Indian Journal of Chemical TechnologyVol. I. July 1994. pp. 225- 229

Corrosion of mild steel in hypochlorite solution - An electrochemical andweight-loss study

BGaurCorrosion 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 solu-tion. Mechanism of corrosion reaction has been discussed and kinetic electrochemical parametershave been derived from polarisation curves. Corrosion rates, obtained from linear polarisation andweight-loss data, are observed to be independent of chloride but increase with hypochlorite concen-tration. These variations have been discussed Or! the basis of reaction mechanism.

Hypochlorites are alkaline oxidising agerts andare among the most corrosive chemicals I. Cal-cium hypochlorite solutions are encountered dur-ing production and in other chemical processesfor bleaching, sanitizing, etc. One such area isbleach plant in paper mill where the processmachinery is attacked by pitting/crevice corro-sion because of which their life reduces consider-ably!".

In order to minimise corrosion problems, it isimportant to identify the mechanism of corrosionreactions and the factors which influence thesereactions. Further, protection of steel equipmentsfrom corrosive media by electrochemical corro-sion control is emerging as a useful alternative-".Consequently, a study is required on electroche-mistry of corrosion behaviour and kinetic electro-chemical parameters of steel in hypochlorite solu-tion. The literature cites some work on thisaspecr'-". This paper reports the work performedon electrochemical and weightloss behaviour ofmild-steel in calcium hypochlorite solution.

Experimental ProcedureThe experiments were performed on commer-

cial grade mild-steel. Its composition was deter-mined by chemical methodsl'l!'. Amount of themain constituents (wt%) in the tested steel areC(0.18), Mn(0.66), Si(0.04) and Fe(Balance).

For electrochemical tests, the samples were cutin cylindrical form of the size 1.0 em length and1.5 ern diameter. For weight loss measurements,rectangular coupons of size 2.5 ern x 3.75 ernwere prepared. Heat treatment was given to these

samples at 750°C for 2 h. The heat treated cou-pons were wet ground upto 600 grit and polishedupto 4/0 grade. These polished samples were de-greased in benzene and washed by 50% acetonesolution.

The samples were tested in sodium chloride so-lutions with chloride ion concentration rangingfrom 200- 3000 ppm (pH Z 7) and in calcium hy-pochlorite solutions having free available chlorine(FACl2) 150-600 ppm (pH Z 9) and chloride1000-3000 ppm. The selected concentrationranges are such as normally observed in liquorsof hypo washers in bleach plants. For weight-lossmeasurements, the coupons were kept immersedfor one day in test solution. Corroded couponswere then cleaned in a solution of hydrochloricacid, 50 gpl stannous chloride and 20 gpl anti-mony(III) chloride 12 and we.;~hed.

Experimental set-up for polarisation measure-ments consisted of model K 47 corrosion cell,173 potentiostat, 376 logarithmic current conver-ter, 175 universal programmer and RE-0074 x-vrecorder all of Princeton Applied Research, USA.Saturated calomel electrode was used as, referenceelectrode. The samples were polarised upto± 250 mV with respect to corrosion potential at ascan rate of 2 mY/so For linear polarisation mea-surements, the samples were scanned ± 25 mVwith respect to corrosion potential at a scan rateof 1 m'V/s.

ResultsCorrosion rate of mild steel in test solutions

was calculated from decrease in weight observed

226 INDIAN J. CHEM. TECHNOL.,JULY 1994

,-~

Table 1 - Corrosion rate in chloride solutionChloride Weight loss Corrosion rate

ppm mg mpy200 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 - Corrosion rate in hypochlorite solutionsCl- Parameter Free available chlorine, ppmppm

150 300 450 600

1000

Weightloss, mg 30.60 51.59 76.72 76.61

Corrosionrate, mpy 28.00 47.20 70.20 70.10

Weightloss, mg 27.87 48.42 76.72 85.79

Corrosionrate, mpy 25.50 44.30 70.20 78.50

Weightloss, mg 29.95 57.27 74.97 99.45

Corrosionrate, mpy 27.40 52.40 68.60 91.00

2000

3000

by. coupons ill weightloss tests using followingformula I 3

534x WCorrosion rate (in mpy) = ----

DxAxTwhereW= 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 mildsteel in chloride and hypochlorite solutions, re-spectively. The data have been depicted graphi-cally in Fig. 1.

Corrosion rates in hypochlorite solutions (1000ppm Cl ') were also calculated from linear polari-sation data. Following formula was used for thispurpose 13

. . 0.129 x icorr x EWCorrosion rate (m mpy) = D

Xa

1000 1000C( (ppm)

3000 1.000

250

200

150

,..Q.

E 100

~ 100'1'--------:---- -,---.......-10:co

:::80....ou

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 .r·

Fig. 1 - Variation of corrosion rate with chloride and hypoch-lorite concentration (weight loss and linear poiarisation data)

whereicorr = 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 linearpolarisation curves. Corrosion rates thus calcula-ted alongwith icorr are shown in Table 3. The dataon corrosion rate are also depicted in Fig. 1.

The cathodic part of polarisation curves of mildsteel in hypochlorite and chloride (1000 ppmCI- ) solutions are shown in Fig. 2.

DiscussionCorrosion reaction in calcium hypochlorite so-

lution is influenced mainly by pH, free availablechlorine and chloride ion concentration. Since pHof these solutions is neutral or slightly alkaline(~7.5-9), the dominating form of free availablechlorine will be hypochlorite ion ( OCI- )14. As

GAUR et al.: CORROSION OF MILD STEEL 227

Table 3 - Corrosion rate in hypochlorite solutions (Cl- 1000ppm) (linear polarisation data)

Reciprocal of ico",polarisation f.1Aresistance

1/ Ror di/ se23.96

32.44

29.54

47.52

Freeavailablechlorine

Corrosionratempy

ppm150 2.084 X 10'

2.820 X 103116

157300

450

600

2.569 X !OJ 143

4.132 x 10' 230

such corrosion will be governed by chloride andhypochlorite ions.

In view of the above, corrosion rates of mild-steel were measured in sodium chloride solutions.These are observed almost unaftected by varia-tion in chloride concentration (Fig. 1, Table 1). Si-milar trend is also shown by weight-loss measure-ments (Fig. 1, Table 2) in calcium hypochloritesolutions, on comparing corrosion rate at varyingchloride concentration. It was not possible tocompare present results with earlier studiesl+" asthe latter correspond to higher chloride concentr-ations. Since corrosion in neutral sodium chlorideis controlled by concentration polarisation of oxy-gen reduction, as depicted by Fig. 2, corrosioncurrent (hence rate) for iron dissolution will re-main unaffected even if exchange current for Fe/Fe2 + increases with CI- concentration 16.

Corrosion rate of mild-steel is observed toincrease with free available chlorine (Fig. 1,Table 2). To understand this and electrochemicalmechanism of corrosion in the present metal-envi-ronment system, polarisation curves of mild steelwere measured in calcium hypochlorite solutionhaving chloride ion concentration fixed at 1000ppm.

Following redox systems are relevant for under-standing corrosion mechanism in present metal-environment system:

Fe/Fel. OHh - Equilibrium potential for this sys-tem varies according to following equation:

EFe/Fe(OH)2 = - 0.293 - 0.059 pH

Thus this potential (with respect to SCE) rangesfrom -794 to - 853 mV for present hypochlo-rite solutions (pH"" 8.5-9.5). Exchange currentdensity, 10' for this system can be considered asaround 3 x 10-5 Azcm? (ref. 17). f3a has been re-ported between 200-320 mY/decade in alkalinesolutions!", its value was considered 200 mV/de-cade in present studies ..

-560

-610 -555

-710 -655

-810 -755

> -910 -855E @).

W-955u - 1010

'"0 10- 5 10-4 103 Hl0-3 10-5

C..'0a.

- 553-525

-653-625

-753-725

-853 ® @-825

105 104 1()3 4Xl0-3 10-6 105

Curr~ntJA

Fig. 2 - Cathodic polarisation curves in chloride and hypoch-lorite solutions [(--) Experimental, (--------) Theoretical

filled curve J

1000crIT1o::r)O-C---:I-:-'C--:t-:------------------.

750ocr-s cr

500

E 250w 0u

'"-;: -2502C -500"~ -750

-1000

-1250

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 oxy-gen varies with pH as per following equation

EO,IOW = 1.23 - 0.059 pH

10"" 10- IO Azcm", f3a = 40 mY/decade and cathod-ic Tafel slope f3c = 110 mv/decade'". Limiting cur-rent density, IL = 80 JiNcm2 considering saturatedconcentration of oxygen in water at 25°C as it va-ries only slightly with dissolved salts?",

228 INDIAN J. CHEM. TECHNOL., JULY 1994

Table 4 - Electrochemical parameters

Fe/Fe(OH)2 redox system id(i.Jo, + (i,Joc1-lE o. 4, fJa fJc - AmY 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.CI2 300 ppm - 857 0.9 x 10- 4 200 200

Chlorite EA.CI2 450 ppm - 835 1.5x 10 4 200 200

• With respect to saturated calomel electrode (SCE) potential

Solution(Cl- •• 1000 ppm)

Ecorr*, mY

0.11 X 10-3Theoretical

-625

-617

-627

-600

Experimental-634

-610

-667

-563

1.16xlO-3

1.30 X 10-3

1.70 x 10-3

OC/- /C/- - Hypochlorite ions present in cal-cium hypochlorite solution reduce as below

EO for this system varies in the following manner

EOCI-/Cl- = 1.69 - O.059pH + O.0295Iog(OCl- /Cl ")

f3c=103 mY/decade and 10""'10-12 A/cm2(ref.8,9). IL"'" 10-4 A/cm2 for the concentration ofOCl- in present solutions",

These three systems are shown together in Fig.3, according to which corrosion of iron in presentsolutions will be governed by the concentrationpolarisation of OCl- and O2, While in case of so-dium chloride solutions, the same will be go-verned by the presence of O2 only.

From the estimated values of electrochemicalparameters, for the three redox systems, cathodicpart of polarisation curves were fitted. The fittingis acceptable (Fig. 2) in all cases except that formild steel in 600 ppm FACl2 hypochlorite solu-tion considering reversible potential for ironaround - 800 mV (with respect to SCE). Table 4shows parameters which gave reasonable fit withexperimental curves. Thus, for Fe/Fe(OHlz, theexchange current density increases with additionof OCI-. The anodic and cathodic Tafel slopesare 200 mY/decade except for sodium chloridesolution. As expected the limiting current densityincreases with addition of OCI-. Ecorr derived the-oretically, after fitting, matches well with the ex-perimental value.

Difference in EO and 10 of Fe/Fe(OHlz andOCl- .ci- are so large (Fig. 3, Table 4) that L;is expected to be very high ("'" 10-3 A'cm"). Somass transfer effects are likely to be present. Toavoid them, solution was stirred while measuringTafel plots. However, stirring appears to changethe metal surface characteristics so rapidly that nomeaningful curve could be drawn. Also presenceof oxygen, in the test solutions, is not expected to

influence the corrosion reaction significantly. Thiswas checked by recording polarisation curve afterpurging N2 gas.

For calculated corrosion rates from linear po-larisation curves, their slope was obtained. Sincecorrosion of iron in present solutions is governedby concentration polarisation, the corrosion cur-rent may be obtained by following equation". Pa ~iI =-x-corr 2.3 ~E

Corrosion rates (Table 3) were obtained frompolarisation data and f3a (Table 4). The rates aregreater than those derived from weight-loss mea-surements (Fig. 1). This is expected since corro-sion rate is maximum in the beginning and it de-creases, due to formation of corrosion products,with time to reach an equilibrium value.

Corrosion rates are ohserved to increase withfree available chlorine concentration (Fig. 1).From the mechanism of corrosion it is observedthat exchange current density for Fe/Fe(OH)z andlimiting current density for OCl- /Cl : both in-crease with free available chlorine concentration.These changes result in increase of corrosion ratewith free available chlorine.Conclusion

The experiments performed to study corrosionon mild steel in hypochlorite solutions indicatethat corrosion rates increase with free availablechlorine concentration and are independent ofchloride ions. Polarisation curves indicate thatreactions can be understood in terms of three re-dox systems namely Fe/Fe(OHh, O/OH - andOCl- /Ct. The rate of corrosion is controlled byconcentration polarisation of the latter two sys-tems, though effect of dissolved oxygen is notsignificant. Electrochemical parameters have beenderived by fitting the polarisation curves. It hasbeen observed that exchange current density forFe/Fe(OH)2 and limiting current density forOCl- /Cl : increase with free available chlorineconcentration.

GAUR et al.: CORROSION OF MILD STEEL 229

AcknowledgementThe authors wish to acknowledge the financial

support of Department of Science and Techno-logy, Government of India, New Delhi. One of us(AKS) is thankful to Prof. Mohd Ajmal, AligarhMuslim University for extending the facilities ofhis laboratory and to Prof Desmond Tromans,University of British Columbia, Vancouver, Cana-da for some useful suggestions.

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