Investigation Of Electroplating Nanocrystalline Zinc-nickel Alloy Coatings

Nanocrystalline zinc–nickel alloy coating on steel obtained from analkaline zincate bath containing a laboratory-made additive and Ni–P/Zn-Nicompound coating on the AZ91D magnesium alloy were investigated in thispaper.We use alkaline bath in order to get uniform nickel content Zn-Nicoating. The effects of the additive content and the cathode current density onthe grain sizes of the coatings were studied.SEM and TEM observations andXRD analysis were performed to examine the microstructure (grain size) andphase composition of the coatings. The experimental results indicated that theaddition of the additive leads to a much brighter and a more uniformnanocrystalline deposition with the grain sizes varying from 14 to 33 nm.The Hull cell test was used to determine the optimum ranges of both thecathode current density and the additive content. Salt spray tests andpolarization measurements indicated that the nanocrystalline zinc–nickelalloy coatings have a better corrosion resistance than the zinc coatingsdeposited from a simple alkaline zincate bath. Direct electroless Ni–P plating on the AZ91D magnesium alloy from aplating bath containing sulfate nickel,then electroplating Zn-Ni coating on theAZ91D magnesium alloy.The nucleation mechanism of Ni–P deposits on theAZ91D magnesium alloy was studied by using XRD and SEM. Theelectroless Ni–P deposits were preferentially nucleated on the β(Mg17Al12)phase and extended to the primary and eutectic α phases of the AZ91Dmagnesium alloy. The effect of the acid pickle treatment time of the AZ91Dmagnesium alloy substrate on the electroless Ni–P plating was alsoinvestigated.With prolonging the acid pickle treatment time, the cluster ofelectroless Ni–P deposition became smaller and the deposits were found to bemore compact. The deposition rate of electroless Ni–P plating wasproportional to the acid pickle treatment time. The hardness values of theNi–P coatings were about 660 VHN and were not influenced by thepretreatments.We investigated micro configuration and component of theNi-P/Zn-Ni compound coatings by SEM and XRD.We also test the adhesionability and anticorrosion property by adhesion test,salt spray test andpolarization curve. The adhesion test result indicate that the adhesion of theNi-P/Zn-Ni compound coatings is finer.The salt spray test result indicate thatthe anticorrosion property of the Ni-P/Zn-Ni compound coatings is excellent.The experiments and theory analysis in this study lead to the followingconclusions:(1) The grain size 14.6~19.8nm nanocrystalline Zinc-Nickel alloycoatings obtained from alkaline zincate bath in the test. The optimum bathcomposition of the bright nanocrystalline Zinc-Nickel alloy coatings was:Zinc oxide 12 g/l, Sodium hydroxide 120 g/l, Nickel ion 1.5 g/l, Heliotropin0.5 g/l, PEAA 30g/l and the additive 8ml/l.(2) The grain size reduces with the increasing of the cathode currentdensity and abruptly at low current density and tends to quite constant after8A dm-2. The grain size of crystals is linear with cathode potentialapproximately. The test results of the grain size of crystals and compositionof phase are same using TEM and XRD. The composition of phase is singleγ-phase (Ni5Zn21). The coating exhibited better corrosion resistance and finergrain size in contrast to Zn–Ni coating reported before. The brightnanocrystalline Zinc-Nickel alloy coatings has shown excellent corrosionresistance and less brittleness and in comparison with the zinc coatings. Theservice life of nanocrystalline Zn-Ni electrodeposits is longer, it provideimproved anticorrosion protection for steel in extremely aggressiveenvironments it is a suitable replacement for zinc. The excellent properties ofan electrodeposited nanocrystalline Zn–Ni coating make it a promisingmaterial for many industrial applications.(3) Direct electroless Ni-P plating on the AZ91D magnesium alloy froma plating bath containing sulfate nickel was first investigated in this paper.The effect of the acid pickle treatment time of the magnesium alloy substrateon the electroless Ni-P plating was also investigated. It was found that withprolonging the acid pickle treatment time, the electroless Ni-P depositsbecame more compact and defect free. The deposition rate of the electrolessNi-P plating was proportional to the acid pickle time. The hardness of theNi-P coating was not influenced by the pretreatments and was about 660VHN.(4) The nucleation mechanism of the electroless Ni-P plating on theAZ91D magnesium alloy was studied by using XRD and SEM. Thedeposition motivation at the initial deposition stage was attributed to theelectrochemically heterogeneous surface of the AZ91D magnesium alloy.From the XRD results and the determinations of the phosphorus contents inthe Ni-P deposits at different plating intervals, it can be seen that theelectroless Ni-P deposits had preferentially nucleated on the β (Mg17Al12)phase and then extended to other phases of the AZ91D magnesium alloy.(5) From the adhesion tests results and the salt spray test results,it can beseen that the adhesion and the corrosion of the Ni-P/Zn-Ni compoundcoatings is excellent.