Surface & Coatings Technology 228 (2013) 84–91

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Surface & Coatings Technology

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Effect of bath pH and stabilizer on electroless nickel plating of magnesium alloys

Bonian Hu a,b, Ruixue Sun a, Gang Yu a,⁎, Lingsong Liu a, Zhihui Xie a, Xiaomei He b, Xueyuan Zhang a,⁎a State Key Laboratory of Chemo/Biosensing and Chemo metrics, College of Chemistry and Chemical Engineering, Hunan University, Changsha, 410082, Chinab Department of Materials and Chemical Engineering, Hunan Institute of Technology, Hengyang, 421002, China

⁎ Corresponding authors.E-mail addresses: yuganghnu@163.com (G. Yu), uwox

0257-8972/$ – see front matter © 2013 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.surfcoat.2013.04.011

a b s t r a c t

a r t i c l e i n f o

Article history:Received 6 December 2012Accepted in revised form 6 April 2013Available online 12 April 2013

Keywords:Magnesium alloysElectroless nickel platingpH stabilizerPlating solutionBath life

In order to develop the new bath solutions with long cycles and high stability for electroless nickel (EN) plat-ing of magnesium alloys, effect of bath pH and pH stabilizers on plating rate and coating quality as well asbath life were studied in detail. Gravimetry was used to determine the deposition rate of coating. The cover-age of coating was evaluated by 2 h of immersion test for magnesium alloys in 3.5 wt.% NaCl solution. Theresults showed that the bath pH not only changed the reactivity of the bath, but also had a strong impacton the microstructure and electrochemical properties of coatings. A highly uniform and dense coating wasformed via EN plating in the new bath containing ammonium acetate. The bath life was extended when ap-propriate amount of ammonium acetate was added. It was effective for ammonium acetate to stabilize pH inthe plating solution, to control coating quality and bath life.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

Extensive application of magnesium alloys in automobile, aero-space, electronic communication and other fields is due to theirhigh strength-to-weight ratio, low density, high damping, good cast-ability and machinability [1–4]. However, their widespread applica-tions have been limited because of high chemical reactivity andpoor corrosion resistance [5–7]. Hence, it is necessary to increasethe corrosion resistance of magnesium alloys via surface treatments.Although electroless plating process is dated from 1944, the studyof electroless nickel (EN) plating on magnesium alloys has been in-vestigated over the last two decades. Compared to ordinary chemicalplating, EN plating on magnesium alloys employs same main salt andreducing agent. However, magnesium alloys have their own charac-teristics. Application of magnesium alloys promotes the study of ENplating process on magnesium alloys. There are many baths reportedfor EN plating on Mg substrate [1,8–12]. The main nickel salts in thebath usually include nickel sulfate, basic nickel carbonate, nickelacetate. Huo et al. [10] found the stable bath through orthogonal ex-periments and the corrosion problem of magnesium alloys wassolved when nickel acetate was used as the main salt. Nickel sulfatewas widely used as the main salt of electroless nickel plating solution.Li et al. [11] and Liu et al. [12] developed a new process of electrolessnickel plating for magnesium alloys using nickel sulfate as nickelresource. The results showed that the micro-hardness, the adherenceand the corrosion resistance of the deposits was significantlyimproved.

yzhang@gmail.com (X. Zhang).

rights reserved.

The chemical bath is a thermodynamically unstable system andthe concentration of each component in the bath varies drasticallyduring EN plating process. It is difficult to control the change. Manyplating baths on magnesium alloys have the problems of lower stabil-ity and high cost of manufacturing, severe pollution to the environ-ment due to the discharge of large amount of wastewater. Theresults of EN plating on magnesium alloys reported in most literaturewere generally obtained through once bath and seldom involved inthe experiments with multi-cycle [13,14]. It is necessary to developa new steady bath for EN plating process on magnesium alloys.

The quality of an EN coating depends on many factors, including thecomposition of the bath, operating conditions of the EN plating and thenature of the substrate. In order to stabilize the bath, stabilizer is usuallyadded in the bath. Thiourea (TU) and maleic acid (MA) are frequentlyused in EN plating bath as stabilizers [15]. Thiourea was determined tobe a good bath stabilizer through our previous study [16]. Complexingagents had some effects on the quality of coating and bath stability[17,18]. Consumption of Ni salt and reducing agent of hypophosphiteobviously influenced thequality of coating andbath stability. Xie studiedthe influence of accumulation of SO4

2− on coating quality [19]. Neverthe-less, the variations of the bath composition, especially, the decrease ofthe pH valuewould cause strong corrosion ofmagnesiumalloys. Ying in-vestigated the effects of NH4F on the deposition rate and buffering capa-bility of electroless Ni\P plating solution [20] and reported that NH4Fhad the maximum buffering capability in an alkaline bath without con-sidering ammonium acetate. The corrosion rate of Mg alloys significant-ly decreased in the solution with enough buffer capability [21]. Inaddition, the pH value variation not only changed the features of thebath, but also had a strong impact on themicrostructure, microhardnessand electrochemical properties of EN coating [22]. It is important tomaintain the stability of the bath pH value by adding appropriate pH

Table 2The composition of solutions and the operating conditions of EN processes.

Process* [26] Solution composition Solution composition

Alkaline cleaning NaOH 50 g · dm−3 60 °CNa3PO4 · 12H2O 10 g · dm−3 10 min

Acid pickling HNO3(68%) 30 cm3 · dm−3 Room temperatureH3PO4(85%) 605 cm3 · dm−3 30–40 s

Activation 1 K4P2O7 120–200 g · dm−3 70 ± 5 °CNa2CO3 10–30 g · dm−3 2–3 minKF · 2H2O 11 g · dm−3

Activation 2 NH4HF2 95 g · dm−3 Room temperatureH3PO4 180 cm3 · dm−3 2–3 min

EN plating NiSO4 · 6H2O 20 g · dm−3 pH: 5.5HF(40%) 12 cm3 · dm−3 85 °CC6H8O7 · H2O 5 g · dm−3 60 minNH4HF2 10 g · dm−3

NH3 · H2O(25%) 30 cm3 · dm−3

NaH2PO2 · H2O 20 g · dm−3

H2NCSNH2 1 mg · dm−3

pH adjustment Discussion

* Insert washing between each step.

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stabilizer. There are few systematical researches about the pH and thestabilizers of EN plating on magnesium alloys in this field [23–25].Some research studied the influence of pH values on the appearancesand the properties of the coatings [23]. Others inspected the influenceof pH in the bath solution on the content of phosphorus, microhardnessand corrosion resistance of the deposits on the steel and polypropylenesubstrate [25]. So the influences of H+ ions accumulation on coatingquality in an acidic bath need be systematically investigated.

In the present paper, the influences of pH and pH stabilizer on thecoating performance were investigated in detail. The bath life with orwithout ammonium acetate was discussed for a novel bath system ofEN plating on magnesium alloys.

2. Experimental procedures

2.1. Experimental materials

The specimens were made of die-cast AM 60 magnesium alloys,whose chemical compositions were listed in Table 1. The testingspecimens were cut into rectangular pieces with the size of30 mm×20mm × 4mm. The substrates were mechanically polishedwith emery papers to ensure an appropriate surface roughness. Allexperimental data were obtained from the average values of threeduplicate specimens or three duplicate tests. The deviations of ex-periments were expressed by error bars.

2.2. EN process

The process of alkaline cleaning, acid pickling, twice activations,EN plating (loading: 10 cm3/1 cm2 plating area) was introduced totreat specimens in sequence. The operating conditions of the compo-sition of bath solutions and various process operations are given inTable 2, which is a novel process characterized by twice activationsand a new whole process flow developed by our group [26]. The pur-pose of alkaline cleaning was to remove greasy dirt on the surface ofthe Mg substrates. Through acid pickling, the substrate could be uni-formly exposed to EN solution after oxides were removed. Activationwas effectively used to prevent Mg substrate from corrosion and toincrease the activity of Ni electroless deposition [27].

2.3. Evaluation on the bath and coating performances

(1) Plating rate and coating thickness measurement: The speci-mens were dried via blowing hot air before and after platingprior to weighing. An electronic analytical balance (FA2104,Hengping of Shanghai in China) with 0.0001 g precision wasused to determine the specimen mass by weighing beforeand after plating.The plating rate (v, μm⋅h−1) and coating thickness were deter-mined and calculated according to the following formula [13]:

v ¼ wt−w0

ρ� S� t� 104 ð1Þ

where wt (mg) is the mass of the specimen plated for a lengthof time,w0 (mg) is the initial weight of specimen, S (cm2) is thesurface area of specimen, ρ (g⋅cm−3) is the density of the Ni\Pdeposit, using a density of 7.9 g · cm−3 in our calculation, andt (h) is the plating duration.

Table 1Chemical composition of magnesium alloys AM60 (in wt.%).

Al Zn Mn Si Cu Fe Ni Others Mg

5.5–6.5 b0.22 0.24–0.60 b0.10 b0.01 b0.004 b0.002 b0.01 >91

The coating thickness (δ: μm) was calculated by the followingequation [28]

δ ¼ 10 wt−w0ð ÞSρ

ð2Þ

The coating thickness was also directly measured via SEM im-aging of the coating cross section.

(2) Buffer capacity characterization: The evaluation of a buffer sys-tem was represented by the buffer capacity. Mallory et al. [29]proposed the concept of buffer capacity. Its mathematical for-mula was defined as follows,

β ¼ Δ Hþ� �ΔpH

ð3Þ

where ΔpH = pHinitial − pHfinal, Δ[H+] = [H+]initial − [H+]final.β in Eq. (3) represents the strong acid consumption in 1 dm3

bath after reducing 1 pH value. 250.0 cm3 bath was titrated with3.0 mol⋅dm−3 H2SO4 under magnetic stirring in order to deter-mine the buffer capacity (β). A pH glass electrode withhydrofluoric resistance was used in a pH meter (HF405-60-P-PA/120, Mettler Teledo Co., LTD of Shanghai) to determine pH valueof bath. Before experiment, pH meter was firstly calibrated withstandard buffer solutions.

(3) Bath stability test: The bath stability was usually evaluatedthrough PdC12 acceleration test reported in the literature [30].The estimation of the bath stability is more reasonable by stabilityconstant, b, defined as follows [31]:

b ¼ wt−w0ð Þ � Ni%

c0;NiV0−ct;NiV t

� �MNi

� 100% ð3Þ

where Ni % is the Ni content (wt.%) in coatings. MNi (g⋅mol−1) isthe molar mass of nickel. Vt (dm3) is the volume of plating solu-tion after plating and V0 (dm3) is the volume of plating solutionbefore plating. c0,Ni and ct,Ni (mol⋅dm−3) are the concentrationof the nickel ions before and after plating, respectively.

(4) MTO test: This operational time is normally expressed as “MetalTurn Overs” (MTO), which refers to how often the original nickelconcentration in a given volume has to be replenished. In mostcases, before the critical limit is reached, the operating solutionin the bath has to be disposed of [32]. The EN solutions were pre-pared as that described in part 2.2. Solutions A and B to be

86 B. Hu et al. / Surface & Coatings Technology 228 (2013) 84–91

replenished were also prepared, Solution A was 200 g · dm−3

NiSO4 · 6H2O. B was 200 g · dm−3 NaH2PO2 · H2O. The nickeland hypophosphite ionswere consumed during the deposition re-action, so it needed be replenished periodically. Ni2+ in the bathwas analyzed by EDTA method [19] for every hour andreplenished. Solutions A and B were replenished with the ratioof 1:1.2 (V/V).

(5) Evaluationon the coverage and corrosion resistance of the coatingsThe coverage on the coating was evaluated by immersing speci-mens in 3.5 wt.% NaCl solution for 2 h with the ratio of specimenarea to solution volume of 1 cm2:10 cm3. The number of corrosionspots unit area on the coating was observed to characterize thecoverage of coatings [33–35].The corrosion resistance of the coating was evaluated by immer-sion test in 3.5 wt.% solution for 100 h at 18–22 °C according tothe testing method introduced in Reference [36] and neutral saltspray (NSS) test in 5 wt.% NaCl solution at 35 ± 2 °C for 96 hbased on ISO 3768-1976 [37]. The setup to measure the volumeof evolved hydrogen due to the corrosion on samples was shownin Fig. 1.Electrochemical polarization experiment was carried out in a con-ventional cylindrical glass cell filled with 250 cm3 3.5 wt.% NaClsolution at room temperature with a scanning rate of 1 mV⋅s−1

(scanning potential zone:−0.5 to +0.5 V versus the open circuitpotential) controlled by an electrochemicalwork station (Interface1000, Gamry Co., USA). A platinum sheet (4 cm2) and a saturatedcalomel electrode (SCE) were used as counter electrode and refer-ence electrode, respectively. The Mg electrodes (1 cm2) with orwithout Ni\P coating were used as working electrodes.

(6) Surface characterization: surface morphologies and compositionswere characterized by scanning electron microscopy (SEM, aJSM-5610matchingwith an energy dispersive X-ray (EDX)micro-analysis system. The specimen was notched before EN process

Fig. 1. The setup of evolved hydrogen detection on specimens immersed in 3.5 wt.%NaCl solution.

and then the specimen coated by Ni\P alloy was broken fromthe notched position to observe the cross section morphology ofcoating.

3. Results and discussion

3.1. Effects of pH value on EN plating

3.1.1. Effect of pH value on EN plating rateThe influence of pH on EN plating rate is shown in Fig. 2. The plat-

ing rate almost linearly increased with pH in the range from 4 to 12. Itclearly demonstrated that pH value in the bath had a significant effecton plating rate.

According to EN plating reaction, the overall reaction in EN platingcould be expressed as follows [38]:

2 Ni2þ þmL−nh i

þ 8H2PO2− þ 2H2O→6H2PO3

− þ 2Hþ þ 3H2↑þ 2Pþ 2Niþ 2mL−n

ð4Þ

where [Ni2+ + mL−n] denotes the nickel complex and mL−n, the“free” complexing agent (ammonium citrate in the bath). As the reac-tion proceeds, the concentration of H+ increases. So the pH value ofthe bath decreases with EN plating.

From the previous reports, nickel deposition rate can be expressedas follows [16]:

d Ni½ �dt

¼ kNi2þh i0:67

H2PO2−½ �0:47

L½ �0:32 Hþ½ �0:09 ð5Þ

The rate of nickel deposition increases with H+ concentrationabating (i.e. pH value elevating) from Eq. (5).

3.1.2. Effects of pH value on the crystal structure and coverage of ENcoating

Fig. 3 shows the XRD patterns of coatings obtained from the bathwith different pH values. The XRD pattern of EN coatings exhibiteda strong peak (at around 2θ = 45°), which was designated as Ni\Pcoating [39,40]. Similar peaks have been reported in a similar pHbaths [41,42]. A broad strong peak (at around 2θ = 45°) in Fig. 3appeared due to the amorphous structure of the Ni\P alloy coatingin pH 5 bath, but a narrow peak occurred in the coating formed inpH 11 bath. With varying pH within the range of 5 to 11, the peakheight decreased, and the peak width increased significantly. Thisphenomenon was ascribed to crystal transformation due to P contentvariation in Ni\P coatings. The chemical compositions of coatings

Fig. 2. Dependence of pH value in the bath on the deposition rate.

Inte

nsity

/(a.

u.)

2θ

pH 5

pH 7

pH 9

pH 11

10 20 30 40 50 60 70 80

Fig. 3. XRD pattern of Ni\P coatings obtained from various pH baths.

Fig. 4. Dependence of coating corrosion spots on bath pH value via soaking in 3.5 wt.%NaCl solution for 2 h.

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obtained in different pH baths are listed in Table 3. Raising pH valueof bath led to the decrease of P content in the coatings. P variationwith pH value of bath could be explained from the following empiri-cal kinetic equation [29]

d P½ �dt

¼ k H2PO−2½ �1:91 Hþh i0:25 ð6Þ

Themicrostructure of theNi\P alloy coatingswould be transformedfrom crystalline phase with low P alloy in high pH bath to amorphousphase with high P alloy formed in a lower pH bath [29,43].

The corrosion spots on EN coatings obtained in various pH bathsare shown in Fig. 4. No corrosion spots were detected on EN coatingin pH 5 or 6 bath. There were many corrosion spots on EN coatingformed in pH 7 or 9 bath, while the number of corrosion spots onthe coating obtained in pH 11 bath decreased because of a higherplating rate and better coverage of Ni\P alloy deposits. Fig. 5(A–B)shows the surface morphology and cross section of compact EN coat-ings formed in pH 5.5 bath. From Fig. 5(A), the coating was compactand 10 μm thickness coating was obtained from the cross section asshown in Fig. 5(B). No corrosion spots were observed through 2 hof NaCl solution immersion test. In Fig. 5(C), there were some pits un-covered by Ni\P alloy in pH 7 bath. Incompact plating as shown inFig. 5(D) was obtained from pH 7 bath, so it produced large corrosionspots through the pores in the coating in NaCl solution due to a gal-vanic corrosion between Ni\P coating and Mg substrate.

3.2. Influence of pH stabilizers on EN plating

3.2.1. Influence of pH stabilizers on the deposition rateAccording to the literature, it was expected that the ideal operating

pH for an acid Ni\P plating bath would be in a range from 5.0 to 7.0[29]. Only a few of pH stabilizers' pKa could meet this requirement,such as, acetic acid (pKa = 4.76 [44]), succinic acid (pKa1 = 4.21,pKa2 = 5.64 [44]). Ammonium acetate and ammonium succinateexhibited better buffer performance in pH 4–6 range.

The influence of pH stabilizers on the plating rates is shown inFig. 6. It is obvious that the plating rate gradually decreased with am-monium succinate increasing. Much free Ni2+ was further chelated

Table 3Chemical composition of coatings formed in the bath with different pH values.

Composition pH = 5 pH = 7 pH = 9 pH = 11

w(Ni)/% 91.68 92.47 95.57 96.45w(P)/% 8.32 7.53 4.43 3.55

with adding ammonium succinate, Ni deposition reaction wasdebased, so that Ni deposition rate gradually declined. However, Nideposition rate significantly increased with addition of ammoniumacetate and reached a maximum at an addition of 15 g⋅dm−3. Beyond15 g⋅dm−3 addition, the deposition rate decreased gradually fromFig. 6. Ammonium acetate below 15 g⋅dm−3 addition could acceler-ate plating rate due to the presence of delocalized π electron bondin their structure [45]. Nevertheless, beyond 15 g⋅dm−3 addition,Nuzzi [46] and Hung [47] considered that the decrease of depositionrate was originated from a phenomenon of stabilizers overcrowdingadsorption.

3.2.2. Influence of pH stabilizers on coating qualityThe 3.5 wt.% NaCl immersion test was used to assess the coverage

of coating obtained according to operation in Table 2 and Section 2.3(5). Fig. 7 shows the dependence of coating corrosion spots on theconcentration of pH stabilizers in the bath.

No corrosion spots appeared on the coating produced only in thebath with the addition of 15 g⋅dm−3 ammonium succinate or15–20 g⋅dm−3 ammonium acetate. It indicated that the coating wascompact and the Mg substrate was completely covered with Ni\Palloy when appropriate pH stabilizers were added in the bath.

The corrosion spots of coating formed with an addition of ammo-nium succinate was inferior to that of the ammonium acetate fromFig. 7. The plating rate in the bath with the addition of ammonium ac-etate was faster than that with ammonium succinate because awell-covered plating was produced. Therefore, ammonium acetateas pH stabilizer in the bath was chosen as an ideal additive for ENcoating.

3.3. Dependence of pH variation in the bath on MTO

Periodic EN plating in the bath with and without ammonium ace-tate was carried out at 85°C based on Table 2. The dependence of pHvalue in the bath on MTO is shown in Fig. 8. From Curve A of Fig. 8, thepH value in the bath without stabilizers dramatically dropped frominitial 5.5 to final 3.5 after four MTO. Many deposits appeared at thebottom of the vessel and the bath lost the activity of EN platingafter five MTO. It was very important to explore an effective way tomaintain the pH value stable in order to guarantee a compact coatingwithout corrosion spots. From Curve B in Fig. 8, after adding ammoni-um acetate, pH value in the bath solution decreased slowly with MTO.The bath could extend to 10 MTO until the pH value reached 3.5 afteradding 20 g⋅dm−3 ammonium acetate. Ammonium acetate could ef-fectively improve the stability of the bath.

Fig. 5. SEM images of morphology, cross section and corrosion spots of coatings: (A) Surface morphology of coating obtained from pH 5.5 bath. (B) Cross section of coating obtainedfrom pH 5.5 bath. (C) Surface morphology of coating obtained from pH 7 bath. (D) Corrosion spots morphology on coating obtained from pH 7 bath through 2 h NaCl solution im-mersion test.

88 B. Hu et al. / Surface & Coatings Technology 228 (2013) 84–91

3.4. Variation of H+ concentration in reactions

During EN plating process, the chemical composition and the con-centration in the bath would change, including the main salt, reducingagents, other agents and the acidity of the solution. The reactions relat-ing to the change of hydrogen ions were discussed and summarized inthe following three aspects:

(1) Complex reactionWhen complexing agents were added into a solution with nickelion (Ni2+), reaction equilibrium was established and shownschematically:

Ni2þ þ nL⇌ NiLnð Þ2þ ð7Þ

Fig. 6. Dependence of the deposition rate on pH adjust agent concentration.

where L was citrate in the bath system. Complexing agents asso-ciated and disassociated with nickel ions, so they could controlthe number of free nickel ions and preventNi from fast precipita-tion. Complexing agents played an important role in stabilizingthe bath.Complexing agent also exhibited an affinity to hydrogen ions,which could be expressed by the following equilibrium:

C6H7O−7 þHþ⇌C6H8O7 ΔrG

⊖m ¼ −17:86kJ⋅mol−1 ð8Þ

C6H6O2−7 þHþ⇌C6H7O

−7 ΔrG

⊖m ¼ −27:16kJ⋅mol−1 ð9Þ

C6H5O3−7 þHþ⇌C6H6O

2−7 ΔrG

⊖m ¼ −36:52kJ⋅mol−1 ð10Þ

Fig. 7. Dependence of corrosion spots on pH stabilizer concentration.

-1 0 1 2 3 4 5 6 7 8 9 10

3.5

4.0

4.5

5.0

5.5

6.0

6.5

7.0

A: pH curve(no stabilizer)B: pH curve(with stabilizer)C: Stability curve(no stabilizer)D: Stability curve(with stabilizer)

MTO

pH v

alue

A

C

B

D

40

45

50

55

60

65

70

75

80

85

90

95

Stability constant/%

Fig. 8. Dependence of pH value and stability constant on MTO with and withoutstabilizer.

89B. Hu et al. / Surface & Coatings Technology 228 (2013) 84–91

Reactions (8)–(10) consumed H+ in the bath. In the bath ofTable 2, the total concentration of citric acid and nickel ionswas 0.02 mol · dm−3 and 0.07 mol · dm−3, respectively. Freecitrate was very small owing to its strong coordination withNi2+, so it played a minor part in hydrogen ions consumed inthe bath.

(2) Redox reactionThe oxydoreduction reactions related to H+ are all listed asfollows:

H2PO−2 þ H2O→H2PO

−3 þ 2Hþ þ 2e− E⊖ ¼ −0:5V ð11Þ

H2PO−2 þ 2Hþ þ e−→Pþ 2H2O E⊖ ¼ −0:51V ð12Þ

2Hþ þ 2e−→H2 E⊖ ¼ 0V ð13Þ

Ni2þ þ 2e−→Ni E⊖ ¼ −0:23V ð14Þ

Reactions (11)–(14) can combine into an overall platingreaction

Ni2þ þ 4H2PO2− þH2O→

k3H2PO3

− þHþ þ 32H2 þ PþNi ð15Þ

Reaction (12) produced phosphorus and consumed H+,which led to form Ni\P alloy coating. Reaction (13) evolvinghydrogen gas could consumemuch H+, but Hydrogen evolu-tion caused pinhole formation on the coating. In general, thetotal redox Reaction (15) generated hydrogen ions duringEN process. The H+ accumulation in the plating bathlowered the pH of the bath solution.

(3) Acid–base NH4+ equilibrium related to hydrogen ions

The acid–base reaction (non-oxidation reduction reaction) ofammonia and ammonium ions in the bath could be representedby:

NH3 þHþ⇌NHþ4 ΔrG

⊖m ¼ 62:86kJ⋅mol�1 ð16Þ

Ammonia could accept hydrogen ions and convert into ammoni-um ions, so hydrogen ions were consumed. When pH rangedfrom 4 to 5,mainly existed in the solution according to the disso-ciation equilibrium of ammonium, while NH3 was less in the so-lution. So it could not be used as a pH stabilizer.

F− in the bath could associate with H+ and formed weak acid ofhydrofluoric acid.

F� þ Hþ⇌HF ΔrG⊖m ¼ 5:59kJ⋅mol�1 ð17Þ

F− occupied nearly 86.8% (percentage of F− existed to the F totalquantity) in the bath with pH 4 based on the dissociation equi-librium of HF weak acid. The buffer couple of HF–F− had veryweak capacity.The bath pH decreased with the proceeding of EN plating reac-tions. A base substance was required to add in the bath andconstructed a strong buffer couple to maintain pH. pH stabilizerscould effectively control bath pH in a required range.

3.5. The dependence of bath stability on MTO

Fig. 8 (Curve C and D) shows the dependence of bath stability con-stants on MTO. The stability constant of the original bath was 86.32%at the beginning of EN plating, but it gradually decreased with MTOextending. The stability constant was reduced to 54.4% in the fourthMTO due to a drastic change of pH in the bath. Over five MTO, thebath lost the activity of Ni deposition. From Curve D in Fig. 8, whenammonium acetate was added in the bath, the new bath stability con-stant decreased slowly and kept about 78.5% after five MTO. The newbath still held some activity of Ni deposition even after ten MTO. Am-monium acetate played an important role in stabilizing pH of thebath, improving the bath stability and extending the bath life.

3.6. Corrosion resistance of coatings

3.6.1. Electrochemical polarization curvesThe typical polarization curves of the EN coatings obtained in pH 5.5,

7.0 baths and the bare Mg substrate in 3.5 wt.% NaCl solutions areshown in Fig. 9. The corrosion potential of the EN coatings formed inpH 5.5 bath was higher than that of AM60 substrate and the coatingfrom pH 7.0 bath. On the other hand, the compact EN coatings formedin the pH 5.5 bath lowered the corrosion current density and protectedMg substrate from the corrosion in NaCl solution. The EN coatingformed in pH 7.0 bath still exhibited larger corrosion current andmore negative corrosion potential. It showed that the dense Ni\P coat-ing formed in pH 5.5 bath exhibited the best corrosion resistance.

3.6.2. Immersion test for 100 hThe dependence of hydrogen evolution for various specimens in

3.5 wt.% NaCl solution with time is shown in Fig. 10. The volume ofevolved hydrogen on the three specimens of bare Mg substrate andthe three specimens plated in pH 7.0 bath for 1 h gradually increasedwith immersion time. The hydrogen evolved by corrosion on bare Mgsubstrate was initially larger than that on the specimens withincompact coating since most of Mg substrate was covered by Ni\Palloy and less area of Mg substrate was directly exposed with NaCl so-lution, but corrosion was accelerated in the specimens withincompact coating after immersion for 65 h. Afterwards, the volumeof evolved hydrogen on the specimens with incompact Ni\P coatingexceeded that from bare Mg substrate. As for the three specimensplated in the 5.5 bath with pH stabilizer for 1 h, the evolved hydrogenwas not detected after immersion for 100 h. It implied that the hydro-gen evolution did not occur due to the protection of compact Ni\Pcoating on corrosion of Mg substrate. It demonstrated that the com-pact coatings with good corrosion resistance were produced in thepH 5.5 bath with pH stabilizer.

3.6.3. Neutral salt spray test for 96 hNSS test was conducted for 96 h on the three specimens plated for

1 h in the pH 7.0 bath and plated for 1 h in pH 5.5 bath with pH sta-bilizer, respectively.

Fig. 9. Typical polarization curves of the EN coatings obtained from pH 5.5, 7.0 bathsand the bare Mg substrate in 3.5 wt.% NaCl solution.

Fig. 11. SEM image of compact coatings formed in pH 5.5 bath with pH stabilizer after96 h NSS test.

90 B. Hu et al. / Surface & Coatings Technology 228 (2013) 84–91

The corrosion spots on the specimens plated in pH 7 bath oc-curred initially after 2 h NSS test. The three duplicate specimenswere severely corroded in 15 h NSS test.

The SEM image of coatings formed in pH 5.5 bath with pH stabilizerafter 96 h NSS test is shown in Fig. 11. Black corrosion products andsome nano holes on the coating after 96 h NSS were observed inFig. 11 comparing with the fresh coating before corrosion test inFig. 5(A). The nano blind holes on the coatings seemed to be the pin-holes formed due to evolution of hydrogen bubbles during the electro-less nickel plating. Theywere unlike corrosion spots because black spotsaround the holes were not observed. There was still a surface morphol-ogy of dense EN coating after 96 h NSS test in Fig. 11. The Ni\P coatingwas slightly eroded, but the Mg substrate was not likely to be attacked.It once more indicated that compact EN coatings with good corrosionresistance were produced in the developed bath with pH stabilizer.

4. Conclusions

(1) Ammonium acetate acted as a pH stabilizer of Mg bath exhibiteda better performance than other pH stabilizers in buffer capacityand EN coating quality as well as corrosion resistance. Properammonium acetate could raise the bath life.

Fig. 10. Dependence of hydrogen evolution from various specimens in 3.5 wt.% NaClsolution on immersion time.

(2) The life of plating solution was extended via stabilizing pH in theplating solution.

(3) Compact EN coating with good corrosion resistance was pro-duced in the bath with pH stabilizer.

Acknowledgment

This study is jointly funded by the National Natural Science Foun-dation (21176061) and by the Key Natural Science Foundation(12JJ2006) and the Construct Program of Key Disciplines in HunanProvince.

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