An investigation on effects of heat treatment on corrosion properties of Ni–P electroless nano-coatingsTaher Rabizadeh,Saeed Reza Allahkaram *,Arman ZarebidakiSchool of Metallurgy and Materials Engineering,University College of Engineering,University of Tehran,P.O.Box 11155-4563,Tehran,Irana r t i c l e i n f o Article history:Received 19January 2010Accepted 15February 2010Available online 17February 2010Keywords:C.CoatingC.Heat treatment E.Corrosiona b s t r a c tElectroless Ni–P coatings are recognized for their excellent properties.In the present investigation elec-troless Ni–P nano-crystalline coatings were prepared.X-ray diffraction technique (XRD),scanning elec-tron microscopy (SEM),potentiodynamic polarization and electrochemical impedance spectroscopy (EIS)were utilized to study prior and post-deposition vacuum heat treatment effects on corrosion resis-tance together with the physical properties of the applied coatings.X-ray diffraction (XRD)results indicated that the As-plated had nano-crystalline structure.Heat treat-ment of the coatings produced a mixture of polycrystalline phases.The highest micro-hardness was achieved for the samples annealed at 600°C for 15min due to the formation of an inter-diffusional layer at the substrate/coating interface.Lower corrosion current density values were obtained for the coatings heat treated at 400°C for 1h.EIS results showed that proper heat treatments also enhanced the corrosion resistance,which was attributed to the coatings’structure improvement.Ó2010Elsevier Ltd.All rights reserved.1.IntroductionSince the invention of electroless plating technology in 1946by A.Brenner and G.Riddell,electroless nickel (EN)coatings have been actively and widely studied [1,2].Nano-crystalline Ni–P alloys show a high degree of hardness,wear resistance,low friction coefficient,non-magnetic behavior and high electro-catalytic activity.Today such Ni–P alloys are widely used in the electronic industry as under-layer in thin film memory disks and in a broad range of other evolving technological applications.It is generally accepted that only nano-crystalline al-loys –irrespective of the way of production –show high corrosion resistance.Indeed,electrodeposited Ni–P alloys with crystalline structure (6–11at.%P)showed anodic dissolution in 0.1M NaCl.On nano-crystalline samples (17–28at.%P)a current arrest was found instead [3–5].To explain high corrosion resistance of Ni–P electroless coatings different models have been proposed,but the issue is still under discussion:a protective nickel phosphate film,the barrier action of hypophosphites (called ‘‘chemical passivity”),the presence of phosphides,a stable P-rich amorphous phase or the phosphorus enrichment of the interface alloy-solution were proposed.Note that such phosphorus enrichment at the interface was reportedby some of the authors to explain the outstanding corrosion resis-tance of Fe70Cr10P13C7amorphous alloys [5].Electroless Ni–P alloys are thermodynamically unstable and eventually form stable structures of face-centered cubic (fcc)Ni crystal and body-centered tetragonal (bct)nickel phosphide (Ni 3P)compounds.Different results have been reported regarding the microstructures in the As-deposited condition and the stable phases after heat treatments.For low P and medium P alloys,nickel crystal precipitated firstly and Ni 3P followed;however,Ni 3P and (or)Ni x P y compounds such as Ni 2P,Ni 5P 2,Ni 12P 5,and Ni 7P 3occur firstly in high P alloys [6–8].In general,the hardness of the electroless Ni–P coatings can be improved by appropriate heat treatment,which can be attributed to fine Ni crystallites and hard inter-metallic Ni 3P particles precip-itated during crystallization of the amorphous phase [8–10].The main reasons for heat treatment are:(1)to eliminate any hydrogen embrittlement in the basic metal,(2)to increase deposit hardness or abrasion resistance,(3)to increase deposit adhesion in the case of certain substrate and (4)to increase temporary corro-sion resistance or tarnish resistance [11].The crystallization and phase transformation behavior of elec-troless-plated Ni–P deposits during thermal processing has also been the subject of various investigations;it has been shown that different alloy compositions and heat treatment conditions could affect both the corrosion resistance and crystallization behavior of the deposit [8].0261-3069/$-see front matter Ó2010Elsevier Ltd.All rights reserved.doi:10.1016/j.matdes.2010.02.027*Corresponding author.Tel./fax:+982161114108.E-mail address:akaram@ut.ac.ir (S.R.Allahkaram).Materials and Design 31(2010)3174–3179Contents lists available at ScienceDirectMaterials and Designj o u r n a l h o m e p a g e :w w w.e l s e vier.c om/loc ate/matdesThe aim of this work is to study the post-deposition heat treat-ment effects and corrosion behavior of electroless deposited nano-crystalline Ni–P alloys.The temperature dependence of the coating structures and compositions were also evaluated and discussed.2.Experimental procedures2.1.Deposition of electroless Ni–P coatingThe deposition was performed on API-5L X65steel substrates (30Â25Â15mm)with a composition of(Fe:base,Mn:1.42,Si: 0.199,Cu:0.144,Mo:0.132,C:0.061,Nb:0.0538,Al:0.0417,Sn: 0.0167,Ti:0.0142,Cr:0.0126,P:0.01).The substrate surface was carefully polished with SiC emery papers(from grades#100to #400).All the specimens were subjected to the following pre-treat-ment and plating procedure:1.Ultrasonic cleaning in acetone.2.Rinsing by immersion in distilled water at room tempera-ture(RT)for2min.3.Cleaning in20vol.%H2SO4at RT for30s.4.Rinsing by immersion in distilled water at RT for2min.5.Cleaning in5vol.%H2SO4at RT for30s.6.Rinsing by immersion in distilled water at RT for2min.7.Electrocleaning in solution containing75g/l sodiumhydroxide(NaOH),25g/l sodium sulfate(Na2SO4),75g/lsodium carbonate(Na2CO3),at room temperature for20min.The current density applied was10mA/cm2inaccordance to ASTM G1.8.Rinsing by immersion in distilled water at RT for30s.For deposition,the substrates were dipped into commercial electroless nickel bath(SLOTONIP70A from Schlotter)with sodium hypophosphite as reducing agent for2h.This bath provided Ni–P deposits with a medium phosphorous content,9–10%P.Tempera-ture changed within88–93°C and pH changed within 4.5–4.7 range,during coating process.2.2.Heat treatment and hardness measurement of Ni–P coatingsIn order to study thefilm properties,coated samples were ther-mally treated in a vacuum environment.The coatings were isother-mally heat treated at different conditions that gave maximum hardness,i.e.200°C(for2h),400°C(for1h),600°C(for 15min).During heating process,the total pressure in the chamber was maintained below1mbar.Then the samples were allowed to cool down,for at least15min in the high vacuum environment prior to their exposure to the atmosphere.The hardness of coatings was measured using an(AMSLER D-6700)Vickers diamond indenter at a load of100g for a loading time of20s.The average offive repeated measurements is reported.2.3.Morphology and microstructure of Ni–P coatingsThe morphology and microstructure of the coatings were stud-ied using scanning electron microscopy SEM,(CAMSCAN MV2300). X-ray diffraction(XRD)patterns were obtained using Philip’s Xpert pro type X-ray diffractometer with a cobalt target and an incident beam mono-chromator(k=1.7889Å).2.4.Electrochemical measurementsCorrosion behavior of As-plated and heat treated electroless Ni–P coatings was studied by using potentiodynamic polarization test and electrochemical impedance spectroscopy(EIS)in3.5wt.%NaCl solution.The tests were carried out in a standard three-electrode cell using an EG&G potentiostat/galvanostat,model273A.Plati-num plate and Ag/AgCl electrode were used as counter and refer-ence electrodes,respectively.Potentiodynamic polarization test was carried out by sweeping the potential at a scan rate of 1mV sÀ1within the range of±400mV vs.open circuit potential (OCP).The EIS tests were undertaken using a Solartron Model SI 1255HF Frequency Response Analyzer(FRA)coupled to a Princeton Applied Research(PAR)Model273A potentiostat/galva-nostat.The EIS measurements were obtained at(OCP)in a frequency range of0.01Hz–100kHz with an applied AC signal of 5mV(rms)using EIS software model398.The equivalent cir-cuit simulation program(ZView2)was used for data analysis,syn-thesis of the equivalent circuit andfitting of the experimental data.3.Results and discussion3.1.Micrograph and structureFig.1shows the SEM images of the surface morphologies of Ni–P coatings before and after vacuum heat treatment at different con-ditions.As it can be seen heat treatment at different temperatures has not had any significant effect on morphology of the coatings.Fig.2shows the cross section image with line scan analysis of Ni–P coating heat treated at600°C(for15min)which shows the formation of an inter-diffusional layer and its elemental distribu-tion that affects the coating properties.Atoms under low temperature heat treatment(below400°C) can have short-range movement which is called structural relaxa-tion such as annihilation of point defects and dislocations within grains and grain boundary zones rather than long range diffusion [4].The higher the temperature is,the greater the atomic vibration energy is.As the temperature of the metal increases,more vacan-cies are present and more thermal energy is available,and so the diffusion rate is higher at higher temperatures[11].Hence,production of an inter-diffusional layer,formed as a re-sult of inter-diffusion of nickel and phosphorous from the coating to the substrate and iron in the reverse direction from the sub-strate will develop upon heating at600°C.3.2.XRD analyses of Ni–P coatingsFig.3shows XRD patterns of As-deposited and heat treated Ni–P coatings.The results show that both the phase composition and phase transformation behavior of the electroless Ni–P deposits de-pended on the heating temperatures.There was no significant change in XRD patterns observed for the samples treated at200°C and only a single broad amorphous profile was found,indicating that no phase transition took place at this tem-perature.When the heat treatment temperature was increased to 400°C,new XRD peaks corresponding to crystalline fcc Ni and Ni x P y appeared,indicating that the second phase precipitation was initi-ated.At this temperature,the diffraction peaks corresponding to the metastable Ni8P3,fcc nickel and stable Ni3P phases in the XRD profile can be seen.At600°C the fcc Ni and Ni3P peaks intensities in-creased with the heat treatment temperature.Therefore,the Ni8P3 metastable phase was decomposed completely at this temperature. Similar behavior has also been observed by Huangs et al.[12].Regarding the intensities of fcc nickel diffraction peaks,they are increased and the full width at half maximum(FWHM)became narrower with increasing the heat treatment temperature to 600°C.In the case of As-plated condition,using FWHM of 6.4676°and Scherrer equation,the grain size was estimated as1.5nm.T.Rabizadeh et al./Materials and Design31(2010)3174–31793175With regards to the FWHM of 5.496°at 200°C,0.9774at 400°C and 0.4027°at 600°C for fcc nickel peaks,the grain sizes were esti-mated as 1.7,10.9and 26.6nm,respectively.Therefore it can be deduced that the grain size of the crystalline nickel increases sub-stantially with increasing temperature,i.e.reaching to the maxi-mum size of 26.6nm at 600°C.This effect has also been confirmed by other investigators exposing Ni–P coatings to various heat treatment conditions [13,14].3.3.Hardness of electroless Ni–P depositsHardness was measured for the deposits at various annealing temperature.The variations in micro-hardness with annealing temperatures are shown in Fig.4.Heat treatment in the region of 200°C does not bring about any significant changes in the deposit properties.It can be observed that there is a marginal decrease of hardness from 612.3Hv to 525.3Hv between room temperature and 200°C.This small decrease in the hardness may be due to hydrogen embrittlement and internal stress relieving [11].Significant increase of hardness from 625.3Hv to 875.7Hv is observed after haet treatment at 400°C.The dispersion hardening effect may also be a reason for increase in hardness due to heat treatment above 200°C.Between 200°C and 400°C,the precipita-tion of Ni x P y compounds occurs.The precipitation of nickel phos-phides (Ni 3P)can be seen in the heat treated condition (Fig.3).The mechanism involved in the steep increase in hardness may be due to the precipitation hardening of a typical supersaturated solid solution,for which the atoms of the solute diffuse to a specific crystallography plane with an atomic arrangement that resembles the array of atoms on a plane in the structure of the precipitate.As the atoms attempt to precipitate,they are forced to conform to the structure of the solvent.This forced coherency between atoms of the solvent and atoms attempting to form the precipitate causes severe localized stresses in the matrix.These stresses areresponsi-Fig.1.SEM morphology images of As-plated and heat treated EN coating:(a)As-plated,(b)HT-200°C (2h),(c)HT-400°C (1h)and (d)HT-600°C (15min)in a vacuum environment.3176T.Rabizadeh et al./Materials and Design 31(2010)3174–3179ble for the increase in hardness.When sufficient numbers of atoms are collected,the structure of the precipitate is formed and thelocalized stresses are relieved,since the forced coherency between the matrix and the precipitated atoms reaches a maximum level [11].Intermediate processes during the formation of inter-metallic compounds can result in precipitation or age hardening.Particles of a precipitate form when sufficient atoms diffuse to aparticularFig.2.Line scan analysis and elemental distribution around inter-diffusional layer of Ni–P coating heat treated at 600°C (for 15min).Fig.3.XRD spectra of electroless Ni–P coatings measured before and after heat treated at differentconditions.Fig.4.Microhardness of As-plated and heat treated Ni–P electroless coatings.T.Rabizadeh et al./Materials and Design 31(2010)3174–31793177location to form a volume of material that has the correct stoichi-ometric composition and that is large enough so that a boundary can form around it.Before the volume of material reaches this crit-ical size,its atomic arrangement and that of the surrounding ma-trix must remain continuous.This coherency between regions having different inter-atomic spacing results in severe straining,which hardens the alloy [11].Around 600°C,the hardness reaches to a maximum value of 938Hv.As it can be seen in XRD patterns of the coatings,the inten-sity peaks of the precipitation phases have increased with raising heat treatment temperature above 400°C.This implies that more precipitation phases have been formed.In addition,the formation of an inter-diffusion layer can enhance the hardness of coating.This layer develops upon heating above 600°C and thickens with increasing annealing time.3.4.Electrochemical results3.4.1.Potentiodynamic polarization studiesFig.5shows the potentiodynamic polarization curves obtained for As-plated and heat treated electroless Ni–P coating in 3.5wt.%NaCl solution.Table 1lists the open circuit and corrosion potential E corr together with the corrosion current density i corr of these coatings.By combining Fig.5and Table 1,the corrosion potential of Ni–P deposits is positively shifted from À0.376to À0.25V with increas-ing the annealing temperature to 600°C.Moreover,the corrosion current density (i corr )of heat treated sample at 400°C is 1Â10À5(A/cm 2)which has the lowest corrosion current density among all the heat treated specimens.It is evident from literature reports on Ni–P coatings that pref-erential dissolution of nickel occurs at open circuit potential,lead-ing to the enrichment of phosphorus on the surface layer [15–20].The enriched phosphorus surface reacts with water to form a layer of adsorbed hypophosphite anions (H 2PO À2).This layer in turn will block the supply of water to the electrode surface,thereby pre-venting the hydration of nickel [15],which is considered to be the first step to form either soluble Ni 2+species or a passive nickel film [19,20].3.4.2.Electrochemical impedance spectroscopy studiesFig.6shows the Nyquist plots obtained for As-plated and heat treated coatings in 3.5%sodium chloride solution at their respec-tive open circuit potentials.All the curves appear to be similar (Ny-quist plots),consisting of a single semi-circle in the high frequency regions signifying the charge controlled reaction.However,it should be noted that though these curves appear to be similar withrespect to their shape,they differ considerably in their size.This indicates that the same fundamental processes must be occurring on all these coatings but over a different effective area in each case [19,20].To account for corrosion behavior of As-plated and heat treated electroless Ni–P coatings in 3.5%sodium chloride solution at their respective open circuit potentials,an equivalent electrical circuit model given in Fig.7has been utilized to simulate the metal/solu-tion interface and to analyze the Nyquist plot [19,20].The charge transfer resistance R ct and double layer capacitance C dl obtained for As-plated and heat treated electroless Ni–P coat-ings are compiled in Table 2.The open circuit potential (OCP),a reliable parameter that indicates the tendency of these systems to corrode,is also included in the same table,along with R ct and C dl values for effective comparison.The occurrence of a single semi-circle in the Nyquist plots indicates that the corrosion pro-cess of these coatings involves a single time constant [19].A similar conclusion of the existence of a single time constant has been reported by Zeller [21],Van DerKouwe [22]and Lo et al.[23]for the corrosion of electroless Ni–P coatings in sodium chloride and sodium hydroxide solutions at the respective open circuit potentials [19].The high values of charge transfer resistance (R ct ),in the range 18,172–52,903X cm 2,obtained for the coatings of presentstudyFig.5.Potentiodynamic polarization curves of As-plated and heat treated electro-less Ni–P coatings in 3.5%sodium chloride solution.Table 1Corrosion characteristics of As-plated and heat treated electroless Ni–P coatings in 3.5%sodium chloride solution by potentiodynamic polarization technique.Type of coating E corr (V vs.Ag/AgCl)i corr (A/cm 2)As-platedÀ0.37610Â10À5HT-200°C,2h À0.355 6.3Â10À5HT-400°C,1h À0.321Â10À5HT-600°C,15minÀ0.252.5Â10À5Fig.7.Equivalent electrical circuit model used to analyze the EIS data of the As-plated and heat treated electroless Ni–P coatings.3178T.Rabizadeh et al./Materials and Design 31(2010)3174–3179imply a better corrosion protective ability of heat treated coatings than the As-plated electroless Ni–P coating.The capacitance value obtained for As-plated and heat treated electroless Ni–P coatings are in the order of23–38l F/cm2.The C dl value is related to the porosity of the coating.The low C dl values confirm that the heat treated electroless Ni–P coatings of present study are relatively less porous in nature.By comparing electrochemical parameters and from EIS data of As-plated and heat treated Ni–P electroless coatings in3.5%so-dium chloride solution and from XRD patterns,it can be see that because As-plated coating is nano-crystalline in nature,it has more grain boundaries,hence it is less protective.By increasing heat treatment temperatures from200°C to600°C coatings become denser and less porous that decreasing C dl value proves this.At 200°C,there is grain growth that reduces grain boundaries and its corrosion properties are better than As-plated sample.At 600°C and400°C it is expected that the alloys consist of two con-stituents:crystals of the nickel-rich phase and the Ni x P y phases. Therefore,it can be observed that alloys are not continuous,be-cause they have surface inhomogeneities(grain boundaries and second phase),which are active sites for corrosion attack.Thus, the mixtures of two phases with two different compositions,prob-ably produces active–passive corrosion cells within the alloy,caus-ing it to suffer sever chemical attack.At400°C formation of second phases of Ni x P y have to decrease the corrosion resistance of coating but grain growth at this temper-ature triumph the formation of second phases and the corrosion resistance increases.After heat treatment at600°C because of increasing second phases’quantity,the corrosion resistance de-creases compare to400°C.4.ConclusionsNano-crystalline Ni–P electroless coatings was deposited on X65steel samples.The heat treatment effects on coating properties were systematically studied.The results show that:(1)As-deposited Ni–P coating has a homogeneous nano-crystal-line structure.(2)By applying heat treatment,second phase precipitationoccurred around400°C.(3)The highest hardness was obtained after heat treatmenttemperature at600°C for15min because of formation of an inter-diffusional layer and Ni3P phase and sealing porosities.(4)Proper heat treatment can significantly improve the corro-sion resistance of these coatings.References[1]Hu YJ,Xiong L,Meng JL.Electron microscopic study on interfacialcharacterization of electroless Ni–W–P plating on aluminium alloy.Appl Surf Sci2007;253:5029–34.[2]Dong D,Chen XH,Xiao WT,Yang GB,Zhang PY.Preparation and properties ofelectroless Ni–P–SiO2composite coatings.Appl Surf Sci2009;255:7051–5. 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