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The electroless nickel-plating on magnesium alloy using NiSO4d6H2Oas the main saltJianzhong Lia,*, Zhongcai Shaob, Xin Zhanga, Yanwen TianaaSchool of materials and metallurgy, Northeastern University, Shenyang 110004, ChinabFaculty of Environment and Chemical Engineering, Shenyang Institute of Technology, Shenyang 110168, ChinaReceived 23 July 2004; accepted in revised form 19 December 2004Available online 26 January 2005AbstractIn this paper, the electroless nickel-plating on magnesium alloy was studied, using NiSO4d 6H2O as the main salt in the electroless platingalkaline solutions. The effects of the buffer agent and plating parameters on the properties and structures of the plating coatings onmagnesium alloy were investigated by means of scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD). In addition, the weight loss/gain of the specimens immersed in the test solution and plating bath was measured byusing the electro-balance, to evaluate the erosion of the alloy in the plating solutions. The adhesion between the electroless plating coatingsand the substrates was also evaluated. The compositions of the non-fluoride and environmentally friendly plating bath were optimizedthrough Latin orthogonal experiment. The buffer agent (Na2CO3) added to the plating bath was found to be useful in increasing the growthrate of the plating coating, adjusting the adhesion between the electroless plating coatings and the substrates, and maintaining the pH valuewithin the range of 8.511.5, which is required for the successful electroless nickel-plating on magnesium alloy with NiSO4d 6H2O as themain salt. Trisodium citrate dihydrate was found to be an essential component of the plating bath to plate magnesium alloy, with an optimumconcentration of 30 g L?1. The obtained plating coatings are crystalline with preferential orientation of (111), having advantages such as low-phosphorus content, high density, low-porosity, good corrosion resistance and strengthened adhesion.D 2004 Elsevier B.V. All rights reserved.Keywords: Magnesium alloy; Electroless plating; Buffer; Corrosion resistance; Adhesion1. IntroductionThe use of magnesium alloys in a variety of applications,particularly in aerospace, automobiles, and mechanical andelectronic components, has increased steadily in recent yearsas magnesium alloys exhibit an attractive combination oflow density, high strength-to-weight ratio, excellent cast-ability, and good mechanical and damping characteristics.However, magnesium is intrinsically highly reactive and itsalloys usually have relatively poor corrosion resistance,which is actually one of the main obstacles to theapplication of magnesium alloys in practical environments13.Hence, the application of a surface engineering techniqueis the most appropriate method to further enhance thecorrosion resistance. Among the various surface engineeringtechniques that are available for this purpose, coating byelectroless nickel is of special interest especially in theelectronic industry, due to the possession of a combinationof properties, such as good corrosion and wear resistance,deposit uniformity, electrical and thermal conductivity, andsolderability etc. As far as magnesium alloys are concerned,the main salts of electroless plating solutions mostly focusattentions on basic nickel carbonate or nickel acetate 49,which result in high-cost, low-efficiency, instability ofelectroless plating solutions and little applications. Inaddition, the basic nickel carbonate or nickel acetate ofplating solutions yet including fluoride, are harmful to theenvironment, therefore, it is urgently needed to develop newenvironmentally friendly plating bath. It is difficult to carry0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved.doi:10.1016/j.surfcoat.2004.12.009* Corresponding author. Tel.: +86 24 8368 7731; fax: +86 24 2398 1731.E-mail address: (J. Li).Surface & Coatings Technology 200 (2006) 30103015/locate/surfcoatout electroless plating on magnesium alloys due to the high-corrosion rate of magnesium alloys in the plating bath withNiSO4d 6H2O or NiCl2d 6H2O as the main salt. It is reported10 that the corrosion rate of magnesium and its alloys inNaCl solutions solely depends on the pH of the bufferedchloride solutions. The objective of this study was to find abuffer agent and determine how the buffer agent affects thedissolution of magnesium alloy in NiSO4d 6H2O alkalinesolutions, and the non-fluoride plating solutions for magne-sium alloy with NiSO4d 6H2O as the main salt. Themicrostructure, compositions and corrosion behavior ofthe coatings were investigated in detail.2. ExperimentalThe substrate material used in the research was AZ91Dingot-cast alloy. The chemical composition of the alloy isgiven in Table 1.Substrates with a size of 50 mm?40 mm?20 mm wereused in the research. The substrates were mechanicallypolished with emery papers up to 1000 grit to ensure similarsurface roughness. The polished substrates were thoroughlywashed with distilled water before passing through the pre-cleaning procedure as shown in Table 2.The substrates were air-dried after the fluoride activation(the last step in the pre-cleaning procedure). In a typicalexperiment, the initial weight of a air-dried substrate wasmeasured and then quickly transferred to the plating bath(1000 mL) in a glass container placed in a water bath with aconstanttemperatureof808C.Afreshbathwas usedforeachexperiment to avoid any change in concentration of bathspecies. The bath compositions and other parameters used inthese experiments are given through Latin orthogonalexperiment in Table 3.Final weights of the specimens were determined and thecoating rates in micrometer per hour were calculated fromthe weight gain. At the same time, in order to study the eachbuffers influence on the substrates and find a bufferappropriate for the electroless plating on magnesium alloy,test solutions with compositions similar to those of theplating bath except that sodium hypophosphite was notadded, were prepared to simulate the corrosion rates ofmagnesium alloy in plating bath and the behaviors of thebuffers. Duplicate experiments were conducted in each case,and the coating rate reported is the average of twoexperiments. The growth rates of the plating coating weremeasured using the electro-balance made in America, whichis the 0.1 mg precision. In the research, the pH value ofplating bath was monitored by a pHS-25C model ofprecision pH/mV meter. Morphology of the coatings wasanalyzed using a scanning electron microscope. The energydispersive X-ray spectroscopy analysis was used fordetermining the content of phosphorus in the coatings.Crystallinity of the coatings was investigated by Rigaku D/max-rA X-ray diffractometer with Cu K-alpha radiation.The adhesion strength of the electrolessly deposited nickelcoatings to the magnesium alloy substrates was determinedby scratch test. During the scratch test, the specimen wasmoved at a constant speed of approximately 11.4 mm/min.Scratches were generated on the specimen using a diamondindenter with a spherical tip of 300 Am in diameter.Corrosion potential measurement in 3.5 wt.% NaCl solutionwas carried out to comparatively investigate the corrosionbehaviors of the bare substrate and the nickel-platedsubstrates. The electrochemical cell used for corrosionpotential measurement consisted of a bare substrate or anickel-plated substrate as the working electrode (exposedarea: 1 cm2), a saturated calomel electrode (SCE), and aplatinum-foil counter electrode.Table 1Chemical composition of the AZ91D alloy (in wt.%)AlMnNiCuZnCaSiKFeMg010.0010.64b0.01b0.01b0.01b0.001BalTable 2Optimized pre-cleaning procedureTable 3Optimized bath composition and parametersBath species and parametersQuantityNiSO4d 6H2O25 g/LNaH2PO2d H2O30 g/LC6H5Na3O7d 2H2O30 g/LNa2CO330 g/LNH3d H2OAdjusting pHpH value11Temperature80F2 8CJ. Li et al. / Surface & Coatings Technology 200 (2006) 3010301530113. Results and discussion3.1. The buffers behaviors in the test NiSO4solutions andthe choice of an appropriate bufferFig. 1 shows the variation of weight loss of magnesiumalloy as a function of the immersion time with differentbuffers in the test solutions. The compositions and thecontrolled temperature of the test solutions were similar tothose of the plating bath except that sodium hypophosphitewas not included. The pH values of the test solutions wereadjusted by NH3d H2O to fix at 11. The weight loss increaseslinearly with the immersion time increasing of magnesiumalloys in the Na2CO3, Na2B4O7, and CH3COONa testsolutions. It is revealed in Fig. 1 that the corrosion rateswere constant throughout the examined immersion time.As recognized from the slope of each solid line in Fig. 1,corrosion rate in the test solution containing Na2CO3buffer is the lowest among the three tested buffers. Theobtained slopes are 0.015, 0.022 and 0.056 mg cm?2min?1for Na2CO3, Na2B4O7and CH3COONa buffers,respectively. These results can be explained in terms ofdissociation constants of the corresponding acids, whicharek2=4.7?10?11(k1=4.4?10?7),k2=1?10?9(k1=1?10?4), and k=1.75?10?5for H2CO3, H2B4O7andCH3COOH, respectively. The second dissociation constantof a binary acid decides the buffer capability of the buffer.Obviously, the Na2CO3buffer has the lowest cost and bestbuffer capability among the tested buffers.Fig. 2 shows the weight loss of the substrates versusimmersion time in the test solutions with pH values at 9, 10and 11, using Na2CO3as the buffer. Corrosion of thespecimens in non-buffered test solutions with pH values at9, 10 and 11 was also investigated. The correspondingweight loss curves are shown in Fig. 2. All test solutionsused for these experiments had compositions similar tothose in the plating bath except that sodium hypophosphitewas not included. The weight loss linearly changes with theincrease of the immersion time in all cases shown in Fig. 2.Under the same pH value, the corrosion rate of thesubstrates in the buffer solution is obviously lower thanthat of the substrates in the non-buffered solution, as shownby the slopes of the curves in Fig. 2. This suggests that thebuffer solution has a considerable effect on the corrosionrate of magnesium alloy. In both the Na2CO3buffered andnon-buffered test solutions, the corrosion rates of magne-sium alloy decrease with the increase of the pH value. Thisindicates the weight-loss of the substrates is related to thereaction between the substrate metal and the hydrogen ions.But the corrosion reaction between the substrate metal andthe hydrogen ions goes gradually on, because the lowconcentration of hydrogen ions is presented in the platingalkaline solutions. And then, the concentration of hydrogenions is weakly decreased during the test progress. This leadsto the constant corrosion rates in the short test time, which isshown in Figs. 1 and 2. At the same time, knowing that forMg(OH)2Ksp at 25 8C=8.9?10?12at pH 9, OH?=10?5M,most Mg2+diffused into plating solution to form up to 10?2M. At pH 11, OH?=10?3M, the Mg2+ couldnt exceed10?6M, thus most Mg2+formed Mg(OH)2and stayed nearthe substrate. Mg(OH)2could increase the adsorptionenergy barrier and reduce the corrosion rate. Therefore,higher pH resulted in lower corrosion rate. As to theNa2CO3buffered solutions, for MgCO3Ksp at 25 8C=10?15,in test solutions, Na2CO3N0.1 M, thus the possibleMg2+b10?14M. This means that the driving force forMg to form Mg2+was very low. Instead of dissolving Mg,the CO32?ion would bond or be adsorbed to the substratesurface to form local MgCO32?. In this case, the substratesurface area exposed to H2O or H+was reduced a lot,05101520253035-0.20.00.81.01.8 Na(CH3COO) Na2B4O7Na2CO3Weight loss/mg.cm-2Time/minFig. 1. The variation of weight loss of magnesium alloy in test solutionswith different buffers.05101520253035012345solution pH=9 pH=10 pH=11 pH=9 pH=10 pH=11Weight loss/mg.cm-2Time/minin non-buffered solutionin Na2CO3 bufferedFig. 2. The variation of weight loss of magnesium alloy in test solutionswith different pH values.J. Li et al. / Surface & Coatings Technology 200 (2006) 301030153012leading to lower corrosion rates. The pKa2for Na2CO3is10.33, at pH lower than 10.33 some CO32?ions formedHCO3?. Reaction Mg+2HCO3?=MgCO3+H2potentiallyexisted. At pH higher than 10.33, HCO3? is negligible.Therefore in Fig. 2, we can see that the corrosion rate at pH11 was not reduced as much, compared the rate at 10.H2B4O7and CH3COOH dont have such advantages.3.2. The effects of plating parameters on coatingsThe coating rate, surface appearance, and adhesion of thecoatings at different concentrations of Na2CO3buffer arelisted in Table 4. The critical load (LC) was measured underprogressive loading conditions, which can be used toaccurately characterize the adhesion strength of the deposit/substratesystem13.Theadhesionbetweenthecoatingsandsubstrates decreases obviously with the increase of theconcentration of Na2CO3. Surface appearance of the platingcoatings becomes gradually shining with the increase of theNa2CO3concentration. Grave corrosion of the substrates wasfoundinthenon-bufferedplatingbath.Thegrowth rate ofthecoatingsnoticeablyincreaseswiththeincreaseoftheNa2CO3concentration. Considering the combination of growth rate,surface appearance, and adhesion of the coatings, theoptimumconcentrationoftheNa2CO3bufferwasdeterminedtobe30gL?1.Withthisconcentration,thepurposeofaddingNa2CO3in plating bath is commendably achieved.In the research, it was found that the pH value of platingbath had a considerable effect on the growth rate and thesurface appearance of the coatings. The hydrogen ions inplating bath were not only astricted by the CO32?ionsdissociated from thebufferNa2CO3,butlinkedwiththeOH?ions. When the pH value of the plating bath was below 8.5,point corrosion or dark gray coatings were obtained and thecoating growth rate was low. When the pH value of theplating bath was above 11.5, the adhesion between coatingsand substrates were deteriorated, although the growth rateand the surface appearance of the coatings were satisfying.Duringtheelectrolessplating,thepHvalueoftheplatingbathwas monitored with a pHS-25C model of precision pH/mVmeter. In this research, the preferred pH range of the platingbath for electroless plating on magnesium alloy is 8.511.5.Table 4Coating rate, surface appearance and adhesion of the coatings obtainedfrom the plating bath with different amounts of Na2CO3Concentration ofNa2CO3(g L?1)Coating rate(Am/h)Surface appearanceLC(N)0Grave corrosion1012.32Point corrosion812016.41Dark gray763018.32Shining734018.91Shining615019.26Shining512030405060701314151617181920The coating thickness/mThe trisodium citrate dihydrate content/g.L-1Fig. 3. Relationship between the coating thickness and the trisodium citratedihydrate concentration.3040506010002000300040005000600070008000Intensity2 /( )Fig. 4. XRD patterns of the electroless plating coating.Fig. 5. Surface morphology of a plating coating.J. Li et al. / Surface & Coatings Technology 200 (2006) 301030153013Fig. 3 shows the variation of coating thickness onmagnesium alloy at same plating time as a function of thetrisodium citrate dihydrate concentration at constant temper-ature and pH. The coating thickness decreases with theincrease of the trisodium citrate dihydrate concentration.According to De Minjer and Brenners explanation 11, atlow concentrations the low adsorption of ligand on thecatalytic surface of the substrate accelerates the platingreaction. At higher concentration, there is a high adsorptionof ligand on the surface, which slows down the platingreaction. But when the concentration was below 20 g L?1,the plating bath became destabilized and nickel precipitatewas observed.3.3. Properties of the plating coatings from nickel sulfateThe coating obtained under optimized bath compositionwasprobablypreferentiallycrystallized(seeFig.4).Theonlyand strong diffraction observed in the XRD spectrumcorresponds to the (111) peak of nickel. Fig. 5 shows thesurface morphology of the plating coating. The surface isoptically smooth and of low porosity. No obvious surfacedamage was observed. The compositions of the platingcoating were determined to be 5.39 wt.% P and 94.61 wt.%Ni byenergy dispersive X-ray spectroscopy. Fig. 6 shows thecross section of an electroless plating coating. The coatinghas a good adhesion to the substrate and no cracks or holeswere observed.Fig. 7 shows the curve of the NiP coating free corrosionpotential with time. After the sample was immersed in 3.5wt.% NaCl solution at room temperature for 2 h, the freecorrosion potential of the coated magnesium alloyapproached to about ?0.4 V. The steady-state workingpotential of magnesium electrode is generally about ?1.50V, although its standard potential is ?2.43 V 14. Thisindicates the improved corrosion resistance of the platingcoatings prepared in this research, compared with the barealloy.The adhesion between the coatings and the substrateswas evaluated by means of quenching and the scratch test.The plated specimens were heated at a temperature of 2508CF10 8C for 1 h, and then quenched in the cold water. Thisprocess was repeated for 20 times on each specimen. Nodiscoloration, cracks, blisters, or peeling was observed 12.For the scratch test, the critical load (LC) of 73 N was foundfor the coatings obtained in the optimized bath compositionand parameters. These results suggest the excellent adhesionof the plating coating to the substrate.3.4. Proposed mechanism of the electroless plating nickelEven under the same pH value, the magnesium alloyexhibits better corrosion resistance in the Na2CO3bufferedplating solution than in the non-buffered plating solution.Fig. 6. Cross section view of an electroless plating coating.012345678-0.46-0.44-0.42-0.40-0.38-0.36-0.34-0.32-0.30ESCE/V 103, time/sFig. 7. Curve of the NiP coating free corrosion potential with time.J. Li et al. / Surface & Coatings Technology 200 (2006) 301030153014Fig. 8 gives a simple model to explain this phenomenon.Large amount of H2gas is produced in the electrolessplating process. Most of the H+ions are taken out by the H2gas bubbles and combine with the CO32?, to form HCO3?.Therefore, a very thin layer of dilute H+solution is formednear the surface of substrate. The Ni2+ions react with themagnesium atoms to form the autocatalysis nickel, whichleads to the deposition of the NiP coating. If theconcentration of the CO32?ions is low, more H+ions willbe free and erode the thin NiP coating and the substrate. Ifthe concentration of the CO32?ions is much higher, the H+ions concentration in the thin dilute H+solution layer nearthe substrate surface will be much lower. Therefore almostno corrosion process will exist in the interface between theNiP coating and the substrate, which will lead to thedecrease of the cohesion of the coating to the substrate. Atthe same time, a certain pH value range in the plating bath isnecessary to ensure the electroless plating process tocontinue. So the optimized buffer is important to theelectroless plating on the magnesium alloy.4. Conclusions1.The electroless nickel-plating on AZ91D magnesiumalloy was successfully carried out in an environmentallyfriendly and non-fluoride plating bath with NiSO4d6H2O as the mai