makethebedSelective area deposition of Tin–Nickel alloy coating–an alternative for decorative chromium plating
B.SUBRAMANIAN*,S.MOHAN and SOBHA JAYAKRISHNAN
Central Electrochemical Rearch Institute(CECRI),Karaikudi,630006,Tamil Nadu,India
(*author for correspondence,tel.:+91-4565-227555,fax:+91-4565-227713,e-mail:s.in) Received12April2006;accepted in revid form6September2006
Key words:corrosion resistance,lective area deposition,Sn–Ni alloy
Abstract
Sn–Ni alloy coatings on mild steel substrates produced by lective area deposition process with layer thickness of about14l m were investigated with regard to the structural and corrosion properties.X-ray diffraction analysis revealed that the lective area plated Sn–Ni alloy was heterogeneous and compod of NiSn,Ni3Sn2and Ni3Sn4 phas.Uniform surface coverage of the substrate by granular morphology was obrved from SEM and AFM.The alloy composition was determined by X-rayfluorescence(XRF).The corrosion protection performance of Sn–Ni alloy on mild steel was assd using salt-water immersion and electrochemical corrosion tests.A sharp decreas
e in I corr and high charge transfer resistance indicated improved corrosion resistant behavior of the lective area deposited Sn–Ni alloy.
1.Introduction
Electrodeposited chromium has high hardness,wear and corrosion resistance and reflectivity and has been applied to different industrial products such as auto-mobiles[1,2].However,chromium(VI)electroplating process produce large volumes of chromium-contam-inated toxic waste,air pollution and water contamina-tion which can result in significant induction of cytogenetic damage in electroplating workers[3]and genotoxic effects associated with occupational exposure [4,5]to chromium.
Tin–nickel alloy is resistant to corrosion,resists tarnishing and retains its brightness.It has good contact and wear resistance.The Sn–Ni plating solution has a deep throw so that where there is a problem in plating chromium in deep recess,tin–nickel alloy is an alternative.Tin–nickel alloy also has excellent frictional resistance and has the ability to retain an oilfilm on its surface.Becau of the properties the alloy has found applications in engineeringfields such as automotive breaking systems,heavy-duty switch gears and mixing valves.From this viewpoint,Sn–Ni plating is a good sub
stitute for chromium plating[6].Wear resistance of printed circuit boards was found to be superior when tin–nickel coating was included between copper and gold deposits[7].
Selective area deposition,also known as brush-plat-ing,is a portable process for accurately applying plated deposits onto localized areas.It differs from traditional tank or bath plating in that the work piece is not immerd in a plating solution(electrolyte).Instead,the electrolyte is brought to the part to be plated and applied by a hand held anode or stylus,which incorpo-rates an absorbent wrapping for applying the solution to the work piece(cathode).A DC power pack drives the electrochemical solution,depositing the desired material on the substrate.A schematic of the brush-plating process is shown in Figure1.It offers portability,flexibility and high-quality deposits.The prent work deals with the brush-plating of tin–nickel alloys on steel substrates and to study the influences of solution composition and operating parameters.The morphol-ogy,elemental composition and corrosion resistance of the alloy coating were also determined.
2.Experimental
The alloy coatings were brush-plated on mild steel(MS) substrates.The composition of the low carbon steel substrate ud is shown in Table1.In this work commercially available brush-plating equ
ipment,Selec-tron power pack,USA,Model150A-40V was utilized. The bath composition and bath parameters ud for the alloy deposition are shown in Table2.Solutions were prepared from reagent grade chemicals and distilled water.Mild steel specimens,5Â2.5Â0.5cm,were
水土流失的英文Journal of Applied Electrochemistry(2007)37:219–224ÓSpringer2006 DOI10.1007/s10800-006-9236-6
polished mechanically and degread with acetone. Following ultrasonic cleaning in acetone and washing in DI water the specimens were ud for brush-plating. The deposit thickness was determined using a Mitutyo profilometer.A region of the mild steel substrate was masked before brush-plating.A stainless steel stylus with a diamond tip was drawn across the step from the substrate to the coating and both the vertical and horizontal motion of the stylus was amplified and recorded.The composition of the deposit was analyzed by HORIBA X ray analytical Microscope XGT2000 with a Rh source at20KeV.Structural characterization of the deposit was carried out by XRD using an XÕpert pro Philips X-ray diffractometer.Differential Thermal Analysis(DTA)measurements were carried out by STA 1000/15000at a scanning rate of20°min)1.Surface morphological examinations were carried out using a Hitachi S3000H Scanning Electron Microscope(SEM) and Molecular Imaging Atomic Force Microscope (AFM).The micro-hardn
ess of the brush-plated samples was determined using a Micro hardness Testing Machine Leco DM400with a Vickers indenter and a load of25g.The corrosion resistance of the deposit was assd by electrochemical polarization studies and AC Impedance measurement using a BAS IM6Electro-chemical analyzer.Experiments were carried out using the standard three-electrode configuration with satu-rated calomel as a reference electrode,a platinum foil as counter electrode and the sample as working electrode. The specimen(1.0cm2expod area)was immerd in the test solution of3.5%w/v NaCl.Experiments were carried out at room temperature(28°C).
3.Results and discussion
The brush-plated alloy samples were bent through an angle of180°repeatedly as required by BS5411 standards and no lifting and peeling was found which indicated good adhesion of the coatings to the mild steel substrates.Deposits with a Vickers hardness of 620and540HV(25g)were obtained for the as prepared and annealed Sn–Ni alloy which are compa-rable to that of chromium.The decrea in hardness for the annealed sample may be due to the increa in grain size obrved for thefilms from XRD and SEM.A thickness of14.2l m was obrved from Profilometric roughness and thickness measurement. This is in good agreement with the value of14l m obtained from the weight difference method.Figure2 shows the DTA curve of the brush-plated Sn–
Ni alloy. The ba line was found to decrea with increasing temperature.The formation of NiSn and the stable compounds thereafter are presumed to occur up to about600°C.The endothermic peak at about740°C can be attributed to the melting of Ni3Sn4.It is evident that nofixed melting temperature existed for the alloy. The fusing temperature was neither the melting point of tin nor of nickel.Figure3shows the XRF spectrum of the sample.The L b1and L b2peaks for the Sn and K a and K b peaks appearing for Ni indicated that the Sn rays are less energetic than Ni.The chemical analysis of Figure3gives the composition as65tin and35 nickel in weight percentage.
3.1.XRD Analysis
The X-ray diffractogram(XRD)for the brush-plated tin–nickel alloy as plated from the bath under optimized
Table1.Specified composition of carbon steel substrate
%C%Mn%S%P%Fe 0.0630.230.030.011Balance
Table2.Optimized bath parameters for the brush-plating of Sn–Ni alloy
SnCl2Æ2H2O50g l)1
NiCl2Æ6H2O250g l)1
NH4HF240g l)1
NH3(s.g–0.88)10ml
PH 2.5
Bath temperature28°C(RT) Plating voltage2V
Duration30min
Anode Graphite
橡皮的英语单词stylus
conditions is shown in Figure4(a).Table3correlates the measured results in Figure4(a,b)with the standard peaks from the JCPDS index cards.The data shows the obrved interplanar distance,dÕvalues are in good agreement with the standard,dÕvalues of the corre-sponding phas.The prence of NiSn compound was confirmed although this pha is not found in the tin–nickel pha diagram(Figure5)[8,9].The layer is thermodynamically unstable.
Figure4(b)shows the XRD spectrum for the speci-men heated to400°C and Table3shows the list of obrved peaks and the JCPDS index.The annealing temperature was above the melting point of tin and much lower than that of nickel.The higher temperature enhances tin and nickel diffusion which,in turn,allows thermodynamically stable compounds to form.More stable compounds,Ni3Sn4and Ni3Sn2were formed.The structure and lattice parameters for the phas are indicated in Table4.The crystallite size(D)of thefilms was calculated from the ScherrerÕs formula from the full width at half-maximum(b)of the peaks expresd in radians
december缩写>ertl
D¼0:94k=b cos hð1Þwhere b is the FWHM calculated from the(421)plane. The crystallite size was calculated from line broadening, under the simple assumption that the line broadening is caud by the crystallite size alone[10].The average crystallite sizes were found to be0.7(601)and0.3(421) l m for the as prepared NiSn layers and1.4(601)and1.5 (421)l m for the annealed NiSn layers on mild ste
el substrates.The increa in crystallite size may be due to agglomeration of grains during annealing.
3.2.Surface morphology
Examination of the surface morphology by SEM showed that the as prepared and annealed Sn–Ni alloy brush-plated at the plating voltage of2V(Figure6a,b), were compact and consisted offine grains covering the whole substrate surface.
The average size of the grains was determined to be 1.0and1.5l m by CottrellÕs method[11]from SEM for the as prepared and annealed samples,respectively.The deposits were found to be microcraked and it is known that such cracks form during brush-plating when the tensile stress exceeds the cohesive strength of the deposit.
Surface characterization of the brush-plated Sn–Ni alloy sample was carried out using atomic force micros-copy(AFM).The advantage of AFM is its capacity to probe minute details related to the individual grains and inter-grain regions.A reprentative AFM picture scanned over an area of1Â1l m of the annealed Sn–Ni alloy sample prepared under optimized conditions is shown in Figure7.The deposit consists of many small spherical particles which are characteristic of brush-plated Sn–Ni coa
tings.
Table3.Comparison of obrved inter-planar distances and standard inter-planar distances of XRD pattern of brush-plated Sn–Ni alloy(an-nealed)
d–obrved d–standard h k l Relative intensity/%Pha 2.884 2.886(601)100NiSn 2.843 2.840(310)51.6Ni3Sn4 2.647 2.649(511)21.6NiSn 2.094 2.076(110)51.9Ni3Sn2 2.058 2.047(421)46.6NiSn 2.013 2.033(112)48.9Ni3Sn
4
团圆饭英语
Table4.Crystallite size and lattice parameters of Sn–Ni alloy Pha JCPDS Card No.Lattice parameters
a/A b/A c/A NiSn26-128924.54 5.400 4.051 Ni3Sn404-084612.22 4.062 5.167 Ni3Sn208-0430 4.14– 5.106
221
3.3.Potentiodynamic polarization studies
The potentiodynamic polarization curves obtained for the mild steel(substrate),brush-plated Sn,as prepared Sn–Ni alloy on mild steel and the annealed sample in 3.5%w/v NaCl electrolyte are prented in Figure8. The E corr and I corr values were calculated using the Tafel extrapolation method as given in Table5.There is an appreciable increa in corrosion resistance for the annealed Sn–Ni alloy on mild steel substrate compared to tin on mild steel,chromium on mild steel,as prepared alloy and bare mild steel substrate which may be due to passivefilm formation on the surface[12].E corr and I corr values improve(a less negative value of E corr and lower value of I corr signifies an improvement in corrosion resistance)for the annealed Sn–Ni alloy on mild steel substrate.The E corr values are shifted to near the equilibrium value of the Sn–Ni alloy system,which
indicates a reduction in corrosion.
3.4.Electrochemical impedance
The electrochemical impedance spectra of brush-plated Sn–Ni alloy system were measured with the same three-electrode asmbly as ud for the potentiodynamic polarization experiments.Impedance measurements were made at open circuit potential(OCP)applying an AC signal of10mV in the frequency range of10Hz to 1MHz.The impedance results obtained from Nyquist plots for the samples ud for corrosion tests in3.5%w/v NaCl solution are shown in Table6and Figure9.
The charge transfer resistance R ct can be related to I corr[13]
R ct¼b aÂb c=2:3ðb aþb aÞI corrð2Þwhere R ct is charge transfer resistance,b a and b c are anodic and cathodic Tafel slopes.
The double layer capacitance C dl value is obtained from the frequency at which Z imaginary is maximum [13]
x Z
ðimÞ
max
¼1=C dl R ctð3ÞThe incread R ct values and decread C dl values for the annealed Sn–Ni alloy clearly confirm the better corrosion resistance of the systems compared to as prepared alloy,tin on mild steel,chromium on mild steel and bare mild steel substrate.Also a more pronounced micircular region is obrved in the ca of the annealed Sn–Ni alloy sample indicating that the system has good corrosion resistance as obrved from the high frequency region of the impedance spectra.
The corrosion resistance of the Sn–Ni alloy deposit was also assd by salt water immersion testing.The deposits were dipped in a3.5%w/v NaCl solution at room temperature.The appearance of the surface after immersion in the solution for different duration is prented in Table7.Comparing the corrosion resistance of the blank mild steel panel and Table5.Corrosion parameters obtained from polarization studies in 3.5%w/v NaCl
Sample E Corr/Vs
SCE/mVenjoy的用法
ba/V
dec)1
bc/V
dec)1
I Corr/A cm)2
家庭教育经验MS panel)0.6800.118)0.196 1.26Â10)4 Sn on MS)0.5420.177)0.201 1.80Â10)5 Cr on MS)0.5040.128)0.226 1.25Â10)5 Sn–Ni alloy on MS
(as prepared)
)0.5270.118)0.2260.57Â10)5
Sn–Ni alloy on MS
(annealed)
)0.4850.121)0.1557.43Â10)
6 Fig.6.Scanning electron micrograph of(a)as prepared(b)annealed
服装加工订单
Sn–Ni alloyfilm.
222
brush-plated tin on mild steel,chromium on mild steel and the as prepared Sn–Ni alloy,the annealed Sn–Ni alloy deposit appears to be more corrosion resistant.
4.Conclusions
Adherent,smooth,bright Sn–Ni alloy deposits were brush-plated successfully onto low carbon steel sub-strates from chloride bath.The brush-plated Sn–Ni alloy films are heterogeneous systems comprising inter-metallic compounds (NiSn,Ni 3Sn 4,Ni 3Sn 2).Uniform coverage of spherical nodular morphology is obrved
from microstructure analys.The annealed Sn–Ni alloy deposit obtained from the bath composition and
bath
Fig.7.AFM images (scan size 5Â5l m)showing the topography of annealed Sn–Ni alloy film on mild steel
sample.
Table 6.Corrosion parameters obtained from impedance measure-ments by Nyquist plots Sample
OCP/V R ct /Ohm cm 2C dl /farads cm )2MS Panel )0.65593.8 5.63Â10)3Sn on MS )0.5061032.8 1.40Â10)4Cr on MS
)0.4781100.70.89Â10)4Sn –Ni alloy on MS (as prepared))0.4831382.40.75Â10)4Sn –Ni alloy on MS (annealed)
)0.468
1519.17
9.56Â10)
5
parameters given in Table2demonstrated excellent corrosion protective performance.
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Table7.Results of tarnishing/rusting level after salt water dipping Sample8h1day4days7days Blank MS10%60%100%100% Sn on MS No No25%30% Cr on MS No No3%10% Sn–Ni alloy on MS(as prepared)No No2%5% Sn–Ni alloy on MS(annealed)No No No3% 224
