Conversion coating on stainless steel as a support for electrochemically induced alumina deposit

Elecrrochimica

Pergamon

Acta, Vol. 41. No. 18, pp. 2199.2803, 1996 Copyright G 1996 Elwier Science Ltd. Printed in &eat Bntatn. All rights reserved 0013 -4686/96 $15.00 + 0.00

PII:SOO13-4686(%)OOO!H-6

CONVERSION COATING ON STAINLESS SUPPORT FOR ELECTROCHEMICALLY ALUMINA DEPOSIT

STEEL AS A INDUCED

L. ARIES,* L. ALBERICH,J. ROY and J. SOTOUL Equipe de Metallurgic Physique-Laboratoire des Mattriaux (URA CNRS 445), Ecole Nationale Superieure de Chimie (INPT), 118 route de Narbonne, 31077 Toulouse Cedex, France (Received 16 October 1995; in

revisedform

5 February

1996)

Abstract-Conversion coatings on stainless steel were used as a support for alumina coatings deposited by electrolytic treatment from an aqueous solution of aluminium sulphate. The conductive conversion coating with its particular morphology facilitates the reduction reactions responsible for the increase of the pH at the surface and acts as a porous electrode allowing favourable local pH conditions for aluminium oxyhydroxide deposition. The effects of potential, current density, time, temperature and stirring on coating weight were studied. Copyright 0 1996 Elsevier Science Ltd Key

words:

alumina, electrolytic treatment, stainless steel, ceramic film, electrochemical study

INTRODUCTION Ceramic coatings on metallic substrates are of great interest in protecting metals against oxidation at high temperature, and acting as a thermal barrier. Generally the oxide coatings concerned are zirconia stabilized with yttria, magnesia or calcia and also alumina well known for their hardness and resist-

ance to chemical attack and high temperature. Compared to methods like plasma spraying, chemical vapour deposition and sputtering[ l-31 and sol gel processes[4] available for production of ceramic coatings, the electrochemical route is relatively new[S-71 and has been used, for example, for deposition of oxides like ZrO, on titanium and graphite[S, 91 and ZrO,[lO], composite oxides of Zr(IV)-Cr(III) or Ti(IV)-Cr(III)[l 1] or La, _,M,CrO,[l2] on ferritic stainless steels. This technique has been also used for deposition of Al,O, on TiB, and TIC substrates[l3]. One problem of ceramic coatings on metal is their low adhesion to the substrate at very high temperatures and the thermal shock resistance of the interface. In previous papers[l4, 151 we described an original method[ 161 to obtain electrochemical ceramic films (Al,O, and stabilized ZrO,) with good adhesion at high temperature. In order to strengthen the ceramic-substrate interface, the metal surface is functionalized beforehand by a conversion treatment in an acid bath with suitable additives. This pretreat* Author to whom correspondence should be addressed.

ment of the surface leads to a microporous conversion coating which is very adherent and enables interfacial reactions at high temperature with the secondary electrochemically induced deposit which confers the refractory character. The deposit is produced by cathodic treatment in a suitable bath which induces the deposition of oxides, hydroxides or oxyhydroxides that are more or less hydrated, into the pores and at the surface of the conversion coating. Thermal treatment between 700°C and looo”C according to the nature of the substrate and the ceramic oxide provokes dehydration processes and interface reactions between the electro-deposited oxide and conversion coating compounds; the reaction products act as a bond between the substrate and the ceramic coating. The potential of this method for the fabrication of alumina or zirconia stabilized protective coatings on stainless superalloys has been demonstrated[ 161.

steels and

The present paper deals with the effect of the electrochemical deposition parameters on the formation rate of an as-deposited alumina coating. EXPERIMENTAL

The deposits were obtained on an Fe-17%Cr ferritic stainless steel whose composition is given in Table 1. Specimens were in the form of 0.6 mm x 20mm x 50 mm plates, their original surface state being preserved. The samples were cleaned with ethanol, washed with distilled water and then dried in air at room temperature.

Table 1. Chemical composition of the stainless steel used (wt%) Si

Mn

Cr

MO

Ni

Al

Cu

Nb

Ti

Fe

0.49

0.19

15.99

0.13

0.24

0.12

0.34

0.17

0.02

Bal.

L. ARES et al

2800

The ferritic stainless steel surface was functionalized by a conversion treatment in a sulphuric acid solution (pH = 0) with 130 ppm S*- and acetylenic alcohol as additives[ 141. The bath temperature, around 50°C was kept constant by water circulation from a thermostat to the water jacket of the treatment cell. The treatment time was 20 min. One of the main conditions of the treatment is the fitting of the electrode potential of the sample to the value of the natural corrosion potential of the steel in the active state and not in the passive state; this potential must be lower than the primary passive potential of the steel in the treatment bath. When necessary the adjustment of the potential to the required values was accomplished by cathodic activation of the surface in the treatment bath with the help of a current generator and a platinum counter-electrode as anode. The electrode potential was measured with a high input impedance (lO’*fl) millivoltmeter vs. saturated calomel electrode. After the preparation of the conversion coating specimens were washed with deionised water and then dried in air at 90°C. Aluminium oxyhydroxide was deposited through cathodic reactions using a platinum electrode as anode from a 1 M aqueous solution of aluminium sulphate (Al,(SO,),). The electrolyte temperature was kept around 12°C by means of a cooling bath with ice. Deposits were obtained either at constant potential ranging from - 1 V to - 5 V or at constant current (from 12.5mAcmm2 to 140mAcm-*). The specimens were washed with deionised water and dried in air at 90°C. The weight of dry samples was determined at 20°C before and after deposition.

RESULTS

AND

DISCUSSION

Cathodic polarization curves for the Fe-17%Cr stainless steel with and without the conversion coating, vs. saturated calomel electrode (see) in the aqueous solution of aluminium sulphate are shown in Fig. 1. For the uncoated stainless steel, the cathodic current increased quickly from - 1.1 V; this potential domain represents the reduction reaction associ-

-2

d q

non-coated

l

coated

steel

steel

Fig. 1. Cathodic polarization curves for the Fe-17’KCr stainless steel in 1 M Al,(SO,), with and without the conversion coating.

ated with the activation polarization regime. It fitted with hydrogen evolution at the steel surface. So, the main reaction taking place in this potential domain is the reduction of protons H+ to form H, ; the reduction of dissolved 0, according to: 0,

+ 2H,O

+4e+40H-

which is expected at higher potentials giving very small current densities, was not observed. At very low potentials (high current densities) around - 3 V, no reproducible fluctuations or current drops were noted probably because of the formation of a thick non-adherent deposit damaged by gas bubbles. The curve obtained for the stainless steel functionalized by the conversion treatment, shows 2 polarization domains. At low potential (high current densities, second domain) from -2.3 V or -2.6 V the polarization regime fits with that observed for the uncoated steel, but in the first domain, current densities increased from -O.SV, it is to say from a higher potential than that indicated for the uncoated steel; so in this domain, the current densities are higher than those observed on the stainless steel. This behaviour is attributed to the particular morphology and composition of the conversion coating; probably the surface roughness decreases the overpotentials and makes the cathodic reactions easier (proton and 0, reduction). In this first potential domain the behaviour of the conversion coating is comparable with that reported by other authors[8] for porous graphite in 0.1 M aqueous solutions of ZrO(NO,),nH,O (pH = 2.3); for the graphite substrate the current observed in this first domain has been attributed by these authors to the reduction of dissolved 0, partially overlapping the reduction of the NO; ions. At low potential, in the second domain, the current densities on the conversion coating approaches those observed on the stainless steel probably because the aluminium hydroxide, which precipitates into pores during polarization curve plotting, leads to the levelling of the surface. Nevertheless, the assumption of reduction reactions involving conversion coating compounds cannot be completely rejected. PreviouslyC17, 183 it was shown that the preponderant oxide composing these ferritic stainless steel conversion coatings is a chromium substituted magnetite (Fe,O, type) which coexists in the superficial zone with a y-lacunar phase (y-Fe,O, type). This lacunar phase results from the superficial oxidation of the initial magnetite phase during the drying of the coating in air at 90°C. The reduction of this y-lacunar phase at the conversion coating surface, could partially overlap the H, and 0, reduction from -0.8 V. The reduction of cations in solution, such as Fe3+, resulting from pollution of the electrolyte by a slight, selective dissolution of the conversion coating while the polarization curve is plotted, near the potential at zero current, cannot be excluded. For deposits obtained at constant potential, Fig. 2 shows the dependence of coating weight on the potential for a constant deposition time of 40min. As expected, the weight increased with the greater cathodic potentials. Deposit was observed even in the first polarization domain (around -1 V), showing that the local rise in pH, responsible for the

2801

Conversion coating on stainless steel (Y

i0

04.

.F E 0 = %

03.

Z

O.l-

r

0.2-

z z

0.0,

-6

, -5

.

, -4

.

, -3

.

I -2

.

Fig. 2. Coating

weight vs. potential for a constant tion time of 40 min.

=

0.5

.-E CI m 8

0.0 0

deposi-

precipitation phenomena, is effective at these potentials. This supports the assumption of chargetransfer, reactions that produce OH- ions from -0.8V. Nevertheless, the 2 domains noted on the polarization curve were not clearly represented; a higher rate of deposit formation was noted from 3SV. Moreover, the gas bubbles emerging from the surface, observed at very negative potential treatment are responsible for the detachment of the deposit from the electrode and slows down its formation rate from 4.5 V. The dependence of current density on deposition time for different treatment potentials is shown in Fig. 3. The current is seen to decrease with time and stabilizes after some minutes. This behaviour can be attributed to the progression of the coating process; the deposit becoming thicker, its electrical resistance increases. For deposits obtained at constant current, Fig. 4 shows the dependence of coating weight on the current densities for a constant deposition time for 40min. Coating weight increases with current density. At low current densities of O-70mAcm-‘, first part of the curve, the deposition rate is relatively moderate the slope of the curve is about 0.007mgcm-2mA-t. It increases quickly from Above 70mAcm-’ (0.03-0.05 mgcmm2 mA- ‘). 110 mA cm- 2 the curve slope decreases because of

,

2 .P $

40

60

100

80

120

140

l/mA cm-2 Fig. 4. Coating weight per cm’ vs. current density constant deposition time of 40 min.

for a

the vigorous release of H, gas bubbles which induces the detachment of the deposit. So, the dependence of coating weight on current density and upon potential are similar. For a constant current density (Fig. 5) of 100mAcm-2 the coating weight increases with deposition time in a linear manner; after a certain time (30min) the deposition process decelerates. The cell voltage was found to increase with deposition time to a maximum, as shown in Fig. 6, and to stabilize at higher values for higher current densities. This behaviour is in keeping with the growth of the coating. The increase in the celi voltage can be attributed to the thickening of the low-conducting oxyhydroxide deposit; formation of molecular hydrogen bubbles damages the thick deposit and provokes a decrease in the cell voltage after it has reached the maximum. The dependence of coating weight on temperature is shown in Fig. 7 for a constant deposition time of current density of 40min and a constant 100mAcm-2, an increase of the bath temperature decreases the coating weight probably by increasing the rate of diffusion of OH- away from the cathode and the solubility of aluminium hydroxide. So, it is very important to control the temperature of the electrolyte to prevent its increase induced by the flow of current and to make deposit formation easier. At low temperature (5°C) thick coatings are obtained

I

40,

-4v

II

-5v -3v

0

”

0

20

10

0

density

vs. deposition potentials.

10

20

30

40

50

60

70

Timelmin

Time/min Fig. 3. Current

20

-1

WV

140

1.0

.

time

for

various

Fig. 5. Coating

weight per cm2 vs. deposition constant current density.

time for a

L. ARES et al.

2802

k--

14

135 mA cm-’ . 105 mA cm-* . 95 mA cm‘*

6 ! 0

.

0

10

Fig. 6. Cell voltage vs. deposition time for various current densities.

and coating time must be reduced to increase adhesion to the substrate. Stirring is also seen to cause a decrease of the deposition rate (Fig. 8) by facilitating migration of OH- from the electrode surface into the solution bulk.

-I

,

p1

‘E 0.75 0

150

06.

Fig. 8. Coating weight per cm’ vs. current density in stirred and unstirred solutions.

After drying in air at 90°C for lOmin, the deposit was studied by X-ray diffraction on its metal substrate by means of a diffractometer using Fe Ka radiation, diffractograms showed only steel diffraction peaks, the other compounds being amorphous or in too small proportion. Consequently, powdered as-deposited coating was obtained by scraping the layer from its substrate after drying. X-ray diffraction of this powder (Fig. 9) showed two forms of aluminium hydroxide, gibbsite and bayerite. These phases crystallize from the initial oxyhydroxide gel, probably boehmite gel.

E m

CONCLUSION

3 0.25 5E” z8

100

VmA cm-*

Timelmin

.’

50

!‘:-:> 0.0 0

10 temperature

20

30

/“C

Fig. 7. Coating weight per cm* vs. temperature for a constant deposition time of 4Omin and constant current density of 100mAcm-2.

B

Finally, owing to its conductivity and its particular morphology with pores, inducing locally favourable pH conditions at the cathode surface, the conversion coating allows the formation of aluminium oxyhydroxide deposits from aqueous solutions. The deposit, forming at the surface and in the pores of the conversion coating, leads to a layer with good adhesion on stainless steel. This method offers

b

tl (FeKa)(deg) Fig. 9. X-ray diffraction spectrum of an as-deposited layer; b: bayerite, g: gibbsite.

Conversion coating on stainless steel an easy way to control the thickness of the coating: low bath temperature and moderate stirring allow thick deposits to be formed. Addition of organic compounds and solvents (eg ethanol) decreasing the evolution rate of hydrogen bubbles are also suitable to facilitate deposit growth.

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2803

9. R. Chaim, I. Silberman and L. Gal-Or, J. electrochem. Sot. 138, 1942(1991). 10. M. Atik, J. Zarzycki and C. R’Kha, J. Mat. Sci. Len.

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