electroless nickel-plating on az91d magnesium alloy ... - ni... · electroless nickel-coating of...

Surface and Coatings Technology 179(2004) 124–134

0257-8972/04/$ - see front matter� 2003 Elsevier B.V. All rights reserved.doi:10.1016/S0257-8972(03)00866-1

Electroless nickel-plating on AZ91D magnesium alloy: effect of substratemicrostructure and plating parameters

Rajan Ambat *, W. Zhoua, b

Metallurgy and Materials, School of Engineering, The University of Birmingham, Birmingham B15 2TT, UKa

School of Mechanical and Production Engineering, Nanyang Technological University, Singapore 639798, Singaporeb

Received 17 June 2002; accepted in revised form 14 July 2003

Abstract

Electroless nickel-plating on AZ91D magnesium alloy has been investigated to understand the effect of substrate microstructureand plating parameters. The initial stage of the deposition was investigated using scanning electron microscopy(SEM) and energydispersive X-ray analysis on substrates plated for a very short interval of time. The early stage of growth was strongly influencedby the substrate microstructure. Plating was initiated onb-phase grains probably due to the galvanic coupling ofb and eutectica-phase. Once theb-phase was covered with the coating, it then spread onto eutectica and primarya-phase. The coatingproduced with the optimised bath showed 7 wt.% phosphorus with a hardness of approximately 600–700 VHN. The optimumligand to metal ion ratio was found to be 1:1.5, while the safe domain for thiourea(TU) was in the range of 0.5–1 mgyl.Fluoride was found to be an essential component of the bath to plate AZ91D alloy with an optimum value of 7.5 gyl. Thepresence of 0.25–0.5 mgyl mercapto-benzo-thiosole(MBT) found to accelerate the plating process.� 2003 Elsevier B.V. All rights reserved.

Keywords: Magnesium alloy; Electroless nickel; Microstructure; Plating parameters

1. Introduction

Magnesium and its alloys, with one quarter of thedensity of steel and only two-thirds that of aluminiumand a strength to weight ratio that far exceeds either ofthese, fulfill the role admirably as an ‘ultra light weight’alloy. Hence, these alloys have obviously become thechoice for weight reduction in portable microelectronics,telecommunications, aerospace and automobile applica-tions etc.The magnesium–aluminium system has been the basis

of the most widely used magnesium alloys since thesematerials were introduced in Germany during the FirstWorld War. Most of these alloys contain 8–9% alumin-ium with small amounts of zincw1,2x.A serious limitation for the potential use of several

magnesium alloys and AZ91 in particular, is theirsusceptibility to corrosion. Magnesium alloys, especially

*Corresponding author. Tel.:q44-121-414-5217; fax:q44-121-414-5232.

E-mail address: [email protected](R. Ambat).

those with high purity, have good resistance to atmos-pheric corrosionw3x. However, the addition of alloyingelements modifies the corrosion behaviour in such away that it can be beneficial or deleterious. The standardelectrochemical potential of magnesium isy2.4 V vs.NHE, even though in aqueous solutions magnesiumshows a potential ofy1.5 V due to the formation ofMg(OH) film w4x. Consequently, magnesium dissolves2

rapidly in aqueous solutions by evolving hydrogenbelow pH 11.0, the equilibrium pH value for Mg(OH)2w4x.Although the addition of several alloying elements

such as aluminium, zinc and rare earths have beenreported w5–12x to improve the corrosion resistance,technologically that does not satisfy the requirement forseveral applications. Hence, the application of a surfaceengineering technique is the most appropriate methodto further enhance the corrosion resistance. Among thevarious surface engineering techniques that are availablefor this purpose, coating by electroless nickel is ofspecial interest especially in the electronic industry, dueto its conductivity and several other engineering prop-

125R. Ambat, W. Zhou / Surface and Coatings Technology 179 (2004) 124–134

Table 1Chemical composition of the AZ91D alloy(in wt.%)

Al Mn Ni Cu Zn Ca Si K Fe Mg

9.1 0.17 0.001 0.001 0.64 -0.01 -0.01 -0.01 -0.001 Bal

Table 2Optimised pre-cleaning steps

Ultrasonic degreasing using acetonex

Rinse in 10% NaOH at 608C for 5 minx

Water rinsex

6% chromic acid–5% nitric acid pickling for 45 sx

Water rinsex

Fluoride activation in HF(250 ml 70% HFyl) for 10 minx

Water rinse

Table 3Optimised bath composition and coating parameters

Bath constituents and parameters Quantity

Basic nickel carbonate 9.7 gyl(NiCO 2Ni(OH) 4H O)3 2 2

Citric acid 5.2 gylAmmonium bifluoride 7.5 gylHydrofluoric acid 11 mlylThiourea(TU) 1 mgylSodium hypophosphite 20 gylAmmonium hydroxide To adjust pHTemperature 808CAgitation Mild-

mechanical

erties. Electroless nickel is well known for its corrosionresistance and hardnessw13–18x. However, the nickelyMg system is a classical example of cathodic coatingon an anodic substrate. Hence, the porosity in the coatingmight influence the corrosion behaviour and servicelifetime of the electroless nickel-plated magnesium. Theprotective ability of electroless nickel on many engi-neering materials is limited by the porosity in the coatingw19–26x.Being a highly active metal, electroless plating of

magnesium alloy needs special bath formulations andpre-cleaning treatments. Hence, the direct plating ofmagnesium is still a challenge for the researchers. Theprocess becomes more complicated on AZ91 alloy dueto the microstructural heterogeneity owing to the une-qual distribution of aluminium within the three constit-uent phases namely primarya, eutectica andb phasesw26x. Therefore, the substrate material is electrochemi-cally heterogeneous and each constituent behaves differ-ently to the plating bath. The available information onelectroless nickel-plating of magnesium alloys is verylimited. This paper reports the work carried out onelectroless nickel-coating of AZ91D magnesium alloy,more specifically, the effect of substrate microstructureon coating nucleation and the effect of various bathparameters.

2. Experimental

The substrate material used for the present investiga-tion was AZ91D ingot-cast alloy. The chemical compo-sition of the alloy is given in Table 1. Rectangularcoupons of size 20=40=4 mm were used for theinvestigation. The surface of the substrate material waswet-ground(using water) on 1000 grade SiC paper and

polished on a diamond wheel using 6-mm diamondpaste. The polished specimens were thoroughly washedwith water before passing through the pre-cleaningschedule as shown in Table 2. The initial weight anddimensions of the specimens were measured prior topre-cleaning steps. Immediately after the fluoride acti-vation (last step in the pre-cleaning process), the spec-imen was quickly transferred to the coating bath(500ml) in a glass container placed in a constant temperaturewater bath to maintain the required temperature. A freshbath was used for each experiment to avoid any changein concentration of bath species. The bath compositionand other parameters used in the present work are givenin Table 3. The specimens were coated for the requiredlength of time, removed from the bath, washed withwater and acetone and air-dried. Final weight of thespecimen was determined and the coating rate inmmyhwas calculated from the weight gain. In this work, theinitial weight of the specimen was measured prior topre-cleaning steps. This is to avoid any surface oxidationdue to time delay in transferring the substrate to theplating bath after fluoride activation step. However, toestimate the error in the weight gain calculation as aresult of dissolution of substrate material during the pre-cleaning steps, a control experiment was carried out tomeasure the weight loss during the pre-cleaning process.The loss in weight during the pre-cleaning process wasfound to be insignificant, although the weight gainvalues reported were corrected for this weight loss.Duplicate experiments were conducted in each case, andthe coating rate reported is the average of two experi-ments. The coated specimens were characterised toevaluate the coating performance. Coating morphologywas analysed using optical(Axiolab ZEISS model) andscanning electron microscope(JEOL Model No.

126 R. Ambat, W. Zhou / Surface and Coatings Technology 179 (2004) 124–134

Fig. 1. AZ91 substrate:(a) microstructure and(b) Al and Zn concentration across theb-phase along the line shown in the picture.

Fig. 2. Electroless nickel-plating rate on AZ91 substrate as a functionof time.

5600LV). EDX analysis was used for the determinationof phosphorus. Microhardness measurements were car-ried out using Shimadzu hardness testing machine(HMV 2000 model) using a load of 100 g. The structureof the deposit was determined using X-ray diffractiontechnique(Philips, Model No. PW1830).For studying the coating nucleation, coupons were

deposited for very short internals of time namely 1–5min, and were analysed using SEM and EDX.

3. Results

3.1. Microstructure of substrate material

Fig. 1a shows microstructure of the substrate materialAZ91D ingot. The microstructure consisted of primarya, eutectica and b-phases(marked in the figure). b-phase is an intermetallic with the stoichiometric com-position of Mg Al . Coring during solidification17 12

resulted in considerable variation in the distribution ofaluminium and zinc in the microstructure of AZ91Dalloy. Previously, we have reported the variation ofaluminium and zinc concentration adjacent tob-phasein AZ91 ingot-cast alloyw26x. Fig. 1b gives a typicalconcentration profile of aluminium and zinc in AZ91Dalloy along the line shown in the SEM picture(shownin the inset) measured using EDX. In general, thealuminium and zinc concentration ata regions near tob-phase(eutectica) was high, which decreased withincrease in distance from theb-phasew26x. The widthof this aluminium rich region()8%) adjacent to theb-phase varied at different sitesw26x. The concentrationof aluminium typically varied between;35% at theb-phase to approximately;8–6% near or within theprimary a-phase. This microstructural heterogeneity onthe surface of the substrate complicates the process ofelectroless deposition.

3.2. Electroless nickel-coating

Fig. 2 shows the variation of coating thickness as afunction of time on AZ91D alloy at constant temperatureand pH. The coating thickness was directly proportionalto the plating time.

In the present investigation, coating formed in optim-ised bath composition was found to be amorphouspossibly with microcrystalline areas(Fig. 3a). The broadpeak in the diffractogram corresponds to the dominantpeak for nickel. Although not shown, the X-ray diffrac-tion studies of the deposit formed in bath with TU,MBT and MBTqTU also revealed amorphousymicro-crystalline structure. Fig. 3b shows the microstructureof the electroless nickel-coating along the thicknessdirection. The picture reveals a lamellar structure due tothe variation in the composition of phosphorus. Theconcentration profiling of phosphorus along the thick-ness of the coating(along the vertical line in Fig. 3b)was carried out using EDX, and shown in Fig. 3c. Thephosphorus content showed a sinusoidal variation withaverage phosphorus content decreasing from interior toexterior. The alternate layers contain high amount ofphosphorus with layers in between exhibiting lowervalues. The average phosphorus content in the depositwas 7–8 wt.% with a hardness value ranging between650 and 750 VHN depending on the plating parameters.

3.3. Coating nucleation and growth

To understand the nucleation and growth of electrolessnickel on electrochemically heterogeneous AZ91D sub-


Fig. 3. Structure of electroless nickel-coating on AZ91 substrate after 4 h:(a) diffractogram,(b) cross-section of the coating showing lamellarstructure and(c) phosphorus content across the lamellar structure.

Fig. 4. Effect of pre-cleaning treatment on substrate microstructure:(a) ultrasonic cleaning,(b) alkaline cleaning in 10% NaOH at 608C, (c)6% chromic acid–5% nitric acid pickling and(d) fluoride activation(250 ml 70% HFyl).

strate, plating growth was monitored at very shortintervals of time using SEM and EDX. The studies werecarried out on substrates after the specified pre-cleaningsteps. The effect of various cleaning steps on surfacefeatures is shown in Fig. 4. Although the alkalinecleaning did not etch the surface, the surface afterchrome–nitric pickling and fluoride activation wasetched, so that the different microconstituents could beclearly seen. After hydrofluoric acid cleaning, the sur-face was found to be smooth and flat(Fig. 4), whereflat b phases can be seen as dark phases. The less dark

regions adjacent tob-phase is the eutectica-phasecontaining a higher amount of aluminium. The etchingdifference between these phases is due to the differencein aluminium composition as described earlier(Fig. 1b).Electroless nickel nucleated on the AZ91D substrate

at different intervals of time is shown in Fig. 5. Thecoating has been preferentially nucleated onb-phase,which then spread to eutectica and primarya-phase.The coating on primarya-phase was formed as acontinuation of the deposit onb-phase and eutectica-phase, after theb-phase and eutectica-phase was


Fig. 5. Early growth of electroless nickel on AZ91 as a function of time:(a) and(b) after 1 min, secondary and topographic mode, respectively;and(c) and(d) after 5 min with(d) at higher magnification showing the morphology of deposit aroundb-phase.

Fig. 6. SEM surface morphology and results of EDX analysis of the coating surface attached to substrate after peeling off:(a) secondary,(b)back scattered,(c) magnified picture showing the morphology near theb-phase and(d) results of EDX analysis at locations 1, 2 and 3 in picture(c).

covered with coating(Fig. 5). However, the coating onthe b-phase was discontinuous and grew slowly, whichled to a concentration of defects over this region. TheEDX analysis of the early stage deposits onb-phase atdifferent intervals of time showed slightly loweramounts of phosphorus content, which increased simul-

taneously with coating growth. However, this phenom-enon was observed in a very thin layer of coating at thesurface while on the average, as shown in Fig. 3c, thephosphorus content decreased with coating growth.Fig. 6 shows the surface morphology of the coating

that was attached to the substrate after peeling off from


Table 4Rate, phosphorus content and hardness of coating with different Ligand–nickel(citric acidyNi ) ratio in the coating bath2q

Citric acidyNi2q Coating rate Phosphorus content Hardness(VHN)Ratio (mmyh) (wt.%)

0.16 No deposition0.33 6.00 7.61 6650.64 5.41 6.86 6510.95 No deposition

Table 5Rate, phosphorus content and hardness of coating with different amounts of ammonium bifluoride in the coating bath

Concentration of Coating rate Phosphorus content Hardness(VHN)Ammonium (mmyh) (wt.%)bifluoride (gyl)

0 1.93 1.93 3427.5 8.11 8.11 78015 5.98 5.98 66522.5 No deposition

the surface. The position marked 1 in the photographshows the areas of coating attached tob-phase. It canbe seen that the coating containedb-phases brokenwhile pulling out. The EDX analysis on the surface ofthe peeled off coating showed the following results.EDX analysis at locations 1, 2 and 3 shows a variation

of Al, Mg and P content as shown in Fig. 6d. At location1, the EDX analysis revealed Mg and Al compositioncorresponding to theb-phase, while the phosphoruscontent was slightly lower compared to coating areasattached to primarya and eutectica-phase. Areas ofthe coating attached to eutectica-phase showed a higheramount of Mg than the primarya-phase. However, bothprimary and eutectica showed very little amount ofaluminium. The SEM picture shown in Fig. 6c showsthe surface of the substrate material after the coatinghas been removed from the surface. It can be seen thatthe b-phase was fractured while pulling out. The frac-tured surface is visible on the micrographs(marked 1in the picture). Some corrosion attack is evident oneutectica-phase(position 2), while the primarya-phasewas essentially unattacked(position 3).

3.4. Effect of coating parameters

Both the pre-cleaning steps and the concentration ofbath constituents have an influence on electroless nickel-coating. The effect of these bath parameters on theelectroless nickel deposition of Mg is not available inthe literature. In the present investigation, the effect ofcomplexing agent, fluoride ion concentration, effect ofstabilizer concentration such as thiourea and effect ofMBT on electroless nickel-plating have been investigat-ed by monitoring coating rate, surface hardness, phos-phorus content and surface morphology.

3.4.1. Effect of ligand–nickel ratioTable 4 gives the coating rate, phosphorus content

and hardness of the coating as a function of ligand–nickel ratio. The optimum ratio of ligand–nickel wasbetween 0.33 (Ligand:Ni (1:3) and 0.642q

(Ligand:Ni (1:1.5) (i.e. citrate concentration of 5.2–2q

7.8 gyl). Within this range of concentration, the effectof ligand on coating characteristics was found to besimilar, although in terms of phosphorus content, thebest ligand–nickel ratio was 0.33(Ligand:Ni (1:3).2q

Above or below this concentration, the coating rate wasdrastically reduced or inhibited. The surface morphologyof the coating did not show significant variation withthe change in ligand–nickel ratio.

3.4.2. Effect of fluoride ion concentrationThe fluoride ion is an essential component for elec-

troless nickel-plating bath for magnesium, and its pres-ence in the bath was reported to improve adhesion. Inthe present investigation, ammonium bifluoride was usedas the source of fluoride ion. Variation of coating rate,phosphorus content and hardness with concentration ofammonium bifluoride in the coating bath is given inTable 5. The values show that the optimum concentra-tion of ammonium bifluoride in the bath should beapproximately 7.5 gyl, although within the concentrationrange of 7.5–15 gyl, the difference was found to beinsignificant. However, the coating produced at 7.5 gylshowed a hardness value of;60 VHN higher than thecoating produced with 15 gyl of ammonium bifluoride.Fig. 7a and b show the surface topographic pictures ofthe coatings produced from baths with 0 gyl and 7.5 gyl of ammonium bifluoride, respectively. The depositformed at 7.5 gyl was found to be more combat anddefect free. However, the simple tape-test for adhesion


Fig. 7. Effect of ammonium bifluoride on the morphology of the coating on AZ91:(a) 7.5 gyl and (b) without fluoride.

Table 6Phosphorus content and hardness of coating with different amounts of thiourea in the coating bath

Concentration of Coating rate Phosphorus content in Hardness(VHN)thiourea(mgyl) (mmyh) the deposit(wt.%)

0 8.15 25.00 5270.5 6.70 8.00 5931.0 6.77 8.00 6202.0 1.63 – –

indicated higher adhesion for the coating produced with15 gyl bifluoride in the bath. Absence of ammoniumbifluoride or a concentration above 15 gyl producedvery inferior coating(Fig. 7b) or almost inhibited thecoating.

3.4.3. Effect of stabilizer (thiourea) concentrationThe effect of thiourea concentration on coating rate,

phosphorus content and hardness of the coating is givenin Table 6. It can be seen that the safe range for thioureastabiliser is within the range of 0.5–1 mgyl. Within thisrange, the phosphorus content in the coating remainedthe same. Below 0.5 mgyl, the bath decomposes spon-taneously, correspondingly a higher amount of phospho-rus content can be seen in the coating, possibly due tothe formation of nickel phosphide. However, above aconcentration of 1 mgyl, the coating was totallyinhibited.

3.4.4. Effect of MBT on electroless nickel-platingHeterocyclic organic compounds are widely used as

accelerators in electroless plating processes. MBT is oneamong them, which is found useful in many electrolessplating processes. In the present investigation, the effectof MBT was studied with and without the presence ofthiourea in the bath. The coating properties as a functionof MBT concentration is shown in Fig. 8. Addition of0.5 mgyl of MBT gave the best deposition with maxi-mum hardness, although the effect was similar in theconcentration range of 0.25–0.5 mgyl. However, addi-tion of MBT without thiourea had destabilised the bathin the long run. So it was necessary to optimise the

concentration of MBT in the presence of TU. Fig. 9shows the effect of TUqMBT at different concentra-tions on coating properties. A concentration of 0.5 mgyl TUq0.25 mgyl MBT was found to be the optimumvalue, so that it produced increased coating rate, highphosphorus content and hardness.

4. Discussion

4.1. Early stage growth and coating morphology

Direct plating of magnesium alloys with electrolessnickel is still a challenge. Only a limited amount ofliterature is availablew28,29x on the electroless nickel-plating of magnesium alloys and applications. The pro-cess is more complicated when the substrate containssecond phase particles as for AZ91, which makes thealloy electrochemically heterogeneous. The threemicrostructural constituents in AZ91 alloy(Fig. 1)namelyb, eutectica and primarya-phase have differentelectrochemical potentialsw10x. This is attributed to thedifferent levels of aluminium in these phases. As shownin Fig. 1b, aluminium concentration typically variesfrom ;35% in theb-phase to;12 and;5 wt.% inthe eutectic and primarya-phase, respectively.Electrochemicallyb-phase is reported to be more

cathodic(more noble) to eutectica and primarya w10x.Lunder et al.w10x reported that the corrosion potentialof b-phase in 5% NaCl saturated with Mg(OH) is2

approximatelyy1.2 V vs. SCE, whereas the values forpure Mg and AZ91 arey1.66 V andy1.62 V vs. SCE,respectively. The lower aluminium containing primarya-phase is expected to have most active(more negative)


Fig. 8. Effect of MBT on electroless nickel-coating on AZ91:(a) coating rate, phosphorus content and hardness; and typical surface morphologyof coating at concentrations(b) 1 mgyl, (c) 0.5 mgyl and (d) 0.75 mgyl.

Fig. 9. Effect of MBTqTU on electroless nickel-coating on AZ91:(a) coating rate, phosphorus content and hardness; and surface morphologyof coating at concentrations(b) 0.5 mgyl TUq0.25 mgyl MBT, (c) 0.5 mgyl TUq0.5 mgyl MBT and (d) 1 mgyl TU.

potential in AZ91w26x. In such a situation, one wouldexpect preferential nucleation of electroless nickel onthe a-phase. However, the present study shows prefer-ential nucleation of nickel deposit onb-phase(Fig. 5).Possible explanation is that the initial stage of depositionhas been influenced by a galvanic coupling betweenb-phase and adjacent eutectica-phase. The electronsproduced by the anodic dissolution of magnesium fromthea-phase are consumed by the cathodic deposition of

electroless nickel onb-phase. The presence ofMg(OH) over eutectica-phase and less phosphorus in2

early nickel deposits might support this argument. Theabove explanation could be represented by the chemicalequation as follows:At the a-phase, magnesium dissolved as Mg ions2q

according to the reaction:2q yMgsMg q2e (1)


At the b-phase, the electrons produced from the Eq.(1) are consumed for the reduction of nickel.

2q y oNi q2e sNi (2)

However, more electrochemical evaluation is neededto prove the above mechanism in order to understandthe early stage growth behaviour of electroless nickelon AZ91 alloy. Xiang et al.w30x also have reportedpreferential nucleation of electroless nickel on secondphase particles in magnesium alloys during plating, andno phosphorus was found in the initial deposited nickel.The lamellar structure of the(Fig. 3b) deposit has

been attributed to the compositional variation of phos-phorus within the deposit as observed in Fig. 3c. Thecause of the compositional variation has been explainedin terms of periodic fluctuations in the pH of the platingsolution adjacent to the deposit surfacew31,32x. Theoverall decrease in phosphorus content with coatingthickness might be attributed to the reduction in theconcentration of hypophosphite as coating proceeds. Thestructure of the deposit observed is in agreement withearlier observation that the structure of electroless nickelwith 4–7 wt.% phosphorus is generally amorphousw33x.

4.2. Effect of bath composition

The nickel source in the present investigation wasbasic nickel carbonate(NiCO 2Ni(OH) 4H O) and3 2 2

the reducing agent was sodium hypophosphite. Citricacid monohydrate was added as the complexing agentthat provides a quadridentate anion for chelation withnickel cation. To activate the magnesium surface, fluo-ride was added in the form of hydrofluoric acid andammonium bifluoride, which also increases the adhesionof the coating. The presence of citric acid and ammo-nium bifluoride also serves the purpose of buffers andaccelerators. The pH of the solution was controlledwithin the range of 7–8 using ammonium hydroxide.The autocatalytic electroless nickel deposition was ini-tiated by catalytic dehydrogenation of the reducing agentwith the release of hydride ion, which then suppliedelectrons for the reduction of nickel ionw27x.Electroless deposited nickel usually contains 3–15

wt.% of co-deposited phosphorus originating from hypo-phosphite ion. The quantity of phosphorus in the coatingdetermines several engineering properties of electrolessnickel.The role of the complexing agent on electroless

nickel-coating on AZ91 alloy can be explained on thebasis of three aspects i.e.(a) a reduction in the concen-tration of free nickel ions,(b) preventing the precipita-tion of basic nickel salts and nickel phosphate, and(c)exerting a buffering action.The coating rate on AZ91 alloy increased initially

with increase in ligand–nickel ratio upto a ratio of 0.33,

above which the rate decreases(Table 4). Severaltheories have been proposed to explain the variation incoating rate with ligand concentration. De Minjer andBrennerw34x argued that the maximum is the result ofthe low adsorption of ligand on the catalytic surface atlow concentrations, which accelerates the reaction. Athigher concentrations, there is a high adsorption ofligand on the surface, which poisons the reaction.A more plausible explanation suggests that the

increase in rate is due to the buffering action of thecomplexing agentsw35x. Maximum rates occur whilethere are uncoordinated(solvated) sites remaining onthe nickel ions, i.e. nickel ions are only partially com-plexed or chelated. Partially complexed nickel ionsretain some of the properties of free, solvated nickelions. Hence, for ligand concentrations up to valueswhere the maximum plating rates occur, buffering is thedominant function of the various ligands.An explanation based on pH is inadequate to explain

the decrease in rate with further increase in ligandconcentration beyond maximum. The pH of the platingbath changes very little with continued plating. More-over, the buffer capacity of the complexing agent doesnot parallel the plating rates obtained with them.However, a decrease in plating rate with increase in

ligand concentration(or ligand–nickel ratio), is mostlikely due to the co-ordination of the remaining solvatedsites with ligand atomsw35x. The concentration of freenickel ions decrease and the metal ions take on thecharacteristics of complexed or chelated nickel ions.Fluoride ion is an essential component in electroless

nickel-plating of magnesium alloys. The initial dip inhydrofluoric acid activates the substrate surface, andforms a fluoride film over the surface. Several research-ers studied the reactions between fluoride ion andmagnesiumw36,37x in relation to corrosion protection.Fluoride ion is reported to be a good corrosion inhibitorfor magnesium and its alloys. Further, good corrosionresistance of magnesium base materials in severe envi-ronments such as F gas and aqueous and gaseous HF2

has been reportedw36,37x. The protection is due to theformation of a MgF film on the surface of the metal.2

However, the protective effect in aqueous solutionsseems to depend on solution pH and fluoride concentra-tion. As a result, fluorides are constituents of severalcommon anodic coating baths, known as DOW17 andHAE w37–39x. In electroless nickel-plating, formationof magnesium fluoride film over the substrate surfaceduring the pre-cleaning step prevents the oxidation ofotherwise reactive magnesium during the transfer ofmaterial to plating bath. In the plating bath, the deposi-tion of electroless nickel takes place by replacement ofthe fluoride film. The presence of fluoride in the bathstabilizes the film on the surface, however, an increasein concentration of fluoride above a value makes theremoval of fluoride film for nickel deposition impossible


(Table 5). Xiang et al. w30x have reported that forelectroless nickel-plating of magnesium alloys, differentfluorideyoxide ratio in the surface film caused differentdeposition rates.Although the two heterogeneous partial reactions

require a catalytic surface on which to occur, Gutzeitw40,41x found that they could also easily happen on thesurfaces of solid particles of colloidal dimensions presentin the solution. As the formation of these colloidalparticles is uncontrollable and the nickel depositionprocess is autocatalytic, working electroless nickel bathsmay decompose spontaneously at any time by triggeringa self-accelerating chain reaction on those colloidalparticles. So in practice, all working electroless nickelbath contain some kind of stabiliser(catalytic inhibitor)to prevent such spontaneous decomposition. The strongadsorption of stabiliser ions on a catalytic surface blockthe catalytic sites from being used for the adsorption ofreactants, thus impedes the deposition process. Whenthe surface density of adsorbed stabiliser ions reaches acertain level, the deposition process could be completelyinhibited (Table 6). As the concentration of stabiliser inbulk solution is usually in the ppm range, unlike theadsorption of reactants, which is kinetically controlled,the adsorption of stabiliser is diffusion limited.Although complexing agent is essential in electroless

nickel-plating baths, its presence sometimes reduces thespeed of the plating reaction. To overcome this, accel-erators can be added to electroless nickel-plating solu-tion. For AZ91, presence of MBT alone in the bathwithout thiourea, doubled the plating rate. The presenceof MBT and thiourea together in the bath also gavesignificant increase in plating rate. These acceleratorsare thought to function by loosening the bond betweenhydrogen and phosphorus atoms in the hypophosphitemolecule, allowing it to be more easily removed andadsorbed on to the substrate surface. The most suitablecompounds for this purpose are heterocyclic organiccompounds with oxygen or nitrogen atoms as these havea lone pair of electrons to participate in the bondingprocess. The delocalisation of electrons possible in thesemolecules makes them suitable to use as accelerators inelectroless nickel-plating process. The MBT heterocyclicring consists of two sulfur and a nitrogen atom. Theseatoms have lone pairs of electrons to participate in theelectroless nickel deposition process.

5. Conclusions

1. The electroless nickel-coating deposited on AZ91Dalloy in optimised bath showed amorphous structurewith 7 wt.% P and a hardness value of 600–700VHN. The microstructure of the coating in the trans-verse direction showed lamellar structure with aphosphorus content varying in a sinusoidal manner.

2. A strong influence of substrate microstructure wasfound. Initially, the coating was nucleated preferen-tially on b-phase. The coating spread over to primarya-phase, once theb-phase and eutectica-phases werecovered with electroless nickel.

3. Optimum ligand–nickel ratio was found to be 0.33(Ligand:Ni (1:3), whereas the best concentration2q

of thiourea was 1 mgyl. Fluoride ions were essentialto plate electroless nickel on AZ91D alloy with anoptimum concentration of 7.5 gyl of ammoniumbifluoride.

4. The presence of 0.5 mgyl of MBT in the plating bathdoubled the plating rate. However, for a stable bath,the optimum concentration of thiourea and MBT was0.5 mgyl of former with 0.25 mgyl of the latter.

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