Department of Mechanical Engineering,NIT Calicut,Kerala,India

Characterization of Ni-P coating on AZ91D magnesium alloy with surfactants and nano-additives

Mohammed Sahal

Department of Mechanical Engineering,NIT Calicut,Kerala,India

Direct electroless Ni-P plating was done on AZ91D magnesium alloy by immersing magnesium AZ91D samples into a bath containing Nickel sulphate.The nucleation mechanism of Ni-P deposits on the AZ91D magnesium alloy in the presence of surfactants and nano-additives was studied by using SEM.The electroless Ni-P deposits were preferentially nucleated on the βMg17Al12phase of AZ91D magnesium alloy. Ni-P coating was coated uniformly in the presence of surfactants.Effect of surfactant C-Tab with varying quantities was studied.Addition of surfactant C-Tab homogenized the Ni-P deposition on AZ91D magnesium alloy surface.The effect produced by surfactant C-Tab was maximum with minimum addition(1 g/l)of surfactant C-Tab further increase in the surfactant C-Tab quantity did not brought much changes in morphology.Effect of surfactant SLS was studied using SEM.Surfactant SLS when incorporated in small amounts(6 g/l and 12 g/l)only exerted a slight inf l uence in Ni-P deposition on AZ91D alloy surface.However Ni-P deposition was more uniform and spread throughout the surface with the addition of SLS surfactant(18 g/l).Effect of nano additives Al2O3,ZnO,SiO2were studied.Nano additive Al2O3enhanced the deposition of Ni-P on AZ91D alloy when added in 0.6 g/l quantity.SiO2addition also gave the same results.ZnO addition inf l uenced the Ni-P deposition on AZ91D alloy positively.Ni-P surface coating was coated more uniform and spread throughout the surface with the addition of surfactants and nano-additives.

Surfactants;Magnesium;Nano-additives;Scanning electron microscopy

1.Introduction

1.1.Importance of magnesium alloys

Today's interest in magnesium alloys for automotive applications is based on the combination of high strength properties and low density.For this reason magnesium alloys are very attractiveasstructuralmaterialsinallapplicationswhereweight savings are of great concern.In automotive applications weight reductionwillimprovetheperformanceofavehiclebyreducing the rolling resistance and energy of acceleration,thus reducing the fuel consumption and moreover a reduction of the greenhouse gas CO2can be achieved[1-4].

Magnesium has a long history in automotive applications. The decrease of magnesium use in automotive applications in the seventies was greatly related to its prize volatility and also to lack of knowledge.Stricter legislative rule(CAFE)and voluntary commitments to reduce the average fuel consumption have nowadays revived the interest in magnesium.

Materials selection is thereby determined by economical issues as much as by materials and component characteristics or properties.Increasing interest in light weight construction since the automobile industry's commitment to achieve a 25% reduction in average fuel consumption for all new cars by the year 2005(compared to levels in 1990)[5,6].Magnesium with its good strength to weight ratio is one of the candidatematerials to realize light weight construction,but it has to compete with various other materials.Magnesium shows high potential to substitute conventional materials.Magnesium alloys should be used in applications where low mass and high specif i c properties are required.According to the combination of specif i c Young's modulus and high specif i c Strength magnesium alloys show similar or even better values than aluminium and many commercial steels[7-9].Development of magnesium alloys with less volatility is of prime importance when magnesium is considered for an engineering application.The Ni-P deposition facilitates more usability of magnesium alloys especially for corrosion resistance applications[10].However the requirement of effective deposition of Ni-P coating on magnesium substrate is to be further enhanced with the incorporation of surfactants and nanoadditives.

1.2.Problems of magnesium alloys

Magnesium alloys have two major disadvantages for the use in automotive applications;they exhibit low high temperature strength and a relatively poor corrosion resistance. The major step for improving the corrosion resistance of magnesium alloys was the introduction of high purity alloys [11].Alloying can further improve the general corrosion behaviour,but it does not change galvanic corrosion problems if magnesium is in contact with another metal and an electrolyte[12].The galvanic corrosion problem can only be solved by proper coating systems[13].Our attempt is one on this path.The Ni-P coating provides effective corrosion resistance;however it is required to be coated in the most effective way.For that same the effect of surfactants and nanoadditives on Ni-P deposition on magnesium substrate was studied in this effort.

1.3.Electroless Ni-P coating

Electroless coating is one of the challenged processes for improvement of the coated surface.Specially,it is used for the improvement of mechanical properties such as wear,hardness properties of the coating surface along with enhanced corrosion resistance.In economic point of view electro deposition is an appropriate technique which is used in industries[14].As grain/particle size is of major concern,this type of composite coating should ideally be developed at lower temperature range/room temperature by the process of electro deposition. Again,electro deposition is simple process for operation and by which uniformly deposited on the heterogeneous surface. Electroless Ni-P coating provides an economical and technical solution for the surface problems of magnesium alloys.

2.Experimental

2.1.Preparation of specimen

The substrate material used was AZ91D die cast magnesium alloy with a size 30 mm × 40 mm × 5 mm was suppliedfrom China.The chemical composition of the alloy is given in Table 1.The samples were abraded with emery sheet upto 2000SiC paper before pre-treatmenting the specimen.

Table 1Chemical composition of the AZ91D die cast magnesium alloy.

2.2.Electroless Ni-P plating

The technical f l ow chart of electroless plating on AZ91D magnesium alloy was given in Fig.1.Initially the substrate undergoes alkaline cleaning,acid pickle treatment and f l uoride activation as pre-treatment processes to avoid easy corrosion when dipped into the plating bath.Direct electroless Ni-P plating was done in the bath containing the ingredients given in Fig.1.The electrolyte bath was heated indirectly through an electrically heated oil bath.The temperature of the oil bath was controlled by an ON/OFF relay and Proportional Integral Derivative(PID)controller.Temperature of the electrolyte bath was monitored using a thermometer.The pH of the electrolyte bath was maintained at 6 by adding sodium hydroxide solution. The total volume of the plating bath was 150 ml.The coating duration was carried out for about 2 h(Fig.2).

Fig.1.The technical f l ow chart of electroless Ni-P plating on the AZ91D magnesium alloy.

Fig.2.Ni-P coating on AZ91D alloy without surfactant and nano-additives.

2.3.Scanning electron microscopy

A scanning electron microscope(SEM)was used for studying the morphology of Ni-P alloy coatings.The signals that derive from electron sample interactions reveal information about the sample including external morphology(texture), chemical composition,and crystalline structure and orientation of materials making up the sample.

3.Results and discussions

3.1.Morphology of Ni-P coating with and without surfactant C-Tab

Ni-P coating was agglomerated over βMg17Al12phase without the usage of surfactants.There is no visible difference among the two quantities of C-Tab used.Ni-P coating was evenly distributed overpreferentially nucleated on the βMg17Al12phase due to the usage of surfactants inf l uencing Ni-P Deposition over the βMg17Al12phase.The usage of CTab facilitated better distribution and deposition of Ni-P coating on AZ91D magnesium alloy.The minimal amount of surfactant was enough to produce this result(Figs.3 and 4).

Fig.3.Ni-P coating on AZ91D alloy with surfactant C-Tab(1 g/l).

Fig.4.Ni-P coating on AZ91D alloy with surfactant C-Tab(2 g/l).

3.2.Morphology of Ni-P coating with surfactant SLS (sodium lauryl sulphate)

Addition of SLS surfactant in amounts 6 g/l and 12 g/l was less inf l uential in facilitating Ni-P coating to be distributed uniformly as shown in Figs.5 and 6.However addition of SLS in more amount(18 g/l)improved the distribution Ni-P coating over AZ91D magnesium alloy as shown in Fig.7.

3.3.Morphology of Ni-P coating on AZ91D magnesium alloy with nano additive Al2O3

There was a visible difference between two SEM micrographs containing different amount of quantities of nano additives.Al2O3nano additive (0.5 g/l) indicates the agglomeration of the Al2O3particles over the Ni-P matrix which is distributed uniformly over the βMg17Al12phase.

Al2O3nano additive(0.6 g/l)indicates the uniform distribution of the Al2O3particles over the Ni-P matrix which is distributed uniformly over the βMg17Al12phase.

Effectiveness of nano-additive Al2O3is increased when added as 0.6 g/l.It showed better distribution of Ni-P coating on AZ91D magnesium alloy with minimal agglomeration.

Fig.5.Ni-P coating on AZ91D alloy with surfactant SLS(6 g/l).

Fig.6.Ni-P coating on AZ91D alloy with surfactant C-Tab(12 g/l).

Fig.7.Ni-P coating on AZ91D alloy with surfactant C-Tab(18 g/l).

Fig.8.Ni-P coating on AZ91D alloy with surfactant nano-additive Al2O3(0.5 g/l).

Fig.9.Ni-P coating on AZ91D alloy with surfactant nano-additive Al2O3(0.6 g/l).

The SEM micrographs of Ni-P coating with Al2O3shown in Figs.8 and 9.

3.4.Morphology of Ni-P coating on AZ91D magnesium alloy with nano additive SiO2

There is no visible difference between two SEM micrographs containing different amount of quantities of nano additives SiO2indicates the uniform distribution of the SiO2particles over the Ni-P matrix which is distributed uniformly over the βMg17Al12phase.Addition of SiO2brought similar results as Al2O3(0.6 g/l).SiO2nano additive(0.6 g/l)indicates the uniform distribution of the SiO2particles over the Ni-P matrix which is distributed uniformly over the βMg17Al12phase.The SEM micrographs of Ni-P coating on AZ91D alloy with the addition of nano-additive SiO2was given inFigs.10 and 11.There was less variation of Ni-P deposition pattern with the variation of quantity of SiO2.

Fig.10.Ni-P coating on AZ91D alloy with surfactant nano-additive SiO2(0.5 g/l).

Fig.11.Ni-P coating on AZ91D alloy with nano-additive SiO2(0.6 g/l).

3.5.Morphology of Ni-P coating on AZ91D magnesium alloy with nano additive ZnO2

There was a visible difference between two SEM micrographs containing different amount of quantities of nano additives ZnO2nano additive (0.5 g/l) indicates the agglomeration of the ZnO2particles over the Ni-P matrix which was distributed uniformly over the βMg17Al12phase ZnO2nano additive(0.6 g/l)indicates the uniform distribution of the ZnO2particles over the Ni-P matrix which is distributed uniformly over the βMg17Al12phase.It could be inferred that the quantity if nano additive used has a profound effect on its microstructure(Figs.12 and 13).

Fig.12.Ni-P coating on AZ91D alloy with surfactant nano-additive ZnO2(0.5 g/l).

Fig.13.Ni-P coating on AZ91D alloy with surfactant nano-additive ZnO2(0.6 g/l).

Electroless Ni-P coating on AZ91D was diff i cult without the aid of surfactant and nano-additives.Ni-P coating was agglomerated on AZ91D magnesium alloy without the addition of surfactants and nano additives.

4.Conclusion

Electroless Ni-P coating on AZ91D magnesium alloy was successfullydoneinthepresenceofsurfactantsC-Tab,SLSand nano-additives alumina,silicon dioxide,Zinc oxide.Results obtainedwerevaryingforeachmodule.Theuseofsurfactantsin providingbetter Ni-PdistributiononAZ91D magnesium alloy was demonstrated by Figs.3 and 6.There was a general improvement in the distribution of Ni-P coating with the addition of surfactants.However better distribution was achievedwithminimalamountforC-Tab(1g/l)anditrequired18g/ l for SLS.Pre-treatment process holds severe importance in avoiding galvanic corrosion.All Ni-P coatings was done on magnesium substratewhich was pretreated initially.The effects broughtoutsurfactantscouldbeattributedtoitsabilitytoreduce the surface tension between the particles which def i nitely avoided the possibility of agglomeration.Addition of nanoadditives also inf l uenced the morphological distribution of the Ni-P coatings.All the three nano-additives used facilitated better distribution of Ni-P coating on AZ91D magnesium substrate.Itactedascatalystfortheelectrolessreactiontooccur. Amongthethreemostuniformmorphologicaldistributionswas provided by SiO2.It is to be noted that an amount of 0.5 g/l of SiO2wasabletobringoutuniformdistributionofNi-Pcoating onmagnesiumsubstrate.Itwasobservedthat0.6g/lofAlumina and Zinc oxidewere able to facilitate more uniform distribution of Ni-P coating on magnesium substrate.The effect demonstrated by the presence of nano-additives could be attributed to the ability of nano-additives to create more wider reaction sites on the magnesium substrate in turn facilitating uniform deposition of coating on the substrate.It was understood that surfactants and nano-additives were able to provide uniformdistribution of Ni-P coating which increases the effectiveness of the coating in engineering applications requiring corrosion and wear resistance.

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Received 13 June 2014;revised 17 September 2014;accepted 9 October 2014 Available online 29 November 2014

E-mail address:mohammedsahal160@gmail.com.

Peer review under responsibility of National Engineering Research Center for Magnesium Alloys of China,Chongqing University.

http://dx.doi.org/10.1016/j.jma.2014.10.003.

2213-9567/Copyright 2014,National Engineering Research Center for Magnesium Alloys of China,Chongqing University.Production and hosting by Elsevier B.V.All rights reserved.

Copyright 2014,National Engineering Research Center for Magnesium Alloys of China,Chongqing University.Production and hosting by Elsevier B.V.All rights reserved.