Corrosion characteristics of some aluminium alloys in oxalic acid

Transcription

1 ndianjurnalfchemicaltechnlgy Vl.3,January1996,pp Crrsin characteristics f sme aluminium allys in xalic acid MM Singh& ArchanaGupta DepartmentfAppliedChenustry,nstitutefTechnlgy,BanarasHinduUniversity,Varanasi221005, ndia Received19 May1995; accepted29 August1995 The crrsin behaviur f 1060 and 1100 aluminium allys in different cncentratins (0.1, 0.25, 0.5, 0.75, 1.0, 2.0 and 5.0%) f xalic acid slutins has been investigated by weight lss and ptentistatic plarisatin techniques, at 25 C. The data indicate that 1100 aluminium is lesscrrsin resistant than 1060 aluminium. The crrsin rates f these allys are a functin f the cncentratin f electrlyte in a narrw range f cncentratin, nly. The andic plarisatln curves f bqth the allys exhibit active behaviur at each cncentratin f xalic acid and in the whle range f the applied ptential. The ~athdic plarisatin curves are almst linear in bth the cases, resembling that fr hydrgen evlutin reactin.the crrsin behaviur f aluminium allys in xalic acid has been explained in terms f the acidity f the slutin and self inhibitin due t the adsrptin f xalate ins. Aluminium is highly resistant t mst f the acidic and neutral slutins due t the frmatin fa prtective xide film n its surface. Aluminium equipments, cmmnly used in textile industries, frequently cme in cntact with xalic acid slutins which are used as a neutralizer during the prcessing f fibres 2 Failure f such apparatus due t crrsin in xalic acid is widely reprted in textile and metallurgical industries. Hence, the understanding f crrsin behaviur f aluminium allys in xalic acid slutins is very much desirable. n the present investigatin, an attempt has been made t elucidate the crrsin behaviur f 1060 and 1100 aluminium allys in different cncentratins f xalic acid, at 25 C. Experimental Prcedure Fr the present investigatin, 1060 and 1100 aluminium allys were used. The cmpsitin f aluminium allys are as fllws: Aluminium (1060): Si %, Fe- 0.02%, Mn % ahd rest A. Aluminium (1100): Si %, Fe- 0.32%, Mn %, Mg % Cu % and rest A. Oxalic acid and sdium xalate (BDH, Analar grade) were used fr the preparatin f the slutins. These. were recrystallized frm their aqueus slutins. Specimens used fr weight lss experiments were f 5 x 5 cm in size. These were plished successively with 1/0 t 4/0 grades f emery paper, fllwed by cleaning with sap and finally with acetne. The weight lss experiments were perfrmed in 500 ml crping glass beakers, cntaining 300 ml f the electrlyte. Triplicate experiments were perfrmed in an air thermstat maintained at 25 ~ O.2 C, t ascertain the reprducibility f the data. Ptentistatic plarisatin experiments were carried ut using a ptentistat (Wenking mdel POS73). A three-necked glass assembly cntaining a wrking electrde f size 1 x 1 cm, platinum fil f size 2 x 2 cm as cunter electrde and a Luggin type saturated calmel electrde were used fr plarisatin measurements. Experiments were perfrmed in unstirred slutin, withut deaeratin. Starting frm the pen circuit ptential, different values f ptential were applied n the electrde surlace in steps and the resulting steady state currents were measured. Results and Discussin n the present investigatin, crrsin rate f 1060 and 1100 aluminium allys has been determined by weight lss and electrchemical techniques, at 25 C. Crrsin rates f 1060 and 1100 aluminium allys in different cncentratins f xalic acid (0.10, 0.25, 0.50, 0.75, 1.0, 2.0 and 5.0%) have been determined fr 24 h immersin perid frm weight lss, using the frmula;

2 SNGH & GUPTA: CORROSON BEHAVOUR OF SOME ALUMNUM ALLOYS 33 Crr~nrare(mpy)= W DAT where W=weight lss (mg), D=density f specimen (glcm3), A=area f specimen (square inches) and T= expsure time. (h) The evaluatin f crrsin rate by electrchemical technique has been dne with the help f the.equatin Crrsn. rate ( mpy) = x Ex icrr D where E, icrr and D are electrchemical equivalent, crrsin current density and density f specimen, respectively. The crrsin current in the abve equatin has been determined by extraplatin methd3 Fig. 1 illustrates the variatin in crrsin rate with acid cncentratin evaluated by weight lss and electrchemical techniques. There exists (' gd agreement between the crrsin rates mea sured by the tw techniques. t is nted frm the nature f the curves that initially with increase in the acid cncentratin, crrsin rate increases rapidly up t 1% f xalic acid. On further in~ crease in its cncentratin, crrsin rate becmes almst cnstant fr 1060 aluminium, hwever, rapid increase in crrsin rate f 1100 aluminium cntinues up t 2% f xalic add. Crrsin rate exhibited by 1100 aluminium at each f ~ z ii 10 u 5,01-/1 / 2.0 / Crrsin "QWf 1060 Al -'-'- Byw.lllht Dss wetr ",,-",cl Crrsin "QW f 1100 Al -----By w.lllht D_ --- ~tr ch.mlcl r /' CONCENTRATON OF OXALC ACD, Fig. 1-Crrsin Pate f 1060 and 1100 aluminium allys in different cncentratins f xalic acid, at 25 {: cncentratin f the electrlyte is higher than that fr 1060 aluminium. As the cncentratin. f xalic add is- succes~ sively increased frm 0.1 t 5.0%, the 'ph f the slu'in decreases. gradually. With increase in the acidity f the. electrlytic slutin the crrsin rate f aluminium allys is expected t increase as reprted earliera fr mst f the acidic slutins including thse f xidizing acids. Thus, the initial increase in crrsin rate f different aluminium allys with cncentratin up t 2% r less may be very well explained in terms f the decrease in ph f the electrlyte. Hwever, at higher cncentratin, the increase in the crrsin rate is relatively sluggish althugh the decrease in ph cntinues up t 5% xalic acid. This shws that there are sme factrs ther than the decrease in ph.f the electrlyte which affect the crrsin rate f aluminium allys. t is knwn that xalate ins act as a gd inhibitr fr the crrsin f aluminium in acidic slutins, the inhibitin f crrsin in the present case, as well, cannt be ver ruled. T cnfirm the inhibitive actin f xalate ins, additinal experiments were perfrmed by adding sdium xalate t a slutin f xalic acid f knwn cncentratin. The inhibitive actin f xalate ins is quite evident frm the nature f the curves in Fig. 2. t was bserved that n increasing the xalate in cncentratin in the electrlyte the crrsin rate f aluminium allys decreases appreciably. An inhibitin efficiency f nearly 77 79% was bserved at the cncentratin f 1.00 M sdium xalate (88 x 103 ppm f xalate in). Thus, it is apparent that even such a large cncentratin f xalate ins is nt sufficient t ar-.. Q. E ft e e c: Crrsin r-ft Al 1100 Al _n E f 1010 Al -' Al ~~::;:.::: ~.:.:.:-:.::.:::..:.::-..:.:.:= % t.. l! e u -i 10 ~ :0: :A 2 S Cnc: tl'1ltln 01 sdium _lw.~. Fig. 2-Variatin f crrsin. rate and %E with different cncentratins f sdium xalate in 1% xalic acid, at 25 C 50

3 34 NDAN J, CHEM. TECHNOL.. JANUARY 1<}<}6 rest the crrsin rate cmpletely. Perhaps this is the reasn why n decrease in crrsin rate has been bserved at any cncentratin f xalic acid used during the presentinvesti~atin. Once, the crrsin rate becmes cnstant. Thus, the variatin in crrsin rate f aluminium allys with increasing cncentratin f xalic acid may be explained in terms f bth, the increase in acidity f the slutin with cncentratin as well as the self-inhibitin f crrsin prcess due t adsrptin f xalate ins. t appears frm the data f the present investigatin that in lwer acid cncentratin range the rate f crrsin is prbably higher than the inhibitin achieved due t adsrptin. Hence, the crrsin rate steadily increases with the increasing acid cncentratin. At higher cncentratins,. the increase in crrsin rate may be almst cmpletely cmpensated by the inhibitin f crrsin due t adsrptin f xalic acid and the crrsin rate becmes almst cnstant. The variatin f pen circuit ptential (OCP) f 1060 and 1100 aluminium allys with acid cncentratin has been shwn in Fig. 3. it is' t be nted frm the figure that n increasing the acid cncentratin frm 0.1 t 1%, the OCP f the allys rapidly shifts twards mre active directin. On further increase in acid cncentratin frm '1 t 5% it becmes almst cnstant fr 1060 aluminium and shifts very slightly twards mre active directin fr 1100 aluminium. The cntinuus shift in OCP in active directin in bth the cases, with the increase in the cncentratin f xalic-acid indicates that the crrsivity f xalic acid increases with its cncentratin up t 1%. Almst cnstant values f OCP frm 1 t 5% f the slutin further' indicates that the cr- rsin rate shuld be mre r less independent f cncentratin. t may als be nted that the values f OCP at each cncentratin f xalic acid are mre negative fr 1100 aluminium as cmpared t that fr 1060 aluminium. Thus, 1060 aluminium appears t be mre resistmt xalic acid at each cncentratin. t crrsin in Similar results were btained by ther investigatrs with respect t the relative crrsin resistance f aluminium allys in nitric acid6 Figs 4 and 5 illustrate the andic plarisatin behaviur f 1060 and 1100 aluminium allys respectively, in different cncentratin f xalic acid, at 25 C. An analysis f the plts reveals that active crrsin behaviur is exhibited by bth the allys in the whle range f ptential and at each cncentratin f xalic acid. Well defined tafel regins fllwed by densities were bserved distinct limiting current at all cncentratins f the acid. t is t be nted frm the figures that n increasing the acid cncentratin, the andic plarisatin curves shift twards higher current density regin withut any change in their nature. The shift f curves twards higher current density is gradual till 1% f xalic acid fr 1060 aluminium (Fig. 4) and after.wards the curves fr 1 t 5% slutin almst verlap ~ach ther. Hwever, the andic plarisatin curves fr 1100 aluminium gradually and cntinuusly shift twards higher '0" "10 / -'-'-0.50'" --0 7S" -"--- '"10 -"'-'-Z"l *10,,,,,,,,,,,, --'1060A Al '--'--'--'--. ' > E, 0 a c: -z,q. -'00 -' z.o 3.0 Fig. CONCENTRATON OF OXALC ACD, 3-Variatin f OCP f 1060 and 1100 aluminium allys with cncentratin f xalic acid Fig. 4-Andic OQ CD, J'A/crnZ plarisatin curves-f 1060 aluminium in xalic acid slutins

4 SNGH & GUPTA: CORROSON BEHAVOUR OF SOME ALUMNUM ALLOYS , 'j;j 200 ()!! > E 0 -' c ;: ffi -200 ~ 600~ " % " CD, JA/ern Fig. 6-Cathdic plarisatin curves f 1060 aluminium fu xalic acid slutins U ~ W ~ U. 100 CD,pA/e"?- Fig. 5-Andic plarisatin curves f 1100 aluminium in xalic acid slutins Table -Crrsin A A L icmumncm2 flncm2 EcmmV parameters frm plarisatin.curves f f 1060 xalicand 1100 aluminium f xalic allys acid, at in25 C different cncentratins current density regin up t 5%, nevertheless, the magnitude f shift is nt prprtinal t the cncentratin change. The values f crrsin current (icrr)' crrsin ptential (Ecrr) and limiting current density (il) calculated frm the plarisatin curves at different cncentr.atigns f xalic acid fr 1060 and 1100 aluminium allys have been recrded in Table 1. The data in the table indicate that the values f icrr and il gradually increase with increase in the acid cncentratin. Whereas, Ecrr values becme mre negative in slutins f higher cncncentratin. The values f i,rr and il are larger fr 1100 aluminium ally than the crrespnding values fr 1060 aluminium, at each cncentratin f xalic acid slutin. Frm the cmparisn f these data it is apparent that 1060 aluminium is mre resistant t crrsin in xalic acid than 1100 aluminium, irrespective f the cncentratin f the electrlyte. The shift f Ecrr values twards mre active directin give the indicatin fr the increased prbability f crrsin prcess in lwer cncentratin range. At higher cncentratins, ~mst cnstant Ecrr is attained due t the adsrptin f xalate ins. A critical analysis f icrr and il values mentined in the Table 1, reveals that initially the increase in icrr value with cncentratin is quite appreciable, indicating the dependence f crrsin rate n xalic acid cncentratin. Small increment in icrr and il frm 1 t 5% slutin fr 1100 aluminium: ally again cnfirms the slwing dwn the crrsin prcess at higher cncentratins. The cnclusins arrived at frm the plarisatin experiments are in gd agreement with thse btained frm weight lss d~ta. The cathdic pl~risatin curves fr 1060 and 1100 aluminium allys in slutins cntaining different cncentratins f xalic acid are depicted in Figs 6 and 7 respectiyely,. at 25 C. if is t be nted frm the figures that the cathdic plarisatin curves are iimst linear and parallel t each ther at each cncentratin f xalic acid. The shift in cathdic p'larisatin curves twards higher current density regin with increase in the cncentratin f xalic acid has been bserved fr bth the allys. Hwever, the magnitudes f the shift are different fr different allys., A large shift in cathdic plarisatin curves ccurs as cncentratin f the acid is raised frm 0.1 t 1.0% and

5 36 NDAN 1. CHEM. TECHNOL., JANUARY en ' ' '4 -'- 0.50' ' '4 -"'- 2' '4 cathdic plarisatin curve is nt that marked. On the ther hand, at higher cncentratins f xalic acid, the inhibitin resulted frm the adsrptin being almst the same fr bth the allys, the shift in cathdic plarisatin curves at higher cncentratins f xalic acid is almst identical. Althugh, this interpretatin culd very well explain the results f the present investigatin" it is difficult t crrelate the relative adsrbability f xalate ins with the cmpsitin f the allys chsen ' lt CO, ""/e'/ Fig. 7-Cathdic plarisatin curves f 1100 aluminium in xalic acid slutins clsely spaced curves are btained in the range 1-5% xalic acid, in the case f 1060 aluminium. Fr 1100 aluminium, the curves are almst equidistant in the whle range f experimental cncentratins. The discharge f hydrgen being the nly pssible cathdic reactin in the acidic slutins and the cathdic plarisatin curves being mre r less linear it may be assumed that activatin cntrlled hydrgen evlutin reactin 7 ccurs in the present case, als. The cntinuus shift in the plarisatin curves twards higher current density regin may, therefre, be attributed t the increase in the rate f hydrgen in discharge due t the availability f H+ in large extent at higher cncentratins f xalic acid. Hwever, the relative magnitude f the shift in cathdic plarisatin curves frm 0.1 t 1% f xalic acid is much mre significant in the case f 1060 aluminium. This seems prbably due t the cmpetitin between xalate and H+ ins fr adsrptin at the same sight, xalate ins seem t be Qlre easily adsrbed n 1100 aluminium at their lwer cncentratins, and thus the shift in Cnclusin Frm the present investigatin, fllwing cnclusins can be drawn: Aluminium is mre resistant twards crrsin in xalic acid slutins than 1100 aluminium. 2 The rate f crrsin f the aluminium allys is functin f the cncentratin f xalic acid in a narrw range f cncentratin (up t r belw 2%) and at higher cncentratin the crrsin rate becmes almst independent f cncentratin. 3 Oxalate ins present in the electrlyte slutins seems t act as a self inhibitr. 4 The crrsin f bth the allys fllw the same mechanism. References 1 Sekine & Okan C, Crrsin, 45 (1989) Sekinel, Okan C & YuasaM, Crrs Sc~ 30 (1990) WagnerC& Traud W,ZPhys Chern, 441 (1938).. 4 Evans U R & Har T P, J Electrchem Sc, 99 (1952) Chakrabrty C, Singh M M & Agarwal C Y, ndian J Techn~ 20 (1982) Chakrabrty C, Ph D Thesis, Banaras Hindu University, ndia, Antrpv L, Theretical Electrchemistry (Mir Publisher, Mscw), 1977,440.