Thin Solid Films 420–421(2002) 446–454

0040-6090/02/$ - see front matter� 2002 Elsevier Science B.V. All rights reserved.PII: S0040-6090Ž02.00815-5

Influence of HVOF parameters on the corrosion resistance of NiWCrBSicoatings

L. Gil , M.H. Staia *a b,

Metallurgical Engineering Department, National Polytechnic University (UNEXPO), Puerto Ordaz, Venezuelaa

School of Metallurgy and Materials Science, Central University of Venezuela, Apartado 49141, Caracas 1042-A, Caracas, Venezuelab

Abstract

Experimental design is an effective method for conducting a reduced number of experiments in order to obtain the optimumspraying conditions and enhance the thermally sprayed coatings properties. In the present study, a 3 factorial design experiment3

was used to establish the effects of the variables on the coatings quality in relation to the corrosion behavior of an HVOF Nibased self-fluxing alloys coatings in a 3.5% NaCl solution. Response surface methodology was employed to describe empiricalrelationships among variables as the spraying distance, the fuelyoxygen ratio and the powder feed rate. The maps obtainedallowed the selection of the optimum operating conditions to achieve the desired specifications of the HVOF coatings for theirbest corrosion resistance in the chosen environment. The analysis of the results indicates that the spraying distance, the fuelyoxygen ratio and the powder feed rate have a significant effect on the porosity and corrosion resistance of these coatings.� 2002 Elsevier Science B.V. All rights reserved.

Keywords: Experimental design; Corrosion; HVOF thermal spray coatings; Response surface methodology

1. Introduction

The industrial benefit of thermal spray coatings is theachievement of cost-effective coating solutions to mini-mize wear and corrosion. Today, the thermal spraytechnology allows tailoring coatings with low porosity,higher bond strength and increased properties. For exam-ple, the use of cheaper substrate materials such as lowcarbon steel, with thin layers of a more costly material,such as nickel alloys, was able to provide a cost-effectiveproduct with high corrosion resistance. However, ther-mally sprayed coatings are expected to have an extreme-ly complex corrosion behavior due to theirheterogeneous, non-isotropic structure with lenticular orlamellar grain structure that results from processing.With exception of some explanations given in terms ofinterconnected porosity andyor unmelted particles andthe ability of the corrosive media to penetrate into thecoatingysubstrate interface, little progress has been made

*Corresponding author.E-mail addresses: mstaia@reacciun.ve(M.H. Staia),

lindagil@telcel.net.ve(L. Gil).

in explicitly determining the corrosion mechanisms,which lead to coatings deteriorationw1–10x.

Therefore, successful protection of the corrosion-prone substrate in aqueous environment using thermallysprayed nobler materials still remains as one of thechallenging property to be achieved, and this requires afundamental understanding of the corrosion mechanism.Electrochemical techniques such as, for example, poten-tiodynamic polarization has proved to be an effectivetool in assessing the corrosion resistance of these coat-ings w1–4x and could be employed as a test method intailoring the optimum process parameters. The morpho-logical characterization of the corrosion products, byusing scanning electron microscopy coupled with X-rayelemental mapping, could give an indication of thecorrosion mechanism, which took place during theinteraction coatingysubstrate system and the aqueousenvironment.

The present study was conducted with the aim ofdetermining the relationship between different deposi-tion parameters of HVOF NiWCrBSi coatings and theircorrosion performance in 3.5% NaCl aqueous solution.The empirical relationships amongst these independent

447L. Gil, M.H. Staia / Thin Solid Films 420 –421 (2002) 446–454

Table 1Chemical composition of NiWCrBSi commercial alloy powder

Element W Cr B Fe C NiWt (%) 17.0 15.0 3.0 3.5 0.75 Balance

Table 2Operating conditions of the HVOF process

Gun type JP-5000Barrel length 100 mmN flow (as powder carrier)2 0.708 mymin3

Powder size(mm) q22.00–66.23Substrate roughness(Ra) 17 mmTransverse speed gun 1 myminSubstrate preheating temperature 1208C

Table 3Level of the spraying parameters for the 3 factorial experiment design3

Variables Levels Initial

Lower Standard Highconditions

Powder feed rate(gymin) 60 90 120 90Spraying distance(mm) 380 470 563 435Fuelyoxygen ratio(F )i 0.8 1.0 1.2 0.7

variables was achieved by employing both statisticalexperimental design methods and the response surfacemethodology(RSM), described in detail elsewherew11–15x.

2. Experimental details

Samples of 12.7 mm diameter AISI 1020 steel wereHVOF coated at industrial level with a commercial alloyof NiWCrBSi powder (Colmonoy 88). The powdercomposition and the operational parameters used arepresented in Tables 1 and 2, respectively. Informationon the coating structure and their morphology has beenpreviously presented in the literature by the authorsw1x.

In this research, a 3 factorial design experiment was3

employed to establish the effects of the variables on thecoating quality. Three operational parameters were var-ied: the spraying distance, the powder feed rate and theequivalence ratio(F ) defined as the ratio of((Fyi

O) y(FyO) ), where (FyO) repre-working stoichiometric working

sents the ratio of the mass of fuel to the mass of oxygenunder working conditions and(FyO) repre-stoichiometric

sents the ratio between the mass of fuel to the mass ofoxygen under stoichiometric conditions, i.e. under con-ditions of a complete combustion process. The choiceof these variables was based on the capability ofcontrolling these parameters and manipulating themduring the HVOF industrial practice.

The independent variables were set to three levels,which imply that 27 experiments were necessary toexplore the variation of all variables at the chosen levels.Two replicate experiments were carried out in each case.The level of the spraying parameters for the 3 factorial3

experiment design are displayed in Table 3. Table 3 alsoindicates the initial operating conditions that were exis-tent in the HVOF industrial plant, prior to those usedfor deposition of the coatings corresponding to thisresearch.

The experiments A1 through A27 were carried outfollowing a random sequence and the data were obtainedfor the response properties as: corrosion current density(i ), corrosion potential(E ) and porosity. Thesecorr corr

results were subject to an analysis of variance(ANO-VA), based onF-test, in order to measure whether afactor contributes significantly to the variance of aresponse and to determine the amount of variance thatis due to pure experimental error. The degree of theinfluence, which each independent variable has on theresponse in order of decreasing influence, was repre-

sented as a frequency histogramw11x. RSM was used todescribe empirical relationships among the three inde-pendent variables investigated. The statistical analysisof the result was conducted by using a commercialstatistical analysis packagew12x.

In order to have reproducible results for the metallo-graphic apparent porosity studies, the samples crosssection have been prepared following the procedurerecommended in the literature for the thermal spraycoatingsw16–18x. Samples were sectioned with a low-speed diamond saw and, subsequently, vacuum impreg-nated with an epoxy resin. The cross-sectioned samplewere polished successively using 180–1200 grit abrasiveSiC paper followed by 0.05 silica colloidal. Imageanalysis of twenty fields on both cross-sections of eachsample produced the average value of the porosityreported. The microstructure and the morphology of thecoatings before and after the corrosion tests were exam-ined by using a scanning electron microscope(PhilipsXL30 SEM) coupled with a standard energy dispersiveX-ray microanalysis attachment(EDAX-DX4).

The corrosion behavior of the coatings was evaluatedby conducting anodic potentiodynamic polarizationcurves in a 3.5% NaCl solution. The solution was purgedwith nitrogen for 1 h to remove the dissolved oxygenprior to immersion of the mounted sample for testing.Both an EG&G model 273 potentiostat and a EG&GK105 flat specimen holder were used, the later beingnecessary to support the samples so only 1 cm of its2

area is exposed to the electrolyte. The samples werescanned fromy0.25 to 1 V, at a scan rate of 1 mVys.A standard cell with 3 electrodes(2 carbon electrodesand one reference AgyAgCl electrode) was employed.To activate the surface prior to the test, cathodic cleaningof the samples was performed during 5 min aty0.4mV below the value corresponding to the open circuitpotential. The polarization curves have allowed the

448 L. Gil, M.H. Staia / Thin Solid Films 420 –421 (2002) 446–454

Fig. 1. Frequency histogram showing the amount of influence of thevariables and their interactions on the porosity in order of theirdecreasing influence.

Table 4Summary of the characteristic results of the HVOF coatings

Run Order Fi Spraying Powder Porosity Ecorr icorr

no. no. distance feed rate (%) (V) (mAycm )2

(mm) (gymin)

A1 16 0.8 380 60 0.27"0.19 y0.388 11.22A2 22 0.8 380 90 0.34"0.28 y0.437 59.47A3 4 0.8 380 120 0.66"60.28 y0.541 60.60A4 13 0.8 470 60 0.83"0.44 y0.337 16.54A5 11 0.8 470 90 0.41"0.43 y0.445 41.11A6 23 0.8 470 120 0.44"0.23 y0.414 11.79A7 6 0.8 560 60 0.50"0.34 y0.492 30.99A8 27 0.8 560 90 0.89"0.46 y0.538 76.02A9 14 0.8 560 120 0.50"0.24 y0.507 90.55A10 7 1.0 380 60 0.94"0.60 y0.337 7.47A11 20 1.0 380 90 0.30"0.09 y0.370 26.06A12 21 1.0 380 120 1.10"0.77 y0.295 5.82A13 1 1.0 470 60 1.14"0.61 y0.469 18.61A14 17 1.0 470 90 0.74"0.39 y0.539 5.66A15 15 1.0 470 120 0.81"0.68 y0.429 32.07A16 25 1.0 560 60 1.13"0.60 y0.415 18.49A17 8 1.0 560 90 1.57"0.35 y0.413 10.25A18 18 1.0 560 120 2.02"0.68 y0.434 43.75A19 10 1.2 380 60 0.57"0.47 y0.313 8.41A20 5 1.2 380 90 0.27"0.09 y0.362 4.99A21 3 1.2 380 120 0.36"0.25 y0.476 18.67A22 2 1.2 470 60 0.09"0.06 y0.401 14.67A23 19 1.2 470 90 0.33"0.09 y0.403 8.53A24 9 1.2 470 120 0.42"0.35 y0.366 11.67A25 24 1.2 560 60 1.31"0.60 y0.473 18.55A26 12 1.2 560 90 0.19"0.09 y0.441 33.78A27 26 1.2 560 120 1.18"0.63 y0.415 19.64Initial conditions – 0.7 435 90 5.40"0.70 y0.653 27.80

Steel 1020 substrate:E sy0.68 V; i s10.77mAycm .2corr corr

calculation of the corrosion potential,E and corrosioncorr

current density,i values by using Tafel extrapolation.corr

3. Results and discussion

3.1. Porosity

Porosity values of the coatings, as revealed by imageanalysis, are listed in Table 4, which presents thesummary of the results obtained in this study thatcorrespond to the test matrix used. As it can be observed,the average porosity of these coatings ranged between0.09 and 2.02%. Very high standard deviations valueswere obtained, which were also indicated in this table.The extreme values of the porosity reported, i.e. of0.09–2.02%, respectively are considered to belong tothe tail of the sample means distribution and have a lowfrequency. The expected response of the variable(theporosity) has to belong to the interval of high frequencyof the sample means and this interval could only bedetermined by applying the statistical model.

From the ANOVA it was found that the sprayingdistance (A) is the only variable, which contributessignificantly to the variance of the response, at 95%confidence level. This could be observed in Fig. 1,

where the frequency histogram shows the amount of theinfluence of each factor has on the response function,i.e. on porosity, in order of its decreasing influence.Since no substantial differences in porosity were foundwhen the powder feed rate was varied between 60 and120 gymin, Fig. 2 represents the typical contour ofconstant porosity of the HVOF NiWCrBSi coating, asfunction of the spraying distance for different values ofF ratio. The interval of the high frequency of the samplei

means was found to be between 0.4 and 1.12%, indi-cating that these are the expected porosity values forthe samples produced using the operational conditions

449L. Gil, M.H. Staia / Thin Solid Films 420 –421 (2002) 446–454

Fig. 2. Contours of constant porosity as function of the spraying dis-tance andF ratio. Powder feed rate is maintained at 60 gymin.i

Fig. 4. Anodic potentiodynamic polarization curves function of theF ratio at a constant powder feed rate of 60 gymin for (a) 380 mm;i

(b) 470 mm;(c) 560 mm spraying distance.

Fig. 3. Anodic potentiodynamic polarization curves function of thespraying distance at a constant powder feed rate 60 gymin and (a)F s0.8; (b) F s1.0; (c) F s1.2.i i i

presented in Table 3. For example, Fig. 2 shows that anincrease of the spraying distance from 370 to 560 mmproduced an increase in the average porosity from 0.4

to 1.04 whenF ratio varies from 1.1 to 1.2. This resulti

is expected because the stand-off distance between gunand substrate is an important variable in determiningthe quality of the coatings, since both factors the lossof kinetic energy during flight and the particle temper-ature at impact are related to itw19x. A higher sprayingdistance results in smaller particles velocity toward thesubstrate producing coatings with lower density. Also,by lowering the average impact temperatures of thedroplets with the substrate surface, an increased volumefraction of unmelted particles is produced. Both theseeffects contribute to a substantial increase in the coatingporosity.

As it is observed from Table 4, the measured valuesfor the coatings porosity irrespective to the processing

450 L. Gil, M.H. Staia / Thin Solid Films 420 –421 (2002) 446–454

Fig. 5. Frequency histogram showing the amount of influence of thevariables and their interaction on the corrosion current density(i )corr

in order of their decreasing influence.

Fig. 6. Frequency histogram showing the amount of influence of thevariables and their interaction on the corrosion potential(E ) incorr

order of their decreasing influence.

parameters used are below 2%, indicating for all con-ditions dense coatings were produced.

3.2. Corrosion resistance

Figs. 3 and 4 present the anodic polarization curvesfor the as-sprayed coatings in 3.5% NaCl solutiondifferent conditions of the experimental design used. Itis observed that the coatings were not passivated underthe set of conditions established during the corrosiontest. In general, a sharp transition in the breakdownpotential,(E ), defined as the voltage corresponding tobk

rapid increase in current, indicates the onset of thepitting corrosion, while a gradual transition is morecharacteristic of the crevice corrosion typew8x. In ourcase, a gradual transition of theE was observedbk

approximately 0.1 V. The crevice corrosion mechanism,which took place during the electrochemical test, wasevidenced by the morphological analysis conducted onthe corroded samples using SEM coupled with EDX.The corresponding micrographs will be presented in thenext section.

The corrosion potential,E and corrosion currentcorr

density, i , values estimated by Tafel extrapolationcorr

method are indicated in Table 4. Corrosion currentdensity value, i , for the carbon steel AISI 1020corr

substrate was also included in table, for comparison.The average experimental values ofE andi rangedcorr corr

from y0.29 toy0.54 V and from 4.99 to 90.55mAycm , respectively.2

For example, the increase in the spraying distancefrom 370 to 560 mm for aF s0.8 has produced ai

corrosion current density,i , which is 2.76 timescorr

higher. With this increase of thei values, it can becorr

observed that the corrosion potential,E , changes fromcorr

y0.388 to y0.492 V indicating a higher dissolutionrate, i.e. a smaller corrosion resistance of the wholesystem coatingysubstrate. This behavior is related to theincrease in the coating porosity, which has a deleteriouseffect on the corrosion resistance, and was similar forall the values tested ofF ratio. The results obtained ini

the present study disagree with those reported in Ref.w4x, which claim that in the current HVOF coatings theinterconnected porosity is very low and, therefore, cor-

rosion by penetration of the coating is not the mainconcern. Evidences of this statement will be presentedlater in this paper when the morphology of the corrosionproducts will be discussed.

From the ANOVA, it was found thati depends ofcorr

the spraying distance(A), the powder feed rate(B), theequivalent ratio(F ) and the interaction between thei

powder feed rate and theF ratio (BC), since they werei

determined as being the significative statistical effects(Fig. 5). However, the frequency histogram determinedfor E , shown in Fig. 6, is indicating that the sprayingcorr

distance (A) and F (C) are the only significativei

statistical effects, when they were varied in the intervalsconsidered in the present investigation.

The results of empirical modeling of the corrosioncurrent density,i , and corrosion potential,E , arecorr corr

presented in Figs. 7 and 8, respectively.Each figure presents the contour of a constant

response and it is constituted of three sections(a–c),each one being determined for a constant value of thepowder feed rate: 60, 90 and 120 gymin, respectively.The axes, in all cases, correspond to the sprayingdistance(horizontal axis) andF ratio (vertical axis).i

From the analysis of Fig. 7a–c it could be observedthat the powder feed rate has a marked influence on thecorrosion current density,i . At constantF ratio, thecorr i

corrosion current density starts to increase with thespraying distance, and this increase is much moreaccelerated with the increase in the powder feed rate.For example, for a powder feed rate of 60 gymin andconstantF s1.2 when the spraying distance increasesi

from 380 to 560 mm,i value increases from 5 to 15corr

mAycm . The increase of the powder feed rate to 1202

gymin produces an increase ofi value to 25mAycorr

cm . Another interesting aspect is the significant change2

of the corrosion current density with theF value, wheni

the powder feed rate increases and the spraying distanceis maintained constant. For example, values oficorr

nearly 10 times higher are obtained whenF changesi

from 1.2 to 0.8 at a constant powder rate of 120 gyminand 380 mm spraying distance.

These results suggest that as the value ofF increases,i

the coatings tend to be composed of well flattened splats

451L. Gil, M.H. Staia / Thin Solid Films 420 –421 (2002) 446–454

Fig. 7. Contours of constant corrosion current density(i ) as a func-corr

tion of the spraying distance and theF ratio. Powder feed rate isi

maintain constant at(a) 60 gymin; (b) 90 gymin; (c) 120 gymin.

Fig. 8. Contours of constant corrosion potential(E ) as a functioncorr

of the spraying distance and theF ratio. Powder feed rate is maintaini

constant at(a) 60 gymin; (b) 90 gymin; (c) 120 gymin.

and the amount of particles, which were in partiallymelted state, have decreased considerably due to thehigher temperature achieved in the system. The resultsfrom this study agree with those from previous investi-gations w19–21x, which have shown that the fuelyoxygen ratio controls both the temperature of the flameand the plume oxygen concentration. It was determinedthat the maximum flame temperature occurs atF ratioi

slightly above stoichiometricw21x, and for a fuel richF ratio, significant amounts of fuel exist in the jet,i

which gives oxidation protection to the particles. Thiscondition guarantees a more homogenous phase distri-bution and, hence, a lower porosity and a lower contentof unmelted particles. With aF ratio of 0.8, significanti

amounts of free oxygen exist in the jet and, additionally,the heat liberated will be used to heat the combustionproducts and the excess oxygen, thus producing a muchlower final gas temperature, which in turn will affectthe coating microstructure.

Fig. 8a–c present the contours of constant corrosionpotential,E indicating that for both constant powdercorr

feed rate values of 60 and 90 gymin the E valuescorr

become more negative when the spraying distance isvaried from 380 to 560 mm atF constant and, hence,i

the coating corrosion resistance is decreased. However,when the powder feed rate used is of 120 gymin theonly influence on the corrosion potential is exerted bythe variation of F ratio values. Thus, in order toi

maintain a higher corrosion resistance at higher powderfeed rate it is necessary to increase the temperature.

From the analysis presented above, it is clear that theempirical modeling procedure used in the present inves-tigation was able to fit the data obtained experimentallyfor the three variables: the spraying distance, the powderfeed rate and the equivalence ratio(F ). This could bei

verified by comparison of the response surfaces of eachproperty and the experimental data presented in Table4.

Therefore, this study could be used to define theoptimum processing conditions that result in the HVOFNiWCrBSi desired coating properties.

For example, if only the value ofi smaller thancorr

the value ofi for the carbon steel AISI 1020 substratecorr

is imposed as desired coating specification, by analysingthese experimental data, it is observed that only sixcombinations of the chosen independent variables areproducing coatings with slightly better corrosion resis-

452 L. Gil, M.H. Staia / Thin Solid Films 420 –421 (2002) 446–454

Fig. 9. (a) SEM (backscattering mode) micrograph of the corrodedas-sprayed coating. Processing conditions: spraying distances560mm, F s0.8 and powder feeds120 gymin (b) details of the previousi

image.

tance than the uncoated substrate, i.e. withi andcorr

E values smaller than those obtained for the barecorr

substrate. All the other conditions are indicating thedeleterious effect due to the presence of the coating onthe corrosion performance of the whole system coatingysubstrate.

However, if the desired coatings properties are:icorr

between 5 and 10mAycm , E between 0.32 and 0.352corr

V and porosity-1% it could be concluded that in orderto achieve these values, the optimum processing condi-tions are: F ratio between 1.1 and 1.2, a sprayingi

distance of 380 mm and a powder feed rate of 60 gymin.

The response surfaces presented in Figs. 2, 7 and 8have been applied successfully in the HVOF industrialplant and it was demonstrated that constitutes a powerfultool in assessing the desired coating properties. Forexample, by changing the initial level of the processingconditions, presented in Table 4, substantial improve-ments in both porosity and corrosion resistance havebeen obtained. The porosity has changed from a valueof 5.4% to less than 1% meanwhile the corrosion currentdensityi has changed from 27.8 to less than 10mAycorr

cm .2

3.3. Scanning electron micrographs of the corrodedsamples

The characterization of the corrosion products byusing scanning electron microscopy was conducted onsamples, which belongs to the extreme processingconditions.

Fig. 9a shows the SEM backscattered electron micro-graph of the cross section of the corroded as-sprayedcoating deposited when a spraying distance of 560 mm,F ratio of 0.8 and a powder feed rate of 120 gymini

were employed during the deposition process. Both thepresence of iron oxide, detected by EDS at the coating-steel interface, and the high values determined for thecorrosion current densities,i (90.5mAycm ) support2

corr

the presence of interconnected pores and the existenceof micropores between lamellas, which have allowedthe electrolyte penetration, causing difference in aeration(crevice corrosion). Another interesting aspect to bementioned is the fact that the corrosion initiated prefer-entially at the splat boundaries, possibly due to amicrocrevice corrosion mechanism. This form of corro-sion has been reported in the literature by differentauthorsw4–6,10x.

Additionally, the galvanic cells formed between thehard phases of HVOF NiWCrBSi coatings, which arenobler, and the matrix of the alloy produced a prefer-ential dissolution of the later. The dark area in the SEMmicrograph presented in Fig. 9b is an indication of thisdissolution process and EDX analysis shows the pres-

ence of 26.16 at.% O ; 39.5 at.% Na; 19.12 at.% Ni;2

3.96 at.% Si; 1.59 at.% W; 6.45 at.% Cr and 3.22 at.%Fe.

Fig. 10a is a is a SEM backscattered electron micro-graph showing the microstructure of the as-sprayedcoating cross section produced with a spraying distanceof 380 mm,F of the 1.2 and a powder feed rate of 60i

gymin, parameters that correspond to the optimumprocessing conditions. The micrograph in Fig. 10b cor-responds to the same coating after the corrosion tookplace. It can be observed that the principal form ofcorrosion attack was general corrosion of the matrix. Inthis case, the corrosion was accelerated by the presenceof the hard phases(carbides and boridesw1x), since theinterface between these phases and the matrix provideda favorable site for the microgalvanic andyor the micro-crevice corrosion.

The morphological evidences provided by the SEManalysis support the fact that two mechanisms arecooperating during the corrosion process of the HVOFNiWCrBSi coatings: micropitting—when the hard phas-es are lost due to the corrosion of the adjacent matrixthat supports them, and macropitting—when individual

453L. Gil, M.H. Staia / Thin Solid Films 420 –421 (2002) 446–454

Fig. 10.(a) SEM (backscattering mode) micrograph of the cross sec-tion of the as-sprayed coating;(b) SEM (backscattering mode)micro-graph of the cross-section of the corroded coating.

splats are removed due to the strong corrosive attackgenerated at the splat boundaries. The later, providedpaths for the penetration of the corrosive fluid acrossthe coating thickness causing serious problems of cor-rosion and lost of adherence in the interface coatingysubstrate.

4. Conclusions

(1) HVOF process can be investigated efficientlyusing methods based on factorial experimental designs.Empirical modeling could be successfully used to iden-tify parameters that have a significant influence oncoating properties.(2) Analysis of the coatings characteristics has

showed that the spraying distance, the equivalence ratio(F ) and the powder feed rate as well as their interactioni

had significant effects on the corrosion resistance andporosity. The best coatings properties, for the level ofthe variables studied in the present investigation, couldbe obtain if theF ratio is between 1.1 and 1.2, thei

spraying distance is of 380 mm and the powder feedrate is of 60 gymin.(3) It was determined that, under the experimental

conditions stated above, the polarization test is a sensi-tive and simple test able to detect the effects producedon coating properties and microstructure due to changesin the deposition parameters.

(4) The heterogeneity of the as-sprayed HVOF coat-ings such as porosity, microcracks and microporebetween lamellae and unmelted particles, affects theircorrosion resistance since they can act as intercommu-nicating paths between the substrate and theenvironment.(5) It has to be taken into account that the presence

in the microstructure of nobler species likes carbidesand borides, generated microgalvanic and microcrevicecorrosion.(6) What it is necessary to point out is the fact that

although coatings with higher corrosion resistance thanthe substrate have been produced using the experimentalconditions indicated above, their industrial use is limiteddue to their inherent mechanisms of corrosion, whichtake place and seriously compromise their integrity inan aqueous environment. In these conditions, for anoptimum performance in corrosive media the coatedmaterials need to be subjected to post depositiontreatments.

Acknowledgments

The authors wish to acknowledge the financial supportreceived from the Venezuelan National Council forScientific and Technological Research(CONICIT)through both projects S1-2000000525 and LAB9700644. Mariana Staia wish to acknowledge the finan-cial support received from CDCH-UCV.

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