,, 26 (1970) 58-69. 16 Uhlig H H & Revie R W, and corrosion control (John Wiley, New York), 1985,74. 17 Pourbaix M, Lectures on electrochemical corrosion (Plenum Press, New York), 1973,248. 18 Hausler R H,, 33 (1977) 117-128. 19 Bockris J O'M & Reddy A K N, Modem electrochemistry, Vol. 2 (Plenum Press, New York), 1970, 145-150. 20 Shreir L L, Ed,, Vol. 1 (Newnes-Butterworths, London), 1979., 1:96-1:98. 21 Bandy R & Jones D A,, 32 (1976) 126-134.