Influence of the quality and uniformity of ceramic coatings on corrosion resistance

CERAMICS

INTERNATIONAL

Available online at https://www.360docs.net/doc/064760144.html,

Ceramics International 41(2015)5138–

5146

In ?uence of the quality and uniformity of ceramic coatings

on corrosion resistance

V.Encinas-Sánchez a ,A.Macías-García a ,n ,M.A.Díaz-Díez a ,Paulo Brito b ,David Cardoso b

a

Departamento de Ingeniería Mecánica,Energética y de los Materiales,Universidad de Extremadura,Avda de Elvas s/n,E-06071Badajoz,Spain

b

C3i –Coordena??o Interdisciplinar para a Investiga??o e Inova??o,Instituto Politécnico de Portalegre,Lugar da Abadessa,Apartado 148,7301-901Portalegre,

Portugal

Received 10November 2014;received in revised form 6December 2014;accepted 15December 2014

Available online 23December 2014

Abstract

Pre-treated stainless steel substrates were dip-coated with yttria-doped zirconia,and the quality of the coatings was studied by a newly proposed non-destructive method,comparing the results with those obtained with other techniques.The objective was to analyze the effect of the coating's quality on the corrosion rate.To this end,the corrosion rate was determined by means of the linear polarization resistance method.The results showed coatings deposited on the polished substrate at a withdrawal rate of 25mm min à1to be the most uniform and homogeneous,and to present the best resistance to corrosion.

&2015Elsevier Ltd and Techna Group S.r.l.All rights reserved.

Keywords:C.Corrosion;Stainless steel;Coating;Yttria-doped zirconia;Characterization

1.Introduction

Austenitic stainless steels have numerous industrial applica-tions because of their mechanical properties and corrosion resistance [1].However,in long-time applications they are prone to localized corrosion from severe biological effects [2].In these cases,in addition to electron transport,Cl à,O 2,and H 2O species play important roles in the corrosion process [3].To prevent this phenomenon occurrence,one or more of the following methods can be used:(i)alloying the metal with elements that enrich the surface with a corrosion-resistant component during the corrosion process [4];(ii)adding aqueous inhibitors which adsorb strongly on the metal surface and prevent it from reacting with the oxidizing agent [5];and (iii)depositing protective coatings [6].The last one is the most commonly used method,especially with the application of ceramic ?lm coatings to the surface [7].The techniques involved have been extensively studied [8–12].

There are numerous methods for the preparation of coatings to deposit on metals.They include chemical vapor deposition (CVD)[13],physical vapor deposition (PVD)[14],electro-chemical deposition [15],plasma spraying [16],and the sol –gel process [17].Sol –gel coatings have many advantages,[18]including their generally low processing temperature (fre-quently close to room temperature)and the fact that thin ?lms can be produced without the need for machining or melting and by means of “green ”coating technologies (i.e.,the compounds used do not introduce impurities into the end product)[19].There have been many studies which have used the sol –gel route [20–23].

Thin ?lms can be formed by several oxides,such as Al 2O 3[24],CeO 2[25],ZrO 2[26],and Cr 2O 3[27].Zirconia (ZrO 2),for example,has good chemical stability and high hardness.It also has a high coef ?cient of thermal expansion that is very close to that of stainless steels,which can reduce the formation of cracks during high-temperature curing processes.Its proper-ties make it a good protective material [28].It has three polymorphic phases according to the temperature.The stable form is monoclinic at room temperature,but undergoes transi-tions to a tetragonal lattice between 9401C and 12501C and to

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n

Corresponding author.Tel.:t342428960086747;fax:t3424289601.E-mail address:amacgar@unex.es (A.Macías-García).

a cubic?uorite structure at23701C[29].Since the tetragonal

phase exhibits optimal mechanical properties,in order to avoid

undesirable phase transitions,zirconia needs to be stabilized

[30].A common system used for this purpose is yttria-stabilized

zirconia because its high ionic conductivity and thermal stability

improve the material's adhesion and corrosion resistance proper-

ties[31].

In sol–gel processes,the deposition of the solutions can be

performed by various techniques,including spin-coating[32],

electrodeposition[33],and dip-coating[34].Two of the

advantages of dip-coating are that it allows large areas to be

coated simply and inexpensively[35].It consists of dipping

the substrate into the sol–gel solution and then withdrawing it

at an appropriate rate to allow deposition of the?lm on the

substrate surface[36].

The quality of the coatings can be studied by electrical

characterization,measuring the impedance response[37].The

elemental composition and the oxidation state of the elements

are analyzed quantitatively by X-ray photoelectron spectro-

scopy(XPS),a surface-sensitive technique which can detect

changes in the surface's chemical composition[38].Time-of-?ight secondary ion mass spectroscopy(ToF-SIMS)is the closest technique to the ideal of chemical microscopy,provid-

ing simultaneously chemical information from surface mass

spectra down to depths of1nm and the surface distribution of

secondary ions at a lateral resolution of200nm[39].Finally,

scanning electron microscopy(SEM)is used to examine the

microstructure and surface morphology of the thin?lms[40].

The objective of the present work was to study how the

uniformity and thickness of yttria-doped zirconia coatings

deposited on AISI304stainless steel substrates in?uence the

action of various corrosive media.

2.Experimental procedure

2.1.Material

The substrate used was AISI304stainless steel.This is one

of the most versatile and most commonly used austenitic steels

in engineering.It is characterized by a low carbon content,

with its composition in weight percent being0.08%C,2.0%

Mn,18.7%Cr,11.0%Ni,0.045%P,and0.03%S.The test

pieces were fabricated to a size of20?30?2mm3.

2.1.1.Surface preparation and characterization of the material The test pieces were mechanically modi?ed by sanding and polishing,using sandpapers of240,600,and800grains cmà2and polishing cloths of9m m,6m m,and3m m,following the procedure presented in Table1.Each surface was treated for 5min at a force of30N,followed by ultrasound cleansing with alcohol and de-ionized water.The result was specimens with surfaces of different textures that were evaluated in terms of the average surface roughness,R a,measured using a Dataview TR-200portable roughness tester.

2.2.Substrate coating

2.2.1.Manufacture and characterization of the sol–gel solution The yttria-doped zirconia sol–gel solution was prepared in accordance with the literature[30]by mixing a solution of N-propoxide/1-propanol/H2O/HNO3of molar ratio1/15/6/1 with another of yttrium acetate/2-propanol/H2O/HNO3of molar ratio1/15/6/1.The mixture was mechanically stirred for30min,and then?nished by adding H2O followed by mechanical stirring for further1h.

The resulting sol–gel solution was characterized in terms of its pH,density,and viscosity at room temperature,using a Crison pH-meter,a Gay-Lussac pycnometer,and a modi?ed Ostwald capillary viscometer,respectively.A digital thermo-meter with a0.31C precision was used to measure the temperature of the solutions and the distilled water.

2.2.2.Coating and sintering

In the dip-coating technique,a clean substrate is immersed in a liquid bath and then withdrawn vertically at a constant rate, leaving a thin liquid?lm adhered to its surface[41].In the present case,the substrate was stainless steel in the form of a test piece specimen,and the bath was a sol–gel solution.We tested various withdrawal rates:135mm minà1,50mm minà1,and25mm minà1.After coating,the samples were dried at1001C for 60min,and then sintered at5001C for120min in a Nabertherm furnace,applying31C minà1heating and cooling rates.

2.2.

3.Characterization of the coating

The structural quality and morphology of the coated surfaces were studied by optical microscopy using a Nikon s Epiphot 300re?ected-light optical microscope,and by scanning electron microscopy(SEM)using an FEI Quanta s3D FEG scanning electron microscope.The latter was also used to determine the coating's thickness.

The uniformity of the coating was characterized by measur-ing the impedance response of the surface using a1920 Precision LCR meter with a four terminal,1mm diameter probe,applying alternating current in a frequency range of

Table1

Sequence of mechanical surface treatments applied to the samples.

First sandpaper Second sandpaper Third sandpaper1st polishing cloth2nd polishing cloth3rd polishing cloth Polished samples P240P600P8009μm6μm3μm

Type A samples P240...

Type B samples P240P600....

Type C samples P240P600P800...

Type D samples P800.....

V.Encinas-Sánchez et al./Ceramics International41(2015)5138–51465139

102–106Hz.In this method,a current is passed through the two outer probes,and the voltage across the two inner probes is recorded,yielding the substrate's resistance and capacitance [42].A scheme of the equipment used is shown in Fig.1.Since the capacitance is inversely proportional to the coating's thickness,a map of that thickness is obtained by taking measurements on a grid of equidistant points covering the coated area of the substrate.

The coating's chemical composition was studied by XPS using a Thermo Scienti ?c K-Alpha photoelectron spectrometer equipped with a monochromated Al K αX-ray source.The source was monochromatic radiation at 1486eV.The beam was perpendicular to the specimen's surface,the spot size was 100m m,and the take-off angle was 601relative to the normal surface.The XPS was combined with Ar tion sputtering at an energy of 2000eV,a 17m A current,and a raster area of 1mm 2in a quasi-vacuum of 4?10à8mbar pressure.For samples coated at a 50mm min à1withdrawal rate,16levels were measured,and 12levels for those coated at 25mm min à1.The sputter etching time was 90s per level.Level 0is taken to be a strictly surface assay because of the null sputtering.The etching-time dependence of the atomic percentage distribution of the various elements present was studied on the basis of the XPS data.

Further surface analyses were performed using a time-of-?ight secondary ion mass spectrometer (ToF-SIMS)equipped with a bismuth ion gun.Pulsed primary ion beams were used to desorb and ionize species from the surface of the specimens,operating with a 25kV Bi tpulsed beam for analysis and a 2kV Cs tpulsed beam for sputtering.For each scan,the total primary-ion dose was greater than 1?1012ions cm à2,and the area of each scan was 250?250m m 2with a spot size of 20.8?20.8m m 2.Negative spectra were recorded,the initial spectrum calibration being with the species H à,C à,CH à,CH 2à,CH 3à,OH à,C 2à,C 3à,C 4à,C 5à,and C 7à.Positive spectra were not recorded as this would have meant altering the oxygen signal since positive ions can only be monitored by applying sputtering with oxygen.For this reason,we applied Cs tsputtering in order to obtain a stable,real oxygen signal.To obtain the spectrum,it was necessary to use a reference sample of yttria-doped zirconia and measure the degree of reduction generated by the sputtering on the stable homo-geneous area studied.The sensitivity factor was recalculated on the basis of this degree of reduction,and then this factor was used in all the following calculations to determine the

percentage of each element in each layer.Since the main objective was to verify the homogeneity of the elemental distribution in the layer,this use of a constant reduction factor causes no problems when checking the distribution since the entire study was carried out under identical sputtering conditions.

2.2.4.Study of coating linear polarization resistance

Linear polarization resistance (LPR)measurement is a technique commonly used to determine corrosion rates.The method assumes that electrochemical corrosion is described by the Stern –Geary equation [43]i corr ?B =R p

e2:1TB ?b a b c =?2303eb a tb c T

e2:2T

where i corr is the corrosion current density,R p is the polariza-tion resistance,and b a and b c are the anodic and cathodic Tafel slopes,respectively.The LPR of the deposits was measured in three solutions:(a)3%(w/w)NaCl,(b)1M NaOH,and (c)1M HCl solutions,with an immersion time of 1h (potential stabilization time).The backs of the coated plates were painted with epoxy to keep the surface area constant and to insulate the copper wire connections.Prior to immersion in the electrolyte bath,the electrodes were rinsed with distilled water and dried.The electrochemical tests were performed in a three-electrode cell.A 316L stainless steel mesh with an apparent surface area of 100cm 2was used as the auxiliary electrode.The working electrode potential was measured against an Ag/AgCl in 1M KCl reference electrode.The LPR was determined with a Gamry 900potentiostat,applying a polarization range of 710mV and a scanning rate of 1mV/s.3.Results and discussion 3.1.Material

There have been numerous papers published on ceramic coatings with stainless steel test pieces used as substrates [44].We selected AISI 304stainless steel mainly because of its excellent properties that have led to its widespread use in industry.These properties include low thermal conductivity,and high values of ductility,tensile strength,fracture tough-ness,and work hardening rate.

3.1.1.Characterization of the material

In the characterization of the test pieces,the R a parameter varied from 0.153m m in the original specimens to 0.004m m in the polished specimens (Fig.2).Other studies have used substrates with an R a value of 0.050m m [45].3.2.Substrate coating

3.2.1.Characterization of the sol –gel solution

The ?nal pH of the yttria-doped zirconia sol –gel solution was kept at around 0.5,the density was 0.966g cm à3,the ?nal precursor/alcohol/H 2O/HNO 3mole ratio was 1/15/6/1,and

the

Fig.1.Equipment used for the electrical measurements.

V.Encinas-Sánchez et al./Ceramics International 41(2015)5138–5146

5140

yttria concentration was 3mol%.The solution's viscosity decreased from 1.3cP to 0.8cP as its temperature was increased from 101C to 301C.Various workers have used similar sol –gel solutions in the synthesis of stabilized or unstabilized ZrO 2[46–49].3.2.2.Characterization of the coating

During the immersion phase of the dip-coating method,the initial adherence of the yttria-doped zirconia thin ?lm to the substrate is due to Van der Waals type forces.Subsequently,stronger chemical bonds are formed by condensation reactions between OH groups linked to atoms of the substrate surface and OH groups or OR groups in the solid skeleton of the gel.During the sample extraction and solvent evaporation phase,and depending on the adhesion of the coating with the substrate surface,there arise tensions in the coating.These tensions are due to both contraction of the thin ?lm forming the coating during the evaporation phase and the ongoing condensation reactions.This would explain how the surface roughness of the sanded specimens (types A –D),re ?ected in their high R a values,affects negatively the homogeneity of the sintered coatings,since the sanding of the surface,the coating process,and the sintering of the ?lms leave a series of irregularities on the surface.These effects are attenuated in the polished specimens which had very low R a values.

With respect to the different withdrawal rates,the higher the value of R a ,the smaller the amount of coating that was deposited.This is because more ROH and H 2O are released during the sintering for higher values of R a .It was also observed that the amount of coating deposited on the substrates decreased with decreasing withdrawal rate.

The coatings were characterized texturally by optical microscopy.Fig.3shows the optical micrographs for some selected samples,illustrating the progressive improvement of the coating as the values of R a decrease with increasing severity of the mechanical treatment of the substrates.Simi-larly,the surface termination also becomes progressively more uniform and homogeneous.

The SEM characterization of the morphology of the coatings showed that the cracking of the ?lms (Fig.4)increases with

increasing withdrawal rate.This may be attributed to the amount of deposited material,which is directly related to the withdrawal rate,and to the increased ROH and H 2O removal during sintering as the deposition grows.

One clearly concludes from the results that the best coatings are attained with polished substrates (lowest values of R a )and lower withdrawal rates.Henceforth therefore,we shall report ?ndings only with the polished samples coated at the 25and 50mm min à1withdrawal rates.The results reported in this section indicate that the coating's homogeneity is probably closely related to the low R a value of the polished samples,and to the fact that,in draining,the receding drying line speed is very close to the withdrawal rate [50].

As described in Section 2.2.3,the uniformity of the coating's thickness was characterized electrically,with appropriate soft-ware converting the experimental data into the corresponding coating maps.These show the measured capacitances (Z axis)as a function of the position on the coated surface (X and

Y

Fig.2.R a parameter values obtained for the samples before

coating.

Fig.3.Optical micrographs of the coated samples:(a)original (R a ?0.153μm);(b)type A (R a ?0.083μm);and (c)polished (R a ?0.004μm).

V.Encinas-Sánchez et al./Ceramics International 41(2015)5138–51465141

axes),with the capacitance being inversely proportional to the coating's thickness.The maps are presented using a color scale for the different capacitance intervals.We measured the capacitance at 16points distributed over the coated area of the substrate.Fig.5shows the two maps for sanded and polished samples coated at the same withdrawal rate (25mm min à1).One clearly observes the improvement in the coating with the greater severity of the mechanical treatment of the substrates,as was to be expected,with the coating's thickness

being uniform across the polished sample.Fig.6shows the two maps for polished samples coated at different withdrawal rates.Again,one clearly observes the improvement in the coating at the lower withdrawal rate.This is attributable to the formation of thinner ?lms at the lower rates,hence release of less ROH and H 2O during the sintering process,and conse-quently greater uniformity in the coating's thickness.The observed area of irregularities in coating thickness is due to edge effects during the coating process which result from inhomogeneities created during the substrate preparation.Fig.7shows SEM observations of the coating thickness of the polished specimens at the withdrawal rates of 25mm min à1and 50mm min à1.One observes that the thicknesses are roughly the same (around 500nm),implying that a critical value has been attained.

Fig.8shows the results of the XPS analysis for these same two samples.One observes that there is a clear similarity of the patterns for the different etching times,con ?rming the homo-geneity of the coating thicknesses observed in the capacitance maps.Quanti ?cation of the Zr(3d),O(1s),Y(3d),and Fe(2p)peaks in the two spectra yielded the elemental atomic percentages as functions of the etching time for the two specimens (Fig.9).The depth pro ?les indicate a uniform and homogeneous central area consisting of the oxides of zirconia and

yttria.

Fig.4.Scanning electron micrographs of the evolution of the ?lms'cracking on the polished substrate for different withdrawal rates:(a)25mm min à1;(b)50mm min à1;and (c)135mm min à1

.

Fig. 5.The coating's uniformity for the 25mm min à1withdrawal rate according to the mechanical treatment applied to the substrate:(a)type C and (b)polished.

V.Encinas-Sánchez et al./Ceramics International 41(2015)5138–5146

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The ToF-SIMS technique used allowed a maximum depth of analysis of about 2nm.The sequence followed was a scan analysis followed by a 1-s sputtering time,and then allowing a period of stabilization (0.5s),with this sequence being repeated several times.Fig.10shows the results of monitoring the ions of interest in the two samples.There is clear con ?rmation of the XPS results.The central area (Zone II),basically zirconia and yttria,is very homogeneous,with essentially ?at plateaux being found for these compounds.The plateaux are ?atter in the 25mm min à1coated sample,implying that this withdrawal rate leads to a more uniform and homogeneous coating.One also sees from the two pro ?les that the resistance to the application of Cs ions is greater in the specimen coated at 50mm min à1,coherent with the thicker coating indicated by the SEM study.

3.2.3.Study of the coating's linear polarization resistance Table 2presents the results of the normalized linear polarization resistance study for each sample in the three types of solution used:HCl (1M),NaCl (1M),and NaOH (1M).The resistances are greatest in the basic medium,intermediate in the saline medium,and very low in the acidic medium.

High

Fig. 6.The coating's uniformity on polished substrates according to the withdrawal rate:(a)25mm min à1and (b)50mm min à1

.

Fig.7.Coating thickness of polished specimens coated at withdrawal rates of (a)25mm min à1(448.5nm)and (b)50mm min à1(544.0

nm).

Fig.8.XPS depth pro ?les for samples coated at withdrawal rates of (a)25mm min à1and (b)50mm min à1.

V.Encinas-Sánchez et al./Ceramics International 41(2015)5138–51465143

values of the resistance indicate a low corrosion rate,i.e.,good anti-corrosion behavior of the coated sample (Eq.(2.1)).

With respect to the specimen's surface treatment and the withdrawal rate,the best results were obtained with the polished samples and the higher withdrawal rate,i.e.,50mm min à1.Thus,considering ?rst the treatment of the surface,the polished samples presented greater coating uniformity than the type C samples (Fig.5),and the ToF-SIMS analysis showed their coatings to be very homogeneous (Fig.9a).These two aspects,uniformity and homogeneity,are closely related to linear polarization resistance,and consequently to corrosion resistance.Considering second the withdrawal rate,the sam-ples coated at 25mm min à1had greater uniformity and homogeneity than those coated at 50mm min à1(Figs.6and 9).Although the 50mm min à1withdrawal rate coatings were thicker,and one might have expected greater thickness to correspond to greater corrosion resistance,their R p values were lower (Table 2),so that these samples would have faster corrosion rates.To understand this behavior,it is important to bear in mind that the critical phase of the sol –gel coating process is the formation of the thin ?lm when the wet gel is drying.In this phase,fractures can appear as a result of both the high capillarity forces that arise when the solvent

evaporates and of the contraction tensions.This is seen in Fig.4b which corresponds to the polished sample coated at a withdrawal rate of 50mm min à1.This clearly shows that the coating's thickness has a major in ?uence on the linear polarization resistance,and hence lower the corrosion resistance,greater will be the thickness.4.Conclusions

Using optical and scanning electron micrographs,an elec-trical method previously developed by our research group,and XPS and ToF-SIMS characterization techniques,we found the coatings on polished specimens to be very regular in uni-formity and thickness,in concordance with the values mea-sured for the R a roughness parameter.The discontinuities in the coating were greater as R a increased.Also,low withdrawal rates were found to yield coatings that were more homoge-neous and uniform.

From the results,one may conclude that the thickness,homogeneity,and uniformity of the coatings,and consequently the R a value of the substrates and the withdrawal rate used in the coating process,have a major effect on linear polarization resistance,and therefore on the corrosion rate.In

particular,

Fig.9.Atomic percentage of elements depending on the etching

time.

Fig.10.ToF-SIMS analysis of the samples coated at withdrawal rates of (a)25mm min à1and (b)50mm min à1.

V.Encinas-Sánchez et al./Ceramics International 41(2015)5138–5146

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greater uniformity and homogeneity of the coatings,and hence lower values of R a and the withdrawal rate,imply greater corrosion resistance.

The linear polarization resistances were lower in the thicker samples,obtained at a withdrawal rate of50mm minà1.This greater thickness seems to in?uence the appearance of cracks, since these samples presented the beginnings of cracking, which is closely related to the tensions created during the sintering process and to the greater coating mass deposited on the sample.Although the cracks actually observed were slight, they seemed to reach the substrate,and would thus lead to poor anti-corrosion behavior.Since the coating's thickness is closely related to corrosion resistance,the straightforward electrical method used here to measure the surface's electrical capaci-tance in order to evaluate the uniformity of that thickness would seem to be of particular interest in these applications. References

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Table2

Normalized linear polarization resistance of the coated samples.

Solution

HCl (1M)NaCl

(1M)

NaOH

(1M)

R p(kΩ/cm2)

Original sample0.20.4 2.2 Type C sample coated at

50mm minà1

0.4 4.432.7

Polished sample coated at

50mm minà1

0.410.058.7 Type C sample coated at

25mm minà1

0.726.2112.4

Polished sample coated at 25mm minà18.9108.2169.3

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