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Chapter 4
Viera Zatkalíková1, Lenka Markovičová2, Monika Oravcová3
THE EFFECT OF CHEMICAL SURFACE TREATMENT ON CORROSION BEHAVIOUR
OF AUSTENITIC STAINLESS STEEL
Abstract: Austenitic stainless steels are considered to be materials with excellent corrosion resistance, and acceptable mechanical properties and therefore they are recommended for various industrial and biomedical applications. However they are prone to pitting corrosion in aggressive chloride environments. The aim of this study is to evaluate the effect of temperature on the corrosion resistance of AISI 316Ti stainless steel with chemically treated surface (nitric acid passivated surface) in 1 M chloride solution.
Evaluation of the corrosion resistance is based on the results of exposition immersion tests (visual and microscopic observation of attacked surfaces, mass losses of specimens) and the results of the electrochemical impedance spectroscopy (EIS) tests.
Key words: AISI 316Ti stainless steel, pitting corrosion resistance, immersion test, electrochemical impedance spectroscopy (EIS)
4.1. Introduction
Due to their passive surface film, austenitic stainless steels have high resistance to uniform corrosion in oxidation environments (SZKLARSKA- SMIALOVSKA Z. 2005, LIPTÁKOVÁ T. 2009, ORAVCOVÁ M., PALČEK P.
2015).
However, aggressive ions present in environment cause local breakdown of this protective film and a progress of the pitting corrosion.
This form of corrosion is very destructive and dangerous because of the possibility of material perforation (LIPTÁKOVÁ T. 2009, ORAVCOVÁ M., PALČEK P. 2015).
1 RNDr. Viera Zatkalíková, PhD., University of Žilina, viera.zatkalikova@fstroj.uniza.sk
2 Ing. Lenka Markovičová, PhD., University of Žilina, lenka.markovicova@fstroj.uniza.sk
3 Ing. Monika Oravcová, University of Žilina, monika.oravcova@fstroj.uniza.sk
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Nitric acid
passivation (ASTM A380) is one of the methods of the chemical surface treatment, used for an improvement of corrosion resistance of stainless steels. Application of nitric acid solution enables to strengthen the natural protective oxide film on the surface of a stainlesssteel (
DA-CUNHA-BELO M., RONDOT B., COMPERE C., MONTEMOR M.F., SIMOES A. M. P., FERREIRA M. G. S. 1998, NOH J. S., LAYCOCK N. J., GAO W., WELLS D. B. 2000). Nitric
acid acts as an oxidising agent which reacts with the surface of the steel. Nature of nitric acid passivation process is based on oxidation of chromium (Cr0 → Cr+III) and reduction of nitrogen (N+V → N+IV, N+II)(
DA-CUNHA-BELO M., RONDOT B., COMPERE C., MONTEMOR M.F., SIMOES A. M. P., FERREIRA M. G. S. 1998, NOH J. S., LAYCOCK N. J., GAO W., WELLS D. B. 2000).
ASTM A380 standard method recommends 20 – 50 % nitric acid for passivation treatment (BABOIAN R. 1995). Noh et al. (NOH J. S. 2000) investigated influence of the nitric acid passivation on the corrosion resistance of AISI 316 stainless steels. They found, that concentration of nitric acid markedly affected some electrochemical characteristics of the tested steels. The pitting potential Ep (in 1M NaCl solution at 70 °C) reached the highest values by using 20 % - 25 % nitric acid solution.
Increase of nitric acid content over 25 % caused strong decrease of Ep. When 50 % nitric acid solution was used for passivation, Ep value was very similar to that obtained for samples without nitric acid passivation (NOH J. S. 2000).
This paper concentrates on the effect of temperature (22 ± 0,5 °C, 50 °C, 80 °C) on the corrosion resistance of AISI 316Ti stainless steel with chemically treated surface (nitric acid passivation) in 1 M chloride solution. Evaluation of the corrosion resistance is based on the results of exposition immersion tests (visual and microscopical observation of attacked surfaces, mass losses of specimens) and on the results of the electrochemical impedance spectroscopy (EIS) tests.
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4.2. Experimental material
AISI 316Ti stainless steel, with the chemical composition shown in Table X.1., was used as an experimental material. It was bought in ITALINOX and its treatment (marked as 2B) was based on annealing and pickling after smoothing rolling.
Due to molybdenum additive, the AISI 316Ti stainless steel has good plasticity and high resistance to acids and deep local corrosion. It is nonferromagnetic steel, with higher yield stress and strength. After welding of thin plate heat treatment is not necessary, because the steel is stabilized by Ti. Mechanical properties of studied steel are shown in Table 4.2.
Table 4.1. Chemical composition of experimental material (wt. %)
Cr 16.5
Ni 10.6
Mo 2.12
Mn 1.69
N 0.012
Ti 0.41 C
0.04
Si 0.43
P 0.026
S 0.002
Fe balance
Table 4.2. Mechanical properties of experimental material
Rp0.2 [MPa]
Rp1.0 [MPa]
Rm
[MPa]
A5
%
A50
%
HB 30/10/10
282 315 585 65 56 162
Microstructure of the original material was observed on the optical metallographic microscope Neophot 32 in transversal and longitudinal cut (Fig. 4.1).
Microstructure is created by polyedric austenitic grains with observable twins (Fig. 4.1a). Twins could be created by annealing or rolling. Strong lines visible in longitudinal cut (Fig. 4.1b) arose by the rolling during the technologic process.
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4.3. Experimental conditions and methods
1 M chloride solution was used as the basic corrosion environment for both immersion and electrochemical impedance spectroscopy (EIS) test. 1 M chloride solution was represented by 5 % FeCl3 (pH=1.2) for immersion tests and by 0.9 M NaCl + 0.1 M HCl (pH=1.1) for EIS test.
Aggressive FeCl3 solution is recommended for testing of pitting corrosion resistance of stainless steels according to ASTM G48 standard method (BABOIAN R. 1995). (FeCl3 solution is too aggressive for use in corrosion cell, therefore for EIS test it was replaced by 0.9 M NaCl + 0.1 M HCl solution with the same concentration of chlorides and almost the same pH. However, redox potential of 5 % FeCl3 is 690.8 mV, redox potential of 0.9 M NaCl + 0.1 M HCl is lower, 509 mV.)
a) b)
Fig. 4.1. Microstructure of the steel AISI 316Ti, etch. 10ml HNO3, 30ml HCl, 30ml glycerine: a) detail of austenitic grains in cross section, BWI, b) general
view, longitudinal section
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The specimen’s shape for immersion tests was rectangular with one nominal dimension (Fig. 4.2.) for simplification of the exposed area definition (30 mm x 80 mm x 1.5 mm).
Fig. 4.2. Shape and dimensions of specimens for immersion tests
The surface of the specimens was not mechanically treated, but the edges were grinded by abrasive paper grain 600. The grease from the tested specimens was removed by diethyl ether. They were consequently passivated in 20 % nitric acid solution (30 minutes, 50 °C) (BABOIAN R.
1995). After passivation the specimens were weighted out (analytical balance Mettler Toledo XS 205, accuracy ± 0,000 01g).
Immersion tests (ASTM G 48) were performed for 24 hours at the temperature 22 ± 0.5 °C, 50 °C, and 80 °C. After exposure the specimens were carefully brushed, washed by demineralized water, freely dried up and weighted up again.
Pitted surfaces of specimens after immersion tests were observed by the optical microscope NIKON AZ 100. Average corrosion rates (g.m-2.h-1) were calculated from corrosion losses (g.m-2) during the immersion tests.
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Electrochemical impedance spectroscopy (EIS) tests:
The electrochemical EIS tests were performed at the same temperatures as immersion tests. The surface of specimens was chemically treated in the same way as the surface for immersion tests.
EIS method allows establishing the value of polarization resistance of less conductive corrosion systems, for example when a passive film with suitable adhesion is created on the metal surface. EIS experiments were performed in a conventional three-electrode cell system (Fig. 4.3.) with a calomel reference electrode (SCE) and a platinum auxiliary electrode (Pt) using Voltalab 10 corrosion measuring system with PGZ 100 measuring unit.
The time for potential stabilization between the specimen and electrolyte was set to 5 min. The exposed area of specimen was 1 cm2. The measurement frequency ran in a range from 100 kHz to 1 mHz, the amplitude was 10 mV (ŠKUBLOVÁ L., ŠKORÍK V., MRÁZIKOVÁ R., HADZIMA B. 2010). Results of EIS measurements were displayed as Nyquist diagrams, which are plotted in coordinates of real and imaginary impedance components.
Fig. 4.3. Three electrode corrosion cell (capacity 300 cm3)
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A representative curve for a given temperature was selected from at least three measurements, which were performed at each temperature.
Polarization resistance values were obtained on the basis of the analysis of these representative Nyquist curves. The measurements were evaluated by EC-LAB software using a circuit with one loop (Fig.4.4.).
Fig. 4.4. Illustration of a circuit with one loop
4.4. Experimental results and discussion
The specimens were locally damaged by local corrosion during the immersion tests in 1 M chloride solution. Very remarkable change in appearance of pitted surfaces is observed between temperatures 22 and 50 °C. At 22 °C the pits are strongly bigger than at 50 °C (Fig. 4.5., 4.6.), but density of pitting is markedly lower (Fig. 4.5.).
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a) b) c)
Fig. 4.5. The pitted surfaces after 24-hours immersion test in 1M chloride solution ( 5 % FeCl3), a – 22 °C, b – 50 °C, c – 80 °C
At 22 °C, the combination of the low density of pitting and the “big”
pits can be probably related to the sporadic penetration of Cl- anions through the passive film and to the follow intensive corrosion by the
“small” area of anode and the “big” area of cathode. The local failure of these specimens is especially situated to their edges and to the holes for hanging (Fig. 4.5.a) with higher surface roughness. The pits in these places seemed to grow under the passive film and they were visualized by cleaning of specimens after the exposure. The authors (DA-CUNHA-BELO M., RONDOT B., COMPERE C., MONTEMOR M.F., SIMOES A. M. P., FERREIRA M.
G. S. 1998). describe the similar oblong pits created under the passive film on the surface of AISI 316 stainless steel.
At 50 and 80 °C the surfaces of the specimens are uniformly pitted (Fig. 4.5b, 4.5c). The typical pit shapes are shown and can be compared in Fig. 4.6. This comparison confirms differences of pit shapes observed visually (Fig. 4.5).
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a) b)
c)
Fig. 4.6. Typical shape of pits after 24-hours immersion test (a – 22 °C, b – 50 °C, c – 80 °C)
Average corrosion rates (g.m-2.h-1) calculated from corrosion losses during the immersion tests in dependence on temperature are shown in Fig. 4.7. The run of corrosion rates cannot be generally considered the essential factor of pitting corrosion evaluation. However, it helps make an idea about probable changes in the pitting corrosion kinetics (SZKLARSKA-SMIALOVSKA Z. 2005. ZATKALÍKOVÁ V. 2008.).
The highest average corrosion rates are observed at 50 °C. The increase of corrosion rates between 22 and 50°C clearly correlates with the change in appearance of pitted surfaces (Fig. 4.5., 4.6.) and with the increase of the pitting density in the same range of temperature (Fig.
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4.5.). A process which controls the pitting corrosion kinetics probably changes from diffusion to combination control in mentioned range of the temperatures (ZATKALÍKOVÁ V. 2008).
Fig. 4.7. Average corrosion rates in dependence on temperature The polarization resistance Rp is an electrochemical property characterizing the material resistance to polarization in the experimental environment. A higher value of polarization resistance Rp represents better corrosion resistance of the material in corrosion environment (ŠKUBLOVÁ L.,ŠKORÍK V.,MRÁZIKOVÁ R.,HADZIMA B.2010).
A polarization resistance Rp valuecan be assessed from diameter of semicircle shaped Nyquist diagram. The larger is the diameter, the higher value of the polarization resistance.
Nyquist curves for nitric acid passivated AISI 316Ti specimens in 1 M chloride solution are shown in Fig. 4.8. As can be seen, diameters of curves decrease with temperature. The decline is most pronounced at 80 °C. It is clear, that at 80 °C, the value of polarization resistance, and thus the resistance of the surface passive film is several times lower than at the two lower temperatures. This fact can be considered from comparison of polarization resistances Rp shown in Fig. 4.9. as well.
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Fig. 4.8. Nyquist curves of tested steel in 1 M chloride solution
From above mentioned it can be concluded, that the temperature in the range from 22 to 80 °C is a very important factor of corrosion resistance of tested steel in 1 M chloride solution. This is consistent with previous research studies (SZKLARSKA-SMIALOVSKA Z. 2005.
ZATKALÍKOVÁ V.2008).
However EIS results are not in full coherence with the results of exposition immersion tests. According to EIS result, stability and resistance of passive surface film is the lowest at 80 °C. According to immersion test, in terms of the corrosion kinetics, the highest corrosion rate was recorded at 50 °C. This discrepancy in results could be partly caused by lower aggressiveness of 1M chloride solution (0.9 M NaCl + 0.1 M HCl) used for EIS test in comparison with 1M chloride solution (5
% FeCl3) used for immersion tests (as explained above, chap. 4.2.).
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Fig. 4.9. Comparison of polarization resistances Rp in dependence on temperature
4.5. Conclusion
Based on
the
results of performed immersion and EIS tests can be concluded:- Despite chemically treated surface, specimens were attacked by local corrosion during the immersion tests. Very remarkable change in appearance of pitted surfaces is observed between temperatures 22 and 50 °C. At 22 °C the pits are strongly bigger than at 50 °C but density of pitting is markedly lower.
-
Polarization resistances
Rp decrease with temperature. The value of Rp, and thus the resistance and quality of the surface passive film is several times lower at 80 °C than at two lower temperatures.On the basis of the above mentioned facts it is clear, that temperature in the range from 22 to 80 °C is a very important factor of corrosion resistance of tested stainless steel in aggressive chloride solutions.
Because of the possibility of initiation of pitting corrosion even by a short exposure at limit conditions, a caution is required when using stainless
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steels in aggressive conditions (Cl- concentration, elevated temperature).
It is suitable to perform at least two independent tests of corrosion resistance before application of stainless steel into operating conditions.
Aknowledgments
The research was supported partially by Scientific Grant Agency of Ministry of Education, Science and Sport of Slovak Republic and Slovak Academy of Science grant VEGA No. 1/0683/15 and by project KEGA No. 044ŽU-4/2014.
Bibliography
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