Conversion coating modified by alumina has been studied as a way for improving the resistance to thermal oxidation of an austenitic stainless steel. Conversion coating, characterized by a particular morphology and strong interfacial adhesion with the substrate, facilitate the electrochemical deposition of ceramic layers and enhance their adhesion to the substrate. The influence of the current density and treatment time on alumina deposit was studied using statistical experimental designs like Doehlert uniform shell design. After heating, coatings present a continuous composition gradient with refractory compounds at the surface. The behavior at high temperature (1000 8C) of the alumina coating was investigated. The presence of alumina increases the oxidation resistance of an austenitic stainless steel at 1000 8C. The morphology and the chemical composition of the deposit are analyzed. Results on the thermal stability of coating on austenitic stainless steel are presented

A. Guenbour

Ben Bachir

D. Mathieu

E. Puech-Costes

El Alaoui

El Hajjaji

L. Aries

M.-T. Maurette

P. Simon

Science and Technology of Advanced Materials

English

International audienceConversion coating modified by alumina has been studied as a way for improving the resistance to thermal oxidation of an austenitic stainless steel. Conversion coating, characterized by a particular morphology and strong interfacial adhesion with the substrate, facilitate the electrochemical deposition of ceramic layers and enhance their adhesion to the substrate. The influence of the current density and treatment time on alumina deposit was studied using statistical experimental designs like Doehlert uniform shell design. After heating, coatings present a continuous composition gradient with refractory compounds at the surface. The behavior at high temperature (1000 8C) of the alumina coating was investigated. The presence of alumina increases the oxidation resistance of an austenitic stainless steel at 1000 8C. The morphology and the chemical composition of the deposit are analyzed. Results on the thermal stability of coating on austenitic stainless steel are presented

El Hajjaji, Souad

El Alaoui, Sidi Mohammed

Simon, Patrice

Guenbour, Abdellah

Ben Bachir, Ali

Puech-Costes, Edith

Maurette, Marie-Thérèse

Ariès, Lucien

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Preparation and characterization of electrolytic alumina deposit on austenitic stainless steel

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   Open Archive Toulouse Archive Ouverte (OATAO)  OATAO is an open access repository that collects the work of Toulouse researchers and makes it freely available over the web where possible.  This is an author-deposited version published in: http://oatao.univ-toulouse.fr/  Eprints ID :  2750  To link to this article :  URL : http://dx.doi.org/10.1016/j.stam.2005.01.004  To cite this version : El Hajjaji, S. and El Alaoui, M. and Simon, Patrice and Guenbour, A and Ben Bachir, A. and Puech-Costes, E. and Maurette, M.-T. and Aries, L. ( 2005) Preparation and characterization of electrolytic alumina deposit on austenitic stainless steel. Science and Technology of Advanced Materials, vol. 6 (n° 5). pp. 519-524. ISSN 1468-6996 Any correspondence concerning this service should be sent to the repository administrator: staff-oatao@inp-toulouse.fr   Preparation and characterization of electrolytic alumina depositon austenitic stainless steelS. El Hajjajia,*, M. El Alaouia,b, P. Simonb, A. Guenboura, A. Ben Bachira,E. Puech-Costesc, M.-T. Maurettec, L. AriesbaLaboratoire d’Electrochimie—Corrosion, Universite´ Med V, Faculte´ des Sciences, Av Ibn Batouta, BP 1014 Rabat, MoroccobCIRIMAT, LCMIE, Universite´ Paul Sabatier, 118 Route de Narbonne, 31064 Toulouse Cedex 4, FrancecIMRCP, Groupe POM, Universite´ Paul Sabatier, 118 Route de Narbonne, 31064 Toulouse Cedex 4, FranceAbstractConversion coating modified by alumina has been studied as a way for improving the resistance to thermal oxidation of an austeniticstainless steel. Conversion coating, characterized by a particular morphology and strong interfacial adhesion with the substrate, facilitate theelectrochemical deposition of ceramic layers and enhance their adhesion to the substrate. The influence of the current density and treatmenttime on alumina deposit was studied using statistical experimental designs like Doehlert uniform shell design. After heating, coatings presenta continuous composition gradient with refractory compounds at the surface. The behavior at high temperature (1000 8C) of the aluminacoating was investigated. The presence of alumina increases the oxidation resistance of an austenitic stainless steel at 1000 8C. Themorphology and the chemical composition of the deposit are analyzed. Results on the thermal stability of coating on austenitic stainless steelare presented.Keywords: Electrolytic deposit; Statistical experimental design; Surface analysis; Conversion coating; Alumina; Oxidation; Austenitic stainless steel1. IntroductionMetals and alloys often have to be protected againstoxidation at high temperatures. This is usually done by theuse of ceramic oxides such as alumina [1–6]. Numerousmethods can be used to obtain these coatings [1–12]. Themost interesting ones is the electrochemical depositionwhich present several advantages [1–2,4,6,7].In order to obtain a strongly adherent ceramic layer, anoriginal method has been developed [2,7,13] which involvesthree steps. The first step consists of a controlled conversiontreatment in acid solution, in order to functionalize thesurface. The particular morphology of the conversioncoating obtained present micropores and a high specificdoi:10.1016/j.stam.2005.01.004* Corresponding author. Tel./fax: C212 3777 5440.E-mail addresses: selhajjaji@hotmail.com (S. El Hajjaji), hajjaji@fsr.ac.ma (S. El Hajjaji).area, which makes the surface suitable for the subsequentdeposit.In the second step, the ceramic layer is deposited by acheap and easy electrochemical method which allows thecontrol of the thickness, the morphology, and thecomposition of the coating, by varying the electrochemicalparameters and bath composition.In the third step, heating induces coat dehydration andleads to a reaction at the interface, between the conversioncoating compounds and the electrochemical deposit, thatimproves coating adhesion.The purpose of this paper is the study of alumina coatingprepared by cathodic treatment of the functionalizedaustenitic stainless steel in an aqueous solution.This study was undertaken to prepare and to investigatethe composition, the morphology, and the thermal behaviorof alumina coating. The parameters of alumina deposit havebeen studied using statistical experimental designs. Theelectrochemical process of deposit and the Doehlert shelldesign are briefly reviewed before the experimental resultsare presented.Table 1Chemical composition of austenitic stainless steel (wt%)C Cr Ni Mo Cu Si S Ti Fe0.031 18.20 10.30 0.10 0.06 0.77 0.007 0.01 Bal.2. Experimental procedure2.1. Preparation of conversion coatingDeposits were prepared on an austenitic stainless steel,its composition are given in Table 1. Substrates wereprepared in the form of 0.5 mm!10 mm!10 mm speci-mens. Samples were cleaned with tetrahydrofurane(ACROS ORGANIC, purityZ99%), washed with distilledwater and then dried in air at room temperature.Stainless steel conversion coatings can be obtained eitherby electrolytic or chemical treatment in acid bath containingsuitable additives and particularly substances containingchalcogenides such as sulphur (sulphides, thiosulphates)[13]. Corrosion inhibitors like acetylenic alcohols are alsorequired to facilitate the control of film growth in order toobtain coats with specific proprieties [13].In this study, the conversion treatment involved twosteps: firstly the austenitic stainless steel was dipped insulphuric acid solution at 20% (purity 98%) for 10 s andthen was dipped in acid solution containing additives suchas thiosulphate and propargyl alcohol, in order to control thegrowth of the coating and to obtain properties such asmicroporosity with the resulting high surface area.Conditions of the conversion treatment are presented inTable 2.2.2. Preparation of alumina depositsSecondly, the electrochemical deposition of aluminacompounds was performed in aqueous solution usingplatinum electrode as anode and a saturated calomelelectrode as reference (SCE).The alumina deposit was induced by cathodic reactions,according to a process which involves both electrochemicaland chemical reactions [13]. The cathodic reactions, withspecies such as protons and dissolved oxygen induces alocal rise in pH at the electrode2H2O C2eK/H2 COHK O2 C2H2O C4eK/4OHKThe pH increase provokes precipitation in the pores andat the surface of oxides or hydroxides compounds withvarying degrees of hydration [1,3]Table 2Conditions of the conversion treatmentAcid bath Hydrated sodium thiosulphate Propargyl alcoholSulphuric acid: 0.2% Na2S2O3$5H2O: 2 g/L C3H4O: 1.25 mL/LAl3C C3OHK$AlðOHÞ3In process optimization (or evaluation) there has alwaysbeen a strong belief that to determine how one factorinfluences a response, all other factors must be held constantwhile only varying that particular factor. As a result, manyinvestigations are still carried out using the inefficient‘changing one separate factor at a time’. This approachmight lead to incomplete mapping of the behavior of thesystem, often resulting in poor understanding, as well asincorrect conclusions. This approach is applicable onlywhen there are no interactions among the studied factors.Interaction effects are measures of how the response ychanges, with respect to a factor, as another factor changes.So, optimization benefit the use of statistical designs[14–16]. These are informationally optimal mathematicalschemes in which all important factors are changedsimultaneously, thereby facilitating the identification ofprocess relations as well as the location of the real processoptimum. In order to determinate the conditions of aluminadeposit, we used a Doehlert uniform shell design. A fullsecond-order polynomial empirical model is often sufficientfor adequately describing an electrolytic alumina depositprocess, at least in a selected limited experimental domain.This strategy allows an estimate (using the least squareslaw) to be made of the coefficients of a quadratic polynomialmodel (response y versus X1 and X2). In the present work,two factors were studied, treatment time (X2) and currentdensity (X1), requiring that six coefficients be determined asfollows:y Z b0 Cb1X1 Cb2X2 Cb11X21 Cb22X22 Cb12X1X2With this strategy, seven experiments were necessaryand, in a coded space X1X2, were distributed as follows: sixpoints were the vertices of a regular hexagon and theseventh one was its center [14,15]. The experimental designand the corresponding scheme are presented in Table 3 andFig. 1. Their variation domains were determined inpreliminary experiments (Table 4).To minimize the effect of uncontrolled factors and timevariations, all experiments were performed in random orderand the calculation was obtained by the NEMROD program[16]. In order to show the effect of each factor, the studiedresponse is the mass gain of alumina deposit.Bath temperature (8C) Treatment time (min) Current (mA)55–60 20 300Table 5Conditions for the synthesis alumina films on austenitic stainless steelSolution Temperature (8C) AgitationAl2(SO4)3$14H2O: 1 mol/L 12 YesTable 3Doehlert design: theoretical values of coded variablesExperiment no. X1 X21 1 0.02 K1 0.03 0.5 0.8664 K0.5 K0.8665 0.5 K0.8666 K0.5 0.8667 0.0 0.0Table 4Factors and their levels for the experimentsi (mA/cm2) (X1) Treatment time (min) (X2)Level (K1) K30 10Level (C1) K100 30The other conditions of deposits are presented in Table 5.After the preparation, samples were washed with water,dried in air at 70 8C and then heated at 900 8C for 2 h.2.3. Characterization methodsThe coatings were analyzed by secondary ions massspectroscopy (SIMS) and the surface morphology wasexamined by scanning electron microscopy (SEM). Iso-thermal oxidation tests were performed on a TAG 24SETARAM thermobalance. Oxidation kinetics of uncoatedand coated samples were studied at temperature 1000 8C for8 h in dynamic air at atmospheric pressure as a function oftime. The total corrosion was evaluated by measuring theweight-gain as a function of heating time. The sensitivity,accuracy, and stability of the balance signal permitted acareful and accurate analysis of the oxidation kinetics.3. Results and discussion3.1. Morphology and chemical composition of conversioncoatingVisually, the conversion coating appeared homogeneous.Examination by scanning electron microscopy (SEM) of theobtained conversion coating showed that the layer had auniform appearance. The surface was rough which suggeststhat the conversion coating has a fractal character (Fig. 2).Fig. 1. Location of seven points of two-factor Doehlert uniform shell designin coded factor space.This specific morphology is well suitable to facilitate thedeposition and the ‘anchoring’ of the alumina deposit.Secondary ion mass spectroscopy analysis shows aconcentration gradient of metallic elements in the coatings(Fig. 3). The coating is characterized by a continuousvariation of the chemical composition from the metallicsubstrate to the superficial zone. SIMS profiles indicate thatthe surface contains only nickel compounds. Fe and Crpresent identical profiles, which suggests that they are partof the same chemical compounds.3.2. Preparation of alumina depositAluminium hydroxide was deposited through cathodicreactions in aqueous solution of aluminium sulphate. Inorder to determine the electrolytic alumina depositconditions, we have used Doehlert uniform shell design.Two variables studied were current density (X1) andtreatment time (X2)The experimental design according to Doehlert’s matrixand response values are given in Table 6. Processing of thedata led to the estimation of six coefficients for thepolynomial equation for response as follows:Fig. 2. SEM image of conversion coating on austenitic stainless steel.Fig. 4. Response surface of the weight-gain versus X1 (current density) andX2 (treatment time).00E+002.0E+064.0E+066.0E+068.0E+061.0E+071.2E+071.4E+070 200 400 600 800Arbitrary Units (u.a)NiCrFeOSputtering time (s)Fig. 3. Distribution profiles of elements in the conversion coating againstbombardment time.Dm Z 1:183 C0:102X1 C0:217X2 K0:11X1X1C0:154X2X2 K0:502X1X2(X1: current density, X2: treatment time)Fig. 4 represents the variation of response Dm accordingto current density and treatment time. Results show that theincrease in treatment time and the decrease in currentdensity increase the mass gain of alumina deposit. Themathematical resolution for this equation led to an areawhere the mass gain of the alumina deposit is the highest.The high mass gain of alumina deposit has been obtained forthe conditions given in Table 7.3.3. Morphology and chemical composition of the aluminadepositAfter electrolytic preparation (before heating), visually,coatings appeared homogeneous. SEM examination showedthat it was uniform. Alumina deposit presents the ‘cracked-mud’ morphology induced on drying the wet coating in airat 70 8C (Fig. 5).SIMS profiles show a continuous concentration gradientof metallic elements in the coating from the metallicTable 6Doehlert design: natural variables and result for each experimentNo. X1, current density(mA/cm2)X2, treatmenttime (min)Mass gain, Dm(mg/cm2)1 K100 20 1.21 K100 20 1.12 K30 20 1.12 0 K30 20 0.93 K82.5 30 1.13 0 K82.5 30 1.034 K47.5 10 0.74 0 K47.5 10 0.45 K82.5 10 1.15 0 K82.5 10 1.26 K47.5 30 1.46 0 K47.5 30 1.47 K65 20 1.27 0 K65 20 1.1substrate to the superficial zone (Fig. 6). Profiles of iron andchromium from the initial conversion coating are notmodified by the cathodic post treatment. A continuousvariation of the aluminium profile is observed and suggeststhat aluminium is present in the deep layer up to theaustenitic stainless steel. We noted the absence of nickelwhich probably has been dissolved in the solution duringcathodic treatment.3.4. Oxidation kinetics, morphology, and composition ofcoatings after heat treatmentA high temperature oxidation was carried out in dynamicair at 1000 8C for 8 h. Fig. 7 shows the weight-gain kineticsfor the uncoated and coated austenitic stainless steel. Theresistance of the coated alloy to oxidation is high comparedto that of the uncoated alloy. The studied coating induced areduction in mass gain during thermal oxidation; the massgain is very low. The thermogravimetric curves, corre-sponding to the coated samples, can be perfectly fitted to aparabola. The coated alloy exhibits a low oxidation ratecompared to that of the uncoated alloy owing to thepresence of the alumina coating. The thermal shockresistance of the coating has been verified and no surfacedamage has been noted. Irregular kinetics is observed forTable 7Conditions of alumina depositSolution Tempera-ture (8C)Agita-tionCurrentdensity(mA/cm2)Treatmenttime (min)Al2(SO4)3$14H2O:1 mol/L12 Yes K30 300.0E+002.0E+064.0E+066.0E+068.0E+061.0E+071.2E+070 500 1000 1500 2000 2500Sputtering time (s)Arbitrary Units(u.a)OAlCrFeNiFig. 6. Distribution profiles of elements in the obtained alumina depositagainst bombardment time.Fig. 8. SEM images of alumina coatings after oxidation at 1000 8C in air for8 h.Fig. 5. SEM image of obtained alumina deposit on austenitic stainless steel.uncoated alloy, which can be attributed to the cracking orpartial detachment of the oxide scale. For the uncoatedalloy, the weight-gain increased with temperature and theoxidation is approximately linear.After oxidation in air at 1000 8C, SEM observations ofcoated samples indicate that the surface does not presentscale spalling phenomenon (Fig. 8). Heating induces animportant modification of the initial coating morphology: itled to nodules forming a compact structure and the crackedmud aspect disappears. Treatment at high temperatureFig. 7. Oxidation curves at 1000 8C in air for 8 h for uncoated and coatedaustenitic stainless steel.induced crystallization phenomenon which led to smallcrystals of about 1 mm in size.The X-ray diffraction spectrum of the oxidized coatedsteel show the peaks corresponding to a alumina, mixediron–chromium oxide and smaller peaks of nickel oxide.4. ConclusionThe subject of this paper was the preparation and thecharacterization of electrolytic alumina coating on an iron–chromium–nickel alloy. The method involves a cathodictreatment of the samples in a solution containing aluminumsalts; the austenitic stainless steel must be initiallyfunctionalized by a suitable conversion treatment. In thispaper, alumina coating process has been studied bystatistical experimental design; the increase of treatmenttime and the decrease of current density increase the massgain of alumina deposit. Observations and analysis,respectively, by SEM and SIMS of the alumina coatingshow that the surface was uniform and the profiles werecontinuous. The study of thermal behavior of this coatedaustenitic stainless steel at 1000 8C shows that the aluminahas a protector character at high temperature. The lowoxidation rate is due to the presence of a alumina whichreinforces the protector character of mixed iron–chromiumoxide.AcknowledgementsWe like to thank the ‘comite´ mixte franco-marocain, A.Ino. MA/03/67’ for financial support for this work.References[1] R. Chaim, I. Silberman, L. Gal-Or, J. Electrochem. Soc. 138 (1991)1942–1946.[2] L. Aries, L. Alberich, J. Roy, J. Sotoul, Electrochem. Acta 41 (18)(1996) 2799–2803.[3] L. Gal-Or, I. Silberman, R. Chaim, J. Electrochem. Soc. 138 (1991)1939–1942.[4] R. Chaim, G. Stark, L. Gal-Or, H. Bestgen, J. Mater. Sci. 29 (1994)6241–6248.[5] H. Honno, M. Tokita, A. Furusaki, R. Furuichi, Electrochem. Acta 37(1992) 2421–2425.[6] D. Tench, L. Warren, J. Electrochem. Soc. 130 (1983) 869–872.[7] L. Aries, J. Roy, J. Sotoul, V. Pontet, P. Costeseque, T. Aigouy,J. Appl. Electrochem. 26 (1996) 617–622.[8] S. El Hajjaji, A. Ben Bachir, L. Aries, Surf. Eng. 17 (2001) 201–204.[9] S. El Hajjaji, S. Manov, J. Roy, T. Aigouy, A. Ben Bachir, L. Aries,Appl. Surf. Sci. 180 (2001) 293–301.[10] S. El Hajjaji, A. Guenbour, A. Ben Bachir, L. Aries, Corros. Sci. 42(2000) 941–956.[11] L. Aries, J. Roy, F. Senocq, S. El Hajjaji, Mater. Corros. 51 (2000)496–501.[12] S. El Hajjaji, M.-T. Maurette, E. Puech-Costes, A. Ben Bachir,L. Aries, Br. Corros. J. 34 (1999) 273–279.[13] L. Aries, J. Appl. Electrochem. 24 (1994) 554–558.[14] D.H. Doehlert, Appl. Stat. 19 (1970) 231–241.[15] D.H. Doehlert, V.L. 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Preparation and characterization of electrolytic alumina deposit on austenitic stainless steel

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