Effects of time and temperature on the mild steel corrosion inhibition by molybdate and nitrite

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1 IndianJurnalfCheiDicalTechnlgy Vl. 3, January 1996, pp ' Effects f time and temperature n the mild steel crrsin inhibitin by mlybdate and nitrite Department C M Mustafa*, A K M Obaydur Rahman & D A Begum f Applied Chemistry & Chemical Technlgy, Rajshahi University, Rajshahi 6205, Bangladesh Received 8 May 1995; accepted 8 August 1995 An investigatin has been cnducted t find ut the effect f saking time and temperature n the CGrrsininhibitin f mild steel by sdium mlybdate and sdium nitrite in simulated cling water (SeW). Increase f saking time increases crrsin rate f the mild steel in sew. Hwever, when sdium mlybdate and sdium nitrite are present crrsin rate decreases r per cent inhibitin efficiency (% IE) increases with the saking time. High cncentratin f inhibitrs r stirring f the slutin results in the attairnnent f the crrsin rate t the cnstant lw value at a shrt saking perid. Mlybdate lses its inhibitin capacity cmparatively rapidly than nitrite with the rise f temperature, and nne f them alne is effective at the increased temperature f sew. When these tw inhibitrs are used cmbinedly effectivecrrsin inhibitin f mild steel is btained up t 55 e temperature f Sew. In recent years, the steel crrsin inhibitin by mlybdate and nitrite has been emphasized frm varius perspectives fr the cling water system with a view t find an alternative f the txic and eclgically unacceptable chrmate based inhibitrs. Investigatins1,2 shw that disslved xyge~ is necessary fr the crrsin inhibitin by mlybdate. Effects f aggressive chlride and sulphate ins have been investigated fr the inhibitin f steel crrsin by mlybdate3,\ and attempts have been made t imprve the inhibitin efficiency by using it with ther chemicals5-s. Mechanisms f inhibitr actins have been prpsed by suitable experiments.9,1o. Nitrite, an xidizing substance, pssesses crrsin inhibitin prperty fr steel. Effect f aggressive chlride in has been studied n the crrsin inhibitin behaviur f nitrite 11, Attempts have been made t imprve the inhibitin actin f mlybdate by using it with nitrite12-16 Inhibitr perfrmances are dependent n time. During the evaluatin f the inhibitr efficiency due cnsideratin shuld be given n this factr. Inhibitr actin may be slw r fast. Surface layer f the inhibited metal may underg slw mdificatin due t crrsin, and accumulatin r depletin f inhibitrs r crrsive agents n the surface may ccur with time. Investigatins5 shw that cnditining time is necessary fr mlybdate t exert crrsin inhibitin actin. Hwever, bth fr mlybdate and nitrite time factr is ver- *Authr t whm crrespndence. shuld be addressed lked during the evaluatin f inhibitin activity. A reprt17 based n the plarizatin study shws that rise f temperature frm 21 t 60 C was detrimintal fr the crrsin inhibitin by mlybdate. Hwever, the degree f temperature effect was nt evaluated. Such experiments are als absent fr nitrite. On the abve perspectives the time and temperature effects have been investigated in this prject fr the inhibitin f mild steel crrsin by mlybdate and nitrite. Simulated cling water (sew), a realistic aqueus system has been used as the test medium. Spectrphtmetric methd fr the slutin irn cntent due t crrsin is mre suitable and quicker than the 'cupn weight lss' methd. Mrever, fr well inhibited systems accurate measurement f the weight lss is difficult by the latter methd. Cnsidering this spectrphtmetric measurement has been used fr the estimatin f crrsin rate in this wrk. Experimental Prcedure 1 mm thick and 2 em by 3 em sized cmmercial grade mild steel cupns f the cmpsitin C, 0.16%, Mn, 0.77%; Si, 0.60%; P, 0.039% and S, 0.062% were used in this investigatin. The cupns were plished with Griffin's fast cutting alumina pwder (grade 5/50) n a plishing wheel, carefully washed and degreased with acetne. The air-dried cupns were stred in a desiccatr fr experiment. Simulated cling water (SeW) f the cmpsitin3,7 Cl- 304 ppm; S ppm; HC03-, 128 ppm and COj-, 14 ppm

2 MUSTAFA et af.: MIlD STEEL CORROSION INHIBmON 45 me, was prepared frm analytical grade NaCl, NazS04, NaHC03 and NazC03 ph was adjusted by dilute slutin f H2S04 r NaOH. Other chemicals used in this wrk were reagent grade chenucals. The crrsin experiment was carried ut withut fllwing the ASTM prcedure. The cupn was saked fr predetermined perids by hanging in 300 ml inhibitr slutin in the stppered glass bttle taking care that it was well immersed and did nt tuch side walls r bttm. Saking was carried ut in stagnant slutin r by stirring the slutin with a magnetic stirrer in a thermstated air chamber. After the cmpletin f the saking perid rusts r adhered crrsin prducts were _ remved frm the surface int the slutin by careful scrubbing with a glass rd. Irn cntent in the slutin due t crsin was determined with the Mdel ANA-75 Spectrphtmeter (Tky Phtelectric C. Ltd.) using thicyanate methdl8 Crrsin rate was reprted in mdd (mg! dm2 day). Per cent inhibitive efficiency (% IE) was determined as19 w,-w % IE= 0 I X 100 W where W is the weight lss per day fr 3 days saking in ph 8.5 SCW, and WI in the presence f inhibitr in Sew. Results and Discussin Fig. 1 shws the effect f saking time n the crrsin rate f mild steel in ph 8.5 sew bth under stagnant and stirring cnditins. The crrsin rate increased with time, arid the cupn in days - fli ~ 40 "C E <3.2.; '" stc: gnant cnditin crrded mre during lng saking. Thick blisters with uter darkish brwn crust cvered mst f the surface.after lng saking perid in the stagnant cnditin. A ph value f as lw as 1.7 was measured by the ph paper in the trapped slutin inside the blister after 25 days f saking. The decrease f ph inside the pit <:>rtrapped zne was als reprted earlierzo,21. In the chlride cntaining envirnment, as in sew, hydrlysis f irn chlrides generates hydrgen ins, which, in stagnant cnditin, cannt dessipate easily; and 'fesults in an increase f acidity. During stirring n such crust with trapped slutin was bserved n the surface. One f the reasns f high crrsin rate in the stagnant cnditin may be due t very high acidic envirnment inside the crust cvered surface. In Fig. 2 the effect f saking time is shwn when mlybdate is present in ph 8.5 sew. -Experiments were carried ut in the stagnant cnditin except ne in stirred slutin in the presence f 500 ppm sdium mlybdate. A gradual decrease f crrsin rate was btained with the increase f saking time in the stagnant cnditin, and n cnstant rate was bserved even after 25 days when 250 and 500 ppm sdium mlybdate ~as present. When cncentratin was raised t 1000 ppm cnstant rate was bserved within 7 days. An early attainment f the cnstant value (within 7 days) was als btained fr 500 ppm sdium mlybdate in the stirring cnditin. In the presence f the tw highest cncentratins f sdium mlybdate used in this wrk (1500 and 2000 ppm) crrsin rate increased slightly with '0E c0...!:! '" ~ 3 5, ~ '0 4 -s i Fig. }-Effect f saking time n the crrsin behaviur f mild steel in sew at.ph 8.5-(0) stagnant cnditin and (0) stirred cnditin Saking time, days Fig. 2-Effect f saking time in tlle plcsence f mlybdate in SCW at ph 8.5: sdium mlybdate cnc.-(o) 250 ppm, (el) 500 ppm, (6) 750 ppm, ('V) 1000 ppm, (0) 15~0 ppm and (0) 2000 ppm in stagnant cnditin; and (a) 500 ppm in stirred cnditin

3 46 INDIAN J. CHEM. TECHNOL.,JANUARY 1996 saking and attained cnstant value after 10 days. This may be due t slw attainment f equilibrium thrugh the passive film. The effect f saking time in the presence f mlybdate at ph 10.0 is shwn in Fig. 3. The pattern f the change f crrsin rate with mlybdate cncentratin in this case is quite similar t thse shwn in Fig. 2. Hwever, reduced rate at this ph may be due t enhanced stability f the surface xide at highph. It is bvius frm these results that depending n ph certain minimum cncentratin f mlybdate is essential n the surface fr reducing crrsin rate t the lw value (crrsin rate thrugh passive film). High cncentratin f mlybdate r stirring f the slutin reduces the time fr this cncentratin. Fig. 4 shws the effect f saking time when nitrite is present in sew at ph 8.5. The crrsin rate decreased with the increase f saking time in stagnant cnditin at 100, 250, 500 and 750 ppm cncentratins f sdium nitrite. At the tw lwest cncentratins the rate did nt becme cnstant up t 25 days f saking. When cncentratin was 1000 ppm crrsin rate was lw and independent f saking time frm 3 days and abve. Stirring f the slutin at 250 ppm cncentratin reduced the time fr cnstant crrsin rate t 5 days. At higher cncentratin f nitrite reduced crrsin rate was btained at all saking cnditins. These results shw that nitrite is quicker in actin than mlybdate. It is shwn in Fig. 5 that in the presence f 500 ppm sdium mlybdate crrsin rate f mild steel was independent f saking time after a perid f three days at sdium nitrite cncentratins 500, 750 and 1000 ppm respectively, and was al- s significantly lw. This suggests that mlybdate and nitrite, when present tgether, usefully interact fr the inhibitin. f mild steel crrsin in sew. This type f behaviur has als been reprted earlier1z-16 The effect f saking time n the per cent inhibitive efficiency (% IE) fr ph 8.5 stagnant sew is pltted in Fig. 6. % IE increased with saking tune bth fr mlybdate and nitrite, and under identical cncentratin levels sdium mlybdate pssessed higher %IE.than sdium nitrite in sew. %IE is significantlyhigh and independent f saking time frm 3 days when 500 ppm sdium mlybdatewas used with 250 ppm sdium nitrite. A rapid increase f %IE is bserved in Figs 7 and 8 with the increase f mlybdate and nitrite cncentratins. At ph 8.5 in stagnant SCW 1500 ppm sdium mlybdate r 1000 ppm sdium nitrite was required fr 95% IE. Hwever, in the presence f 500 ppm sdium mlybdate l~ss than 250 ppm sdium nitrite was sufficient fr this value. This shws synergistic crrsin inhibitin by the inhibitrs. Cling water temperature ften rises abve 50 C. In Fig. 9. a rapid increase f the crrsin rate f mild steel is bserved with the rise f temperature in stagnant sew at ph 8.5. The effect f temperature was nt s significant up t 3QC bth at ph 8.5 and 10.0 when 500 sdium mlybdate was present. Hwever, abve this temperature the crrsin rate increased rapidly especially at ph 8.5. As shwn in Fig. 10 the crrsin rate increases less rapidly with temperature when nitrite was present in sew. When sdium mlybdate and nitrite were used cmbihedly, each at CL.e e Q e e 0 I _ :g 15 a-.. C e e 2.0 u El u El Q _ -. CI :! c 1.5 A.-",A'".... A c 10 ~ ~.~ 1.0 v v ~ V V V ~... ~ L.5 c:=-: :--:: 8 51 :;: ~ ; -: I Saking time. days Saking time. days Fig. 3-Effect f saking time in the presence f mlybdate in stagnant sew at ph 10: sdium mlybdate cnc.-(o) 250 ppm, (0) 500 ppm, (A) 750 ppm, (\1) 1000 ppm, (<» 1500 ppm and (.) 2000 ppm Fig. 4-Effect f saking time in the presence f nitrite in sew at ph 8.5: sdium nitrite cnc.- (0) 100 ppm, (0) 250 ppm, (v) 500 ppm, (<» 750 ppm and (.) 1000 ppm in stagnant cnditin; and (6) 250 ppm in stirred cnditin illi/ll '~J If'I I~, II 1 ~"1" I

4 MUSTAFA et J.: MIlD STEEL CORROSION INHIBmON ~ & $ $ aking time, days E.;.... c 'Mọ... Q V25,. $.~ c > :c c> ~ 10 0 M 60 [:{ 0, / ph10.0o,ph 8.5!80 ~ ~ ~ 0.4 f ~: ~ ~ Fig. 5-Effect f saking time in the presenc~ f 500 ppm sdium mlybdate with -(0) 100 ppm, (O) 250 ppm, (A) 500 ppm, (v) 750 ppm and (<» 1000 ppm sdium nitrite in stagnant sew at ph Sdium mlybdate cne., ppm Fig. 7-Effect f mlybdate cncentratin n the % IE fr 3 days saking in sew >- > ;;:.;; 'u - c>c :.c I> :c 20 M Saking time, days Sdium nitrite cne., ppm Fig. 6-Effect f saking time n the % IE in stagnant sew at ph 8.5 in the presence f (0) 500 ppm sdium mlybdate. (0) 500 ppm sdium nitrite and (A) 500 ppm sdium mlybdate and 500 ppm sdium nitrite ppm cncentratin level, a much slwer increase f crrsin rate was bserved with the rise f temperature bth at ph 8.5 and At rh 8.5 a crrsin rate f apprximately 2 mdd at 55 e, in this case, shws their effectivityat the high perating temperature f Sew. In neutral r alkaline medium the crrsin f irn is cntrlled by the diffusin f xygen t the surface. As xygen diffusin rate increases with the rise f temperature, therefre, accelerated crrsin is quite likely in sew. The mechanism f crrsin inhibitin by mlybdate has been explained due t its adsrptin n the mild steel by Fig. 8-Effect f nitrite cncentratin-(o} alne and (O) with 500 ppm sdium mlybdate n the % IE fr 3 day saking in sew at ph 8.5 hydrgen bnding between xygen atm f mlybdate and hydrgen atm f the hydrxy xide f the surface8,lo. Increased thermal mtin f the in with the rise f temperature can easily disrupt this bnd making mlybdate in incapable f prtecting the surface frm aggressive in attack. Ni: trite inhibits the steel crrsin by enhancing xide frmatin n the surfaces,10, as a result temperature has less effect n its actin. When used tgether nitrite takes dminating rle fr crrsin inhibitin at the increased temperature. Cnclusins In sew mild steel shws increased crrsin rate n saking.due t gradual destructin f the

5 48 INDIAN 1. CHEM. TECHNOL. JANUARY 1996 c - :! ~ ; E 8.~ ~ Temperature,Oc Fig. 9--Effect f temperature n the crrsin rate fr 3 days saking cnditin: (0) in SCW at ph 8.5; and in SCW cntaining 500 ppm sdium mlybdate at (0) ph 8,5 and (6) ph Temperature.c Fig. t-effect f temperature fr 3 days saking in SCW cntaining (0) 500 ppm sdium nitrite alne at ph 8.5; and with 500 ppm sdium mlybdate at ph (0) 8.5 and (6) 10.0 xide and increased acidity f the surface. Saking time has beneficial effect when mlybdate r nitrite is present in sew. Less time is necessary fr attaining cnstant crrsin rate when inhibitr cncentratin is high r the slutin is stirred. Nitrite is mre effective in this regard than mlybdate. When the inhibitrs are used tgether lw and cnstant crrsin rate is btained even earlier. %IE increases with the increase f cncentratins f mlybdate and nitrite, r f saking perid. Early and high %IE is als btained when inhibitrs are cmbinedly present. Mlybdate and nitrite alne are less effective fr inhibiting mild steel crrsin in sew with the rise f temperature. This is mre nticeable fr the frmer. Hwever, mlybdate and nitrite cmbinedly can be used in sew t effectively inhibit the mild steel crrsin up t 55~C. Refer:ences 1 Pryr M J & Chen M, J Electrchem Sc, 100 (1953) Armur A W & Rbitaille D R, J Chern Technl Bitechnl, 29 (1979.l619. :3 Lizlvs E A, Crrsin, 32 (1976) Kdama T & Ambrse J R, Crrsin, 33 (1977) Farr J P G & Saremi M, Prc 4th int cnf n the chemistry and uses f mlybdenum, Glden, C, 1982, Climax Mlybdenum C, Ann Arbr, MI, 1982, Rbitaille D R & Bilek J G, Chem Eng, 83 (1976) Farr J P G & Saremi M, Surf Technl, 19 (1983) Mustafa C M, Crrsin inhibitin f mild steel by mlybdate in the presence f rganic cmpunds and xidising agents, PhD Thesis, Birmingham University, Great Britain, Sakashita M & Sat N, Crrs Sc~ 17 (1977)473. lo Bentley A J, Earwaker L G, Farr J P G, Saremi M & Seeney A M, Plyhedrn,S (1986) Jseph G & Perret R, Crrs Sc~ 7 (1967) Rbitaille D R, Mater Perfrm, 15 (1976) 40. [3 Vukasvich M S & Rbitaille D R, J Less cmmn Metals, 54 (1977) Mustafa C M & Shahinr Islam Dulal S M, Br crrs J (submitted). 15 Stranick M A, Prc 4th int cnf chemistry and uses f mlybdenum (AMAX Inc, Greenwich CT), 1982, Weber T R, Stranick M A & Vukasvich M S, Crrsin! 85, Paper n 122, NACE, Hustn, TX, Stranick M A, Crrsin, 40 (1984) Vgel A I, A text bk f quantitative inrganic analysis (Lngman, Lndn), 1971, Mr E D & Wruble C, Br crrs J, 11 (1976) Schwenk W, Crrsin, 20 (1964) Greene N D& Fntana M G, Crrsin, 15 (1959) 321. Iii ~;lr'l i' t4! 11111,1)1..! ~I I r I '" ii"" 1 t 'I' I