Sol-gel thin coatings of ZrO2, SiO2, 70SiO2-30TiO2 and 88SiO2-12Al2O3 compositions (mole %) have been prepared from sonocatalyzed sols and deposited by dip-coating technique on 316L stainless steel foils. The influence of the coatings on the chemical corrosion of the substrate has been measured through potentiodynamic polarization curves in aqueous 15% H2SO4 solution between 25 and 50°C. The values of the corrosion potential, polarization resistance and corrosion rate have been determined. Analysis of the data combined with scanning electron microscopy studies indicate that the films act as a geometric blocking against exposure to the corrosive media and increase the lifetime of the substrate up to a factor 8.5

Aegerter, Michel A.

Atik, Mohamed

Avaca, Luis A.

De Lima Neto, Pedro

Universaar

Sol-gel coatings for chemical protection of stainless steel

Scientific documents from the Saarland University,

Journal of Sol-Gel Science and Technology, 2, 529-534 (1994) 
© 1994 Kluwer Academic Publishers, Boston. Manufactured in The Netherlands. 
Sol-Gel Coatings for Chemical Protection of Stainless Steel 
Code: EP7 
PEDRO DE LIMA NETO, MOHAMED ATIK, LUIS A. AVACA AND MICHEL A. AEGERTER 
Institute of Physics and Chemistry of S~o Carlos, University of S~o Paulo, Cx. Postal 369 - 13560-970 - Sdo Carlos (SP), Brazil 
Abstract. Sol-gel thin coatings of ZrO2, SiO2, 70SiO2-30TiO2 and 88SiO2-12A12Oa compositions (mole %) have 
been prepared from sonocatalyzed sols and deposited bydip-coating technique on 316L stainless teel foils. The 
influence of the coatings on the chemical corrosion of the substrate has been measured through potentiodynamic 
polarization curves in aqueous 15% H2SO4 solution between 25 and 50°C. The values of the corrosion potential, 
polarization resistance and corrosion rate have been determined. Analysis of the data combined with scanning 
electron microscopy studies indicate that the films act as a geometric blocking against exposure to he corrosive 
media and increase the lifetime of the substrate up to a factor 8.5. 
Keywords: thin film, ZrO2, SiO~, SiO2 TiO2, SIO2-A1203, corrosion protection, stainless teel 316L 
1. Introduction 
Sol-gel methods are now widely used for the prepara- 
tion of films of oxide materials from solutions and offer 
potential advantages over traditional techniques [ 1] [2] 
[3] [4]. The chemical processing [5] and the technical 
applications [6], which encompass optical,electronic, 
optoelectronic, photonic, biotechnology, chemical and 
mechanical fields have been reviewed recently. 
Inorganic oatings having a vitreous tructure are 
widely used as protective coatings for metals and al- 
loys [7] [8]. They improve the chemical and physical 
properties of the metal surfaces relative to corrosion, 
friction and wear without altering the original proper- 
ties of strength and toughness of the substrate. 
The preparation and characterization of sol-gel films 
having these specific chemical and mechanical func- 
tions have been so far scarcely studied. A1203 [9], 
Si-O-N, Si3N4 [10] [11] films have been proposed for 
semiconductor passivation. SiO2 [12] and TiO2-SiO2 
[13] are known to prevent alkali diffusion. The pre- 
vention of chemical corrosion and oxidation of mild 
steel, carbon steel and stainless teels has been tested 
with SiO2 [14], SiO2-B2Oa [15] [16], mullite [17], 
ZrO2 [15, 18-22], MTOS [23] and ZrO2-CeO2 [24] 
coatings. They all increase the protection of metal 
substrates from air corrosion (tested up to 800°C) and 
acid attack (tested up to 80°C).The most promising 
prevention for stainless teel has been so far reported 
by our group using sol-gel film of ZrO2, SiO2-TiO2, 
SIO2-A1203 and TiO2-CeO2 prepared by dip-coating 
using sonocatalyzed sols [18, 19, 21, 25]. 
In this paper we present the r sults of a system- 
atic electrochemical corrosion study in 15% aqueous 
H2SO4 up to 50°C of sol-gel films of ZrO2, SiO2- 
TiO2, SiO2-AI~O3 (mole%) deposited on stainless 
steel 316L by dip coating, a material widely used for 
marine and chemical industry environments. 
2. Experimental 
316L stainless steel substrates (Caseurop, France) were 
cut from large foils and degreased ultrasonically in ace- 
tone. This austenitic material was chosen because ithas 
a low carbon content (<0.03%) and is therefore less 
susceptible to sensitization during the heat treatment 
necessary for the densification of the coatings. 
The zirconia sol was prepared by dissolving the 
alkoxide Zr(OC3HT)4 in isopropanol (CaHTOH) with 
a small amount of acetic acid (CHaCOOH). Excess 
water was then added to complete the hydrolysis. 
The concentration of the alkoxide was varied be- 
tween 0.025 and 0.9 mole/1 ZrO2. The volume ratios 
H20/CaHTOH and H20/CHaCOOH were respectively 
equal to 1 and 2. The silica sol was prepared by dis- 
solving tetraethylorthosilicate Si(OC2H5)4 in absolute 
530 de Lima Neto, Atik, Avaca and Aegerter 
. I000  
/' I I 
600  
J!il, 200 - (c) '.(a) [ (b)  . :.... ) 
: 
- -200- -  _ ___  ~ . . . .  .+ .,;,']( 
~' . . .  
-600  - ., ~,..... 
- IOOO , ,,i,,,,I . , , I , . , I  1 l lhl l l l  i l lh[l l l  i , INHI[ i , lhl l l l  I , , . , , , I  , , , , , ,  
- -3  - -2 - - I  0 I 2 5 4 .6 
I 0  10 I0  IO I0  I0  I0  I0  10 
T ( l aA /cm 2 ) 
Fig. 1. Potentiodynamic polarization curves measured in deareated 
15% aqueous H2SO4 at 25°C for 316L stainless teel a) uncoated 
and untreated, b) uncoated but heat treated at 800°C for 2 h., c) 
coated with ZrO2 (800 ° C/2 h), d) coated with SiO2 (800 ° C/2 h). 
ethanol C2HsOH (0.43 mole/mole of alkoxide) to 
which a small amount of acetic acid CHaOOH (0.087 
mole/mole of alkoxide) was added. Binary composi- 
tions in the system SiO2-TiO2 and SIO2-A1203 have 
been prepared using the same protocol and concentra- 
tion using Si(OC2Hs)4, tetraorthotitanate Ti(OC2Hs)4 
and aluminum sec-butilate AI(OC4Hs)3 as source of Si, 
Ti and A1 respectively. The composition of the solu- 
tions were zSiO2(1-z)MvOz where M = Ti or AI with 
z = 70 mole% (70SiO2-30TiO2) and z = 88 mole% 
(88SiO2-12A1203). However a complete study with 
0 < z < 100 for both binary systems is underway 
and will be published elsewhere. All the sols were 
submitted to intense ultrasonic irradiation (Heat Sys- 
tems Ultrasonics W385). They became homogeneous 
and transparent after 25 minutes and were stable up 
to 4 weeks at room temperature when kept in a close 
vessel. 
Single layer coatings have been deposited by dip 
coating technique at a constant withdrawal speed of 
10 cm min -1, dried at 60°C for 15 min. and then 
treated in air at a slow rate of5°C min -1 up to 800°C, 
with an intermediate isothermal holding at 450°C dur- 
ing 1 h and further isothermal treatments at800°C up 
to 20 h. The coatings have a thickness of about 0.3 
to 0.6 #m. 1 ° glancing incidence X-ray diffraction 
(Rigaku Rotaflex) shows that the heat reated SiO2 and 
88SIO2-12A1203 coatings are amorphous; however 
ZrO2 coatings crystallized at ,-~450°C into a tetragonal 
structure [21] while 70SiO2-30TiO2 coatings partially 
,ooo- / / : i  
600 ,dy./ , 
a) (b) 
v 
LU _ 200  -- 
-6oo  - ~-L :  . . . .  
--3 --2 - - I  O I 2 3 4 
I 0  I0  I0  I0  I0  I0  I0  I0  IO 
I ( laA /cm 2 ) 
Fig. 2. Same as figure 1 for T= 25°C a) and b) same as in Figure 
1 (for reference); c) coated with SiO2-TiO2 (800°C/2 h); d) coated 
with SIO2-A1203(800°C/2 h). 
crystallized at ~650°C showing anatase crystallites in 
an amorphous silica matrix [21]. 
Electrochemical measurements are convenient and 
precise methods to evaluate the corrosion of a mate- 
rial in a given medium [26]. They have been car- 
ried out with freshly prepared samples in deareated 
15% H2SO4 (Merck p.a.) aqueous olutions (Milli-Q) 
up to 50°C using a computerized PAR 273 Potentio- 
stat/Galvanostat. A saturated calomel electrode (SCE) 
was used as reference and a Pt foil served as the auxil- 
iary electrode. The working electrodes were immersed 
1 cm into the solution. The potentiodynamic measure- 
ments were initiated at -0 .7  V and scanned continu- 
ously toward the anodic direction at 1 mV s-1. The 
data were analyzed with the PAR 342 Corrosion Mea- 
surement Software. A detailed explanation of the pro- 
cedure and analysis of the results can be found in [25]. 
3. Results and Discussion 
Potentiodynamic polarization curves have been mea- 
sured at 25, 40 and 50°C. Figures I and 2 show typ- 
ical results obtained at 25°C for stainless teel (ss) 
without coating and untreated, ss without coating but 
heat treated at 800°C during 2 h, ss with ZrO2 coat- 
ing (800°C/2 h), ss with SiO2 coating (800°C/2 h), 
ss with SIO2-A1203 coating (800°C/2 h), and ss with 
SiO2-TiO2 coating (800°C/2 h). Similar data have 
been obtained at the other temperatures. 
The protective ffect is noticeable for all coat- 
ings in both the cathodic and anodic branches of the 
Chemical Protection of Stainless Steel 531 
Table 1. Corrosion parameters determined from the potentiodynamic curves measured for stainless steel 316L (ss) uncoated and untreated, 
uncoated but heat reated at 800°C/2 h, coated with ZrO2, SiO2, SIO2-A1203, and SiO2-TiO2 (800°C/2 h): corrosion potential Ecorr (mV); 
polarization resistance Rp (kf2 cm 2) and corrosion rate CR (mills per year, MPY). 
Sample T(°C) -Ecorr (mV) Rp (kf~ cm 2) CR (MPY) 
SS 316L 25 286 1.80 11.5 
un~e~ed 40 261 0.37 35.2 
50 280 0.35 53.8 
SS 316L hem 25 306 1.45 5.0 
tre~ed 40 304 0.57 19.8 
50 307 0.14 41.2 
SS + ZrO2 
SS + SiO2 
25 308 10.50 3.4 
40 328 8.50 4.2 
50 299 0.99 12.9 
25 289 8.56 3.1 
40 307 1.21 6.5 
50 314 0.31 14.2 
SS -}- 88SIO2- 25 240 5.92 4.2 
12A12Oa 40 297 0.93 8.4 
50 301 0.89 13.9 
SS + 70SiOz- 25 291 10.7 2.2 
30TIO2 40 297 5.70 5.7 
50 293 3.10 6.3 
curves. The cathodic part corresponds to the hydrogen 
evolution reaction 2H + + 2e- --+ H2. The slope of 
the curves for uncoated and untreated stainless teel is 
ca. 2.3 × 2 RT/F, where R is the universal gas con- 
stant and F the Faraday constant a value coherent with 
a Volmer-Heyrovsky sequence, a current mechanism 
accepted for this reaction [27]. In the presence of the 
coatings, the value of the slope is maintained suggest- 
ing that the mechanism remains unchanged. However 
there is a downward shift in the current densities (in the 
sequence: untreated ss, heat treated ss, SiO2-A12Oa, 
SiO2-TiO2, ZrO2) indicating that the films act as a 
geometric blocking; they act as an inert obstacle to 
exposure of the metal surface to its environment and 
therefore the reaction rates fall in proportion to the ex- 
tent of surface coverage. As an example the active area 
for the reaction is typically reduced by a factor of 10 
for the best coating (SiO2TiO2). The chemical attack 
is probably done via a small remaining open poros- 
ity of the film (not observed in SEM measurements) 
left after the densification heat treatment. The posi- 
tion and the slope of the SiO2-TiO2 potentiodynamic 
curves measured in the cathodic range at 25°C do not 
change for heat treatment up to 15 h, all being identical 
to curve d of Figure 2 indicating that the film surface 
coverage is not affected by these heat treatments. The 
measurements performed at 40°C and 50°C present 
similar behavior and consequently the blocking prop- 
erties remain unchanged at these temperatures. 
An intermediate behavior is observed for the heat 
treated substrates without coating (curves b of Figures 1 
and 2). It is difficult to interpret it without a detailed 
knowledge of the processes occurring in the oxide lay- 
ers growing during the heat treatment on the stainless 
steel surface as well as in its bulk structure. 
The anodic branches of the polarization curves how 
that the coatings have a strong effect on the current den- 
sity in the passive region, diminishing the current value 
of uncoated and untreated stainless teel by roughly one 
order of magnitude. In the same region, the heat treated 
uncoated ss samples (curve b of the Figures 1 and 2) 
show current densities omewhat higher than those of 
the untreated substrates (curve a) revealing again sur- 
face changes during the treatment. 
The combined cathodic and anodic effects of the 
coatings on the corrosion behavior of 316L stainless 
steel are summarized in Table 1, which presents the 
relevant corrosion parameters derived from computer 
analysis of the potentiodynamic curves. The values of 
532 de Lima Neto, Atik, Avaca and Aegerter 
10000 
1000 
EL 
n," 100 
0 
[] 
r'l 
10 
[] 
[] 
O 
O 
O 
2.7 218 '  219 ' 310 '  311 '  3~.2' 313 ' 3.4 
1000/T (K -1) 
Fig. 3. Temperature dependence of the corrosion rate (MPY) in 
deareated 15% aqueous H2SO4 solution of 316L stainless steel: 
(n, o) uncoated and untreated; the high temperature data (D) are- 
from [19] and have been obtained by weight loss in non deareated 
solution; (V) coated with SiO2; (A) ZrO2; (~-) SIO2-A1203 and 
(©) SiO2-TiO2. 
the corrosion potential (Ecorr) are very little affected by 
the presence of the films indicating abalance between 
cathodic and anodic effects. However the polariza- 
tion resistances (Rp) which reflects the kinetics of the 
process at the equilibrium potential and the corrosion 
rate (CR), measured in mill per year, are drastically 
changed. At all tested temperatures all coatings how 
a lower corrosion rate than the untreated and uncoated 
substrate. The lifetime of the coated substrates i in- 
creased typically by a factor 3,5 to 5,5 at 25°C. The 
effect is even more pronounced at 40°C (4 to 8) and 
reaches 8.5 for SiO2-TiO2 at 50°C but decreases to 4 
for the other coatings. Even in relation to the heat- 
treated uncoated ss samples, which appear to be more 
acid resistant than the untreated samples, the effect of 
the coating is still considerable. 
The corrosion rates determined for the different 
coating compositions heat treated at 800°C for 2 h. 
increase with the temperature of the electrolyte (Fig- 
ure 3). The low temperature data show the same over- 
all Arrhenius behavior (activation energy E = 27-28 
kcal mole -1) for all coatings as well as for the un- 
coated stainless teel. For the oxide coatings how- 
ever the preexponential f ctor is smaller. This confirms 
that the layers act as a geometric blocking. The data 
at high temperatures have been determined by weight 
loss measurements in non deareated H2SO4 solution 
[19]. In order to truly compare these values to those de- 
termined electrochemically the measurements should 
have been extrapolated atzero time which was not pos- 
sible due to the lack of sensitivity of the method. Non 
deareated acid solutions are also known to accelerate 
the corrosion phenomena [26]. 
The corrosion rates have been also studied as a func- 
tion of the heat reatment time at 800 ° C (Figure 4). The 
protection action is basically the same up to around 
15 h. This shows that the film surface coverage of 
the substrate and probably the structure of the pores 
are not changed uring the heat treatment. There is 
however a drastic change observed at t = 20 h, where 
the corrosion rates reach the same very high value (CR 
~39 MPY) for all systems (higher than that of the as 
received stainless teel (11.5 MPY at t = 0). SEM 
measurements show that the films are dense with no 
visible porosity but are completely broken leaving an 
easy access of the electrolyte to the substrate (Figure 5). 
The physical reasons of this decohesion is the growth 
of a layer of Cr203 single crystals (containing also 
mainly Mn and Fe) at the interface metal-coating caus- 
ing significant stresses in the film and leading eventu- 
ally to its fracture under prolonged high temperature 
treatment. The growth of the single crystals already 
occurs at 450°C as confirmed by Rutherford Back Scat- 
tering measurements [28]. 
4. Conclusion 
Sol-gel coatings of ZrO2, SiO2, 70SiO2-30TiO2 and 
88SiO2-12A1~O3 compositions have been prepared 
from sonocatalyzed sols and deposited by dip coating 
technique of 316L stainless teel foil. All the coat- 
ings protect he metallic substrate against acid (15% 
H2SO4) corrosion. The films are shown to act as a ge- 
ometric blocking layer and increase the lifetime of the 
substrate by a factor up to 8.5, the effect being more 
pronounced at 50°C than at room temperature. The 
electrochemical parameters such as the corrosion po- 
tential, polarization resistance and corrosion rate have 
been determined. These results, together with those 
already published on the corrosion obtained in NaC1 
solutions [25] confirm the outstanding corrosion pro- 
tection behavior of thin sonocatalyzed sol-gel oxide 
Chemical Protection of Stainless Steel 533 
n 
20 
tY 
O 
10 [ 
T=25°C,H2S04 
[] 0 A A 
0 I 
0 5 ilO ll5 2; 
Time(h) 
Fig,. 4. Corrosion rate (MPY) versus time of heat reatment a 800 ° C 
measured at 25°C in 15% H2SO4 for (O)ss, (o)SiO2, (A) SiO2- 
Ti02, V (Si02-A 1203). 
Fig. 5. SEM micrograph of a SiO2-TiO2 coating heat treated at 
800°C during 20 h showing the fracture of the coating due to the 
growth of Cr203 single crystals at the metal/coating interface. The 
lower part is a 5 times amplification ofthe inset shown in the micro- 
graph. 
films which can be practically used by the chemical 
and construction industries. 
Acknowledgments 
This research was supported by Fapesp, Finep, CNPq, 
CAPES/PICD and the Program RHAE-New materi- 
als (Brazil). 
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Acronym

Journal of Sol-Gel Science and Technology, 2, 529-534 (1994) © 1994 Kluwer Academic Publishers, Boston. Manufactured in The Netherlands. Sol-Gel Coatings for Chemical Protection of Stainless Steel Code: EP7 PEDRO DE LIMA NETO, MOHAMED ATIK, LUIS A. AVACA AND MICHEL A. AEGERTER Institute of Physics and Chemistry of S~o Carlos, University of S~o Paulo, Cx. Postal 369 - 13560-970 - Sdo Carlos (SP), Brazil Abstract. Sol-gel thin coatings of ZrO2, SiO2, 70SiO2-30TiO2 and 88SiO2-12A12Oa compositions (mole %) have been prepared from sonocatalyzed sols and deposited bydip-coating technique on 316L stainless teel foils. The influence of the coatings on the chemical corrosion of the substrate has been measured through potentiodynamic polarization curves in aqueous 15% H2SO4 solution between 25 and 50°C. The values of the corrosion potential, polarization resistance and corrosion rate have been determined. Analysis of the data combined with scanning electron microscopy studies indicate that the films act as a geometric blocking against exposure to he corrosive media and increase the lifetime of the substrate up to a factor 8.5. Keywords: thin film, ZrO2, SiO~, SiO2 TiO2, SIO2-A1203, corrosion protection, stainless teel 316L 1. Introduction Sol-gel methods are now widely used for the prepara- tion of films of oxide materials from solutions and offer potential advantages over traditional techniques [ 1] [2] [3] [4]. The chemical processing [5] and the technical applications [6], which encompass optical,electronic, optoelectronic, photonic, biotechnology, chemical and mechanical fields have been reviewed recently. Inorganic oatings having a vitreous tructure are widely used as protective coatings for metals and al- loys [7] [8]. They improve the chemical and physical properties of the metal surfaces relative to corrosion, friction and wear without altering the original proper- ties of strength and toughness of the substrate. The preparation and characterization of sol-gel films having these specific chemical and mechanical func- tions have been so far scarcely studied. A1203 [9], Si-O-N, Si3N4 [10] [11] films have been proposed for semiconductor passivation. SiO2 [12] and TiO2-SiO2 [13] are known to prevent alkali diffusion. The pre- vention of chemical corrosion and oxidation of mild steel, carbon steel and stainless teels has been tested with SiO2 [14], SiO2-B2Oa [15] [16], mullite [17], ZrO2 [15, 18-22], MTOS [23] and ZrO2-CeO2 [24] coatings. They all increase the protection of metal substrates from air corrosion (tested up to 800°C) and acid attack (tested up to 80°C).The most promising prevention for stainless teel has been so far reported by our group using sol-gel film of ZrO2, SiO2-TiO2, SIO2-A1203 and TiO2-CeO2 prepared by dip-coating using sonocatalyzed sols [18, 19, 21, 25]. In this paper we present the r sults of a system- atic electrochemical corrosion study in 15% aqueous H2SO4 up to 50°C of sol-gel films of ZrO2, SiO2- TiO2, SiO2-AI~O3 (mole%) deposited on stainless steel 316L by dip coating, a material widely used for marine and chemical industry environments. 2. Experimental 316L stainless steel substrates (Caseurop, France) were cut from large foils and degreased ultrasonically in ace- tone. This austenitic material was chosen because ithas a low carbon content (<0.03%) and is therefore less susceptible to sensitization during the heat treatment necessary for the densification of the coatings. The zirconia sol was prepared by dissolving the alkoxide Zr(OC3HT)4 in isopropanol (CaHTOH) with a small amount of acetic acid (CHaCOOH). Excess water was then added to complete the hydrolysis. The concentration of the alkoxide was varied be- tween 0.025 and 0.9 mole/1 ZrO2. The volume ratios H20/CaHTOH and H20/CHaCOOH were respectively equal to 1 and 2. The silica sol was prepared by dis- solving tetraethylorthosilicate Si(OC2H5)4 in absolute 530 de Lima Neto, Atik, Avaca and Aegerter . I000  /' I I 600  J!il, 200 - (c) '.(a) [ (b)  . :.... ) : - -200- -  _ ___  ~ . . . .  .+ .,;,']( ~' . . .  -600  - ., ~,..... - IOOO , ,,i,,,,I . , , I , . , I  1 l lhl l l l  i l lh[l l l  i , INHI[ i , lhl l l l  I , , . , , , I  , , , , , ,  - -3  - -2 - - I  0 I 2 5 4 .6 I 0  10 I0  IO I0  I0  I0  I0  10 T ( l aA /cm 2 ) Fig. 1. Potentiodynamic polarization curves measured in deareated 15% aqueous H2SO4 at 25°C for 316L stainless teel a) uncoated and untreated, b) uncoated but heat treated at 800°C for 2 h., c) coated with ZrO2 (800 ° C/2 h), d) coated with SiO2 (800 ° C/2 h). ethanol C2HsOH (0.43 mole/mole of alkoxide) to which a small amount of acetic acid CHaOOH (0.087 mole/mole of alkoxide) was added. Binary composi- tions in the system SiO2-TiO2 and SIO2-A1203 have been prepared using the same protocol and concentra- tion using Si(OC2Hs)4, tetraorthotitanate Ti(OC2Hs)4 and aluminum sec-butilate AI(OC4Hs)3 as source of Si, Ti and A1 respectively. The composition of the solu- tions were zSiO2(1-z)MvOz where M = Ti or AI with z = 70 mole% (70SiO2-30TiO2) and z = 88 mole% (88SiO2-12A1203). However a complete study with 0 < z < 100 for both binary systems is underway and will be published elsewhere. All the sols were submitted to intense ultrasonic irradiation (Heat Sys- tems Ultrasonics W385). They became homogeneous and transparent after 25 minutes and were stable up to 4 weeks at room temperature when kept in a close vessel. Single layer coatings have been deposited by dip coating technique at a constant withdrawal speed of 10 cm min -1, dried at 60°C for 15 min. and then treated in air at a slow rate of5°C min -1 up to 800°C, with an intermediate isothermal holding at 450°C dur- ing 1 h and further isothermal treatments at800°C up to 20 h. The coatings have a thickness of about 0.3 to 0.6 #m. 1 ° glancing incidence X-ray diffraction (Rigaku Rotaflex) shows that the heat reated SiO2 and 88SIO2-12A1203 coatings are amorphous; however ZrO2 coatings crystallized at ,-~450°C into a tetragonal structure [21] while 70SiO2-30TiO2 coatings partially ,ooo- / / : i  600 ,dy./ , a) (b) v LU _ 200  -- -6oo  - ~-L :  . . . .  --3 --2 - - I  O I 2 3 4 I 0  I0  I0  I0  I0  I0  I0  I0  IO I ( laA /cm 2 ) Fig. 2. Same as figure 1 for T= 25°C a) and b) same as in Figure 1 (for reference); c) coated with SiO2-TiO2 (800°C/2 h); d) coated with SIO2-A1203(800°C/2 h). crystallized at ~650°C showing anatase crystallites in an amorphous silica matrix [21]. Electrochemical measurements are convenient and precise methods to evaluate the corrosion of a mate- rial in a given medium [26]. They have been car- ried out with freshly prepared samples in deareated 15% H2SO4 (Merck p.a.) aqueous olutions (Milli-Q) up to 50°C using a computerized PAR 273 Potentio- stat/Galvanostat. A saturated calomel electrode (SCE) was used as reference and a Pt foil served as the auxil- iary electrode. The working electrodes were immersed 1 cm into the solution. The potentiodynamic measure- ments were initiated at -0 .7  V and scanned continu- ously toward the anodic direction at 1 mV s-1. The data were analyzed with the PAR 342 Corrosion Mea- surement Software. A detailed explanation of the pro- cedure and analysis of the results can be found in [25]. 3. Results and Discussion Potentiodynamic polarization curves have been mea- sured at 25, 40 and 50°C. Figures I and 2 show typ- ical results obtained at 25°C for stainless teel (ss) without coating and untreated, ss without coating but heat treated at 800°C during 2 h, ss with ZrO2 coat- ing (800°C/2 h), ss with SiO2 coating (800°C/2 h), ss with SIO2-A1203 coating (800°C/2 h), and ss with SiO2-TiO2 coating (800°C/2 h). Similar data have been obtained at the other temperatures. The protective ffect is noticeable for all coat- ings in both the cathodic and anodic branches of the Chemical Protection of Stainless Steel 531 Table 1. Corrosion parameters determined from the potentiodynamic curves measured for stainless steel 316L (ss) uncoated and untreated, uncoated but heat reated at 800°C/2 h, coated with ZrO2, SiO2, SIO2-A1203, and SiO2-TiO2 (800°C/2 h): corrosion potential Ecorr (mV); polarization resistance Rp (kf2 cm 2) and corrosion rate CR (mills per year, MPY). Sample T(°C) -Ecorr (mV) Rp (kf~ cm 2) CR (MPY) SS 316L 25 286 1.80 11.5 un~e~ed 40 261 0.37 35.2 50 280 0.35 53.8 SS 316L hem 25 306 1.45 5.0 tre~ed 40 304 0.57 19.8 50 307 0.14 41.2 SS + ZrO2 SS + SiO2 25 308 10.50 3.4 40 328 8.50 4.2 50 299 0.99 12.9 25 289 8.56 3.1 40 307 1.21 6.5 50 314 0.31 14.2 SS -}- 88SIO2- 25 240 5.92 4.2 12A12Oa 40 297 0.93 8.4 50 301 0.89 13.9 SS + 70SiOz- 25 291 10.7 2.2 30TIO2 40 297 5.70 5.7 50 293 3.10 6.3 curves. The cathodic part corresponds to the hydrogen evolution reaction 2H + + 2e- --+ H2. The slope of the curves for uncoated and untreated stainless teel is ca. 2.3 × 2 RT/F, where R is the universal gas con- stant and F the Faraday constant a value coherent with a Volmer-Heyrovsky sequence, a current mechanism accepted for this reaction [27]. In the presence of the coatings, the value of the slope is maintained suggest- ing that the mechanism remains unchanged. However there is a downward shift in the current densities (in the sequence: untreated ss, heat treated ss, SiO2-A12Oa, SiO2-TiO2, ZrO2) indicating that the films act as a geometric blocking; they act as an inert obstacle to exposure of the metal surface to its environment and therefore the reaction rates fall in proportion to the ex- tent of surface coverage. As an example the active area for the reaction is typically reduced by a factor of 10 for the best coating (SiO2TiO2). The chemical attack is probably done via a small remaining open poros- ity of the film (not observed in SEM measurements) left after the densification heat treatment. The posi- tion and the slope of the SiO2-TiO2 potentiodynamic curves measured in the cathodic range at 25°C do not change for heat treatment up to 15 h, all being identical to curve d of Figure 2 indicating that the film surface coverage is not affected by these heat treatments. The measurements performed at 40°C and 50°C present similar behavior and consequently the blocking prop- erties remain unchanged at these temperatures. An intermediate behavior is observed for the heat treated substrates without coating (curves b of Figures 1 and 2). It is difficult to interpret it without a detailed knowledge of the processes occurring in the oxide lay- ers growing during the heat treatment on the stainless steel surface as well as in its bulk structure. The anodic branches of the polarization curves how that the coatings have a strong effect on the current den- sity in the passive region, diminishing the current value of uncoated and untreated stainless teel by roughly one order of magnitude. In the same region, the heat treated uncoated ss samples (curve b of the Figures 1 and 2) show current densities omewhat higher than those of the untreated substrates (curve a) revealing again sur- face changes during the treatment. The combined cathodic and anodic effects of the coatings on the corrosion behavior of 316L stainless steel are summarized in Table 1, which presents the relevant corrosion parameters derived from computer analysis of the potentiodynamic curves. The values of 532 de Lima Neto, Atik, Avaca and Aegerter 10000 1000 EL n," 100 0 [] r'l 10 [] [] O O O 2.7 218 '  219 ' 310 '  311 '  3~.2' 313 ' 3.4 1000/T (K -1) Fig. 3. Temperature dependence of the corrosion rate (MPY) in deareated 15% aqueous H2SO4 solution of 316L stainless steel: (n, o) uncoated and untreated; the high temperature data (D) are- from [19] and have been obtained by weight loss in non deareated solution; (V) coated with SiO2; (A) ZrO2; (~-) SIO2-A1203 and (©) SiO2-TiO2. the corrosion potential (Ecorr) are very little affected by the presence of the films indicating abalance between cathodic and anodic effects. However the polariza- tion resistances (Rp) which reflects the kinetics of the process at the equilibrium potential and the corrosion rate (CR), measured in mill per year, are drastically changed. At all tested temperatures all coatings how a lower corrosion rate than the untreated and uncoated substrate. The lifetime of the coated substrates i in- creased typically by a factor 3,5 to 5,5 at 25°C. The effect is even more pronounced at 40°C (4 to 8) and reaches 8.5 for SiO2-TiO2 at 50°C but decreases to 4 for the other coatings. Even in relation to the heat- treated uncoated ss samples, which appear to be more acid resistant than the untreated samples, the effect of the coating is still considerable. The corrosion rates determined for the different coating compositions heat treated at 800°C for 2 h. increase with the temperature of the electrolyte (Fig- ure 3). The low temperature data show the same over- all Arrhenius behavior (activation energy E = 27-28 kcal mole -1) for all coatings as well as for the un- coated stainless teel. For the oxide coatings how- ever the preexponential f ctor is smaller. This confirms that the layers act as a geometric blocking. The data at high temperatures have been determined by weight loss measurements in non deareated H2SO4 solution [19]. In order to truly compare these values to those de- termined electrochemically the measurements should have been extrapolated atzero time which was not pos- sible due to the lack of sensitivity of the method. Non deareated acid solutions are also known to accelerate the corrosion phenomena [26]. The corrosion rates have been also studied as a func- tion of the heat reatment time at 800 ° C (Figure 4). The protection action is basically the same up to around 15 h. This shows that the film surface coverage of the substrate and probably the structure of the pores are not changed uring the heat treatment. There is however a drastic change observed at t = 20 h, where the corrosion rates reach the same very high value (CR ~39 MPY) for all systems (higher than that of the as received stainless teel (11.5 MPY at t = 0). SEM measurements show that the films are dense with no visible porosity but are completely broken leaving an easy access of the electrolyte to the substrate (Figure 5). The physical reasons of this decohesion is the growth of a layer of Cr203 single crystals (containing also mainly Mn and Fe) at the interface metal-coating caus- ing significant stresses in the film and leading eventu- ally to its fracture under prolonged high temperature treatment. The growth of the single crystals already occurs at 450°C as confirmed by Rutherford Back Scat- tering measurements [28]. 4. Conclusion Sol-gel coatings of ZrO2, SiO2, 70SiO2-30TiO2 and 88SiO2-12A1~O3 compositions have been prepared from sonocatalyzed sols and deposited by dip coating technique of 316L stainless teel foil. All the coat- ings protect he metallic substrate against acid (15% H2SO4) corrosion. The films are shown to act as a ge- ometric blocking layer and increase the lifetime of the substrate by a factor up to 8.5, the effect being more pronounced at 50°C than at room temperature. The electrochemical parameters such as the corrosion po- tential, polarization resistance and corrosion rate have been determined. These results, together with those already published on the corrosion obtained in NaC1 solutions [25] confirm the outstanding corrosion pro- tection behavior of thin sonocatalyzed sol-gel oxide Chemical Protection of Stainless Steel 533 n 20 tY O 10 [ T=25°C,H2S04 [] 0 A A 0 I 0 5 ilO ll5 2; Time(h) Fig,. 4. Corrosion rate (MPY) versus time of heat reatment a 800 ° C measured at 25°C in 15% H2SO4 for (O)ss, (o)SiO2, (A) SiO2- Ti02, V (Si02-A 1203). Fig. 5. SEM micrograph of a SiO2-TiO2 coating heat treated at 800°C during 20 h showing the fracture of the coating due to the growth of Cr203 single crystals at the metal/coating interface. The lower part is a 5 times amplification ofthe inset shown in the micro- graph. films which can be practically used by the chemical and construction industries. Acknowledgments This research was supported by Fapesp, Finep, CNPq, CAPES/PICD and the Program RHAE-New materi- als (Brazil). References 1. Brinker, C.J. and Scherer, G.W., Sol-gel Science: the Physics and Chemistry ofSol-gel Processing (Academic Press, San Diego, 1990). 2. Sakka, S., in Sol-Gel Science and Technology, edited by Aegerter, M.A., Jr., M.J., Souza, D.E, Zanotto, E.D., (World Scientific, Singapore, 1991) pp. 346-374. 3. Dislich, H., in Sol-Gel Technologyfi)r Thin Films, Fibers, Pre- fi)rms, Electronics, and Specialty Shapes, edited by L.C. Klein (Noyes Publications, Park Ridge, W, 1988) pp. 50-79. 4. Pettit, R.B., Ashley, C.S., Reed, S.T., and Brinker, C.J., in Sol- gel Technology Jbr Thin Films, Fibers, Pretbrms, Electronics and Specialty Shapes, edited by L.C. Klein (Noyes Publica- tions, Park Ridge, 1988) pp. 80-109. 5. Schmidt, H., in Chemistry, Spectroscopy and Applications q/" Sol-Gel Glasses, edited by R. Reisfeld, C.K. Jorgensen (Springer-Verlag, Berlin, 1992) pp. 119-152. 6. Sakka, S. and Yoko, T., in Chemistry, Spectroscopy andAppli- cations qfSol-Gel Glasses, edited by R.R., J.C.K. (Springer- Verlag, Berlin, 1992)pp. 89-118. 7. Jones, D.A., Principles and Prevention [" Corrosion (Maxwell-MacMillan, 1992). 8. Schreir, L.L., Corrosion (Newnes-Butterworths, 1976), vol. 2. 9. Schlichting, J., and Neumann, S., J. Non-Cryst. Solids 48 (185), (1982). 10. Martinsen, J., Figat, R.A., and Shafer, M.W., Mat. Res. Soc. Symp. Proc. 32 (361), (1984). 11. Brow, R.K. and Pantano, C.G., Mat. Res. Soc. Syrup. Proc. 32 (361), (1982). 12. Ogiwa~a, S. and Kinugawa, K., Yogyo-Kyokai-Shi (in Japanese) 90 (157), (1982). 13. Matsuda, A., Matsuno, Y., Katagama, S., Tsuno, T., Tohge, N., and Minami, T., J. Ceram. Soc. Jpn., Int. Ed. 100, 79-1082 (1992). 14. Sanctis, O., Gomez, L., Pelligri, N., Parodi, C., Marajofsky, A., and Duran, A., J. Non-Cryst. Solids 121 (338), (1990). 15. Tohge, N., Matsuda, A., and Minami, T., J. Am. Ceram. Soc. 70 (C13), (1987). 16. Guglielmi, M., Festa, D., Innocenzi, RC., Colombo, E, and Gobain, M., J. Non-Cryst. Solids 147-148, 474-477 (1992). 17. Di Giampaolo, A.R., Puerta, M., Lira, J., and Ruiz, N., J. Non- Cryst. Solids 147-148, 467--473 (1992). 18. Atik, M. and Aegerter, M.A., J. Non-Cryst. Solids 147-148, 813-819 (1992). t9. Atik, M. and Zarzycki, J., J. Mat. Sci. Lett. (accepted), (1993). 20. Tomasi, A., Scardi, R, and Marchetti, E, Mat. Res. Soc. Symp. Proc. 271,477-483 (1992). 21. Atik, M. and Aegerter, M.A., in Better Ceramics Through Chemistry V, edited by Hampden-Smith, M.J., Klemperer, W.G., Brinker, CJ. (MRS, Pittsburgh, 1992), vol. 271, pp. 471476. 22. Ganguli, D., and Kundu, D., J. Mat. Sci. Lett. 3 (503), (1984). 534 de Lima Neto, Atik, Avaca and Aegerter 23. Izumi, K., Tanaka, H., Uchida, Y., Tohge, N., and Minami, T., J. Non-Cryst. Solids 147-148, 483--487 (1992). 24. Maggio, R.D., Scardi, P., and Tomasi, A., Mat. Res. Soc. Symp. 180 (481), (1990). 25. Lima Neto, P., Atik, M., Avoca, L.A., and Aegerter, M.A., J. Sol-Gel Science and Tech. (accepted), (1993). 26. Uhlig, H.H. and Revie, R.W., Corrosion and Corrosion Con- trol. J.W. & Sons, Eds., New York, 1985). 27. Bockriss, J.O., and Reddy, A.K.N., Modern Electrochemistry (Plenum Press, 1970), vol. 2. 28. Lima Neto, E, Atik, M., Acquadro, J.C., Avaca, L.A., and Aegerter, M.A., (to be published), (1993). 

ZrO2 coatings deposited on 316 L stainless steel sheets were synthesized by sol-gel method using Zr(OC3H7)4 as precursor and isopropanol, glacial acetic acid, and water as solvents for application with ultrasounds. Different solutions for dip-coating were prepared with compositions varying between 0.025 and 0.9 mol/dm(3) of ZrO2. X-ray diffraction shows that the films densified at 800°C are crystalline with a tetragonal structure. The thickness of the coatings varied from 0.35-0.75 µm. The influence of the ZrO2 coatings on the corrosion behavior of stainless steel substrates in aqueous NaCI was studied through potentiodynamic polarization curves at 1 mV/s. The values of the electrochemical parameters allow for an explanation of the role of the films in the increased resistance of steel against corrosion in moderately aggressive environments

Sol-gel ZrO2 coatings for chemical protection of stainless steel

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Sol-gel coatings for chemical protection of stainless steel

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