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Characterisation of electrolytically deposited alumina and yttrium modified alumina coatings on steel
Surface and Coatings Technology 162 (2003) 154–160
Characterisation of electrolytically deposited alumina and yttrium modified alumina coatings on steel A. Lgamria, A. Guenboura, A. Ben Bachira, S. El Hajjajia,*, L. Ariesb a
´ Laboratoire d’Electrochimie – Corrosion, Departement de Chimie, Faculte´ des Sciences, Universite´ Med V, Av Ibn Battouta, BP 1014 Rabat, Morocco b CIRIMAT-LCMIE, Universite´ Paul Sabatier, 118 Route de Narbonne, 31064 Toulouse Cedex 4, France Received 24 January 2002; accepted in revised form 23 July 2002
Abstract Conversion coatings modified by deposits of alumina added or not with yttrium have been studied in order to improve the resistance to thermal oxidation of steel. The proposed process leads to the formation of a coating characterised by strong interfacial adhesion with the steel. Conversion coating, characterised by a very porous morphology and strong interfacial adhesion to the substrate, facilitates the electrochemical deposition of ceramic layers and enhances its adhesion. After heating, the coatings present continuous composition gradients with refractory compounds at the surface. All the studied coatings increase the oxidation resistance of the steel at 750 8C; they reduce the oxidation parabolic rate constant and prevent a scale spallation phenomenon. The extent of beneficial effect depends on the nature of cathodic treatment bath. Yttrium improved the protecting character of alumina coating; reduces the rate of oxidation and increase the adhesion of deposits. The electrochemical behaviours, morphology, chemical composition and thermal stability of the different deposits are analysed. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Electrolytic deposit; Conversion coating; Alumina; Yttrium; Oxidation; Steel
1. Introduction Numerous methods have been proposed to protect metals against oxidisation at high temperature w1–14x. The most interesting ones consist of deposition of ceramics which are currently attracting much interest for high temperature applications and in severely corrosive environments. In order to obtain a strongly adherent ceramic layer, we have used an original method, which includes a pre-treatment so as to make the surface suitable for the subsequent deposit. This pre-treatment of the metal consists of the formation of a functional conversion coating characterised by strong interfacial adhesion with the substrate and enable to induce inter*Corresponding author. Tel.yfax: q212-37-77-54-40. E-mail address: [email protected] (S. El Hajjaji). [email protected] (S. El Hajjaji), [email protected] (S. El Hajjaji).
facial reactions at high temperature with the ceramic coating. The functional conversion coating presents a morphology with micropores and a high level of roughness. This texture contributes to the anchoring of the ceramic layer and facilitates the interfacial reactions with the electrodeposit. It contains oxides of the metal substrate promoting the formation of a favourable phase at high temperature. For Fe–17%Cr stainless steel, it has been demonstrated w15x that the conductive conversion coating owing to its particular morphology with pores and cavities, facilitates the reduction reactions responsible for the increase of the pH at the surface and acts as a porous electrode allowing local pH conditions favourable for aluminium oxyhydroxide precipitation. In the second step, the ceramic layer is deposited by electrochemical method in an suitable baths according to the considered deposit. The thickness, morphology
0257-8972/03/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 2 . 0 0 5 1 6 - 9
A. Lgamri et al. / Surface and Coatings Technology 162 (2003) 154–160
(i) Generation of OHy
Table 1 Conditions for the conversion treatment Sulphuric acid concentration Thiosulphate concentration Propargyl alcohol concentration Temperature Treatment time
155
2.04 M 6=10y3 M 2=10y2 M 60 8C 15 min
and composition of the coating can be easily controlled by varying the electrochemical parameters and bath composition. In previous papers w8–14x, it was shown that an alumina coating prepared in this way slows the oxidation rate and protects 17% Cr stainless steel up to 1000 8C. Indeed, it was well known that very small additions of reactive elements such as Ca, Y, to the bulk or to the surface of Cr2O3 and Al2O3 also forming alloys to the pure metals could improve the oxidation resistance significantly w16–20x. The purpose of this paper is the study of aluminay yttrium coatings prepared by cathodic treatment of functionalised steel in solution containing Y andyor alumina salts. The morphology and composition of deposits are presented. The thermal behaviour of such coatings will be compared with that of steel.
2H2Oq2ey™H2q2OHy If the cathode is a porous electrode, like the conversion coating, this reaction may occur at the bottom of the pores with a resulting rise in local pH. Hydroxyl ions OHy can also be generated by reduction of dissolved oxygen according to the following reaction: O2q2H2Oq4ey™4OHy (ii) The pH increase provokes precipitation in the pores and at the surface of oxide or hydroxide compounds with varying degrees of hydration. The deposition of alumina modified by the yttrium has been achieved while adding the latter to the bath of alumina deposit; conditions of deposition are the same as that for alumina.
2. Experimental details 2.1. Conditions of chemical pre-treatment Functional conversion coating is obtained by chemical treatment in an aqueous solution of sulfuric acid with suitable additives such as sodium thiosufate as an accelerator and propargyl alcohol as an inhibitor in order to facilitate the control of the film growth (Table 1). After the pre-treatment, the samples were rinsed in deionised water, and then dried at 70 8C for 10 min. Scanning electron microscopy (SEM) examination of conversion coating w13x showed that the surface was very rough and had pores and cavities with a range of dimensions (Fig. 1). The real surface area of optimal conversion coating was very high (real surface area ;200 m2 my2) w13x. This conversion coating will facilitate the electrolytic deposition of alumina and will increase anchoring of the deposit. 2.2. Conditions of the ceramic deposits The deposits were obtained on a low-carbon steel containing 0.2% C. The samples were cleaned with tetrahydrofuran, washed with distilled water and then dried in air at room temperature. Oxides and hydroxides can be obtained through cathodic reactions with a process involving both electrochemical and chemical reactions w1,2x. The hydroxide my be formed in two successive steps:
Fig. 1. SEM image of initial conversion coating w13x.
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Table 2 Conditions for the alumina and aluminayyttrium deposits Current density (I) Temperature wAl2(SO4)3Ø14 H2Ox YCl3 pH Treatment time
y100 mA cmy2 13 8C 1M 0.15 M 3 15 min
Conditions of the alumina deposition modified or not by yttrium have been optimised and they are regrouped in Table 2. 2.3. Characterisation methods The polarisation curves (20 mV sy1) and open circuit potential were obtained using a Tacussel potentiostat PRT 20-02. A saturated calomel electrode was used as the reference electrode and a platinum electrode was used as the counter electrode. Three-dimensional profiles were obtained using a Tencor P2 profilometer. The measurements were made with a laser beam of 1 mm radius. The conventional way of presenting microgeometry data is to use statistical criteria established from surface state profiles. For the definition of the texture of coatings, we selected the arithmetical mean of the measured heights or the roughness, Ra. High temperature oxidation experiments were carried out in air at 750 8C using a Setaram TAG24 Thermobalance. Coating compositions were analysed by SIMS, using an IMS300 Cameca analyser. The microstructures were characterised using SEM (Jeol model 200 CX). 3. Results and discussion 3.1. Electrochemical study of the functionalised steel in the bath of aluminium sulfate In Fig. 2, we illustrate the evolution of the electrode potential of the steel with conversion coating according to the treatment time in aqueous solution of aluminium sulfate. We noted that the potential increases during the first minutes of the treatment and offers to stabilise to surroundings of y3 VyE.C.S. This evolution of potential can be attributed to the deposition of aluminium hydroxide. Cathodic polarisation curves for steel with and without conversion coatings in the aqueous solution of aluminium sulfate are shown in Fig. 3. In the case of steel with conversion layer (Fig. 3B), current densities increase from higher potential than that observed for the uncoated steel. This behaviour can be
Fig. 2. Electrode potential versus treatment time during alumina coating growth.
assigned to the roughness of the surface that gives back easier reactions at the interface. This electrochemical reaction induces a pH increase suitable to the hydroxide precipitation into the porous and at the conversion coating surface. 3.2. Morphology of deposits SEM examination of samples with alumina deposits (Fig. 4a) shows a homogeneous morphology with the superficial micro cracks, whose width is of the order of 1 mm (like ‘cracked-mud’ morphology). The deposit of aluminayyttrium observed by SEM (Fig. 4b), presents a regular and homogeneous morphology with the identical microcracks to those observed in the case of the alumina deposit. Therefore, the addition of yttrium did not modify the general morphology of conventional alumina deposit. As shown in Table 3, the application of ceramic deposits on the functionalised steel produces the smoother surface inducing a reduction of roughness parameters.
Fig. 3. Polarisation curves of (a) steel and (b) functionalised steel in cathode alumina deposit bath.
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Fig. 5. Gain in weight of steel against thermal treatment time in air at 750 8C. (a) Uncoated steel; (b) steel functionalised by conversion treatment; (c) steelqalumina deposit; and (d) steelqyttriumyalumina deposit.
Fig. 4. SEM images of coatings (a) alumina deposit and (b) yttriumyalumina deposit.
The evolution of these parameters translated into good penetration of the constituent of the deposit in pores and cavities of the conversion layer, inducing a good adherence. 3.3. Oxidation behaviour 3.3.1. Oxidation curves at 750 8C in air In this study, in order to analyse the behaviour at high temperature of steel with an alumina layer, the oxidation curves are compared to those of the uncoated and coated steel in order to put in evidence the protective character of the coatings achieved. Fig. 5 shows the isothermal variation as a function of time of weight changes per unit area (Dm) for the oxidation tests performed at 750 8C in air.
The curve of the uncoated steel (Fig. 5a) reveals three important stages in its oxidation kinetics. During the first minutes of heating, the oxidation is very fast. After 20 min of treatment, it becomes slower. This reduction of oxidation rate maybe attributed to the formation on the surface of the steel of a film of iron oxide that prevents the diffusion of gas oxidising through the substratum. This layer flakes at the end of 2 h of heating; the phenomenon of scaling appears by losses of mass that one observes on the kinetic curves. For steel with conversion layer, the kinetics of oxidation follows a parabolic law characterised by a fast oxidation kinetic during the first minutes of heating (Fig. 5b). The formed layer even resists after 3 h 30 min of heating. It can be explained by the fact that the layer of conversion previously achieved, assure a good continuity between the substratum and products of oxidation to the surface that present a better adhesion. In the case of steel with alumina deposit (Fig. 5c), oxidation curve reveals a parabolic behaviour. After 1 h of heating, the oxidation rate almost became stationary, due to the formation at the surface of the substratum of a thick and very adhesive layer that protects the same material after 11 h of thermal treatment. No phenomenon of scaling is observed. The introduction of the yttrium to the deposit of alumina (Fig. 5d) reduces distinctly the oxidation kinetics of steel. After 2 h 30 min of treatment, the material
Table 3 Parameters of the roughness of coatings
Ra (mm)
Untreated steel
Steelqconversion coating
Steelqconversion coatingqalumina
Steelqconversion coatingqYyalumina
0.04
1.29
0.48
0.46
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Table 4 Calculated parabolic constants of corresponding thermogravimetric curves Kp (mg2 cmy4 sy1) Steel Steelqconversion coating Steelqconversion coatingqalumina Steelqconversion coatingqaluminayY
9=10y3 6=10y3 2=10y4 2=10y5
does not present any phenomenon of scaling and thermogravimetric curve presents a landing lasting which the mass of the sample is constant in the time. However, irregular kinetics are observed which can be attributed to local cracking of the oxide layer, without scaling. However, the mass gain as well as the general kinetics is always weaker translating a better behaviour to the oxidation in relation to steel without coating. The thermogravimetric curves can be fitted to a parabola: tscqbDmqaDm2 Where t denotes the time, m the mass-gain per unit area, a, b and c are constants calculated by a leastsquare method. This equation takes into account the complete oxidation rate equation and permits a calculation of apparent steady-state parabolic rate constant Kp: Kps1ya.
The constant a can be accurately determined by means of a fitting procedure using more the 2000 experimental data points (m, t). To compare the parabolic rate constant of the different samples, the transient oxidation has been excluded. The Table 4 regroups values of the constant Kp for the whole set of samples. These values have been calculated from the best kinetic curve adjustment on periods of tests w20x. The Kp values show that the composition of the bath treatment influences significantly the oxidation kinetics. The coatings reduce the oxidation rate to different degrees, coating with aluminayY has the most effective oxidation. 3.3.2. Morphology of oxide scales SEM images corresponding to the different samples are illustrated by Fig. 6 and described below: (1) Surface examination of the uncoated steel oxidised at 750 8C shows that the oxide scale is constituted by very small crystallites (Fig. 6a). Oxide scales formed spalled off partially or completely during cooling, the specimen surface show, therefore, an heterogeneous aspect. The presence of pores containing oxides when the scaling is intense, the layer is very fragile and one attends a phenomenon of scaling. (2) Steel with conversion coating (Fig. 6b) presents a continuous oxide layer presenting little relief. One
Fig. 6. SEM images of steel after thermal treatment at 750 8C in air. (A) Uncoated steel; (B) functionalised steel; (C) steel Coated with alumina deposit; and (D) steel coated with yttriumyalumina deposit.
A. Lgamri et al. / Surface and Coatings Technology 162 (2003) 154–160
159
Fig. 7. Distribution profiles of the elements in the coatings against bombardment time after heat treatment at 750 8C in air. (a) steel; (b) steelq conversion coating; (c) steelqduplex conversionyalumina coating; and (d) steel coated with yttriumyalumina deposit.
notes the presence of small size of crystallites that are scattered on the surface. (3) On steel with alumina deposit (Fig. 6c) the layers of oxides formed are different; they consist of a colony of crystallites and smaller pores from above (1 mm). The layer of oxide appears compact and adhesive without any excrescence, no crack is observed in the surface and no phenomenon of scaling is visible. (4) Steel with yttrium modified alumina deposit (Fig. 6d) presents a homogeneous surface, similar to the one obtained on steel with alumina; this layer is composed of crystallites organised in a structure of ‘nest of bee’. 3.3.3. Study of element distribution by SIMS The analysis of the oxide layer formed on the uncoated steel after thermal treatment to 750 8C (Fig. 7a), shows that this layer is very rich in oxygen and in iron, the concentration of this latter increases while going from the external surface toward metal.
For steel with conversion coating (Fig. 7b), after 2500 s of analysis, the concentration of iron is always lower then that of metal. As in the case of the uncoated steel, this layer is very rich in iron and in oxygen. In presence of alumina deposit, the analysis by SIMS showed the presence of the aluminium in the surface and in depth (Fig. 7c). The concentration of this element passes then by a minimum offers to increase, what is a proof of a good diffusion of the aluminium in the layer of conversion and by consequence a good adhesion of this deposit. Thus, one always detects the aluminium even after 3000 s of analysis; the deep zone corresponds to the layer of conversion modified by the aluminium deposit. Aluminium intensity increases with sputtering time; which means that Al distributed toward the interfacing lays down a conversion layer–steel substrate. For yttriumyalumina deposits, the SIMS analysis (Fig. 7d) show the presence of the aluminium and the yttrium in the layer achieved on steel after treatment to 750 8C.
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Concentration profiles are parallel while going from the external surface toward the substratum because they have been put down simultaneously and in the same conditions. The element profiles indicate that the coating was composed of two domains: a superficial domain mainly containing iron compounds and a deep domain rich in aluminium and yttrium compounds. 4. Conclusions The results and observations presented in this paper reveal the beneficial effect of the application of a ceramic deposit on the oxidation of steel. This effect an appreciable reduction of the kinetics of oxidation. It indicates that the functionalisation of steel by a layer of conversion protects shows against the phenomenon of scaling. The application of an alumina deposit slows appreciably the kinetics of oxidation, forming on the surface of steel an adhered layer that opposes all ulterior attack. In order to improve the adhesion of this deposit on the one hand, and to bring a better resistance to the oxidation on the other hand, we thought about addition of elements other than yttrium to the deposit of alumina. The modification of this deposit by the yttrium reduces the rate of oxidation and improves the morphology of alumina deposits. The ordering by increasing of the effect of every deposit type is the following: Steel-Steelqconversion layer -Steelqalumina deposit -Steelqaluminayyttrium deposit This method, which involves a cathodic treatment of
functionalised samples in an aqueous solution, allows other additions to adjust coating properties. References w1x L. Gal-Or, I. Silberman, R. Chaim, J. Electrochem. Soc. 138 (1991) 1939. w2x R. Chaim, I. Silberman, L. Gal-Or, J. Electrochem. Soc. 138 (1991) 1942. w3x D.E. Clarck, W.J. Dalzell, D.C. Folz, Ceram. Eng. Sci. Proc. 9 (1988) 1111. w4x C.R. Aita, Mater. Sci. Technol. 8 (1992) 666. w5x N.K. Huang, H. Kheyrandish, J.S. Colligon, Phys. Stat. Sol. (a) 132 (1992) 405. w6x H. Konno, J. Electrochem. Soc. 134 (1987) 1034. w7x L. Aries, ´ L. Alberich, J. Roy, J. Sotoul, Electrochem. Acta 41 (No. 18) (1996) 2799. w8x L. Aries, J. Appl. Electrochem. 24 (1994) 554. w9x M. Atik, J. Zarzycki, C. R’Kha, J. Mater. Sci. Lett. 13 (1994) 266. w10x H. Konno, R. Furuichi, J. Met. Finish Soc. Jpn 39 (1988) 29. w11x L. Aries, A. Laaouini, J. Roy, P. Kressmann, Br. Corr. J. 33 (No. 4) (1998) 281. w12x L. Aries, J. Roy, S. El Hajjaji, L. Alberich, P. Vicente Hernandez, J. Mater. Sci. 33 (1998) 429. w13x S. El Hajjaji, M.-T. Maurette, E. Puech-Costes, A. Guenbour, A. Ben Bachir, L. Aries, Surf. Coat. Technol. 110 (1998) 40. w14x S. El Hajjaji, S. Manov, J. Roy, T. Aigouy, A. Ben Bachir, L. Aries, Appl. Surf. Sci. 180 (2001) 293. w15x L. Aries, J. Roy, J. Sotoul, P. Costeseque, T. Aigouy, J. Appl. Electrochem. 26 (1996) 617. w16x J.L. Smialek, Metal Trans. 22A (1991) 739. w17x A.W. Funkenbusch, J.G. Smeggil, N.S. Bornstein, Metal Trans. 16A (1985) 1164. w18x P.Y. Hou, J. Stringer, Mater. Sci. Eng. 87 (1987) 295. w19x L.V. Ramanathan, Corr. Sci. 35 (1993) 871. w20x A. Strawbridge, R.A. Rapp, J. Electrochem. Soc. 141 (1994) 1905.
Characterisation of electrolytically deposited alumina and yttrium modified alumina coatings on steel A. Lgamria, A. Guenboura, A. Ben Bachira, S. El Hajjajia,*, L. Ariesb a
´ Laboratoire d’Electrochimie – Corrosion, Departement de Chimie, Faculte´ des Sciences, Universite´ Med V, Av Ibn Battouta, BP 1014 Rabat, Morocco b CIRIMAT-LCMIE, Universite´ Paul Sabatier, 118 Route de Narbonne, 31064 Toulouse Cedex 4, France Received 24 January 2002; accepted in revised form 23 July 2002
Abstract Conversion coatings modified by deposits of alumina added or not with yttrium have been studied in order to improve the resistance to thermal oxidation of steel. The proposed process leads to the formation of a coating characterised by strong interfacial adhesion with the steel. Conversion coating, characterised by a very porous morphology and strong interfacial adhesion to the substrate, facilitates the electrochemical deposition of ceramic layers and enhances its adhesion. After heating, the coatings present continuous composition gradients with refractory compounds at the surface. All the studied coatings increase the oxidation resistance of the steel at 750 8C; they reduce the oxidation parabolic rate constant and prevent a scale spallation phenomenon. The extent of beneficial effect depends on the nature of cathodic treatment bath. Yttrium improved the protecting character of alumina coating; reduces the rate of oxidation and increase the adhesion of deposits. The electrochemical behaviours, morphology, chemical composition and thermal stability of the different deposits are analysed. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Electrolytic deposit; Conversion coating; Alumina; Yttrium; Oxidation; Steel
1. Introduction Numerous methods have been proposed to protect metals against oxidisation at high temperature w1–14x. The most interesting ones consist of deposition of ceramics which are currently attracting much interest for high temperature applications and in severely corrosive environments. In order to obtain a strongly adherent ceramic layer, we have used an original method, which includes a pre-treatment so as to make the surface suitable for the subsequent deposit. This pre-treatment of the metal consists of the formation of a functional conversion coating characterised by strong interfacial adhesion with the substrate and enable to induce inter*Corresponding author. Tel.yfax: q212-37-77-54-40. E-mail address: [email protected] (S. El Hajjaji). [email protected] (S. El Hajjaji), [email protected] (S. El Hajjaji).
facial reactions at high temperature with the ceramic coating. The functional conversion coating presents a morphology with micropores and a high level of roughness. This texture contributes to the anchoring of the ceramic layer and facilitates the interfacial reactions with the electrodeposit. It contains oxides of the metal substrate promoting the formation of a favourable phase at high temperature. For Fe–17%Cr stainless steel, it has been demonstrated w15x that the conductive conversion coating owing to its particular morphology with pores and cavities, facilitates the reduction reactions responsible for the increase of the pH at the surface and acts as a porous electrode allowing local pH conditions favourable for aluminium oxyhydroxide precipitation. In the second step, the ceramic layer is deposited by electrochemical method in an suitable baths according to the considered deposit. The thickness, morphology
0257-8972/03/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 2 . 0 0 5 1 6 - 9
A. Lgamri et al. / Surface and Coatings Technology 162 (2003) 154–160
(i) Generation of OHy
Table 1 Conditions for the conversion treatment Sulphuric acid concentration Thiosulphate concentration Propargyl alcohol concentration Temperature Treatment time
155
2.04 M 6=10y3 M 2=10y2 M 60 8C 15 min
and composition of the coating can be easily controlled by varying the electrochemical parameters and bath composition. In previous papers w8–14x, it was shown that an alumina coating prepared in this way slows the oxidation rate and protects 17% Cr stainless steel up to 1000 8C. Indeed, it was well known that very small additions of reactive elements such as Ca, Y, to the bulk or to the surface of Cr2O3 and Al2O3 also forming alloys to the pure metals could improve the oxidation resistance significantly w16–20x. The purpose of this paper is the study of aluminay yttrium coatings prepared by cathodic treatment of functionalised steel in solution containing Y andyor alumina salts. The morphology and composition of deposits are presented. The thermal behaviour of such coatings will be compared with that of steel.
2H2Oq2ey™H2q2OHy If the cathode is a porous electrode, like the conversion coating, this reaction may occur at the bottom of the pores with a resulting rise in local pH. Hydroxyl ions OHy can also be generated by reduction of dissolved oxygen according to the following reaction: O2q2H2Oq4ey™4OHy (ii) The pH increase provokes precipitation in the pores and at the surface of oxide or hydroxide compounds with varying degrees of hydration. The deposition of alumina modified by the yttrium has been achieved while adding the latter to the bath of alumina deposit; conditions of deposition are the same as that for alumina.
2. Experimental details 2.1. Conditions of chemical pre-treatment Functional conversion coating is obtained by chemical treatment in an aqueous solution of sulfuric acid with suitable additives such as sodium thiosufate as an accelerator and propargyl alcohol as an inhibitor in order to facilitate the control of the film growth (Table 1). After the pre-treatment, the samples were rinsed in deionised water, and then dried at 70 8C for 10 min. Scanning electron microscopy (SEM) examination of conversion coating w13x showed that the surface was very rough and had pores and cavities with a range of dimensions (Fig. 1). The real surface area of optimal conversion coating was very high (real surface area ;200 m2 my2) w13x. This conversion coating will facilitate the electrolytic deposition of alumina and will increase anchoring of the deposit. 2.2. Conditions of the ceramic deposits The deposits were obtained on a low-carbon steel containing 0.2% C. The samples were cleaned with tetrahydrofuran, washed with distilled water and then dried in air at room temperature. Oxides and hydroxides can be obtained through cathodic reactions with a process involving both electrochemical and chemical reactions w1,2x. The hydroxide my be formed in two successive steps:
Fig. 1. SEM image of initial conversion coating w13x.
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Table 2 Conditions for the alumina and aluminayyttrium deposits Current density (I) Temperature wAl2(SO4)3Ø14 H2Ox YCl3 pH Treatment time
y100 mA cmy2 13 8C 1M 0.15 M 3 15 min
Conditions of the alumina deposition modified or not by yttrium have been optimised and they are regrouped in Table 2. 2.3. Characterisation methods The polarisation curves (20 mV sy1) and open circuit potential were obtained using a Tacussel potentiostat PRT 20-02. A saturated calomel electrode was used as the reference electrode and a platinum electrode was used as the counter electrode. Three-dimensional profiles were obtained using a Tencor P2 profilometer. The measurements were made with a laser beam of 1 mm radius. The conventional way of presenting microgeometry data is to use statistical criteria established from surface state profiles. For the definition of the texture of coatings, we selected the arithmetical mean of the measured heights or the roughness, Ra. High temperature oxidation experiments were carried out in air at 750 8C using a Setaram TAG24 Thermobalance. Coating compositions were analysed by SIMS, using an IMS300 Cameca analyser. The microstructures were characterised using SEM (Jeol model 200 CX). 3. Results and discussion 3.1. Electrochemical study of the functionalised steel in the bath of aluminium sulfate In Fig. 2, we illustrate the evolution of the electrode potential of the steel with conversion coating according to the treatment time in aqueous solution of aluminium sulfate. We noted that the potential increases during the first minutes of the treatment and offers to stabilise to surroundings of y3 VyE.C.S. This evolution of potential can be attributed to the deposition of aluminium hydroxide. Cathodic polarisation curves for steel with and without conversion coatings in the aqueous solution of aluminium sulfate are shown in Fig. 3. In the case of steel with conversion layer (Fig. 3B), current densities increase from higher potential than that observed for the uncoated steel. This behaviour can be
Fig. 2. Electrode potential versus treatment time during alumina coating growth.
assigned to the roughness of the surface that gives back easier reactions at the interface. This electrochemical reaction induces a pH increase suitable to the hydroxide precipitation into the porous and at the conversion coating surface. 3.2. Morphology of deposits SEM examination of samples with alumina deposits (Fig. 4a) shows a homogeneous morphology with the superficial micro cracks, whose width is of the order of 1 mm (like ‘cracked-mud’ morphology). The deposit of aluminayyttrium observed by SEM (Fig. 4b), presents a regular and homogeneous morphology with the identical microcracks to those observed in the case of the alumina deposit. Therefore, the addition of yttrium did not modify the general morphology of conventional alumina deposit. As shown in Table 3, the application of ceramic deposits on the functionalised steel produces the smoother surface inducing a reduction of roughness parameters.
Fig. 3. Polarisation curves of (a) steel and (b) functionalised steel in cathode alumina deposit bath.
A. Lgamri et al. / Surface and Coatings Technology 162 (2003) 154–160
157
Fig. 5. Gain in weight of steel against thermal treatment time in air at 750 8C. (a) Uncoated steel; (b) steel functionalised by conversion treatment; (c) steelqalumina deposit; and (d) steelqyttriumyalumina deposit.
Fig. 4. SEM images of coatings (a) alumina deposit and (b) yttriumyalumina deposit.
The evolution of these parameters translated into good penetration of the constituent of the deposit in pores and cavities of the conversion layer, inducing a good adherence. 3.3. Oxidation behaviour 3.3.1. Oxidation curves at 750 8C in air In this study, in order to analyse the behaviour at high temperature of steel with an alumina layer, the oxidation curves are compared to those of the uncoated and coated steel in order to put in evidence the protective character of the coatings achieved. Fig. 5 shows the isothermal variation as a function of time of weight changes per unit area (Dm) for the oxidation tests performed at 750 8C in air.
The curve of the uncoated steel (Fig. 5a) reveals three important stages in its oxidation kinetics. During the first minutes of heating, the oxidation is very fast. After 20 min of treatment, it becomes slower. This reduction of oxidation rate maybe attributed to the formation on the surface of the steel of a film of iron oxide that prevents the diffusion of gas oxidising through the substratum. This layer flakes at the end of 2 h of heating; the phenomenon of scaling appears by losses of mass that one observes on the kinetic curves. For steel with conversion layer, the kinetics of oxidation follows a parabolic law characterised by a fast oxidation kinetic during the first minutes of heating (Fig. 5b). The formed layer even resists after 3 h 30 min of heating. It can be explained by the fact that the layer of conversion previously achieved, assure a good continuity between the substratum and products of oxidation to the surface that present a better adhesion. In the case of steel with alumina deposit (Fig. 5c), oxidation curve reveals a parabolic behaviour. After 1 h of heating, the oxidation rate almost became stationary, due to the formation at the surface of the substratum of a thick and very adhesive layer that protects the same material after 11 h of thermal treatment. No phenomenon of scaling is observed. The introduction of the yttrium to the deposit of alumina (Fig. 5d) reduces distinctly the oxidation kinetics of steel. After 2 h 30 min of treatment, the material
Table 3 Parameters of the roughness of coatings
Ra (mm)
Untreated steel
Steelqconversion coating
Steelqconversion coatingqalumina
Steelqconversion coatingqYyalumina
0.04
1.29
0.48
0.46
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Table 4 Calculated parabolic constants of corresponding thermogravimetric curves Kp (mg2 cmy4 sy1) Steel Steelqconversion coating Steelqconversion coatingqalumina Steelqconversion coatingqaluminayY
9=10y3 6=10y3 2=10y4 2=10y5
does not present any phenomenon of scaling and thermogravimetric curve presents a landing lasting which the mass of the sample is constant in the time. However, irregular kinetics are observed which can be attributed to local cracking of the oxide layer, without scaling. However, the mass gain as well as the general kinetics is always weaker translating a better behaviour to the oxidation in relation to steel without coating. The thermogravimetric curves can be fitted to a parabola: tscqbDmqaDm2 Where t denotes the time, m the mass-gain per unit area, a, b and c are constants calculated by a leastsquare method. This equation takes into account the complete oxidation rate equation and permits a calculation of apparent steady-state parabolic rate constant Kp: Kps1ya.
The constant a can be accurately determined by means of a fitting procedure using more the 2000 experimental data points (m, t). To compare the parabolic rate constant of the different samples, the transient oxidation has been excluded. The Table 4 regroups values of the constant Kp for the whole set of samples. These values have been calculated from the best kinetic curve adjustment on periods of tests w20x. The Kp values show that the composition of the bath treatment influences significantly the oxidation kinetics. The coatings reduce the oxidation rate to different degrees, coating with aluminayY has the most effective oxidation. 3.3.2. Morphology of oxide scales SEM images corresponding to the different samples are illustrated by Fig. 6 and described below: (1) Surface examination of the uncoated steel oxidised at 750 8C shows that the oxide scale is constituted by very small crystallites (Fig. 6a). Oxide scales formed spalled off partially or completely during cooling, the specimen surface show, therefore, an heterogeneous aspect. The presence of pores containing oxides when the scaling is intense, the layer is very fragile and one attends a phenomenon of scaling. (2) Steel with conversion coating (Fig. 6b) presents a continuous oxide layer presenting little relief. One
Fig. 6. SEM images of steel after thermal treatment at 750 8C in air. (A) Uncoated steel; (B) functionalised steel; (C) steel Coated with alumina deposit; and (D) steel coated with yttriumyalumina deposit.
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Fig. 7. Distribution profiles of the elements in the coatings against bombardment time after heat treatment at 750 8C in air. (a) steel; (b) steelq conversion coating; (c) steelqduplex conversionyalumina coating; and (d) steel coated with yttriumyalumina deposit.
notes the presence of small size of crystallites that are scattered on the surface. (3) On steel with alumina deposit (Fig. 6c) the layers of oxides formed are different; they consist of a colony of crystallites and smaller pores from above (1 mm). The layer of oxide appears compact and adhesive without any excrescence, no crack is observed in the surface and no phenomenon of scaling is visible. (4) Steel with yttrium modified alumina deposit (Fig. 6d) presents a homogeneous surface, similar to the one obtained on steel with alumina; this layer is composed of crystallites organised in a structure of ‘nest of bee’. 3.3.3. Study of element distribution by SIMS The analysis of the oxide layer formed on the uncoated steel after thermal treatment to 750 8C (Fig. 7a), shows that this layer is very rich in oxygen and in iron, the concentration of this latter increases while going from the external surface toward metal.
For steel with conversion coating (Fig. 7b), after 2500 s of analysis, the concentration of iron is always lower then that of metal. As in the case of the uncoated steel, this layer is very rich in iron and in oxygen. In presence of alumina deposit, the analysis by SIMS showed the presence of the aluminium in the surface and in depth (Fig. 7c). The concentration of this element passes then by a minimum offers to increase, what is a proof of a good diffusion of the aluminium in the layer of conversion and by consequence a good adhesion of this deposit. Thus, one always detects the aluminium even after 3000 s of analysis; the deep zone corresponds to the layer of conversion modified by the aluminium deposit. Aluminium intensity increases with sputtering time; which means that Al distributed toward the interfacing lays down a conversion layer–steel substrate. For yttriumyalumina deposits, the SIMS analysis (Fig. 7d) show the presence of the aluminium and the yttrium in the layer achieved on steel after treatment to 750 8C.
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Concentration profiles are parallel while going from the external surface toward the substratum because they have been put down simultaneously and in the same conditions. The element profiles indicate that the coating was composed of two domains: a superficial domain mainly containing iron compounds and a deep domain rich in aluminium and yttrium compounds. 4. Conclusions The results and observations presented in this paper reveal the beneficial effect of the application of a ceramic deposit on the oxidation of steel. This effect an appreciable reduction of the kinetics of oxidation. It indicates that the functionalisation of steel by a layer of conversion protects shows against the phenomenon of scaling. The application of an alumina deposit slows appreciably the kinetics of oxidation, forming on the surface of steel an adhered layer that opposes all ulterior attack. In order to improve the adhesion of this deposit on the one hand, and to bring a better resistance to the oxidation on the other hand, we thought about addition of elements other than yttrium to the deposit of alumina. The modification of this deposit by the yttrium reduces the rate of oxidation and improves the morphology of alumina deposits. The ordering by increasing of the effect of every deposit type is the following: Steel-Steelqconversion layer -Steelqalumina deposit -Steelqaluminayyttrium deposit This method, which involves a cathodic treatment of
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