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1 Anti corrosive electroless Ni-P films for mild steel mterils Deprtment of Chemistry, Kuvempu University, Shkrghtt, Indi Astrct Purpose The purpose of this pper is to evlute the corrosion resistnce of the electroless Ni-P cotings in two ggressive medi 3.5 wt.% NCl nd Synthetic industril wste wter. Also to study the effect of Phosphorous content in the electroless Ni-P deposits on its surfce nture, morphology nd corrosion resistnce. Design/methodology/pproch The corrosion ehvior of electroless Ni-P cotings generted on mild steel coupons from n cidic nd n lkline ths nd their nti-corrosion performnce of ws compred systemticlly in 3.5 wt.% of NCl solution nd lso in synthetic industril wste wter. Microstructure nd surfce composition of cotings were nlyzed using X-ry diffrction, scnning electron microscopy nd energy dispersive spectroscopy techniques, respectively. The Ni-P coted mild steel specimens were sujected to corrosion nd the rte of corrosion ws studied y chemicl nd electrochemicl methods. The liner sweep voltmmetry, Tfel nd electrochemicl impednce spectroscopy were employed to otin corrosion dt. Findings The electroless Ni-P cotings with higher P content possess homogeneous, uniform nd morphous surfce nture nd exhiited higher corrosion resistnce in the ggressive corrosive medi chosen. Originlity/vlue This pper provides corrosion ehvior of electroless Ni-P cotings in 3.5 wt.% NCl nd synthetic industril wste wter, nd estlishes the importnce of phosphorous content on nture nd properties of the cotings. Keywords Steels, Cotings, Corrosion resistnce, Films (sttes of mtter), Electroless coting, Ni-P, Synthetic industril wste wter, Impednce, Polriztion Pper type Reserch pper Introduction The electroless plting is chemicl reduction process; the cotings otined re of uniform thickness ll over the oject. The qulity of the deposit, nmely, physicl nd mechnicl properties is lso uniform ecuse the composition of the coting is uniform t ll the thicknesses. Recent dvnces in the electroless nickel system offer extremely right deposit, which re comprle with electroplted right nickel, while retining the dvntges of uniformity of thickness, especilly for prts of complex geometry (Hri Krishnn et l., 2006). The electroless Ni-P cotings performed etter ginst corrosion compred to ulk Ni nd even electrodeposited Ni. Ese of pssive film formtion in the cse of Ni-P deposits due to the presence of P lloyed with Ni held responsile for this nole chrcter (Bi et l., 2003). The physicl nd chemicl properties of electroless Ni-P deposits re dependent on the composition of the lloy in question. When the ph decreses, the noticele chnge in the plting process is concurrent decrese in the rte of deposition. The most drmtic effect of lowering ph is on the composition nd concomitntly, the properties of the Ni-P deposit. Moreover, rising or lowering the ph effects on the process itself s well s the resultnt deposits. The higher ph leds to incresed deposition rte nd the deposits contin The current issue nd full text rchive of this journl is ville t 59/2 (2012) q Emerld Group Pulishing Limited [ISSN ] [DOI / ] decresed P content nd exhiit poorer dhesion on steel. On the other hnd, lower ph results in decresed deposition rte nd corresponding deposits possess higher P content nd shows improved dhesion on the steel mterils nd good corrosion resistnce property (Glenn Mllory nd Jun Hjdu, 1990). So the ddition of P to the nickel mtrix significntly enhnces the corrosion resistnce property in vrious corrosive environments (Bi et l., 2003). The P content cn e mostly controlled y ph of the plting th (Glenn Mllory nd Jun Hjdu, 1990; Ashssi-Sorkhi nd Rfizdeh, 2004). Furthermore, the significnt increse in nti-corrosion ehvior cn e oserved with decresing the ph there y enhnced P content in the deposit (Ashssi-Sorkhi nd Rfizdeh, 2004; Petukhov et l., 2004). The superior corrosion resistnce of high P morphous deposits is ttriutle to the extreme homogeneity, sence of defects nd corrosion pths such s grin oundries, s crystlline mterils do (Chngdong et l., 2005; Krolikowski et l., 2006; Ping-Ho et l., 1994). Due to unique properties of electroless Ni-P deposits, such s wer resistnce, prmgnetic chrcteristics, hrdness nd electroctlytic ctivities, these mterils hve ttrcted much ttention of the reserchers. The nticorrosive ehvior of these deposits found greter importnce nd pplicility (Bi et l., 2003). The electroless Ni-P cotings re well known for high corrosion resistnce in demnding, ggressive medi (Krolikowski et l., 2006). The present work ims to produce electroless Ni-P deposits over steel specimens with vrying mount of P content nd to evlute their microstructure, surfce morphology nd corrosion resistnce ehvior in two different ggressive environments like neutrl 3.5 wt.% NCl nd in synthetic industril wste wter (SIWW). 69

2 Experimentl method The electroless Ni-P cotings were deposited onto the mild steel pltes. The plting th composition is given in Tle I. Anlyticl grde chemicls nd distilled wter were used to prepre the plting solution. The cidic nd sic ph ws djusted y using dil.h 2 SO 4 nd NOH, respectively. The plting durtion ws 2 hours with working temperture t 85 ^ 58C under continuous mgnetic stirring ( rpm). The mild steel pltes of 4 6cm 2 size were polished mechniclly using rsive SiC 2000 nd 4000 emery pper, degresed with trichloroethylene nd ctivted in dil.hcl followed y wter wsh efore sujecting to plting process. The electrochemicl experiments were performed in CH Instrument 660C USA, using sturted clomel electrode s the reference nd pltinum wire s uxiliry electrode. The Ni-P coted smples were used s working electrodes nd were emedded into epoxy resin with free exposed re of 1cm 2. The corrosive medi were 3.5 wt.% NCl nd the SIWW with composition 0.1 M NCl, 0.1 M H 3 PO 4 nd 0.1 M H 2 SO 4. The nodic polriztion curves were mesured t sweep rte of 0.01 V/st room temperture. The impednce mesurements were crried out in the frequency rnge of mhz, sinusoidl voltge excittion with mplitude ^5 mv. The specimens efore sujecting to electrochemicl test were immersed in the electrolyte for out 45 minutes to estlish the equilirium potentil. Ech electrochemicl test ws repeted to verify the reproduciility of the results. The weight loss experiments were crried out in oth corrosive medi for the cotings with immersion time of 12 hours. Bsed on the mount of weight loss of cotings the corrosion rtes were clculted. The surfce morphology of the cotings ws exmined using scnning electron microscope nd the composition ws determined using n energy dispersive spectrometer (EDS) coupled with the SEM on different loctions t the surfce. The X-ry diffrction (XRD) ptterns of the cotings were recorded using n X-ry diffrctometer with Cu K s the rdition source nd Ni s the filter. Results Anti corrosive electroless Ni-P films for mild steel mterils Chrcteriztion of deposit The XRD ptterns of Ni-P cotings otined from Bth-I nd -II with plting durtion of 2 hours were recorded nd re shown in Figure 1. The pek t 2u ¼ 458 represents the pure Ni phse. The surfce morphology of the cotings ws exmined y mens of scnning electron microscopy equipped with the EDS nlysis which ws used for the determintion of Tle I The compositions of the ths Components Bth-I Bth-II NiSO 4 6H 2 O 35 g/l NiCl 2 6H 2 O 30 g/l N 2 H 2 PO 2 H 2 O 10 g/l 20 g/l N 3 C 6 H 5 O 7 2H 2 O 10 g/l 50 g/l CH 3 COON 7 g/l NH 4 Cl 50 g/l ph Temperture (8C) 85 ^ 58C 85 ^ 58C Figure 1 XRD ptterns of s plted Ni-P from Bth-I nd -II Intensity Volume 59 Numer Ni-P coting from th-i --- Ni-P coting from th-ii θ chemicl composition of the deposits. The EDS results show the presence of Ni, P nd Fe (se metl) in the coting. The P content in the coting from Bth-I is wt.% nd tht in the coting from Bth-II is 9.21 wt.% (Figure 2). Corrosion ehvior nd its mesurements The electrochemicl properties of the Ni-P coting re sensitive to their structurl stte nd P content. Good corrosion resistnce seems ttriutle to high P content nd the corresponding morphous structure (Chngdong et l., 2005). The results of nodic polriztion of the morphous structure Ni-P coting from Bth-I nd -II in 3.5 wt.% NCl nd solution of SIWW re shown in Figure 3. The profiles revel tht, initilly the current vlue remins constnt followed y n increse in the nodic polriztion of cotings. Cotings from Bth-I hve wider pssivtion plteu nd lower current vlues compred to Bth-II, in oth the corrosive medi. The coting from Bth-I, strts the dissolution t þ0.4 V wheres Bth-II coting t þ0.1 V in 3.5 wt.% NCl, nd in nother corrosive medi, Bth-I coting strts dissolution t nd V for coting from Bth-II. EDS spectr showed tht the coting from Bth-I possess higher P content compred to the coting from Bth-II which leds to the formtion of P rich film t the lloy-solution interfce fster nd controls the dissolution of nickel in lrger mount (Tle II). Figure 4 shows the Tfel curves for Ni-P cotings from Bth-I nd -II in 3.5 wt.% NCl nd SIWW. The corrosion prmeters were clculted nd they re given in Tle III. In 3.5 wt.% NCl solution, the deposit from Bth-II hs E corr vlue V nd i corr A/cm 2, wheres E corr vlue for the deposit from Bth-I shows positive shift to V nd I corr, is A/cm 2. In SIWW solution lso, the E corr vlue of the deposit from Bth-I is shifted to V compred to the V for the deposit from Bth-II, lso the i corr for deposit from Bth-I is A/cm 2 nd tht for Bth-II is A/cm 2. The corrosion ehvior of Ni-P deposits ws investigted y using the electrochemicl impednce spectroscopy (EIS). The mesured impednce spectr for the Ni-P cotings from Bth-I nd -II in two corrosive medi 3.5 wt.% NCl nd Ni(111) Ni(111) 70

3 Anti corrosive electroless Ni-P films for mild steel mterils Volume 59 Numer Figure 2 SEM imges of the surfce 1 µm X µm X5000 () Notes: () Ni-P coting from Bth-I; () Ni-P coting from Bth-II () Figure 3 Anodic polriztion curves of Ni-P cotings in (I) 3.5 wt.% NCl solution nd (II) in SIWW Current A/cm Ni-P coting from th-i --- Ni-P coting from th-ii I Current A/cm Ni-P coting from th-i --- Ni-P coting from th-ii II Potentil V vs SCE Potentil V vs SCE Tle II Electrochemicl prmeters of cotings derived from Tfel plots Medium Specimen I corr A/cm 2 E corr V c V 2 1 V 2 1 Corr rte Mil/yer 3.5 wt.% NCl Bth-I Bth-II SIWW Bth-I Bth-II SIWW re shown s Nyquist digrms in Figure 5. The equivlent circuit is shown in the Figure 6 ws used to simulte the EIS dt for the coting system. As the mesured cpcitnce response usully will not idel, constnt phse element (CPE) is introduced for etter dt fitting insted of n idel cpcitnce prmeter. From the chnges in impednce nd semicircle dimeters inferred the electricl impednce of the cotings from Bth-I nd tht of the Bth-II. It is oserved tht, the impednce rnge for the cotings from Bth-II re smller thn for cotings from Bth-I. When the polriztion resistnce vlues re oserved, it cn e noted tht the dissolution process is more intense for the cotings from Bth-II. Moreover, the polriztion resistnce is inversely proportionl to the corrosion rte of the system. This prmeter provides n estimtion of the protective efficiency of the cotings (Tle IV). The corrosion velocity of the Ni-P deposits otined from Bth-I nd -II with different th ph ws clculted from weight loss method. The coted steel smples specimens were utilized for the study. The re of the specimen ws 4 4cm 2 2. The corrosive medi were 3.5 wt.% NCl nd SIWW t 298 K. The coted (oth sides) smples were immersed for 12 hours in solution nd pltes were removed, wshed in running wter for out 5 minutes, dried nd weighed. The loss in weight so otined ws used to determine the corrosion velocity: Corrosion rte ¼ Dm St Where, Dm ¼ weight lost, S ¼ surfce re exposed for dissolution, t ¼ time in seconds kept for dissolution. The SEM imges of corroded (weight loss) Ni-P cotings from Bth-I nd -II in oth the corrosive medi (immersion time 71

4 Anti corrosive electroless Ni-P films for mild steel mterils Volume 59 Numer Figure 4 Tfel plots of Ni-P cotings from Bth-I nd -II compred (I) in 3.5 wt.% NCl solution nd (II) in SIWW Log I (A/cm 2 ) Ni-P coting from th-i --- Ni-P coting from th-ii Potentil V vs SCE I Log I (A/cm 2 ) 1.5 c--- Ni-P coting from th-i d--- Ni-P coting from th-ii II c Potentil V vs SCE Tle III Electrochemicl prmeters of cotings derived from EIS Medium Smple R S V R P V CPE Error 3.5 wt.% NCl Bth-I , Bth-II , SIWW Bth-I Bth-II hours) re shown in Figures 7 nd 8. The imges shown in Figures 7() nd 8() exhiited more corrosion products nd dmged surfce compred to imges shown in Figures 7() nd 8() which elong to Ni-P cotings from Bth-II nd -I, respectively. Discussion Electroless Ni-P cotings were deposited from two different plting th solutions of different ph. The Ni-P deposit with higher P content ws produced y the plting th of lower ph. The P content of the deposit gretly influences the deposit nture nd structure. With incresed P codeposition coting exhiits morphous nture nd tht with lower P coting will e crystlline. The XRD dt show the presence of Ni (111) phse t 2u ¼ 44 (Snkr Nrynn et l., 2006; Blrju et l., 2006). The SEM pictures show lrger hemispheres or spheroids tht for the coting from Bth-I reveling morphous nture. Thus, depending on the concentrtion of non-metllic component P, chemiclly deposited nickel cotings exhiit different microstructure (Petkhov, 2007). To summrize, morphous structure could not e otined s no sufficient P toms re present to distort the nickel lttice to such n extent tht morphous nickel could e otined with respect to the non-morphous deposit otined from Bth-II. These re in ccordnce with previous results. The corrosion ehvior of the electroless Ni-P cotings eing our min interest, electrochemicl studies hd een crried out y potentiodynmic polriztion nd EIS techniques. Also the incresing demnd of the protective cotings in different ggressive medium, mde us to study the chemicl stility of the cotings in NCl nd SIWW. The shifts in corrosion potentils to positive vlues nd lower corrosion rtes of the deposits from Bth-I with high polriztion resistnce revel high corrosion resistnce. Also this coting exhiited Nyquist Figure 5 Impednce spectr of Ni-P cotings in (I) 3.5 wt.% NCl solution nd (II) SIWW 1, Ni-P coting from th-i --- Ni-P coting from th-ii Ni-P coting from th-i --- Ni-P coting from th-ii Imginry prt 1,200 1, I Imginry prt II ,000 1,500 2,000 2,500 3,000 Rel prt Rel prt 72

5 Anti corrosive electroless Ni-P films for mild steel mterils Figure 6 Equivlent circuit R S Tle IV Corrosion dt from weight loss method Corrosion rte in gcm 2 2 s 2 1 Medi Bth-I Bth-II 3.5 wt.% NCl SIWW loops with lrger re compred to tht of coting from Bth-II. The difference in chrge trnsfer resistnce of oth the cotings support ove results. The sme corrosion trend ws found to follow in oth corrosive environments. Weight loss experimentl results once gin pronounced the sme corrosion ehvior of the deposits. This is in ccordnce with reports (Bi et l., 2003; Chngdong et l., 2005; Krolikowski et l., 2006; Blrju et l., 2001). R P CPE Volume 59 Numer This corrosion ehvior of the cotings is minly dependent on deposit structure nd surfce morphology. The deposit composition in turn surfce nture gretly influenced y P content. The deposit derived from Bth-I, possess high P content. This P content mkes nickel to dissolve preferentilly t open circuit potentil leding to enrichment of P on the surfce lyer. The enriched P surfce rects with wter to form lyer of dsored hypophosphite nions. This lyer lock the supply of wter molecules to the electrode surfce, therey preventing hydrtion of nickel, which is considered to e first step to form the solule Ni 2þ species (Krolikowski et l., 2006; Snkr Nrynn et l., 2006; Petkhov, 2007; Bernhrd et l., 2008; Fryd nd Seed Rez, 2009; Ping-Ho et l., 1994; Guojin nd Giovnni, 2002): P þ 2H 2 O! H 2 PO 2 2 þ 2Hþ þ e A work y Der Kouwe y using Glow Dischrge Opticl Emission Spectrometry, confirmed the presence of P rich surfce lyer in high P electroless nickel deposit even efore it is sujected to ny corrosive medium, leding to higher corrosion resistnce compred to the deposit with lower P content (Blrju et l., 2001). This oservtion mkes us to conclude tht P rich Ni-P deposits re chemiclly stle in ggressive medi compred to tht possessing lower P content. By compiling ll the results otined y potentiodynmic polriztion, EIS nd weight loss experiments, we cn oserve the Ni-P deposits with higher mount of ws chemiclly most stle in oth the medium. Also, the corrosion rte is high in the Figure 7 SEM imges of the corroded smples in 3.5 wt.% NCl for 12 hours 1 µm X µm X5000 () Notes: () Ni-P coting from Bth-I; () Ni-P coting from Bth-II () Figure 8 SEM imges of the corroded smples in SIWW for 12 hours 1 µm X µm X5000 () Notes: () Ni-P coting from Bth-I; () Ni-P coting from Bth-II () 73

6 Anti corrosive electroless Ni-P films for mild steel mterils strong corrosive medi, i.e. synthetic industril wste wter s it contins SO 22,Cl 2 nd PO 23 4 ions wheres the rte of corrosion is lower in the cse of 3.5 wt.% NCl wherein only Cl 2 ions exists. These results re further confirmed y the SEM imges in Figures 7 nd 8, which were cptured for the corroded smples in oth the corrosive medi. According to literture it is widely ccepted tht, hlides destroy the existing pssivity nd hinders the pssivtion of mny metls. Some uthors commented on the ctivtion of pssive metl y hlides to the replcement of dsored oxygen or hydroxyl ions y hlide nions. This effect hs een ttriuted to the higher soluility of hydrted metllic surfce compounds (NiO, NiSO 4 ). The Cl 2 ions re most rective nd led to the formtion of surfce compounds which then results pitting corrosion. The effect of sulfte ion on the corrosion of vrious metls depends on ph of the solution. In generl sulfte ion increses the corrosion rte of ferrous mterils t lower ph vlue. Generlly, phosphtes re used s inhiitor. The protective property of the phosphtes depends on ph of the medium. The dded phosphte forms the metl phosphte complex film on the metl surfce nd these compounds re springly solule t higher ph, ut re unstle t lower ph. This my end up with the pitting corrosion (Ammr nd Drwish, 1968; Kilincceker et l., 1999; Acost et l., 1985). Conclusion The study highlighted the influence of P content on the corrosion ehvior of Ni-P electroless cotings. The SEM imges showed high morphous, uniform nd homogeneous surfce nture for the cotings of higher P content. Higher P content in coting hs strong effect on nodic polriztion ehvior. A significnt decrese in the nodic current density nd positive shift of corrosion potentil were found for the Ni- P cotings with higher P contents. EIS mesurements indicted tht incresing the P content cuses n increse in Rp of the cotings. Among the two ggressive corrosive environments, the cotings performed etter in 3.5 wt.% NCl solution thn in SIWW. References Acost, C.A., Slvrezz, R.C., Videl, H.A. nd Arviy, A.J. (1985), The pitting of mild steel in phosphte-orte solutions in the presence of sodium sulphte, Corrosion Science, Vol. 25, p Ammr, I.A. nd Drwish, S. (1968), Effect of hlogens on pssivity of nickel-i, chloride ions, Electrochimic Act, Vol. 13, p Ashssi-Sorkhi, H. nd Rfizdeh, S.H. (2004), Effect of coting time nd het tretment on structures nd corrosion chrcteristics of electroless Ni-P lloy deposit, Surfce nd Cotings Technology, Vol. 176, p Bi, A., Chung, P.Y. nd Hu, C.C. (2003), The corrosion ehvior of Ni-P deposits with high phosphorous contents in rine medi, Mterils Chemistry nd Physics, Vol. 82, p. 93. Blrju, J.N., Snkr Nrynn, T.S.N. nd Seshdri, S.K. (2001), Evlution of the corrosion resistnce of electroless Ni-P nd Ni-P composite cotings y electrochemicl impednce spectroscopy, Journl of Solid Stte Electrochemistry, Vol. 5, p Blrju, J.N., Snkr Nrynn, T.S.N. nd Seshdri, S.K. (2006), Structurl nd phse trnsformtion ehvior of electroless Ni-P composite cotings, Mterils Reserch Bulletin, Vol. 41, p Bernhrd, E., Mur, C., Mrino Andre, S. nd Antonell, R. (2008), Electroless deposited Ni-P lloys: corrosion resistnce mechnism, Journl of Applied Electrochemistry, Vol. 38, p Chngdong, G., Jinshe, L., Gungyu, L., Liyun, N. nd Zhongho, J. (2005), High corrosion-resistnt Ni-P/Ni/Ni- P multilyer cotings on steel, Surfce nd Cotings Technology, Vol. 197, p. 61. Fryd, B. nd Seed Rez, A. (2009), An investigtion on corrosion resistnce of s-plted nd het treted Ni-P/nnoSiC cotings, Mterils & Design, Vol. 30, p Glenn Mllory, O. nd Jun Hjdu, B. (Eds) (1990), Electroless Plting: Fundmentls nd Applictions, AESF Pulishing, Orlndo, FL. Guojin, L. nd Giovnni, Z. (2002), Corrosion resistnce of ternry Ni-P sed lloys in sulfuric cid solutions, Electrochimic Act, Vol. 47, pp Hri Krishnn, K., John, S., Srinivsn, K.N., Prveen, J., Gnesn, M. nd Kvimni, P.M. (2006), An overll spect of electroless Ni-P deposition review rticle, Metllurgicl nd Mterils Trnsctions A, Vol. 37, p Kilincceker, G., Yzici, B., Eril, M. nd Glip, H. (1999), The effect of phosphte ions (PO 23 4 ) on the corrosion of Iron in sulphte solutions, Turkish Journl of Chemistry, Vol. 23, p. 41. Krolikowski, A., Krownick, B. nd Jklewicz, O. (2006), Anodic dissolution of morphous Ni-P lloys, Electrochimic Act, Vol. 51, p Petkhov, I.V. (2007), Of the mechnism governing the growth of electrolessly deposited nickel-phosphorus cotings, Russin Journl of Electrochemistry, Vol. 43, p. 34. Petukhov, I.V., Shchern, M.G., Skryin, N.E. nd Mlinin, L.N. (2004), Corrosion nd electrochemicl ehvior of Ni-P cotings in 0.5M H 2 SO 4, Protection of Metls, Vol. 38, p Ping-Ho, L., Wen-T, T., Ju-Tung, L. nd Ming-Pn, H. (1994), Role of phosphorus in the electrochemicl ehvior of electroless Ni-P lloys in 3.5 wt.% NCl solutions, Surfce nd Cotings Technology, Vol. 67, p. 27. Snkr Nrynn, T.S.N., Bskrn, I., Krishnveni, K. nd Prthin, S. (2006), Deposition of electroless Ni-P grded cotings nd evlution of their corrosion resistnce, Surfce nd Cotings Technology, Vol. 200, p Aout the uthors Volume 59 Numer Thimmpp Venktrngih Venktesh received the PhD degree in Chemistry from Bnglore University, Indi. He is currently with the School of Chemicl Sciences, Kuvempu University, Indi, s Professor. He is the uthor or co-uthor of more thn 70 journl rticles nd 25 invited nd regulr conference presenttions. He is the principl investigtor for four completed nd two ongoing reserch projects sponsored y DST nd UGC, Government of Indi. He is serving s reviewer nd referee for vrious reputed journls (Elsevier nd Tylor & Frncis). He hs guided ten PhD students nd hs 25 yers teching nd reserch experience in Physicl Chemistry. His current reserch interests 74

7 Anti corrosive electroless Ni-P films for mild steel mterils include conducting polymers, polymer nnocomposites, electrodeposition of nnocomposites nd electrochemicl engineering. Thimmpp Venktrngih Venktesh is the corresponding uthor nd cn e contcted t: drtvvenktesh@yhoo.co.uk Volume 59 Numer Sudhkr Rngnth completed the MSc degree in Chemistry from Kuvempu University, Indi. He is currently with the Kuvempu University s Reserch Scholr nd his reserch interests include corrosion, electroless cotings, electrodeposition of nnocomposites nd surfce engineering. To purchse reprints of this rticle plese e-mil: reprints@emerldinsight.com Or visit our we site for further detils: 75