Studies of Several Pickling and Activation Processes for Electroless Ni-P Plating on AZ31 Magnesium Alloy

Journal of The Electrochemical Society, 162 (3) D115-D123 (2015) D115

Studies of Several Pickling and Activation Processesfor Electroless Ni-P Plating on AZ31 Magnesium AlloyZhi-Hui Xie,a,z Fang Chen,a Shu-Rong Xiang,a Jun-Li Zhou,a Zheng-Wei Song,band Gang Yub

aChemical Synthesis and Pollution Control Key Laboratory of Sichuan Province, College of Chemistry and ChemicalEngineering, China West Normal University, Nanchong, 637002 Sichuan, People’s Republic of ChinabState Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering,Hunan University, Changsha, 410082 Hunan, People’s Republic of China

Several chromium-free pickling and activation processes were investigated in order to develop an eco-friendly pretreatment forelectroless Ni-P plating on magnesium alloy. The coatings obtained from etching only in H3PO4 solution exhibit poor adhesiondue to the existance of an interlayer. Meanwhile, a corrosion model of Mg alloy in H3PO4 solution was proposed. When fluoridewas added into the pickling solution, the adhesion of the coatings was also unsatisfied because of the bad mechanical interlockingaction. By comparison, the coatings obtained from pickling in H3PO4 solution followed by NH4HF2 activation displayed wonderfulproperties in both adhesion and corrosion resistance.© The Author(s) 2014. Published by ECS. This is an open access article distributed under the terms of the Creative CommonsAttribution Non-Commercial No Derivatives 4.0 License (CC BY-NC-ND, http://creativecommons.org/licenses/by-nc-nd/4.0/),which permits non-commercial reuse, distribution, and reproduction in any medium, provided the original work is not changed in anyway and is properly cited. For permission for commercial reuse, please email: [email protected]. [DOI: 10.1149/2.0601503jes]All rights reserved.

Manuscript submitted September 8, 2014; revised manuscript received December 12, 2014. Published December 30, 2014.

Magnesium (Mg) alloy is one of the lightest structural metalsand a potential candidate to replace the heavier aluminum or steelin manufacturing of mechanic components due to its low density,excellent physical and mechanical properties.1–6 However, the use ofMg alloy for structural applications has been limited owing to theirpoor corrosion resistance and high chemical reactivity.7–12

Electroless Ni-P plating (ENP) is one of the most effective surfacetreatment techniques to overcome the defect of Mg alloy, because ENPcan provide excellent properties of hardness, good wear resistance andgreat corrosion resistance by forming a uniform deposit on the Mgalloy substrate surface.13–16 However, ENP on Mg alloy presents manyproblems still because of the rapid formation of oxide film upon Mgalloy exposed to air/water, and forming loose immersion layers onthe substrate surface by replacement reactions at the initial depositionstage. The loose oxide film and immersion layers hinder successfulENP and are detrimental to the adhesion and uniformity of subsequentcoatings.17–20 Therefore, appropriate pretreatment steps must be takenbefore ENP to remove the natural oxide tarnish, embedded sand, heavymetal impurities and burned-in lubricants on the surface of the Mgalloy, and to activate the deposition of nickel on the matrix.21–23

There are mainly two types of preferred pretreatments for Mg alloy,including zinc immersion and direct ENP.18,24 The direct ENP is sim-pler and more convenient.11 However, acid pickling in a chromium-containing (VI) solution, followed by activation in a hydrofluoric acidsolution is usually employed in the conventional pretreatment pro-cesses of direct ENP.25–30 Several modified pretreatments have beendevised to avoid the use of such process, which contains some speciesdetrimental to the environment and harmful to human health.11,31–33

A method of avoiding the use of toxicant was carried out by employ-ing a pre-plating step with an alkaline ENP bath prior to subsequentcommon ENP in an acidic bath.31 Molybdate (MoO4

2−) chemical con-version coating was also attempted for replacing the chromium acidpickling.32–33 In the work of Ref. 32, strong volatile hydrogen fluorideacid was applied in the activation process and the molybdate con-version bath contained carcinogenic NaNO2.32 The adhesion, whichis a very important parameter for ENP coatings, was not interpretedparticularly in Ref. 33, although they obtained relatively uniform andcompact coatings with good anticorrosion properties.33

So far, only one study has been seen that reports the formationof ENP coatings on Mg alloy without any pickling or activationprocess.34 The authors state that continuous, defect-free coatings were

zE-mail: [email protected]

obtained in alkaline baths.34 Ni-P coatings from an alkaline solu-tion may contain less P content and so have many useful properties,such as excellent wear resistance.18 Regrettably, the detail data aboutthe adhesion and the corrosion resistance of the coatings were notpresented.

Obviously, most of these alternative pretreatments were based ondifferent processes and technologies, which are either complex or con-tain detrimental substances. In the present study, a simpler process wasused to make ENP on Mg alloy; and more environmentally-friendlychemicals, such as H3PO4 and NH4HF2 were selected as the picklingand activation substances based on our own previous studies and theliteratures.35,36 The effect of various pickling and activation processeson the surface morphologies of the substrate and the characteristicsof Ni-P coatings were investigated. The morphology and compositionof the samples were characterized with scanning electron microscopy(SEM) and energy-dispersive X-ray spectroscopy (EDX). Corrosionbehavior and adhesion of the coatings were assessed with potentiody-namic polarization measurements, scribe and grid test, respectively.

Experimental

Materials and ENP procedure.— AZ31 Mg alloy with the compo-sition (wt%) of Al 2.5∼3.0, Zn 0.7∼1.3, Mn >0.2, and Mg balancewas cut into about 30 mm × 20 mm × 2.0 mm rectangular shapecoupons and used as the substrate for the experiment. The specimenswere mechanically ground with SiC waterproof abrasive papers of400 and 1200 grit and then washed in distilled water, and finally driedin air. The compositions of various solutions and the operation condi-tions of the processes are given in Table I.13,37 All solutions used werefreshly prepared from analytical grade reagents and double distilledwater.

Property evaluation.— The etching weight loss of AZ31 Mg al-loy substrate in acidic pickling bath was evaluated with a weighingmethod. The weight loss, WL (mg · cm−2 · s−1), can be expressed as

W L = �m

St[1]

where �m (mg) is the weight loss after etching in a H3PO4 solution;S (cm2) is the area of Mg alloy samples; t (s) is the pickling time.

The coverage of the Ni-P coating was evaluated by calculatingthe corrosion spots in each square centimeter after immersion in 3.5%(wt%) NaCl aqueous solution for 2 h.38 The corrosion spots per square

) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 54.237.57.119Downloaded on 2016-03-30 to IP

http://creativecommons.org/licenses/by-nc-nd/4.0/

mailto:[email protected]

http://dx.doi.org/10.1149/2.0601503jes

mailto:[email protected]

http://ecsdl.org/site/terms_use

D116 Journal of The Electrochemical Society, 162 (3) D115-D123 (2015)

Table I. Pretreatment solutions, plating bath and their operation conditions

Processes∗ Bath compositions Conditions

1. Ultrasonically degreased Acetone 25◦C, 10 min2. Alkaline cleaning NaOH 50g/L 60 ± 5◦C

Na3PO4 · 12H2O 10g/L 10 min3. Method I (MI): Pickling only H3PO4 50∼700 mL/L 25◦C, 1 min

Method II (MII): Pickling and activationwith one step:MIIA: H3PO4 400 mL/L 25◦C, 1 min

HF 10∼300 mL/LMIIB: H3PO4 400 mL/L 25◦C, 1 min

NH4HF2 5∼150 mL/LMethod III (MIII): Pickling followed byactivationMIIIA: Pickling H3PO4 400 mL/L 25◦C, 1 min 25◦C,

Activation HF 10∼300 mL/L 8 minMIIIB: Pickling H3PO4 400 mL/L 25◦C, 1 min

Activation NH4HF2 5∼150 mL/L 25◦C, 8 min4. Electroless Ni-P plating NiSO4 · 6H2O 20g/L 85◦C

NaH2PO2 · H2O 20g/L 90 minC6H8O7 · H2O 2.5∼5g/L pH = 5.5± 0.5HF (40%) 12 mL/LNH4HF2 10g/LCS(NH2)2 1 mg/LNH3 · H2O (25%) App. 30 mL/L pH adjustor

∗Rinse in distilled water in each step.

centimeter (CS) was calculated as follows

C S = n

S[2]

where n is total number of corrosion spots of the Ni-P coating surface.To check reproducibility, the ENP process was repeated at least threetimes for each condition, using a fresh ENP bath.

Adhesion of nickel coating was evaluated by scribe and grid testaccording to GB/T5270-2005 and ISO 2819.13 The coated specimenswere drawn a square with a grid of 1 mm using a hardened steel scribethat had been ground to a sharp 30◦ point. In scribing the square lines,enough pressure shall be applied to cut through the coating to thebasis metal in a single stroke. The quality of the adhesion is classifiedinto three grades. Excellent adhesion is assigned by “O”, in whichno coating could be exfoliated from the sample surface. The symbol“�” indicates a small coating swell occurring but no peeling off.The symbol “×” represents a coating with very poor adhesion andcontinuously sloughed-off.13

The corrosion resistance was evaluated by potentiodynamic polar-ization measurement in 3.5% NaCl aqueous solution at 25◦C. The testwas carried out by CHI618B electrochemical workstation (Chenhua,Shanghai, China) using a classical three-electrode cell with saturatedcalomel reference electrode (SCE), Pt foil as counter electrode, andfreshly ENP coated samples that were sealed by epoxy resin with anexposed area of 1 × 1 cm2 as working electrode.13

The surface and cross section morphologies of the substrate beforeand after deposition, and the elemental composition of the sampleswere observed and inspected, respectively, with scanning electronmicroscopy (SEM, JSM-6510, Japan) and energy-dispersive X-rayanalysis (EDX).

Results and Discussion

Pickling in phosphoric acid solution (Method I).— Fig. 1 showsthe weight loss of AZ31 Mg alloy after pickling for 60 s in variousconcentrations of H3PO4 at 25◦C. With increasing H3PO4 concentra-tion, the weight loss of Mg alloy increases until reaching a maximumvalue of 0.43 mg·cm−2·s−1. Then, the speed of weight loss decreaseswhen H3PO4 concentration is higher than 400 mL/L. The increasein weight loss is attributed to the increasing in the number of hydro-

gen ions (decreased pH) when H3PO4 concentration was lower than400 mL/L. However, further increase in H3PO4 concentration willresult in a higher concentration of PO3−

4 , leading to a higher tendenyto produce insoluble films (mainly Mg3(PO4)2 and AlPO4).22,39 Theseinsoluble films deposited on the surface of the substrate and slowdown the rate of Mg alloy oxidization and dissolution, leading to adecrease in weight loss.

Fig. 2 shows the morphological evolution at the surface of AZ31Mg alloy before and after pickling in various H3PO4 concentrations.Apparently, the grit-blasting of the substrate generates a rough surface(Fig. 2a). It is mainly because of severe plastic deformation causedby SiC particles. Grooves and striations (indicated by black and whitearrows, respectively) are obviously observed on the deformed faces.After etching in 100 mL/L H3PO4 solution for 60 s, these flutes andstripes disappeared and the substrate exhibits a structure containingmany cracks (white arrows) and cavities ranging in size from a fewmicrons (μm) to tens of microns (Fig. 2b). As shown in the upperright corner of Fig. 2b, the whole surface morphology of the Mg al-loy after etching at this concentration is more clearly under a lower

0 100 200 300 400 500 600 700 8000.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

WL

/ mg

cm2

s1

[H3PO

4]/ mL L

1

Figure 1. The weight loss of Mg alloy in H3PO4 solution.




Figure 2. SEM micrographs of Mg alloy (a) before and after pickling in H3PO4 solution (b) 100 mL/L, (c) 400 mL/L, (d) 700 mL/L.

magnification (×200). The eroded pits on the surface of the Mg al-loy substrate seems not obvious for immersion in 400 mL/L H3PO4

solution compared to 100 mL/L (Fig. 2c). However, this does notmean that the corrosion extent of the Mg alloy in 400 mL/L H3PO4

solution is milder than that in lower concentration of H3PO4 solution.As proved by the weight loss curve shown in Figure 1, the corrosionrate of Mg alloy in 400 mL/L H3PO4 solution is higher than that in100 mL/L H3PO4. The porous network structure at Mg alloy surface isattributed to the excessive corrosion. An irregular and uneven surfacewas observed when the concentration of H3PO4 solution increased to700 mL/L (Fig. 2d). Lower magnification (×200) micrographs at theupper left corner in Fig. 2d indicate that many parts of the Mg alloysubstrate surface are not etched, and the size of the erosion pits areranging from a few tens of microns to several tens of microns. Thisis consistent with the result shown in Fig. 1, and hydrogen evolutionobserved during the experiment. All of the three surfaces of the Mg al-loy specimens after pickling in H3PO4 solution exhibit white particlesbecause of corrosion products (black arrows in Fig. 2d) with chemi-cal composition of Mg3(PO4)2, AlPO4, Zn3(PO4)2 and trace amountsof hydroxides.22,35,39–42 In most instances, these incompact corrosionproducts would deposit on the surface of Mg alloy substrate and can-not be rinsed away completely with running water. These residualcorrosion products (often called “sealing smut” in anodic oxidation)remained eventually, leading to an inclusion layer between the coatingand substrate, and a detrimental effect on the adhesion of the deposit.Moreover, with increasing concentration of H3PO4 from 100 mL/L to700 mL/L, the colors of the substrate surfaces changed gradually fromdark black to gray after pickling followed by distilled water rinse. Theadverse effect of the “sealing smut” on the adhesion of Ni-P coatingis more evident in lower concentration of H3PO4. Table II lists theresults of scribe and grid test of Ni-P coatings prepared with various

pretreatments. As shown in column MI of Table II, all those adhesionof the ENP coatings obtained by only H3PO4 pickling are alwaysunacceptable.

Fig. 3 shows the surface and cross sectional morphologies of thedeposited Ni-P coatings by pretreatment MI. As shown in Fig. 3a,when the concentration of H3PO4 is 100 mL/L, the deposits displayspherical microstructure as well as several pores (indicted by blackarrows), which are relevant to the evolution of hydrogen that occursduring ENP.43 The sizes of the spheres in Fig. 3a range from a fewtens of microns to about thirty microns. Some smaller sizes of sphereswere not observed even at higher magnification (×1000), as shown in

Table II. The adhesion of Ni-P coating obtained by differentpre-treatments

H3PO4/ HF/ NH4HF2/(mL/L) MI∗ (mL/L) MIIA MIIIA (g/L) MIIB MIIIB

50 × 10 � O 5 � O100 × 30 � O 10 � O200 × 50 � O 20 � O300 × 100 × O 30 � O400 × 150 × O 60 × O500 × 200 × O 90 × O600 × 250 × O 120 × O700 × 300 × O 150 × O

∗The symbol “O” stands for excellent adhesion which no coating couldbe exfoliated from the sample surface, “�” indicates a small coatingswell occurring but no peeling off and “×” represents a coating withvery poor adhesion and continuously sloughed-off.




Figure 3. SEM micrographs of Ni-P coatings obtained by pickling in various concentration of H3PO4 solution (MI) (a) 100 mL/L, (b, d) 400 mL/L, (c) 700 mL/L.

the inset of Fig. 3a. Figs. 3b and 3c show SEM pictures of the Ni-Pcoatings obtained by pickling in 400 mL/L and 700 mL/L H3PO4

solutions, respectively, followed by plating in the ENP bath. Bothcoatings are continuous and uniform, which are the same as the coat-ings obtained by pickling in 100 mL/L H3PO4 solutions, exhibitingtypical cauliflower-like structure with different dimensions of nodules.These large nodules can be divided into smaller sub-nodules; and thetortuous boundaries between them are observed in Figs. 3b and 3c.The tortuous boundaries have especial distinctness that may evolveinto crevices and cracks eventually.44–45 By comparison, the picklingsolution of Fig. 3b is more suitable for ENP on Mg alloy, that is,400 mL/L H3PO4. At this condition, the substrate could be coarsenedfully, and the oxide layer on the substrate surface after pickling is rel-atively thinner. The cross sectional morphology of the Ni-P coating inFig. 3d indicates that the oxide layer probably consists of MgO, Al2O3

and trace amounts of phosphate, according to EDX results and the re-ports in literature.22,35,39,46 Obviously, this micron-thick interlayer isunfavorable for the adhesion of the Ni-P coating,17 and the picklingprocess needs to be further optimized to overcome this problem.

Fig. 4 shows schematic drawings of the corrosion mechanismof AZ31 Mg alloy in various H3PO4 concentrations. As shown inFig. 4A1, the substrate may corrode like Scheme 1 in a low concen-tration of H3PO4; and the probable reactions may be as follows:22,35,39

Mg → Mg2+ + 2e− [3]

Al → Al3+ + 3e− [4]

2H+ + 2e− → H2 [5]

Meanwhile, the oxidized Mg and Al ions combined with PO3−4 :

3Mg2+ + 2PO3−4 → Mg3(PO4)2 [6]

Al3+ + PO3−4 → AlPO4 [7]

The insoluble phosphate film of the alloying elements (Al, Zn, Fe,etc.) and the base metal (Mg) were possibly formed on the sur-face of the substrate. This might provide some etching resistance(Fig. 4A2).35 An even surface with many small etch pits was observedafter pickling in this concentration of H3PO4 (100 mL/L). With in-creasing H3PO4 concentration from 100 mL/L to 400 mL/L, thosesmall etch pits continue to enlarge and tend to coalesce, resulting insome parts of matrix with insoluble phosphate precipitation detachedfrom the substrate (Scheme 2, Fig. 4B1). The closer the H3PO4 con-centration near to 400 mL/L, the Scheme 2 occurs more possibly. Thissignifies that the etching protection by phosphate films was weak-ened gradually with increasing concentration of H3PO4 (lower than400 mL/L), resulting in a rough and rugged substrate surface af-ter pickling (Fig. 4B2). However, when a large amount of insolublephosphate films were formed and accumulated on the surface of thecorroded and un-corroded areas of the matrix in an instant, a phos-phate film was created when the concentration of H3PO4 increased to700 mL/L (Fig. 4C). This insoluble film can decrease the etching rateof Mg alloy distinctly, although the film may not very compact. Here,it must be mentioned that there are no obvious boundaries betweenScheme 1, Scheme 2 and Scheme 3. In other words, the three formsof corrosion progress simultaneously over the entire H3PO4 concen-tration ranging from 100 mL/L to 700 mL/L. The corrosion progressis presumably dominated by Scheme 1 at a low H3PO4 concentra-tion but by Scheme 2 at higher H3PO4 concentration until reaching




Figure 4. Schematic models for pickling of Mg alloy indifferent concentrations of H3PO4 (Scheme 1: A1, A2)100 mL/L, (Scheme 2: B1, B2) 400 mL/L, (Scheme 3:C1, C2) 700 mL/L.

400 mL/L or by Scheme 3 when the concentration of H3PO4 increasesto 700 mL/L. Apparently, the insoluble phosphate films formed inthe pickling process cannot provide enough thickness of corrosionprotection layer when the concentration of H3PO4 is lower than400 mL/L.

Pickling and activation in H3PO4 solution containing fluoride ions(Method II).— Fig. 5 shows the weight loss of Mg alloy pickling in400 mL/L H3PO4 plus various concentrations of HF or NH4HF2. Theweight loss of these samples either in the mixing solution of H3PO4

and HF or H3PO4 and NH4HF2 first decreases rapidly and then de-creases to a value near zero with increasing concentration of F−. Thevalue of weight loss is decreased to 3.4 μg·cm−2·s−1 when the HF con-centration increases to 60 mL/L (curve a in Fig. 5); whereas the valueis decreased to 6.9 μg·cm−2·s−1 when the concentration of NH4HF2

0 25 50 75 100 125 150 175 200 225 250 275 300

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

MIIA

[NH4HF

2]/ g L

1

WL

/ mg

cm2

s1

[HF]/ mL L1

MIIB

0 10 20 30 40 50 60 70 80 90 100 110 120 130

a

b

Figure 5. The weight loss of Mg alloy in aqueous solutions containing400 mL/L H3PO4 and various concentrations of (a) HF or (b) NH4HF2.

increases to 40 g/L (curve b in Fig. 5). Those variations of weightloss indicate that very little dissolution of Mg alloy occurred whenHF concentration is greater than 60 mL/L or NH4HF2 concentration isgreater than 40 g/L. This phenomenon is attributed to the formation ofa relatively insoluble film of magnesium fluoride, MgF2, which actedas a barrier against active dissolution of the Mg alloy, according to thefollowing reaction:46–47

Mg2+ + 2F− → MgF2 [8]

Fig. 6 shows the surface morphologies of the substrate and Ni-Pcoatings treated by process MII. After the pickling-activation stage inthe mixed aqueous solution of H3PO4 and HF or H3PO4 and NH4HF2

(Figs. 6a and 6c), some grooves or striations (indicated by black ar-rows) formed in the grit-blasting process are still clearly observed onthe surface of the substrate, although the surface of the substrate isduller than before pickling-activation. In addition, only some smalletch pits are emerged on the surface of the matrix (indicated bywhite arrows), which implies that the substrates are hardly attackedwhen concentrated HF or NH4HF2 is added into the H3PO4 solution.Figs. 6b and 6d shows that both substrates are covered by smoothand bright Ni-P coatings with the size of nodules ranging from a fewtenths of a micron to ∼13 μm and ∼8 μm, respectively. The substratein Fig. 6b exhibits regular linear boundaries whereas the substrate inFig. 6d is irregular. The relatively more compact, more uniform andbrighter Ni-P coating in Fig. 6d than in Fig. 6b is attributed to the morelubricous and smoother surface of the microstructure in Fig. 6c thanthat in Fig. 6a.26–27 However, this flat and leveled off surface resultingfrom the protection of the insouble film (MgF2) at high concentra-tions of F− generates a decrease in mechanical interlocking actionbetween the coating and the substrate, resulting in poor adhesion aslisted in column MIIA and MIIB in Table II.48 The adhesions of theNi-P coatings obtained at low concentrations of F− are slightly betterbecause of the interlocking action. However, this adhesion obtained atlow concentrations of F− is still unsatisfactory since the formed oxidefilm on the surface of the substrate cannot be removed adequately andthe substrate cannot be protected by trace quantities of magnesiumfluoride.

The corrosion spots (CS/cm−2) of the Ni-P coatings were acquiredfrom various pretreatments. Fig. 7 shows plots of CS/cm−2 versus




Figure 6. SEM micrographs of substrate (a, c) after pickling-activation and corresponding Ni-P coatings (b, d) prepared by MII (a, b) HF 100 mL/L, (c, d)NH4HF2 60 g/L.

concentration of HF (MIIA) or NH4HF2 (MIIB) for the Ni-P coatingsobtained by pickling and activation with one step in the solutioncontaining H3PO4 and HF or H3PO4 and NH4HF2. It exhibits relativelyhigh porosity and poor corrosion resistance in NaCl solution. It is not

0 50 100 150 200 250 300

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

[NH4HF

2]/ g L

1

CS/

cm

2

[HF]/ mL L1

MIIB

MIIA

MIIIA

MIIIB

0 20 40 60 80 100 120 140 160

Figure 7. Corrosion spots of Ni-P coatings prepared by different methods.

easy to simply obtain a favorable pretreatment method by using theetching solution for direct ENP on Mg alloy.

Pickling in H3PO4 solution followed by F− activation (MIII).—Fig. 8 shows the surface morphologies of the substrate and Ni-P

coatings treated by process MIII. After pickling and activation in theoptimum concentration of H3PO4 (400 mL/L) and aqueous solutionof HF or NH4HF2, respectively, the grooves or striations formed inthe grit-blasting processes were disappeared, and coarse surfaces withhoneycomb-like microstructure were achieved as shown in Fig. 8aand 8c. This microstructure on the substrate surface provides not onlymore chemical active sites for the ENP, but also surface pits for me-chanically interlocking and improving coating adhesion.17,48 Theseideas are consolidated by the data listed in the columns of MIIIA andMIIIB in Table II. The results of scribe and grid test demonstrate thatthe Ni-P coatings prepared by an etching process in H3PO4 solutionfollowed by HF or NH4HF2 activation will give wonderful adhesionof ENP on Mg alloy. Meanwhile, obvious decrease of porosities ofthe Ni-P coatings (compared with that of process MII), especiallywhen the concentraion of NH4HF2 is higher than 30 g/L, was detected(curves of MIIIA and MIIIB in Fig. 7). As shown in Fig. 8b, thesubstrate are fully covered by nodules, and only an abnormal largenodule and one pinhole are found (indicated by white and black ar-row, respectively). The size of most nodules in Fig. 8b is lower than∼10 μm, whereas that in Fig. 8d is ∼13 μm, indicating that theNi-P coating of the former is slightly finer but less uniform than thelater. However, the deposits both in Figs. 8b and 8d are smoothercompared with the deposit obtained without fluoride activation(Fig. 3b).




Figure 8. SEM micrographs of substrate (a, c) and corresponding Ni-P coatings (b, d) prepared by method III (a, b) HF 10 mL/L, (c, d) NH4HF2 30 g/L.

Fig. 9 shows the potentiodynamic polarization curves of the Mgalloy and Ni-P coatings in a 3.5% NaCl aqueous solution at 25◦C.The corresponding corrosion potential (Ecorr) and corrosion currentdensity (Icorr) are summarized in Table III. The cathodic branch is themain reaction corresponding to the evolution of hydrogen; and theanodic branch is the most important features related to the corrosionresistance.49 For substrate of the bare Mg alloy (curve a in Fig. 9),the current density for the anodic polarization increases with increas-ing applied potential; and an activation-controlled anodic process is

-2.0

-1.8

-1.6

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

-8 -7 -6 -5 -4 -3 -2 -1

bc

log[i/(A cm2)]

E/V

SC

E

a: Bare Mg alloyb: MIIIAc: MIIIB a

Figure 9. Potentiodynamic polarization curves of the Mg alloy before andafter deposition of nickel in 3.5% NaCl solution (b) HF10 mL/L, (c) NH4HF230 g/L.

observed, indicating that no obvious passivation film formed.50 Bothcoatings obtained by processes of MIIIA (curve b in Fig. 9) and MIIIB(curve c in Fig. 9) exhibit less negative Ecorr and lower Icorr than that ofthe matrix. As shown in Table III, the value of Ecorr of the samples withdeposits is positively shifted by ∼800 mV and the Icorr is decreased bythree orders of magnitude compared with the bare Mg alloy substrate.Moreover, when the applied potential is increased to about −0.736 V(curve c in Fig. 9), a passive thin film is formed on the surface ofthe coating with the Icorr of about 6.7 μA·cm−2. The passive thinfilm starts to dissolve when the applied potential increased to about−0.464 V, resulting in the Icorr increases rapidly. Obviously, bothcoatings obtained by MIIIA and MIIIB will provide better corrosionresistance compared with the substrate. Furthermore, process MIIIAuses easily volatile HF, which is fatal if inhaled or absorbed throughthe skin or swallowed. Thus, the process MIIIB is more favorable.

Fig. 10 shows the cross section morphology of the deposit preparedby pickling in 400 mL/L H3PO4 solution followed by activation in30 g/L NH4HF2. The ENP layer is well bound to the matrix. From thepresented micrograph, the thickness of the coating is about 30 μm.In addition, both the surface of the coating and the substrate/coating

Table III. The results of potentiodynamic polarization test of theAZ31 Mg alloy before and after deposition of nickel in 3.5% NaClsolution

Sample Ecorr(V) icorr(A·cm−2)

AZ31 Mg alloy −1.65 2.09 × 10−3

MIIIA (HF10 mL/L) −0.87 3.58 × 10−6

MIIIB (NH4HF2 30g/L) −0.83 2.88 × 10−6




Figure 10. Cross section morphology of the as prepared Ni-P coating byprocess MIIIB.

interface are more regular than that of the specimen without F− treat-ment (Fig. 3d).

Conclusions

A new pretreatment process with H3PO4 pickling followed byNH4HF2 activation for direct ENP on AZ31 Mg alloy in an acidic bathhas been developed. The results presented in this work demonstratethat H3PO4 and NH4HF2 are effective for pickling and activation, re-spectively, in the processing of ENP on AZ31 Mg alloy. The phosphatefilm formed on the surface of the Mg alloy substrate in the picklingsolution provides some corrosion resistance unless the concentraionof H3PO4 exceeds 400 mL/L. If the porous film is not removed prior toplating, a poor adhesion of coatings will be generated. When fluorideis added into the etching solution, the surfaces with leveled off andfiner-textured structure are obtained owing to the protection of MgF2.This leads to brighter but poorer adhesion of coatings. Interestingly,the Ni-P coatings prepared by pickling in H3PO4 solution followed byNH4HF2 activation exhibit wonderful characteristics in both adhesionand corrosion resistance. The satisfactory adhesion of the coatings isattributed to the formation of a honeycomb-like microstructure on thesubstrate surface during the pickling stage. Compared with the AZ31Mg alloy substrate, the corrosion potential of the new obtained coat-ing shifts positively by more than 800 mV, and the corrosion currentdensity decreases by three orders of magnitude.

Acknowledgments

The authors thank the Open Project of Chemical Synthesisand Pollution Control Key Laboratory of Sichuan Province (No.CSPC2014∼4∼2), the Scientific Research Fund of Sichuan Provin-cial Education Department (No. 14ZB0147) and the FundamentalResearch Funds of China West Normal University (No. 13E006) forthe financial support.

References

1. S. Schumann and H. Friedrich, in Magnesium Technology, Metallurgy, Design Data,Applications, H. E. Friedrich and B. L. Mordike, Editors, p. 499, Springer–Verlag,Berlin, Germany (2006).

2. Z. H. Xie, G. Yu, T. J. Li, Z. J. Wu, and B. N. Hu, Dynamic behavior of electrolessnickel plating reaction on magnesium alloys, J. Coat. Technol. Res., 9, 107 (2012).

3. Z. W. Song, G. Yu, Z. H. Xie, B. N. Hu, X. M. He, and X. Y. Zhang, Performanceof composite coating on AZ31B magnesium alloy prepared by anodic polarizationtreatment and electroless electrophoresis coating, Surf. Coat. Technol., 242, 83 (2014).

4. R. Supplit, T. Koch, and U. Schubert, Evaluation of the anti–corrosive effect of acidpickling and sol–gel coating on magnesium AZ31 alloy, Corros. Sci. 49, 3015 (2007).

5. Y. W. Song, E. H. Han, D. Y. Shan, C. D. D Yim, and B. S. You, The effect of Znconcentration on the corrosion behavior of Mg–xZn alloys, Corros. Sci. 65, 322(2012).

6. A. L. Rudd, C. B. Breslin, and F. Mansfeld, The corrosion protection afforded by rareearth conversion coatings applied to magnesium, Corros. Sci. 42, 275 (2000).

7. M. Liu, P. J. Uggowitzer, A. V. Nagasekhar, P. Schmutz, M. Easton, and G. L. Song,Calculated phase diagrams and the corrosion of die–cast Mg–Al alloys, Corros. Sci.51, 602 (2009).

8. M. C. Zhao, P. Schmutz, S. Brunner, M. Liu, G. L. Song, and A. Atrens, An ex-ploratory study of the corrosion of Mg alloys during interrupted salt spray testing,Corros. Sci. 51, 1277 (2009).

9. G. Ballerini, U. Bardi, R. Bignucolo, and G. Ceraolo, About some corrosion mecha-nisms of AZ91D magnesium alloy, Corros. Sci. 47, 2173 (2005).

10. L. J. Liu and M. Schlesinger, Corrosion of magnesium and its alloys, Corros. Sci. 51,1733 (2009).

11. Z. C. Shao, Z. Q. Cai, R. Hu, and S. Q. Wei, The study of electroless nickel platingdirectly on magnesium alloy, Surf. Coat. Technol. 249, 42 (2014).

12. E. Correa, A. A. Zuleta, L. Guerra, M. A. Gomez, J. G. Castano, F. Echeverrıa,H. Liu, A. Baron–Wiechec, T. Hashimoto, P. Skeldon, and G. E. Thompson, Coatingdevelopment during electroless Ni–B plating on magnesium and AZ91D alloy, Surf.Coat. Technol. 232, 784 (2013).

13. Z. H. Xie, G. Yu, B. N. Hu, X. P. Lei, T. J. Li, and J. Zhang, Effects of (NH4)2SO4on the characteristics of the deposits and properties of an electroless Ni–P platingsolution, Appl. Surf. Sci. 257, 5025 (2011).

14. D. Seifzadeh and Z. Rajabalizadeh, Environmentally–friendly method for electrolessNi–P plating on magnesium alloy, Surf. Coat. Technol. 218, 119 (2013).

15. E. Georgiza, J. Novakovic, and P. Vassiliou, Characterization and corrosion resistanceof duplex electroless Ni–P composite coatings on magnesium alloy, Surf. Coat.Technol. 232, 432 (2013).

16. W. J. Cheong and D. W. Shoesmith, Protective coating on Mg AZ91D alloy – Theeffect of electroless nickel (EN) bath stabilizers on corrosion behaviour of Ni–Pdeposit, Corros. Sci. 49, 1777 (2007).

17. J E Gray and B Luan. Protective coatings on magnesium and its alloys–a criticalreview, J. Alloys Compd. 336, 88 (2002).

18. J. Sudagar, J. Lian, and W. Sha, Electroless nickel, alloy, composite and nanocoatings–A critical review, J. Alloys Compd. 571, 183 (2013).

19. H. Yang, X. Guo, X. Chen, and N. Birbilis, A homogenisation pre–treatment for ad-herent and corrosion–resistant Ni electroplated coatings on Mg–alloy AZ91, Corros.Sci. 79, 41 (2014).

20. Y. W. Song, E. H. Han, D. Y. Shan, C. D. Yim, and B. S. You, The role of sec-ond phases in the corrosion behavior of Mg–5Zn alloy, Corros. Sci. 60, 238(2012).

21. C. A. Huang, C. K. Lin, and Y. H. Yeh, Corrosion behavior of Cr/Cu–coated Mgalloy (AZ91D) in 0.1 M H2SO4 with different concentrations of NaCl, Corros. Sci.52, 1326 (2010).

22. Y. P. Zhu, G. Yu, B. N. Hu, X. P. Lei, H. B. Yi, and J. Zhang. Electrochemicalbehaviors of the magnesium alloy substrates in various pretreatment solutions, Appl.Surf. Sci. 256, 2988 (2010).

23. U. C. Nwaogu, C. Blawert, N. Scharnagl, W. Dietzel, and K. U. Kainer. Effects oforganic acid pickling on the corrosion resistance of magnesium alloy AZ31 sheet,Corros. Sci. 52, 2143 (2010).

24. N. El Mahallawy, A. Bakkar, M. Shoeib M, H. Palkowski, and V. Neubert, ElectrolessNi–P coating of different magnesium alloys, Surf. Coat. Technol. 202, 5151 (2008).

25. Z. Liu and W. Gao, Electroless nickel plating on AZ91 Mg alloy substrate, Surf. Coat.Technol. 200, 5087 (2006).

26. Z. Liu and W. Gao, The effect of substrate on the electroless nickel plating of Mgand Mg alloys, Surf. Coat. Technol. 200, 3553 (2006).

27. R. Ambat and W. Zhou, Electroless nickel–plating on AZ91D magnesium alloy:effect of substrate microstructure and plating parameters, Surf. Coat. Technol. 179,124 (2004).

28. W. X. Zhang, Z. H. Jiang, G. Y. Li, Q. Jiang, and J. S. Lian, Electroless Ni–P/Ni–Bduplex coatings for improving the hardness and the corrosion resistance of AZ91Dmagnesium alloy, Appl. Surf. Sci. 254, 4949 (2008).

29. A. N. Khramov, V. N. Balbyshev, L. S. Kasten, and R. A. Mantz, Sol–gel coatingswith phosphonate functionalities for surface modification of magnesium alloys, ThinSolid Films, 514, 174 (2006).

30. H. Zhang, S. Wang, G. Yao G, and Z. Hua, Electroless Ni–P plating on Mg–10Li–1Znalloy, J. Alloys Compd. 474, 306 (2009).

31. A. A. Zuleta, E. Correa, M. Sepulveda, L. Guerra, J. G. Castano, F. Echeverrıa,P. Skeldon, and G. E. Thompson, Effect of NH4HF2 on deposition of alkaline elec-troless Ni–P coatings as a chromium–free pre–treatment for magnesium, Corros. Sci.55, 194 (2012).

32. H. H. Elsentriecy and K. Azumi, Electroless Ni–P Deposition on AZ91D magnesiumalloy prepared by molybdate chemical conversion coatings, J. Electrochem. Soc. 156,D70 (2009).

33. L. Yang, J. Li, Y. Zheng Y, J. Wang, and M. Zhang, Electroless Ni–P plat-ing with molybdate pretreatment on Mg–8Li alloy, J. Alloys Compd. 467, 562(2009).

34. R. Petro and M. Schlesinger, Direct electroless deposition of low phosphorous Ni–Pfilms on AZ91D Mg alloy, J. Electrochem. Soc. 159, D455 (2012).

35. U. C. Nwaogu, C. Blawert, N. Scharnagl, W. Dietzel, and K. U. Kainer, Influence ofinorganic acid pickling on the corrosion resistance of magnesium alloy AZ31 sheet,Corros. Sci. 51, 2544 (2009).

36. T. N. Tran, G. Yu, B. N. Hu, Z. H. Xie, R. Tang, and X. Y. Zhang, Effects of pretreat-ments of magnesium alloys on direct electroless nickel plating, Transactions of theIMF, 90, 209 (2012).


http://dx.doi.org/10.1007/s11998-011-9336-z

http://dx.doi.org/10.1016/j.surfcoat.2014.01.022

http://dx.doi.org/10.1016/j.corsci.2007.02.006


http://dx.doi.org/10.1016/S0010-938X(99)00076-1








http://dx.doi.org/10.1016/j.apsusc.2011.01.016





http://dx.doi.org/10.1016/S0925-8388(01)01899-0

http://dx.doi.org/10.1016/j.jallcom.2013.03.107












http://dx.doi.org/10.1016/S0257-8972(03)00866-1


http://dx.doi.org/10.1016/j.tsf.2006.02.023

http://dx.doi.org/10.1016/j.tsf.2006.02.023



http://dx.doi.org/10.1149/1.3040243




http://dx.doi.org/10.1179/0020296712Z.00000000037

http://dx.doi.org/10.1179/0020296712Z.00000000037



37. B. N. Hu, R. X. Sun, G. Yu, L. S. Liu, Z. H. Xie, X. M. He, and X. Y. Zhang. Effectof bath pH and stabilizer on electroless nickel plating of magnesium alloys, Surf.Coat. Technol. 228, 84 (2013).

38. J. L. Chen, G. Yu, B. N. Hu, Z. Liu, L. Y. Ye, and Z. F. Wang, A zinc transition layerin electroless nickel plating, Surf. Coat. Technol. 201, 686 (2006).

39. X. P. Lei, G. Yu, X. L. Gao, L. Y. Ye, J. Zhang, and B. N. Hu, A study of chromium–free pickling process before electroless Ni–P plating on magnesium alloys, Surf.Coat. Technol. 205, 4058 (2011).

40. P. Volovitch, C. Allely, and K. Ogle, Understanding corrosion via corrosion productcharacterization: I. Case study of the role of Mg alloying in Zn–Mg coating on steel,Corros. Sci. 51, 1251 (2009).

41. M. C. Merino, A. Pardo, R. Arrabal, S. Merino, P. Casajus, and M. Mohedano,Influence of chloride ion concentration and temperature on the corrosion of Mg–Alalloys in salt fog, Corros. Sci. 52, 1696 (2010).

42. Y. W. Song, E. H. Han, K. K Dong, D. Y. Shan, C. D. Yim, and B. S. You, Microstruc-ture and protection characteristics of the naturally formed oxide films on Mg–xZnalloys, Corros. Sci., 72, 133 (2013).

43. E. Correa, A. A. Zuleta, L. Guerra, J. G. Castano, F. Echeverrıa, A. Baron-Wiechec,P. Skeldon, and G. E. Thompsonb, Formation of electroless Ni–B on bifluoride–activated magnesium and AZ91D alloy, J. Electrochem. Soc. 160, D327 (2013).

44. N. M. Martyak and J. E. McCaskie, Removal of orthophosphite ions from electrolessnickel plating baths: U. S. Pat. 6,048,585[P]. 2000–4–11.

45. H. H. Abdel–Rahman, A. A. Harfoush, and A. H. E. Moustafa. Effect of new diamineadditives on rate and qualities of nickel deposition, Transactions of the IMF, 91, 32(2013).

46. Y. H. Xiang, W. B. Hu, X. K. Liu, C. Zhao, and W. J. Ding, Initial deposition mecha-nism of electroless nickel plating on magnesium alloys, Transactions of the IMF, 79,30 (2001).

47. X. K. Liu, Z. L. Liu, P. Liu, Y. H Xiang, W. B. Hu, and W. J. Ding, Properties offluoride film and its effect on electroless nickel deposition on magnesium alloys,Trans. Nonferrous Met. Soc. China, 20, 2185 (2010).

48. C. Gu, J. Lian, G. Li, L. Niu, and Z. Jiang, Electroless Ni–P plating onAZ91D magnesium alloy from a sulfate solution, J. Alloys Compd. 391, 104(2005).

49. Y. Gao, C. Liu, S. Fu, J. Jin, X. Shu, and Y. Gao, Electroless nickel plating on ZM6(Mg–2.6 Nd–0.6 Zn–0.8 Zr) magnesium alloy substrate, Surf. Coat. Technol. 204,3629 (2010).

50. C. Gu, J. Lian, J. He, Z. Jiang, and Q. Jiang, High corrosion–resistance nanocrys-talline Ni coating on AZ91D magnesium alloy, Surf. Coat. Technol. 200, 5413,(2006).











http://dx.doi.org/10.1179/0020296712Z.00000000061

http://dx.doi.org/10.1016/S1003-6326(09)60440-4





Documents

Studies of Several Pickling and Activation Processes for Electroless Ni-P Plating on AZ31 Magnesium Alloy