M A T E R I A L S C H A R A C T E R I Z A T I O N 5 9 ( 2 0 0 8 ) 1 4 7 4 – 1 4 8 0

Influence of material structure on the electrochemical behaviorof nickel–titanium carbonitride composites

Mukesh Bhardwaj, R. Balasubramaniam⁎

Department of Materials and Metallurgical Engineering, Indian Institute of Technology, Kanpur 208016, India

A R T I C L E D A T A

⁎ Corresponding author. Tel.: +91 512 2597089E-mail address: bala@iitk.ac.in (R. Balasubr

1044-5803/$ – see front matter © 2008 Elsevidoi:10.1016/j.matchar.2008.01.009

A B S T R A C T

Article history:Received 10 November 2006Received in revised form10 August 2007Accepted 10 January 2008

Ni–TiC0.7N0.3 composites were prepared using direct current electrodeposition and hotpressing routes. Hot pressed Ni and sintered TiC0.7N0.3 were also prepared. In case ofelectrodeposition, the samples contained between 10.4vol.% and 20.9vol.% TiC0.7N0.3. Thehot pressed composite contained 16.7vol.% TiC0.7N0.3. The Ni based materials exhibitedactive–passive behavior in de-aerated 0.1mol/l H2SO4 solution, while TiC0.7N0.3 exhibitedactive behavior. The corrosion rate and passive current density increased and criticalcurrent density decreased with increase in reinforcement volume fraction. Based onmicroscopic examination, the increase in passive current density has been related to theincrease in matrix area due to selective corrosion at metal–ceramic interfaces.

© 2008 Elsevier Inc. All rights reserved.

Keywords:ElectrodepositionNickelTitanium carbonitrideCompositePolarizationElectrochemical behaviorCorrosion rateInterfaces

1. Introduction

Titanium carbonitride based cermets have received greatattention as cutting tool inserts due to highhardness, enhancedwear resistance and chemical stability at elevated temperatures[1–7]. In order to control toughness, nickel, cobalt and/or ironpowder is usually added as binder [3–11]. Thus, cermets areheterogeneous materials and therefore undergoes galvaniccorrosion in acidic environments as the ceramic phase isusually noble with respect to the binder phase [12–19].

Some results pertaining to electrochemical studies ofcermets are as follows. In alkaline and neutral environment,cermets are generally corrosion resistant at room temperature[20]. Attack by chlorides is more severe compared to sulfates[17]. Role of oxygen in enhancing corrosion rate has beenfound to be insignificant in aerated environment [17]. How-ever, usually de-aerated solutions are used for electrochemi-

; fax: +91 512 2590260.amaniam).

er Inc. All rights reserved

cal tests. Stern linear rule of mixture [21] to relate corrosionrate with respect to ceramic and binder area ratio has beenfound valid for cermets [15].

Limited literature is available on electrochemical proper-ties of titanium carbonitride based cermets. Lavrenko et al. [7]prepared TiC0.5N0.5 ceramic using hot isostatic pressing with-out any additive. They then performed potentiodynamicpolarization in 3% NaCl electrolyte at 20°C. After polarization,they characterized oxidized surface through chemical analy-sis, scanning electronmicroscopy, X-ray diffraction and Augerelectron spectroscopy. They concluded that oxidation of theceramic is a multistage process. They further concluded thatthe ceramic has exceptionally high corrosion resistance asthere was no binder phase. In another study by Kumar et al.[22], TiC0.7N0.3-20wt.% Ni cermet was studied by potentiody-namic polarization in 0.2mol/l sulfuric acid. They noticed twopassive regions in the potentiodynamic curve. They attributed

.

Fig. 1 – Schematic of experimental cell assembly utilized forelectrodeposition.

1475M A T E R I A L S C H A R A C T E R I Z A T I O N 5 9 ( 2 0 0 8 ) 1 4 7 4 – 1 4 8 0

passive region at lower potential due to passivation ofTiC0.7N0.3 while the passive region at higher potential due topassivation of nickel.

Twomajor causes of corrosion have been noticed in cermetswith multiple phases. One is galvanic corrosion due tosignificant difference in corrosion potentials of different phasesand other is crevice corrosion in pores. Since the ceramic phase(nobler phase) is usually the major phase in cermets, thecathode to anode area ratio might play an important role.Cermets unavoidably contain pores as they are processedthrough sintering or hot isostatic pressing. In various systems,presence or formation of pores has been found detrimental tocorrosion properties. Some examples showing decrease incorrosion resistance due to porosity are discussed as follows.

Jegannathan et al. [23] found poor corrosion resistance ofphosphate coatings obtained by anodic treatment. Theyattributed this to the creation of pores during anodic treat-ment. Gu et al. [24] found high corrosion resistance ofnanocrystalline nickel coating on AZ91D magnesium alloydue to low porosity and fine grain structure. Liu et al. [25]studied corrosion and wear performance of HVOF-sprayedInconel 625 and WC-Inconel 625 coatings treated with highpower diode laser. They achieved significant improvementof corrosion and wear resistance after laser treatment as aresult of the elimination of discrete splat-structure, micro-crevice and porosity, and also the reduction of microgalvanicdriving force between the WC and the metal matrix. Herrastiet al. [26] obtained polypyrrole coating by applying a constantcurrent on copper with high corrosion resistance due to lowerporosity. Chandramouli et al. [27] performed corrosionstudies of sintered powder metallurgy plain carbon steelpreforms. They attributed enhanced rate of uniform corrosionin both Fe and Fe-1%C carbon steels due to presence of porosity.

Table 1 – Sample preparation and characteristic physical param

Sample no. 1 2 3

Current density (mA/cm2) 25 50 100Temperature (°C) 30 30 30Current efficiency (%) 90.9 93.8 92.6TiC0.7N0.3 (vol.%) 12.3 10.7 15.5

Ahn et al. [28] studied corrosion behavior of TiN hard coatingsproduced by a modified two-grid attachment magnetronsputtering process. They found accelerated localized corrosiondue to presence of pores.

Factors like crevice corrosion due to presence of pores andgalvanic corrosion due to different phases (ceramic and binderphase) determine the electrochemical properties of cermets. Itis therefore important to understand the independent effect ofporosity and the binder phase on corrosion. The present studywas undertaken to understand the effect of material structureon the electrochemical behavior of titanium carbonitride in afully dense binder phase. This was achieved by co-depositionof titanium carbonitride in nickelmatrix. To study the effect ofporosity in the binder phase, hot pressed nickel sample wasprepared. To test the combined effect of both porosity andtitanium carbonitride particles, hot pressed nickel titaniumcarbonitride composite was also prepared. The secondary aimof the present study was to compare the electrochemicalbehavior of Ni-based materials with that of bulk nickel.

2. Experimental

Ni–TiC0.7N0.3 electrodeposited samples were prepared using150ml Watt's bath (of composition NiSO4 · 6H2O 250g/l, NiCl2 ·6H2O 30g/l, H3BO3 40g/l). NaOHwas added to theWatts bath toadjust pH to 4. A 2g/l suspension of TiC0.7N0.3 particles (H. C.Starck, USA) of size 3 to 4μm was used in all experiments.Copper substrate of area 3cm × 3cm, polished up to 1000 gradeSiC paper, served as the cathode. Before each eletrodepositionexperiment, the substrate was ultrasonically cleaned anddegreasedusing acetone. The cathodewaselectrically insulatedat the back face and edges (using perspex sheet and teflon tape)and electrical connectionwas provided at the back, as shown inFig. 1. Nickel anodewas placed 5cmaway from the cathode. Thedepositiondirect current density andbath temperatureusedarementioned in Table 1. The time of electrodeposition wasadjusted such that a charge of 540 C/cm2 was used for everyelectrodeposition. Current efficiency was calculated from theknown increase in weight of the sample after electrodepositionand the amount of TiC0.7N0.3 codeposited.

Hot pressed Ni-16.7vol.% TiC0.7N0.3 composite and pure Niwere prepared using Ni powder (of size 20μm) in a graphitemould, which was maintained at a high temperature by resis-tance heating. The composition of the composite (i.e., 16.7vol.%TiC0.7N0.3 equivalent to 10wt.% TiC0.7N0.3) was chosen such thatit is close to the average amount of TiC0.7N0.3 reinforced inelectrodeposited samples, for comparison of properties. Ahydraulic pressure of 1.5MPa was maintained during hotpressing. Hot pressing was conducted in open atmosphere.TiC0.7N0.3 sintered samplewaspreparedby first coldcompactingthe powder at 102MPa and then sintering in vacuum at 1950°C

eters of electrodeposited composites

4 5 6 7 8 9

25 50 100 25 50 10040 40 40 50 50 5094.1 88.8 87.2 94.6 92.2 88.715.0 20.9 18.1 10.4 11.8 14.7

Table 3 – Parameters obtained from linear polarizationcurves

Sampleno.

ZCP FCP Rp

(Ω cm2)|βc| |βa| icorr

(μA/cm2)(V vs SCE) (V / log(i))

1 −0.292 −0.289 430 0.04 0.05 212 −0.298 −0.295 252 0.05 0.07 473 −0.293 −0.291 329 0.05 0.05 31

Table 2 – Physical parameters of bulk and hot pressedsamples

Sample No. 10 11 12 13

Sample Bulk Ni Hot pressed Ni Ni-10 wt.%TiC0.7N0.3

TiC0.7N0.3

Open porosity(vol.%)

– 7.7 17.9 2.0

1476 M A T E R I A L S C H A R A C T E R I Z A T I O N 5 9 ( 2 0 0 8 ) 1 4 7 4 – 1 4 8 0

for 120min. This provided almost fully dense TiC0.7N0.3 withporosity estimated at 2vol.%. These samples were assignedsample numbers as shown in Table 2.

The electrodeposited samples were characterized for thefraction of TiC0.7N0.3 particles present in the matrix by energydispersive spectroscopy (EDS) for element titanium using FEIQuanta 200 HV scanning electron microscope. The accuracy ofthemethodwas testedbymeasuring thewt.%Ti in ahotpressedNi-16.7vol.% TiC0.7N0.3. The error was approximately 10%. EDSresults for Ti (wt.%) were converted to that for TiC0.7N0.3,knowing the ratio of molecular weight of TiC0.7N0.3 to atomicweight of Ti. The apparent density ρc of hot pressed and sinteredsamples were measured using Archimedes principle. Since thecomposition of samples were known, the theoretical density(ρthc ) of the composite was obtained using Eq. (1).

qcth ¼ x1 þ x2

x1= qthð Þ1þx2= qthð Þ2ð1Þ

where, ωi is component wt.% and (ρth)i is component theoreticaldensity, i being 1 and 2. The volume percent open porosity (ɛ)was calculated using Eq. (2).

e ¼ 1� qc=qcth� �� 100 ð2Þ

The samples were electrochemically characterized by potentio-dynamic polarization experiments using a potentiostat (Versa-stat II, Amtek USA). All experiments were performed in a roundbottompolarizationcell. Graphitewasusedas counterelectrode.Saturated calomel electrode (SCE) was used as referenceelectrode (0.242V vs NHE). Electrolyte of composition 0.1mol/lH2SO4 was de-aerated before start of experiment (by simulta-neously purging and stirring) with nitrogen for 5min. Thetemperature of all the experiments was ambient. The freecorrosion potential (FCP) was monitored as a function of timeupon immersion of the sample. After a stable FCP was obtained,the corrosion rates were evaluated by linear polarizationexperiment. In this experiment, the potential was scannedfrom− 0.020Vwith respect to FCP to 0.020Vwith respect to FCP ata scan rate of 0.167mV/s. Immediately after the linear polariza-tion experiment, the samples were subjected to potentiody-namic polarization experiments. The potential was scannedfrom − 0.250V to 2.0Vwith respect to FCP at a scan rate of 1mV/s.A pure Ni bulk sample was tested for comparison purposes.

4 −0.364 −0.367 512 0.07 0.10 355 −0.313 −0.311 392 0.02 0.03 156 −0.332 −0.316 495 0.07 0.10 367 −0.279 −0.276 524 0.05 0.05 218 −0.302 −0.299 543 0.04 0.05 169 −0.309 −0.306 345 0.05 0.07 3610 −0.256 −0.257 34,080 0.06 0.02 0.1811 −0.323 −0.327 302 0.10 0.10 7212 −0.354 −0.353 51 0.04 0.05 190

3. Results and Discussion

3.1. Physical Characterization

For electrodeposited composites, current efficiency and amountof TiC0.7N0.3 codeposited are provided in Table 1. The amount of

TiC0.7N0.3 in the electrodeposited composites did not reveal anyparticular trendwith respect to current density or temperature ofdeposition. It varied between 10.4vol.% to 20.9vol.% (see Table 1).The current efficiency generally decreased with increase in de-position current density because ofhigher overpotential availablefor thedischargeofH3O+overnickel athigher currentdensity [29].In case of hot pressed samples, open porosity values arementioned in Table 2. Hot pressed Ni–TiC0.7N0.3 composite pos-sessed much higher open porosity compared to hot pressed Ni.

3.2. Electrochemical

3.2.1. Free Corrosion PotentialFree corrosion potential (FCP) achieved steady state between2000s and 4000s. The electrodeposited samples can be assumedto have negligible porosity. Bulk nickel was free from anyporosity. On comparing corresponding FCP data (Table 3), it islowest for sample with highest porosity. The FCP for electro-deposited sampleswas in a relatively close range,whichmaybedue to lack of porosity. The variation noted would arise due tovariation in the amount of reinforcement. Bulk nickel exhibitedthenoblest FCP. It can therefore be concluded that the stabilizedFCP was a decreasing function of both the amount of porosityand the amount of noble particulate reinforcement.

Open pores act as natural crevices which causes splitting oflarge cathodic (open surface) and small anodic areas (insidecrevices due to lack of oxidizing agents transport). After pit for-mation, anode area (pit area) is drastically reduced. Kinks wereobserved in the FCP vs time curves for all the samples, especiallyduring the initial period of immersion. This may be due to local-ized corrosion tendencyduring initial stages of immersionat FCP.

3.2.2. Linear PolarizationLinear polarization data was utilized to determine corrosionrate (icorr). The zero current potential (ZCP) and polarizationresistance (Rp) are provided in Table 3. The corrosion rate (icorr)is related to Rp by the following equation.

icorr ¼ 12:3Rp 1=jbaj þ 1=jbcjð Þ ð3Þ

In order to estimate βa and βc, icorr (Eq. (3)) was substituted inEq. (4). The equation obtained after substitution was used for

Fig. 2 – Potentiodynamic polarization curves in 0.1 M H2SO4

solution. Thenumbers correspond to thesamplesmentioned inTable 1. The data for electrodeposited composites are shown asdotted lines and these are amplified in Fig. 3.

1477M A T E R I A L S C H A R A C T E R I Z A T I O N 5 9 ( 2 0 0 8 ) 1 4 7 4 – 1 4 8 0

numerical fitting of the experimental data [29,30]. Knowing βaand βc, icorr was determined using Eq. (3).

iapp ¼ icorr exp2:3 E� ZCPð Þ

jbaj� exp

�2:3 E� ZCPð Þjbcj

� �ð4Þ

All the measured and derived data are mentioned in Table 3.Rp increased and icorr decreased with nobler ZCP. It wasnoticed earlier that FCP was lowest for sample with highestporosity. The corrosion rates of electrodeposited compositeswere higher than that of bulk Ni and thismust be related to the

Fig. 3 – Potentiodynamic polarization curves in 0.1 M H2SO4

solution for electrodeposited samples. Thenumbers correspondto the samplesmentioned in Table 1. Epp, Ecp, EO2/OH− and icrit aredefined for curve 2 and are marked with horizontal and verticaldotted lineswith labels near them. Similar definitions follow forother curves.

presence of TiC0.7N0.3. Hot pressed Ni exhibited highercorrosion rate due to the porosity present in the sample. Thehot pressed composite possessed highest corrosion rate due topresence of both porosity and TiC0.7N0.3 reinforcement.

3.2.3. Potentiodynamic PolarizationPotentiodynamic polarization curves for all the materials areshown in Fig. 2. The curves for electrodeposited samples areshown separately in Fig. 3, because they are clustered in Fig. 2.The electrochemical parameters like potential for primarypassivation (Epp), potential for completion of passivation (Ecp),oxygen evolution potential (EO2/OH−), critical current densityfor passivation (icrit) and the passive current density (ipass)were obtained from the curves and are tabulated in Table 4.

Bulk nickel and electrodeposited composites exhibitedactive–passive behavior. In case of bulk Ni, the passive currentdensitywas almost constant between Ecp and EO2/OH−. SinteredTiC0.7N0.3 did not exhibit active–passive nature. Compared tobulk nickel, the anodic branch of the curve for TiC0.7N0.3

showed much lower current density at lower overpotential.The current density continuously increased with increase inanodic polarization. The standard aqueous half cell reductionpotential for Ni and Ti are − 0.499V vs SCE and − 1.872V vs SCE,respectively. This indicates that Ni is noble to Ti. The possiblereason for TiC0.7N0.3 to exhibit low anodic current density andnobler ZCP must be due to stable passive film. In case of hotpressed samples, active–passive phenomenon was notobserved. The active nature of the surface in hot pressed Nimay be due to large surface area exposed to the environmentbecause of open porosity. The hot pressed composite showedsome indications of passivity. If one considers the region inthe anodic polarization region where the current densityvaried minimally with potential to represent passive filmconditions, the ipass was much higher (two orders of magni-tude) compared to bulk nickel.

In case of electrodeposited composites, they also exhibitedactive–passive behavior. Their polarization curves were ob-tained in between that of bulk Ni and hot pressed composite. Itappears that there exist two passive zones in the passiveregion (see Fig. 3). In the case of TiC0.7N0.3-based cermets, thishas been related to formation of surface film due to reinforce-ment and binder phases [22].

Table 4 – Parameters obtained from potentiodynamicpolarization curves

Sampleno.

E vs SCE (V) i (mA/cm2)

Epp Ecp EO2/OH− icrit ipass

1 −0.094 0.225 1.113 7.92 1.152 −0.062 0.252 1.126 7.92 1.813 −0.075 0.212 1.133 3.93 1.634 0.115 0.234 1.110 4.75 1.235 −0.092 0.264 1.082 7.67 1.586 0.079 0.178 1.139 3.69 2.707 0.056 0.180 1.146 3.29 0.788 −0.100 0.154 1.093 5.99 0.999 −0.068 0.199 1.133 5.99 1.8710 −0.086 0.176 1.086 6.93 0.3812 0.369 0.499 1.078 60.14 20.8

Fig. 4 – Back scattered electron images after polishing for(a) electrodeposited composite (at 50 °C and 25 mA/cm2)and (b) Hot pressed Ni-16.7 vol.% TiC0.7N0.3.

1478 M A T E R I A L S C H A R A C T E R I Z A T I O N 5 9 ( 2 0 0 8 ) 1 4 7 4 – 1 4 8 0

3.3. Microstructural Characterization

The back scattered electron images of polished electrodepos-ited composite and hot pressed composite are shown in Fig. 4.The dark phase is TiC0.7N0.3 and bright phase is Ni matrix.Electrodeposited samples showed more uniform distributionof particles. In case of hot pressed composite, macro-segrega-tion of TiC0.7N0.3 particles around Ni was noticed. Suchsegregation occurred at a distance of approximately 50μm.The segregation occurred due to limited diffusion of TiC0.7N0.3

particles into Nimatrix as less timewasmade available duringhot pressing.

To understand the electrochemical behavior of the com-posite samples, it was necessary to observe samples that hadbeen polarized to the passive zone. Therefore, in a separate setof polarization experiments, selected samples were potentio-dynamically polarized to 0.67V vs SCE and held at 0.67V for15min. This potential was chosen as it was almost in themiddle of the passive range, i.e. potential almost in themiddleof Ecp and EO2/OH−. After release of potentiostatic control, thesamples were removed from the solution, washed withdistilled water and dried. The surfaces were later observed inthe SEM. The surfacemorphologies are shown in Fig. 5. Bulk Nidid not show any deep pits but only some roughening of thesurface (Fig. 5a). The nature of attack appears to follow thepolishing direction. In case of electrodeposited sample, the

interface between particle and matrix was specifically cor-roded (Fig. 5b). The consequence of this was the appearance ofspherical pits. Particles could be located inside those pits inseveral cases. Such severe corrosion of the matrix leading topittingwas attributed to the high current density during activepassive transition. This important fact must be borne in mindbecause future discussion of pitting around the reinforcementparticles would mean these locations of localized attack.

In case of hot pressed composite (Fig. 5c), enhancedcorrosion occurred in the grain boundary region. This is theregion where TiC0.7N0.3 particles were segregated. The devel-opment of deep pits due to this enhanced corrosion wouldresult in higher corrosion rate and active polarization natureobserved for this case. In case of hot pressed Ni (Fig. 5d),enhanced corrosion attack of the matrix was observed. In thecase of hot pressing, Ni particles tried to coalesce in alldirection with equal probability, which resulted in uniformlydistributed fine open pores with electrochemically active largesurface area. These deep pits on the surface (Fig. 5d) must berelated to the porous structure of hot pressed Ni.

3.4. Simulation of Potentiodynamic Polarization Curve

In order to quantitatively understand the interaction betweenTiC0.7N0.3 particles and Ni matrix on polarization behaviorbased on observed microstructural and electrochemical data,the electrodeposited composite was chosen since the porosityeffect could be eliminated. The sample should have uniformdistribution of particles and lower amount of reinforcement toneglect interaction due to neighboring particles. The electro-deposited sample deposited at 50°C and 25mA/cm2 waschosen based on above mentioned criteria as it possessedrelatively low vol.% of TiC0.7N0.3 particles.

In order to simulate the experimentally obtained potentio-dynamic curve in caseof electrodeposited composite, the exper-imental potentiodynamic curves of bulk nickel and that ofTiC0.7N0.3 cermet were utilized. The simulation was performedby manually adjusting electrochemically active true surfacearea contribution of nickel matrix and that of reinforcedTiC0.7N0.3 particles through trial and error approach in theStern linear rule of mixture [21] of determining net current.

There are three major issues in estimating electrochemi-cally active true surface area of matrix and that of particlesreinforced area contribution in the net current.

1. Continuous variation of electrochemically active true sur-faceareadue to the localizedattackduring the initial polarization.

2. Isolated pits formed around TiC0.7N0.3 particles grow andinterconnect with other pits thus providing complicated mor-phology of corroded surface.

3. Non-uniform current distribution within the pits due topresence of TiC0.7N0.3 particles further complicate the estima-tion of net current.

In order to simplify the problem, the experimental poten-tiodynamic curve was divided into two parts, one below Eppand the other above Epp. In case of part of curve below Epp, itwas assumed that no significant pit growth has occurred andthe true surface area contribution is sameas the apparent area.In case of part above Epp, it was assumed that pit growth isclose to completion and there is no significant change inelectrochemically active surface area on further polarization to

Fig. 6 – Comparison of experimental and simulatedpotentiodynamic curves for electrodeposited sample,using experimentally determined curves for bulk nickeland TiC0.7N0.3.

Fig. 5 – Secondary electron images after potentiodynamic polarization up to 0.67 V vs SCE for (a) bulk nickel, (b) electrodepositedcomposite (at 50 °C and 25 mA/cm2, (c) hot pressed Ni-16.7 vol.% TiC0.7N0.3 and (d) hot pressed Ni.

1479M A T E R I A L S C H A R A C T E R I Z A T I O N 5 9 ( 2 0 0 8 ) 1 4 7 4 – 1 4 8 0

higher potentials. This assumption is valid since we havenoted that localized attack regions form on the surface due toinitial transition from active to passive region on polarization.The equations giving best fitting in the respectivepolarizationsare given as follows.

EbEpp icomp ¼ 0:6 iNi þ 0:4iTiCN ð5Þ

EbEpp icomp ¼ 2:2 iNi þ 0:4iTiCN ð6Þ

where icomp is the total simulated current, iNi is the experi-mentally measured current for bulk nickel and iTiCN is theexperimentally measured current for TiC0.7N0.3, at the respec-tive polarization. The result of such simulation is shown inFig. 6. Good fitting is evident from the figure.

In order to relate the equations utilized for simulation andthe microstructure of the potentiodynamically polarized sur-face up to 0.67 V, the image of Fig. 5b was analyzed usingimage analyzer. It was found that 40% of the area was coveredwith pits. Comparing this with Eq. (5), it can be concluded thatroughly electrochemically active surface area was quite closeto the apparent area for both particles and matrix. However,on comparing with Eq. (6), area contribution of particles re-mained unaffected while that of nickel matrix grew approxi-mately four times.

Under normal immersion conditions, the corrosion rate ofthe composite is more than bulk samples due to possiblegalvanic coupling of active Ni matrix to noble TiC0.7N0.3

reinforcements. The distance effect of microgalvanic corro-sion further implies that the matrix in the immediate vicinityof the reinforcement would be subjected to enhanced attackupon anodic polarization well past the ZCP. The matrixreinforcement areas are locations of enhanced corrosion and

1480 M A T E R I A L S C H A R A C T E R I Z A T I O N 5 9 ( 2 0 0 8 ) 1 4 7 4 – 1 4 8 0

this leads to removal of reinforcements at several locations onthe surface, thereby leading to a larger Ni surface area. Aconsequence of this microstructural change brought about bycorrosion is that passive current densities are higher forcomposites. The passive film is further affected by the presenceof reinforcement. For example, a more defective passive filmwas identified in Mg–SiC composites compared to pure Mg [31]which was related to injection of defects into the passive filmdue to discontinuities at matrix-reinforcement interfaces.

4. Conclusions

Ni–TiC0.7N0.3 composites were produced by electrodepositionand hot pressing. Their microstructures and electrochemicalbehavior were compared with that of bulk nickel, hot pressednickel and sintered TiC0.7N0.3. Microstructural analysisshowed uniformly distributed TiC0.7N0.3 particles in thematrixin case of electrodeposited samples, while the TiC0.7N0.3

particles were segregated in the hot pressed composite. Theporosity was also higher in case of hot pressed samples. Thevolume fraction of TiC0.7N0.3 in the electrodeposited samplesvaried between 10.4 vol.% and 20.9 vol.%.

The passive current density increased on introducingTiC0.7N0.3 particles into nickel matrix. It further increaseddue to the effect of porosity as noted in case of hot pressedsamples. Simulation of polarization curve showed that suchincrease may be due to the increase in electrochemicallyactive surface area of nickel matrix. The effect of porosity wasfound to be more detrimental to corrosion compared toNi–TiC0.7N0.3 interfaces, since the electrochemical propertiesof pore-containing structures were poor.

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