- Email: [email protected]
Electrolytic pickling of stainless steel studied by electrochemical polarisation and DC resistance measurements combined with surface analysis
Electrochimica Acta 46 (2001) 3859– 3866 www.elsevier.com/locate/electacta
Electrolytic pickling of stainless steel studied by electrochemical polarisation and DC resistance measurements combined with surface analysis Jouko Hilde´n a,*, Jorma Virtanen b, Olof Forse´n b, Jari Aromaa b a
VTT Manufacturing Technology, Materials and Manufacturing Technology, P.O. Box 1703, FIN-02044 VTT, Espoo, Finland b Laboratory of Corrosion and Material Chemistry, Helsinki Uni6ersity of Technology, P.O. Box 6200, FIN-02015 TKK, Espoo, Finland Received 18 September 2000; received in revised form 14 March 2001
Abstract A tightly adhering oxide scale is formed on stainless steels when they are annealed. The removal of the oxide scale and chromium-depleted subscale is one of the most important processes during the stainless steel production. Electrolytic pickling in neutral sodium sulphate is widely used for oxide scale removal. This study describes the different stages of the oxide scale removal on Polarit 725 (EN 1.4301) stainless steel in sodium sulphate solution. A mechanism of the scale dissolution is also proposed. The dissolution is proposed to proceed by the electrochemical reactions of the scale in three successive stages. At the beginning of the pickling process chromium and manganese of the outer oxide layer were preferentially dissolved. When the chromium content of the outer layer decreased, the scale was enriched of iron. The electrode potential was then increased and the scale thickness greatly reduced. Finally a steady state was obtained and a thin oxide layer, rich in iron and silicon, was left on the surface. Silicon could not be removed by the electrolytic pickling and post-treatment in nitric– hydrofluoric acid is required. © 2001 Elsevier Science Ltd. All rights reserved. Keywords: Stainless steel; Electrolytic pickling; Contact electric resistance
1. Introduction An oxide scale that mainly consists of spinel-type metal oxides is formed on the surface of the stainless steels when they are heated in annealing processes during the manufacturing cycle. The removal of the oxide scale and also a chromium-depleted subscale, which is formed underneath the oxide scale, is one of the most important processes during the production. The removal of the oxide and the chromium-depleted layer is done by chemical or electrochemical treatment called pickling. A proper pickling procedure passivates * Corresponding author.
the stainless steel surface and ensures full corrosion resistance of the material. Electrolytic pickling in a neutral sodium sulphate solution has been used industrially for more than 30 years [1]. Neutral electrolytic pickling provides relatively fast removal of the oxide scale and in combination with nitric– hydrofluoric acid, i.e. mixed acid pickling, the surface quality is excellent. It is also claimed that the operating and maintenance costs of the system are low and handling of the process is simple [1]. A consensus on the electrochemical mechanism of pickling has not been reached. In the 1980s and the early 1990s several different electrochemical dissolution mechanisms were described in the literature [1 – 4]. Con-
0013-4686/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 3 - 4 6 8 6 ( 0 1 ) 0 0 6 7 3 - 9
J. Hilde´n et al. / Electrochimica Acta 46 (2001) 3859–3866
3860
Table 1 Nominal chemical composition of the test material Material
UNS
Cr
Ni
Mnmax
Cmax
Simax
EN 1.4301
S30400
17–19
8–11
2
0.06
1
trary to the previous reports, Dunaevskii et al. [5] suggested that the oxidation processes in the scale do not play a decisive role in the removal mechanism. The most recent state of the art report [6] describes the electrochemical nature of the scale disintegration, but no common mechanism is agreed upon. This laboratory study was launched to clarify the mechanism of the electrolytic pickling. The advancement of the dissolution process was followed by the successive surface analyses. The surface analysis data were combined with the polarisation and DC resistance measurements to evaluate the process kinetics and to follow the development of the surface properties. Understanding the basic mechanisms and the process kinetics is necessary for the further development of the pickling conditions and for the improvement of the process control and the equipment. Complementary laboratory test methods were used to get a better view of the phenomena and to select the most reliable methods for the industrial tests.
2. Experimental The stainless steel used was EN 1.4301. Nominal chemical composition of the steel is presented in Table 1. The samples were cut from 2 mm steel sheets and the size of each sample was 3 ×3 cm. The samples were polished with 1000 mesh abrasive paper and then they were annealed in a laboratory furnace. The temperature was 1140 °C and the annealing period was 2.5 min in an atmosphere containing 6% oxygen. Electrochemical experiments were carried out with a computer controlled potentiostat. Two different kinds of electrochemical cells were used. The first one was so-called Avesta-cell [7] and it was used for most of the tests (Fig. 1). The sample area in the Avesta-cell was 1.13 cm2. The oxide layers of certain samples were analysed after the set polarisation time by a glow discharge optical emission spectrometer (GDOES). The experiments for these samples were carried out with a conventional electrochemical cell, in which the sample area was 8 cm2. In both cells three-electrode arrangement was used, a saturated calomel electrode (SCE) was used as a reference electrode and a platinum plate as a counter electrode. The test solution was 15 wt% Na2SO4 and the temperature 70 °C.
DC resistance was measured by a contact electric resistance (CER) equipment [8], which is presented in Fig. 2. The specimens were circular plates (diameter 6 mm). An iridium probe (diameter 2 mm) was used as a contacting tip (Fig. 3). First, potentiodynamic tests with the annealed samples and the polished reference samples were carried out. The aim of the potentiodynamic experiments was to compare the general electrochemical behaviour of the annealed and freely passivating stainless steel surface. The tests were started from the open circuit potential, since the cathodic polarisation had no influence on the anodic polarisation curve. Sweep rates 10 and 60 mV/min were used. In the galvanostatic experiments current densities 0.5, 1, 2, 3, 5 and 10 mA/cm2 were used. In this case the tests were continued until the potential stabilised. The oxide composition and oxide thickness of the samples was measured by the GDOES. The GDOES analyses were first made for annealed samples. GDOES analyses were then made for the samples, which were polarised at different times by the galvanostatic method. In these experiments 1 or 10 mA/cm2 current density was used. The DC resistance of the specimens was measured simultaneously with potentiodynamic polarisation. A comparison of resistance change was made to the change in the current density. The DC resistance was also measured potentiostatically by stepping the potential into anodic direction. The potential range was between − 1500 and + 1500 mV (vs. SCE). At the lowest potential hydrogen evolution and at the highest potential oxygen evolution occurred. The rate of the potential change was typically 10 mV/min.
Fig. 1. The Avesta-cell.
J. Hilde´ n et al. / Electrochimica Acta 46 (2001) 3859–3866
3861
Fig. 2. The CER equipment.
3. Results The polarisation experiments were carried out to identify the electrochemical behaviour of the oxide scale and to study the mechanism of scale removal. In Fig. 4, a potentiodynamic polarisation curve of an annealed sample and a polished sample considered as a reference sample are shown. In the passive region the current of the reference sample is higher than the current of the annealed sample. The thick oxide layer formed during annealing is protecting the steel from dissolving. At higher potential there is a current peak in the polarisation curve of the annealed sample before the transpassive dissolution regime of the reference sample. In the galvanostatic experiments the current was kept constant and a decrease of the electrode potential was observed when scale dissolution started (Fig. 5). When the dissolution had proceeded long enough, the process became more difficult and higher potential was required to keep up the fixed current density. If the current is set between 0.1 and 5 mA/cm2, the total area of the peak of the potential drop is suggested to correspond to the charge, which is required for the dissolution of the oxide layer. If the current was lower than 0.1 mA/cm2 the potential was not high enough to dissolve the oxide completely. If 10 mA/cm2 was used, the current efficiency was lower, which is presumed to be a result of the oxygen evolution and other side reactions. Fig. 6 shows how the DC resistance develops, when the annealed stainless steel sample was polarised from cathodic to anodic direction. At low potentials the DC-resistance was only a little higher than the resistance of the polished specimen (Fig. 7). Close to the corrosion potential the resistance increased and remained quite stable until current started to increase due to the dissolving oxide layer. The resistance reaches the maximum point, when the metallic oxides have dissolved and there probably exists a dielectric silicon oxide layer. The resistance decreases, when the polarisa-
tion is continued. With the polished sample the DC-resistance stays very low throughout the potential range used. There are only two minor resistance peaks discernible (Fig. 7). The annealed specimens were also polarised to cathodic direction ( − 1500 mV vs. SCE) and back to the corrosion potential with different sweep rates of 3 – 60 mV/min. The resistance decreased at negative potential, but returned to the original level at the corrosion potential. This means that the oxide layer becomes more conductive at cathodic potential, but it does not detach or dissolve. When the annealed specimens are at the corrosion potential, the DC resistance may vary quite a lot between different specimens. However, the resistance of an individual specimen remains quite stable. When the specimen is polarised to the potential of +1500 mV
Fig. 3. The plate sample for the CER equipment.
3862
J. Hilde´ n et al. / Electrochimica Acta 46 (2001) 3859–3866
Fig. 4. Anodic polarisation curves for the polished and the annealed specimens, EN 1.4301, 70 °C, 10 mV/min.
versus SCE, the resistance decreases a little at first, but then it increases to a higher level (Fig. 8). It will take quite a long polarising time before the resistance decreases to the level of the polished specimen. The experiments were continued by polarising specimens with constant current density of 1 mA/cm2. The polarisation was continued until the potential started to increase and reached the middle between the minimum and the maximum points (Fig. 5). At this point the experiment was aborted and the surface was rinsed with water and analysed with the GDOES. Another set of samples was polarised with same current density and the test was aborted after half a minute from the maximum point of the potential. The changes in the potential were very clear. However, the dissolution kinetics was rather slow compared to the requirements of the industrial process. A current density of 10 mA/cm2 was used in further experiments. First, a 30-min test was performed. It was presumed that a steady state is reached and no more compositional changes occur in the oxide layer after such a long polarisation time. For the material EN 1.4301 the largest changes in potential occurred within 2 min after the polarisation was started. So for the GDOES analyses three polarisation periods below that time (18, 30 and 50 s) were chosen. The fourth point was selected to be 4 min, when the decrease of the potential was still visible.
3.1. Surface analyses
Fig. 5. Galvanostatic experiments for the annealed specimens, EN 1.4301, 70 °C.
The GDOES analysis for the annealed material is shown in Fig. 9. The content of the oxygen is only trend setting in the analysis. The thickness of the oxide layer on the surface of the specimen was measured to be approximately 150 nm. The criterion used for the thickness determination was the decrease of the oxygen content in the analysis below 15%. In the outer oxide layer large amounts of chromium and manganese were found, but the iron content was very low. Below the chromium and manganese rich layer the content of iron was increased and probably an iron –chromium spinel oxide was formed. At the interface of the oxide layer and the base material a slight enrichment of silicon was also found. GDOES analysis results for the samples, which were made by polarising them with 1 mA/cm2 current density, showed that the potential they obtained was high enough to dissolve chromium and iron oxides. The manganese content remained quite high (about 6 wt%) after the shorter polarisation time (8 min), but decreased to about the basic material composition (2 wt%) after the longer polarisation time (10 min). Silicon had enriched at the surface and its content exceeded 10 wt%.
J. Hilde´ n et al. / Electrochimica Acta 46 (2001) 3859–3866
3863
Fig. 6. A schematic presentation of development of DC resistance and current density as a function of potential for the annealed stainless steel.
Fig. 7. A schematic presentation of development of DC resistance and current density as a function of potential for the polished stainless steel.
When the specimen was polarised with 10 mA/cm2 current density, the oxide layer started to dissolve fast. In the beginning the amounts of chromium and manganese decreased and the contents of iron and silicon increased (Fig. 10), but the oxide layer did not become much thinner. Iron starts to dissolve and the layer becomes thinner only when the chromium content has decreased enough. The silicon content at the surface increased until the base material started to dissolve. After that there was a slow decrease in the silicon content.
4. Discussion The current peak in the potentiodynamic polarisation experiment of the annealed stainless steel before the transpassive dissolution region indicates the electro-
Fig. 8. DC resistance after potential step to +1500 mV vs. SCE, 70 °C.
J. Hilde´ n et al. / Electrochimica Acta 46 (2001) 3859–3866
3864
Fig. 9. Depth profile from the surface of the annealed specimen (EN 1.4301) by GDOES.
chemical dissolution of the oxide scale at potentials above +1050 mV versus SCE. In the passive region the current of an annealed specimen is lower than the current of a freely passivating polished sample and the thick oxide layer is protecting the steel from dissolving. After the peak the current density is about equal in both samples, which indicates that the thick oxide layer is completely or partly removed and both specimen suffer transpassive dissolution by similar mechanism. Galvanostatic experiments showed an increase in the dissolution rate when the current density was increased. Comparison of the dissolution time, measured from the voltage peak (Fig. 5) and the current density shows that the charge (A s) is almost equal in all cases. With a current density of 10 mA/cm2 most of the oxide was dissolved in less than 50 s, which is comparable to the pickling times of the industrial pickling lines. The equal charge of the galvanostatic experiments with different current densities indicates that the charge is consumed by the electrochemical oxidation reactions of the scale. Calculating the charge of the current peak of the potentiodynamic experiments also equals the charge consumed in galvanostatic experiments. To take this further a theoretical charge (Q) for dissolution can be calculated. The chemical constitution of the outer oxide layer was close to MnCr2O4 oxide, which is found to exist on stainless steel. The charge of MnCr2O4 dissolution is obtained by, Q=
dZF Vi
+ − − MnCr2O4+8H2O [ 2HCrO− 4 +MnO4 +14H +11e (2)
The theoretical charge, which is required to dissolve 1 nm thick oxide layer (MnCr2O4), was calculated to be 2.32 mC/cm2. This value is in good accordance with the experiments. The shape of the potentiodynamic polarisation curve is not affected by the cathodic starting potential of the experiment. The DC resistance measurements show low resistance at cathodic polarisation, but sweeping the potential back to anodic direction increases the resis-
(1)
where d is the oxide thickness, Z is the number of electrons transferred, F is the Faradaic constant and Vi is the molar volume of the oxide. The molar volume can be obtained by dividing molar mass of oxide (Mox) by oxide density (Dox). To obtain Z, the following dissolution reaction is proposed:
Fig. 10. A schematic presentation of the changes in the depth profile from the surface of the annealed stainless steel specimen during polarisation.
J. Hilde´ n et al. / Electrochimica Acta 46 (2001) 3859–3866
tance to the initial value. Visual appearance of the sample remains dark brown after the potential excursion to cathodic potential. The prolonged cathodic polarisation and many successive cathodic sweeps did not change the contact resistance and the visual appearance of the sample. Low resistance at cathodic potential indicates that the thick oxide layer behaves like an n-type semiconductor. Polarisation and contact resistance measurements are in good accordance. Good conductivity of the oxide layer at the cathodic potential allows hydrogen evolution to occur at the oxide surface. The oxide destruction by mechanical effect of hydrogen evolution is then greatly reduced, which gives explanation for ineffectiveness of the cathodic polarisation. The reduction of the oxide plays also only a small role because the stability of the chromium oxide under reducing conditions [9]. Three different stages were found in the dissolution process. Referring to Fig. 5, it can be seen that in the first stage the potential decreased, in the second stage the potential increased and in the third stage it decreased again. In the beginning the scale was enriched with chromium and also had a high content of manganese in the outer layer. When anodic current was passed through the sample, preferential dissolution of chromium and manganese occurred. As chromium and manganese dissolution proceeded the potential was decreased. As the chromium content decreased the scale was enriched with iron and the potential required to keep the constant current increased. The content of iron was about 25 – 30% at this stage and the chromium content was decreased below 30%. This corresponds to secondary passivation of stainless steels by iron oxides [9,10]. Experiments carried out with artificial oxides show [9] that the dissolution rate of mixed chromium/ iron oxides are pH dependent and that 50% of iron oxide in the layer is able to prevent oxidative dissolution of Cr3 + in a neutral electrolyte. Present findings are in agreement with that ratio, even though the dissolution rate of chromium was not considerably decreased, but the potential was increased. In the second stage the potential increased and the content of chromium and manganese in the outer layer of the scale diminished below 10%. The content of the iron was increased to about 60% and silicon, whose content increased in the first stage, was enriched on the surface. The content of the silicon at the surface reached several percent. After the second stage the oxide layer thickness was diminished below 20 nm and most of the original oxide scale was dissolved. Visual appearance of the samples was still a bit brownish at this stage. In the third stage the potential decreased and finally levelled out to a steady state value (about 1470 mV vs. SCE), where the base material probably dissolved by transpassive mechanism. Potential decrease was sta-
3865
bilised after 2 –3 min. As the dissolution of the base material proceeded, the gradient of iron content at the surface became steeper. Silicon is not electrochemically oxidised to higher valence. The steady state dissolution of the stainless steel samples proceeded through the thin silicon rich oxide layer and amount of silicon gradually decreased but some still remained on the surface after prolonged polarisation. The silicon concentrated layer can be removed by immersion in nitric –hydrofluoric acid pickling bath [11]. In the DC resistance measurements the three different stages of the dissolution process were also seen. In the beginning, when chromium and manganese dissolved fast, the contact resistance decreased. The observed decrease is in accordance with the findings of Bojinov et al. [10] that contact resistance of passive chromium rich alloy decreases as transpassive dissolution starts. But very soon the resistance increased. The increase was probably because of the enrichment of silicon and the formation of thin resistive layer. Increase of iron content and change of the oxide structure may also play a role in resistance change. After a long polarisation period the resistance decreases again, which was possibly due to the dissolution of iron oxide or decrease of silicon content or both.
5. Conclusions Cathodic polarisation down to − 1500 mV versus SCE was not able to dissolve or remove the oxide layer formed in annealing. A general conclusion drawn from the results of electrochemical tests and surface analyses was that oxide scale formed in annealing was protective at low potentials and it dissolved electrochemically at higher potentials over + 1050 mV versus SCE. The electrochemical dissolution of the oxide layer can be detected both by traditional electrochemical measurements and also by DC resistance measurements. Three different stages could be found in the scale dissolution process. In the beginning the scale was chromium rich with high content of manganese in the outer layer. When anodic current was passed through the samples, preferential dissolution of chromium and manganese occurred. As chromium dissolution proceeded the potential was decreased; however, the scale thickness reduced only slightly. When the chromium content decreased the scale was enriched with iron and the potential increased. This phenomenon is similar to secondary passivation of stainless steel by iron [9,10]. In the second stage, when the potential increased, chromium and manganese were almost completely dissolved from the outer layer of the scale. The content of iron increased to about 30% and silicon was also enriched on the surface. In the third stage the potential decreased and finally levelled out to a stable value. A
3866
J. Hilde´ n et al. / Electrochimica Acta 46 (2001) 3859–3866
steady state was obtained and the base material dissolved through the thin oxide layer, which was still rich in silicon. Silicon is not electrochemically oxidised and it remained on the surface after prolonged polarisation. Nitric– hydrofluoric acid pickling or other suitable chemical post-treatment is required to remove the silicon enriched layer.
[4] [5] [6] [7] [8]
References
[9]
[1] E. Braun, Iron Steel Eng. 4 (1980) 79. [2] E.T. Shapovalov, A.P. Shlyamnev, International Conference 21st Century Steel Industry of Russia and CIS, Moscow, 6– 10 June 1994, p. 59. [3] E.T. Shapovalov, A.P. Shlyamnev, E.A. Uly´ anin, V.D.
[10] [11]
.
Nikitin, A.R. Fisher, D.B. Gol%zon, Steel USSR 12 (1982) 215. V.I. Ignate´ v, N.N. Sergeeva, M.A. Shluger, Stal’ 3 (1987) 84. V.I. Dunaevskii, V.T. Stepanenko, M.Y. Shnol’, L.N. Bespal’ko, Protection Metals 21 (1985) 359. D. Henriet, Surface Treatments for Stainless Steel of the Art Developments and Trends, EUR 17248 EN, 1996. R. Qvarfort, Corros. Sci. 28 (1988) 135. T. Saario, Doctoral Thesis, Helsinki University of Technology, 1995. P. Schmuki, S. Virtanen, H.S. Isaacs, M.P. Ryan, A.J. Davenport, H. Bo¨ hni, T. Stenberg, J. Electrochem. Soc. 145 (1998) 791. M. Bojinov, G. Fabricius, T. Laitinen, K. Ma¨ kela¨ , T. Saario, G. Sundholm, J. Electrochem. Soc. 146 (1999) 3238. S. Kiya, T. Kojima, Y. Hyugaji, Rev. Metall. 93 (1996) 1431.
Electrolytic pickling of stainless steel studied by electrochemical polarisation and DC resistance measurements combined with surface analysis Jouko Hilde´n a,*, Jorma Virtanen b, Olof Forse´n b, Jari Aromaa b a
VTT Manufacturing Technology, Materials and Manufacturing Technology, P.O. Box 1703, FIN-02044 VTT, Espoo, Finland b Laboratory of Corrosion and Material Chemistry, Helsinki Uni6ersity of Technology, P.O. Box 6200, FIN-02015 TKK, Espoo, Finland Received 18 September 2000; received in revised form 14 March 2001
Abstract A tightly adhering oxide scale is formed on stainless steels when they are annealed. The removal of the oxide scale and chromium-depleted subscale is one of the most important processes during the stainless steel production. Electrolytic pickling in neutral sodium sulphate is widely used for oxide scale removal. This study describes the different stages of the oxide scale removal on Polarit 725 (EN 1.4301) stainless steel in sodium sulphate solution. A mechanism of the scale dissolution is also proposed. The dissolution is proposed to proceed by the electrochemical reactions of the scale in three successive stages. At the beginning of the pickling process chromium and manganese of the outer oxide layer were preferentially dissolved. When the chromium content of the outer layer decreased, the scale was enriched of iron. The electrode potential was then increased and the scale thickness greatly reduced. Finally a steady state was obtained and a thin oxide layer, rich in iron and silicon, was left on the surface. Silicon could not be removed by the electrolytic pickling and post-treatment in nitric– hydrofluoric acid is required. © 2001 Elsevier Science Ltd. All rights reserved. Keywords: Stainless steel; Electrolytic pickling; Contact electric resistance
1. Introduction An oxide scale that mainly consists of spinel-type metal oxides is formed on the surface of the stainless steels when they are heated in annealing processes during the manufacturing cycle. The removal of the oxide scale and also a chromium-depleted subscale, which is formed underneath the oxide scale, is one of the most important processes during the production. The removal of the oxide and the chromium-depleted layer is done by chemical or electrochemical treatment called pickling. A proper pickling procedure passivates * Corresponding author.
the stainless steel surface and ensures full corrosion resistance of the material. Electrolytic pickling in a neutral sodium sulphate solution has been used industrially for more than 30 years [1]. Neutral electrolytic pickling provides relatively fast removal of the oxide scale and in combination with nitric– hydrofluoric acid, i.e. mixed acid pickling, the surface quality is excellent. It is also claimed that the operating and maintenance costs of the system are low and handling of the process is simple [1]. A consensus on the electrochemical mechanism of pickling has not been reached. In the 1980s and the early 1990s several different electrochemical dissolution mechanisms were described in the literature [1 – 4]. Con-
0013-4686/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 3 - 4 6 8 6 ( 0 1 ) 0 0 6 7 3 - 9
J. Hilde´n et al. / Electrochimica Acta 46 (2001) 3859–3866
3860
Table 1 Nominal chemical composition of the test material Material
UNS
Cr
Ni
Mnmax
Cmax
Simax
EN 1.4301
S30400
17–19
8–11
2
0.06
1
trary to the previous reports, Dunaevskii et al. [5] suggested that the oxidation processes in the scale do not play a decisive role in the removal mechanism. The most recent state of the art report [6] describes the electrochemical nature of the scale disintegration, but no common mechanism is agreed upon. This laboratory study was launched to clarify the mechanism of the electrolytic pickling. The advancement of the dissolution process was followed by the successive surface analyses. The surface analysis data were combined with the polarisation and DC resistance measurements to evaluate the process kinetics and to follow the development of the surface properties. Understanding the basic mechanisms and the process kinetics is necessary for the further development of the pickling conditions and for the improvement of the process control and the equipment. Complementary laboratory test methods were used to get a better view of the phenomena and to select the most reliable methods for the industrial tests.
2. Experimental The stainless steel used was EN 1.4301. Nominal chemical composition of the steel is presented in Table 1. The samples were cut from 2 mm steel sheets and the size of each sample was 3 ×3 cm. The samples were polished with 1000 mesh abrasive paper and then they were annealed in a laboratory furnace. The temperature was 1140 °C and the annealing period was 2.5 min in an atmosphere containing 6% oxygen. Electrochemical experiments were carried out with a computer controlled potentiostat. Two different kinds of electrochemical cells were used. The first one was so-called Avesta-cell [7] and it was used for most of the tests (Fig. 1). The sample area in the Avesta-cell was 1.13 cm2. The oxide layers of certain samples were analysed after the set polarisation time by a glow discharge optical emission spectrometer (GDOES). The experiments for these samples were carried out with a conventional electrochemical cell, in which the sample area was 8 cm2. In both cells three-electrode arrangement was used, a saturated calomel electrode (SCE) was used as a reference electrode and a platinum plate as a counter electrode. The test solution was 15 wt% Na2SO4 and the temperature 70 °C.
DC resistance was measured by a contact electric resistance (CER) equipment [8], which is presented in Fig. 2. The specimens were circular plates (diameter 6 mm). An iridium probe (diameter 2 mm) was used as a contacting tip (Fig. 3). First, potentiodynamic tests with the annealed samples and the polished reference samples were carried out. The aim of the potentiodynamic experiments was to compare the general electrochemical behaviour of the annealed and freely passivating stainless steel surface. The tests were started from the open circuit potential, since the cathodic polarisation had no influence on the anodic polarisation curve. Sweep rates 10 and 60 mV/min were used. In the galvanostatic experiments current densities 0.5, 1, 2, 3, 5 and 10 mA/cm2 were used. In this case the tests were continued until the potential stabilised. The oxide composition and oxide thickness of the samples was measured by the GDOES. The GDOES analyses were first made for annealed samples. GDOES analyses were then made for the samples, which were polarised at different times by the galvanostatic method. In these experiments 1 or 10 mA/cm2 current density was used. The DC resistance of the specimens was measured simultaneously with potentiodynamic polarisation. A comparison of resistance change was made to the change in the current density. The DC resistance was also measured potentiostatically by stepping the potential into anodic direction. The potential range was between − 1500 and + 1500 mV (vs. SCE). At the lowest potential hydrogen evolution and at the highest potential oxygen evolution occurred. The rate of the potential change was typically 10 mV/min.
Fig. 1. The Avesta-cell.
J. Hilde´ n et al. / Electrochimica Acta 46 (2001) 3859–3866
3861
Fig. 2. The CER equipment.
3. Results The polarisation experiments were carried out to identify the electrochemical behaviour of the oxide scale and to study the mechanism of scale removal. In Fig. 4, a potentiodynamic polarisation curve of an annealed sample and a polished sample considered as a reference sample are shown. In the passive region the current of the reference sample is higher than the current of the annealed sample. The thick oxide layer formed during annealing is protecting the steel from dissolving. At higher potential there is a current peak in the polarisation curve of the annealed sample before the transpassive dissolution regime of the reference sample. In the galvanostatic experiments the current was kept constant and a decrease of the electrode potential was observed when scale dissolution started (Fig. 5). When the dissolution had proceeded long enough, the process became more difficult and higher potential was required to keep up the fixed current density. If the current is set between 0.1 and 5 mA/cm2, the total area of the peak of the potential drop is suggested to correspond to the charge, which is required for the dissolution of the oxide layer. If the current was lower than 0.1 mA/cm2 the potential was not high enough to dissolve the oxide completely. If 10 mA/cm2 was used, the current efficiency was lower, which is presumed to be a result of the oxygen evolution and other side reactions. Fig. 6 shows how the DC resistance develops, when the annealed stainless steel sample was polarised from cathodic to anodic direction. At low potentials the DC-resistance was only a little higher than the resistance of the polished specimen (Fig. 7). Close to the corrosion potential the resistance increased and remained quite stable until current started to increase due to the dissolving oxide layer. The resistance reaches the maximum point, when the metallic oxides have dissolved and there probably exists a dielectric silicon oxide layer. The resistance decreases, when the polarisa-
tion is continued. With the polished sample the DC-resistance stays very low throughout the potential range used. There are only two minor resistance peaks discernible (Fig. 7). The annealed specimens were also polarised to cathodic direction ( − 1500 mV vs. SCE) and back to the corrosion potential with different sweep rates of 3 – 60 mV/min. The resistance decreased at negative potential, but returned to the original level at the corrosion potential. This means that the oxide layer becomes more conductive at cathodic potential, but it does not detach or dissolve. When the annealed specimens are at the corrosion potential, the DC resistance may vary quite a lot between different specimens. However, the resistance of an individual specimen remains quite stable. When the specimen is polarised to the potential of +1500 mV
Fig. 3. The plate sample for the CER equipment.
3862
J. Hilde´ n et al. / Electrochimica Acta 46 (2001) 3859–3866
Fig. 4. Anodic polarisation curves for the polished and the annealed specimens, EN 1.4301, 70 °C, 10 mV/min.
versus SCE, the resistance decreases a little at first, but then it increases to a higher level (Fig. 8). It will take quite a long polarising time before the resistance decreases to the level of the polished specimen. The experiments were continued by polarising specimens with constant current density of 1 mA/cm2. The polarisation was continued until the potential started to increase and reached the middle between the minimum and the maximum points (Fig. 5). At this point the experiment was aborted and the surface was rinsed with water and analysed with the GDOES. Another set of samples was polarised with same current density and the test was aborted after half a minute from the maximum point of the potential. The changes in the potential were very clear. However, the dissolution kinetics was rather slow compared to the requirements of the industrial process. A current density of 10 mA/cm2 was used in further experiments. First, a 30-min test was performed. It was presumed that a steady state is reached and no more compositional changes occur in the oxide layer after such a long polarisation time. For the material EN 1.4301 the largest changes in potential occurred within 2 min after the polarisation was started. So for the GDOES analyses three polarisation periods below that time (18, 30 and 50 s) were chosen. The fourth point was selected to be 4 min, when the decrease of the potential was still visible.
3.1. Surface analyses
Fig. 5. Galvanostatic experiments for the annealed specimens, EN 1.4301, 70 °C.
The GDOES analysis for the annealed material is shown in Fig. 9. The content of the oxygen is only trend setting in the analysis. The thickness of the oxide layer on the surface of the specimen was measured to be approximately 150 nm. The criterion used for the thickness determination was the decrease of the oxygen content in the analysis below 15%. In the outer oxide layer large amounts of chromium and manganese were found, but the iron content was very low. Below the chromium and manganese rich layer the content of iron was increased and probably an iron –chromium spinel oxide was formed. At the interface of the oxide layer and the base material a slight enrichment of silicon was also found. GDOES analysis results for the samples, which were made by polarising them with 1 mA/cm2 current density, showed that the potential they obtained was high enough to dissolve chromium and iron oxides. The manganese content remained quite high (about 6 wt%) after the shorter polarisation time (8 min), but decreased to about the basic material composition (2 wt%) after the longer polarisation time (10 min). Silicon had enriched at the surface and its content exceeded 10 wt%.
J. Hilde´ n et al. / Electrochimica Acta 46 (2001) 3859–3866
3863
Fig. 6. A schematic presentation of development of DC resistance and current density as a function of potential for the annealed stainless steel.
Fig. 7. A schematic presentation of development of DC resistance and current density as a function of potential for the polished stainless steel.
When the specimen was polarised with 10 mA/cm2 current density, the oxide layer started to dissolve fast. In the beginning the amounts of chromium and manganese decreased and the contents of iron and silicon increased (Fig. 10), but the oxide layer did not become much thinner. Iron starts to dissolve and the layer becomes thinner only when the chromium content has decreased enough. The silicon content at the surface increased until the base material started to dissolve. After that there was a slow decrease in the silicon content.
4. Discussion The current peak in the potentiodynamic polarisation experiment of the annealed stainless steel before the transpassive dissolution region indicates the electro-
Fig. 8. DC resistance after potential step to +1500 mV vs. SCE, 70 °C.
J. Hilde´ n et al. / Electrochimica Acta 46 (2001) 3859–3866
3864
Fig. 9. Depth profile from the surface of the annealed specimen (EN 1.4301) by GDOES.
chemical dissolution of the oxide scale at potentials above +1050 mV versus SCE. In the passive region the current of an annealed specimen is lower than the current of a freely passivating polished sample and the thick oxide layer is protecting the steel from dissolving. After the peak the current density is about equal in both samples, which indicates that the thick oxide layer is completely or partly removed and both specimen suffer transpassive dissolution by similar mechanism. Galvanostatic experiments showed an increase in the dissolution rate when the current density was increased. Comparison of the dissolution time, measured from the voltage peak (Fig. 5) and the current density shows that the charge (A s) is almost equal in all cases. With a current density of 10 mA/cm2 most of the oxide was dissolved in less than 50 s, which is comparable to the pickling times of the industrial pickling lines. The equal charge of the galvanostatic experiments with different current densities indicates that the charge is consumed by the electrochemical oxidation reactions of the scale. Calculating the charge of the current peak of the potentiodynamic experiments also equals the charge consumed in galvanostatic experiments. To take this further a theoretical charge (Q) for dissolution can be calculated. The chemical constitution of the outer oxide layer was close to MnCr2O4 oxide, which is found to exist on stainless steel. The charge of MnCr2O4 dissolution is obtained by, Q=
dZF Vi
+ − − MnCr2O4+8H2O [ 2HCrO− 4 +MnO4 +14H +11e (2)
The theoretical charge, which is required to dissolve 1 nm thick oxide layer (MnCr2O4), was calculated to be 2.32 mC/cm2. This value is in good accordance with the experiments. The shape of the potentiodynamic polarisation curve is not affected by the cathodic starting potential of the experiment. The DC resistance measurements show low resistance at cathodic polarisation, but sweeping the potential back to anodic direction increases the resis-
(1)
where d is the oxide thickness, Z is the number of electrons transferred, F is the Faradaic constant and Vi is the molar volume of the oxide. The molar volume can be obtained by dividing molar mass of oxide (Mox) by oxide density (Dox). To obtain Z, the following dissolution reaction is proposed:
Fig. 10. A schematic presentation of the changes in the depth profile from the surface of the annealed stainless steel specimen during polarisation.
J. Hilde´ n et al. / Electrochimica Acta 46 (2001) 3859–3866
tance to the initial value. Visual appearance of the sample remains dark brown after the potential excursion to cathodic potential. The prolonged cathodic polarisation and many successive cathodic sweeps did not change the contact resistance and the visual appearance of the sample. Low resistance at cathodic potential indicates that the thick oxide layer behaves like an n-type semiconductor. Polarisation and contact resistance measurements are in good accordance. Good conductivity of the oxide layer at the cathodic potential allows hydrogen evolution to occur at the oxide surface. The oxide destruction by mechanical effect of hydrogen evolution is then greatly reduced, which gives explanation for ineffectiveness of the cathodic polarisation. The reduction of the oxide plays also only a small role because the stability of the chromium oxide under reducing conditions [9]. Three different stages were found in the dissolution process. Referring to Fig. 5, it can be seen that in the first stage the potential decreased, in the second stage the potential increased and in the third stage it decreased again. In the beginning the scale was enriched with chromium and also had a high content of manganese in the outer layer. When anodic current was passed through the sample, preferential dissolution of chromium and manganese occurred. As chromium and manganese dissolution proceeded the potential was decreased. As the chromium content decreased the scale was enriched with iron and the potential required to keep the constant current increased. The content of iron was about 25 – 30% at this stage and the chromium content was decreased below 30%. This corresponds to secondary passivation of stainless steels by iron oxides [9,10]. Experiments carried out with artificial oxides show [9] that the dissolution rate of mixed chromium/ iron oxides are pH dependent and that 50% of iron oxide in the layer is able to prevent oxidative dissolution of Cr3 + in a neutral electrolyte. Present findings are in agreement with that ratio, even though the dissolution rate of chromium was not considerably decreased, but the potential was increased. In the second stage the potential increased and the content of chromium and manganese in the outer layer of the scale diminished below 10%. The content of the iron was increased to about 60% and silicon, whose content increased in the first stage, was enriched on the surface. The content of the silicon at the surface reached several percent. After the second stage the oxide layer thickness was diminished below 20 nm and most of the original oxide scale was dissolved. Visual appearance of the samples was still a bit brownish at this stage. In the third stage the potential decreased and finally levelled out to a steady state value (about 1470 mV vs. SCE), where the base material probably dissolved by transpassive mechanism. Potential decrease was sta-
3865
bilised after 2 –3 min. As the dissolution of the base material proceeded, the gradient of iron content at the surface became steeper. Silicon is not electrochemically oxidised to higher valence. The steady state dissolution of the stainless steel samples proceeded through the thin silicon rich oxide layer and amount of silicon gradually decreased but some still remained on the surface after prolonged polarisation. The silicon concentrated layer can be removed by immersion in nitric –hydrofluoric acid pickling bath [11]. In the DC resistance measurements the three different stages of the dissolution process were also seen. In the beginning, when chromium and manganese dissolved fast, the contact resistance decreased. The observed decrease is in accordance with the findings of Bojinov et al. [10] that contact resistance of passive chromium rich alloy decreases as transpassive dissolution starts. But very soon the resistance increased. The increase was probably because of the enrichment of silicon and the formation of thin resistive layer. Increase of iron content and change of the oxide structure may also play a role in resistance change. After a long polarisation period the resistance decreases again, which was possibly due to the dissolution of iron oxide or decrease of silicon content or both.
5. Conclusions Cathodic polarisation down to − 1500 mV versus SCE was not able to dissolve or remove the oxide layer formed in annealing. A general conclusion drawn from the results of electrochemical tests and surface analyses was that oxide scale formed in annealing was protective at low potentials and it dissolved electrochemically at higher potentials over + 1050 mV versus SCE. The electrochemical dissolution of the oxide layer can be detected both by traditional electrochemical measurements and also by DC resistance measurements. Three different stages could be found in the scale dissolution process. In the beginning the scale was chromium rich with high content of manganese in the outer layer. When anodic current was passed through the samples, preferential dissolution of chromium and manganese occurred. As chromium dissolution proceeded the potential was decreased; however, the scale thickness reduced only slightly. When the chromium content decreased the scale was enriched with iron and the potential increased. This phenomenon is similar to secondary passivation of stainless steel by iron [9,10]. In the second stage, when the potential increased, chromium and manganese were almost completely dissolved from the outer layer of the scale. The content of iron increased to about 30% and silicon was also enriched on the surface. In the third stage the potential decreased and finally levelled out to a stable value. A
3866
J. Hilde´ n et al. / Electrochimica Acta 46 (2001) 3859–3866
steady state was obtained and the base material dissolved through the thin oxide layer, which was still rich in silicon. Silicon is not electrochemically oxidised and it remained on the surface after prolonged polarisation. Nitric– hydrofluoric acid pickling or other suitable chemical post-treatment is required to remove the silicon enriched layer.
[4] [5] [6] [7] [8]
References
[9]
[1] E. Braun, Iron Steel Eng. 4 (1980) 79. [2] E.T. Shapovalov, A.P. Shlyamnev, International Conference 21st Century Steel Industry of Russia and CIS, Moscow, 6– 10 June 1994, p. 59. [3] E.T. Shapovalov, A.P. Shlyamnev, E.A. Uly´ anin, V.D.
[10] [11]
.
Nikitin, A.R. Fisher, D.B. Gol%zon, Steel USSR 12 (1982) 215. V.I. Ignate´ v, N.N. Sergeeva, M.A. Shluger, Stal’ 3 (1987) 84. V.I. Dunaevskii, V.T. Stepanenko, M.Y. Shnol’, L.N. Bespal’ko, Protection Metals 21 (1985) 359. D. Henriet, Surface Treatments for Stainless Steel of the Art Developments and Trends, EUR 17248 EN, 1996. R. Qvarfort, Corros. Sci. 28 (1988) 135. T. Saario, Doctoral Thesis, Helsinki University of Technology, 1995. P. Schmuki, S. Virtanen, H.S. Isaacs, M.P. Ryan, A.J. Davenport, H. Bo¨ hni, T. Stenberg, J. Electrochem. Soc. 145 (1998) 791. M. Bojinov, G. Fabricius, T. Laitinen, K. Ma¨ kela¨ , T. Saario, G. Sundholm, J. Electrochem. Soc. 146 (1999) 3238. S. Kiya, T. Kojima, Y. Hyugaji, Rev. Metall. 93 (1996) 1431.