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Studying the formation process of chromate conversion coatings on aluminium using continuous electrochemical noise resistance measurements
Corrosion Science 44 (2002) 1277–1286 www.elsevier.com/locate/corsci
Studying the formation process of chromate conversion coatings on aluminium using continuous electrochemical noise resistance measurements Yong-Jun Tan a,*, Stuart Bailey b, Brian Kinsella b a
b
Division of Materials Science, School of Materials Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798, Singapore Department of Applied Chemistry, School of Applied Chemistry and Environmental Biology, Curtin University of Technology, P.O. Box U 1987, Perth, Western Australia 6845, Australia Received 16 January 2001; accepted 27 July 2001
Abstract A unique electrochemical technique, namely continuous noise resistance calculation (CNRC), was used to obtain electrochemical kinetic information from the formation process of chromate conversion coatings (CCC) on aluminium electrodes. It was found that the noise resistance (Rn ) of aluminium electrodes remained almost unchanged during electrodes’ immersion in a chromate containing acidic solution where the CCC films were supposed to form rapidly. This result indicates that the formation of CCC was associated with continuous corrosion of the aluminium electrodes and that the CCC films formed on aluminium surface were not intact barrier films, but most likely porous layers. The CCCs became protective only after they were aged in the environment. Based on these findings, the formation and inhibition mechanisms of CCC have been discussed. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Electrochemical methods; Electrochemical noise; Electrochemical noise resistance; Chromate conversion coatings; Surface treatment; Aluminium
*
Corresponding author. Fax: +65-790-9081. E-mail address: [email protected] (Y.-J. Tan).
0010-938X/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 0 - 9 3 8 X ( 0 1 ) 0 0 1 3 1 - 7
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1. Introduction Chromate conversion coatings (CCC) have been used in industry such as the aircraft manufacture and maintenance industries for a long time to inhibit aluminium corrosion. Traditionally CCCs are prepared by immersing aluminium in acidic solutions of chromate and fluoride for a short period of time, usually from a few seconds to a few minutes. Such coatings have been shown to contain both Cr(III) and Cr(VI) [1,2], which provide excellent corrosion inhibition for aluminium and its alloys. Unfortunately, hexavalent chromium is toxic and is environmentally unacceptable, and thus environmentally friendly replacements of hexavalent chromium have to be found. A practical approach of developing chromium-free CCC treatment is to mimic chromium-containing CCC treatment. A prerequisite for doing so is a good understanding on the process and mechanism of CCC formation. The CCC formation is generally believed to involve destabilisation of the passive oxide film by fluoride, followed by reduction of chromate by the exposed aluminium [3]; however, details about the process have not yet been clearly understood. During recent years significant research has been carried out in order to understand the CCC using various surface analytical techniques such as Raman spectroscopy [4] and scanning laser microscopy [5]. A major unanswered question that is frequently asked by researchers and engineers is that ‘what electrochemical processes have really happened during the short immersion of aluminium electrodes in chromate-containing solutions’? To address such question, electrochemical information regarding the rapid electrochemical process occurring at the interface of aluminium electrodes and CCC treatment solution has to be obtained. One technique that could provide such information is corrosion potential monitoring, i.e. to continuously measure corrosion potential vs. time. However corrosion potential measurement can only obtain electrochemical thermodynamic information and it cannot provide electrochemical kinetic information, which is the key to understand the CCC formation process. This present work is a new approach of studying the CCC formation using a unique electrochemical technique, namely continuous noise resistance calculation (CNRC) [6–9], to study the electrochemical kinetics of the CCC formation process. The CNRC technique [6–8] is based on the measurement and calculation of noise resistance (Rn ) [10] under continuous bases. Previously this technique has been used in the continuous monitoring of batch treatment inhibitor performance [6,7] and the monitoring of the formation and destruction of corrosion inhibitor films [8]. It has been shown to be uniquely capable of monitoring rapidly changing electrochemical processes. It appears to be exactly the technique that could be used to detect valuable data from the rapid electrochemical process of CCC formation.
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Fig. 1. The experimental arrangement for electrochemical noise recording and for linear polarisation measurements.
2. Experimental 2.1. Experimental set-up The electrochemical cell and the experimental arrangement for electrochemical noise (EN) recording are schematically shown in Fig. 1. A dual cylinder electrode with two identical cylindrical electrodes (pure aluminium) of the same surface area (3.02 cm2 ) was used for all EN recording and also for linear polarisation (LP) measurements. When carrying out LP measurements, the two identical electrodes (top and bottom electrode) were measured individually. In this way, noise resistance and linear polarisation resistance can be compared directly. The dual cylinder electrode was rotated at 2700 rpm (equivalent to 100 m/min) during the experiment and air sparging was continued in order to simulate the industrial CCC treatment process that solutions are sprayed onto the aluminium surface. A platinum electrode with large surface area (>10 cm2 ) was used as a counter electrode and a 3 M Ag/ AgCl electrode was used as a reference electrode. An automatic zero resistance ammeter (AutoZRA, ACM Instruments, England) was used for EN recording. The AutoZRA enables simultaneous measurements of current noise (325 mA to 10 pA) and potential noise to be taken and displayed automatically. Its data logging and analysis software run in the Microsoft Windows environment complete with a real time Excel link. Thus the analysis of noise data can
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be performed with Microsoft Excel. Potential noise was recorded by measuring the free corrosion potential of the two identical electrodes (short circuited via the AutoZRA) against an Ag/AgCl reference electrode, using an AutoZRA connected to a personal computer. The current flow between the two short-circuited identical electrodes was monitored simultaneously to obtain current noise using the same Auto-ZRA. Very fast sampling rate was applied to both potential and current noise measurements (sampling intervals were 0.02 s, i.e. 50 data points per second). A potentiostat model 273A with software M398A (EG&G Princeton Applied Research) was used for LP measurements. 2.2. Experimental procedures The surfaces of the aluminium electrodes were polished with 400, 800 and 1200 grit silicon carbide paper and cleaned with ethanol and isopropanol. Electrodes were exposed to 600 ml chromium-containing CCC treatment solutions within 2 min of being polished and cleaned. Two commercial CCC treatment solutions were studied in this work. The CCC solutions are used as received. CCC solution A is a commercial chromic acid solution namely IRIDITE 9L6 (Gibson Chemicals Limited, Cheltenham, Victoria, Australia). Its ingredients are: sodium dichromate: 10–30%, chromic acid: <10%, fluoride salt: <10%, water: balance, recommended treatment temperature: 66 °C. CCC solution B is a commercial chromate passivation chemical namely Oakite Okemcoat F1 (Tak Pty Ltd., Bayswater North, Victoria, Australia). It is designed for zinc coated steel with the following ingredients: phosphoric acid: 20%, chromium trioxide: 18%, chromium phosphate: 10–20%, water: balance, recommended treatment temperature: 25 °C. All other chemicals were reagent grade. Electrochemical potential and current noise measurements were performed immediately after electrode immersion in CCC solutions in order to continuously monitor the rapid electrochemical changes during first 3 min of immersion. Following the 3 min electrochemical noise recording, LP measurements on both the dual electrodes were carried out. Electrodes were then withdrawn from CCC solution for air drying at 55 °C for 2 min. The electrodes were used for further ageing treatment test or immersion test in corrosive media. In the case of immersion test, electrodes were immediate immersed in 0.1 M NaCl solution (600 ml, 25 °C under
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steady conditions) to perform regular linear polarisation measurements for evaluating the corrosion prevention performance of the CCC against that of untreated electrodes. In case of ageing treatment test, electrodes were exposed to the air at room temperature (about 25 °C) for three days and then they were immersed in 0.1 M NaCl solution (600 ml, under steady conditions) for immersion test.
3. Data analysis Rn is defined as the ratio of the standard deviation of the potential noise to that of the current noise between two identical working electrodes which are linked by a zero resistance ammeter (Rn ¼ rv =ri ). Rn has been confirmed to be equivalent to polarisation resistance (Rp ) experimentally and also theoretically [10–17]. This similarity allows Rn to be used to determine corrosion rates quantitatively by means of the Stern–Geary equation. The CNRC method uses very small and moving time windows for Rn calculation. This is briefly explained below. If a series of voltage–time records (k data points), fV1 ; V2 ; V3 ; . . . Vi1 ; Vi ; Viþ1 ; . . . Vk g, and corresponding current–time records, fI1 ; I2 ; I3 ; . . . Ii1 ; Ii ; Iiþ1 ; . . . Ik g, are experimentally recorded. For a datum point of voltage and current noise, Vi and Ii , a small data series of 2m þ 1 neighbour points (in time window Dt), can be used to calculate noise resistance Vn;i . Rn;i ¼
rV fVim ; Vimþ1 ; . . . Vi ; Viþ1 ; . . . Viþm g rI fVim ; Vimþ1 ; . . . Vi ; Viþ1 ; . . . Viþm g
ð1Þ
where rV fVim ; Vimþ1 ; Vi ; . . . Viþm g and rI fIim ; Iimþ1 ; Ii ; . . . Iiþm g are the standard deviations of voltage and current noise data (2m þ 1 neighbour points of i). A Rn value can thus be calculated for each data point of voltage and current noise, forming a series of Rn values: fRn;m . . . Rn;i1 ; Rn;i ; Rn;iþ1 ; . . . Rn;km g. In this way, a corrosion process can be continuously monitored by plotting the Rn value series. This technique can thus be used to analyse corrosion systems where parameters are subject to rapid change (i.e. processes before the steady state has been achieved, or where there is a change in the steady-state condition such as during inhibitor film breakdown). In this work, each noise measurement file contains 30,000 data points (i.e. k ¼ 30,000) and the sliding time window for Rn calculation was 0.4 seconds (20 data points, i.e. m ¼ 20).
4. Results and discussion A common belief on the CCC is that a protective and intact surface passive film forms during treatment in CCC solutions. If this is true, the treatment process would lead to a rapid increase in Rn since a protective passive film would significantly increase polarisation resistance and thus a dramatic decrease in corrosion rates. This expected rapid increase in Rn , however, did not occur in experiments. As shown in
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Fig. 2. Electrochemical noise resistance calculated from aluminium electrodes immersed in CCC solution A at 66 °C.
Fig. 2, Rn of aluminium electrodes kept low and remained almost unchanged during 3 min of immersion in CCC solution A at 66 °C. This result is in contrast with previously studied processes that rapid Rn increases were recorded when inhibitor films formed [6–9]. This result was confirmed by linear polarisation measurements carried out immediately after the 3 min noise recording. Linear polarisation resistances of 5.3 X cm2 for the top electrode and 12.5 X cm2 for the bottom electrode, i.e. average 9 X cm2 , were obtained. These polarisation resistance values are reasonably close to the average Rn value shown in Fig. 2 (approximately 15 X cm2 ), which corresponded to a corrosion rate of about 30 mm/year (estimated using the Stern–Geary equation) and indicated rapid corrosion of aluminium electrodes. A logical explanation to this result is that a protective film did not form on aluminium surfaces. Similar characteristics were also observed when aluminium electrodes were immersed in CCC solution B at 25 °C. As shown in Fig. 3, during the first 3 min of immersion Rn of aluminium electrodes did not show significant increases although
Fig. 3. Electrochemical noise resistance calculated from aluminium electrodes immersed in CCC solution B at 25 °C.
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indeed an increase in Rn was observed during the first 30–40 s of immersion. However, the Rn decreased with time to approximately 750 X cm2 after reaching a maximum. This Rn value is in agreement with linear polarisation measurements immediately after the 3 min noise recording. Polarisation resistances measured were 1129 X cm2 for top electrode and 727 X cm2 for bottom electrode, i.e. average 928 X cm2 . The fact that Rn values shown in Fig. 3 are much higher than that in CCC solution A (Fig. 2) may indicate different CCC treatment mechanisms and requires further investigation. These experimental findings suggest that passivation films did not form on aluminium surfaces in the CCC solutions. The films formed on aluminium electrodes were most likely porous layers. To further understand such layers, untreated, treated, treated-aged aluminium electrodes were immersed in 0.1 M NaCl brine solutions in order to monitor and evaluate their corrosion behaviour and their protection ability using linear polarisation (LP) measurements. Fig. 4 summarises the results obtained from LP measurements. At the very beginning of the exposure tests, as shown in Fig. 4, the corrosion rates of CCC treated specimens were similar to those of untreated metal specimens. This result suggests that freshly prepared CCC films did not provide extra protection to the aluminium electrodes and that the CCC layer must be porous. The corrosion rates of the treated specimens dropped significantly with the extension of exposure to 0.1 M NaCl (Fig. 4). This suggests that some form of surface change occurred and corrosion protection was enhanced during exposure to the corrosive environment by some unknown mechanism. Ageing treatment of CCC treated electrode also significantly improved corrosion protective ability. As shown in Fig. 5, after being aged in the air for three days aluminium electrodes exhibited much lower corrosion rates at the beginning of exposure to 0.1 M NaCl and also better protection during
Fig. 4. Comparison of corrosion behaviour of treated and untreated electrodes in 0.1 M NaCl solution. (a) Treated with CCC solution A; (b) treated with CCC solution B.
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Fig. 5. Exposure tests of untreated, freshly treated and aged aluminium electrodes in 0.1 M NaCl solution.
extended exposure. These results confirmed that ageing of treated electrodes can improve the protective ability of CCC and this again supports that CCC formation is not simply a solution process, but more likely involve enhancement during exposure to environment. One explanation to these results is that during the exposure or ageing process, pores in the CCC became blocked with corrosion products since these pore areas could be the anodic sites of corrosion. Another possible explanation is that these anodic pore sites became covered by corrosion inhibiting substance during ageing period due to migration of a Cr(VI) species to pore sites, and subsequent blocking of corrosion. These explanations are in line with the mechanism of corrosion inhibition by CCC films proposed by other researchers [18,19]. Katzman et al. [18] proposed that the inhibition mechanism of CCC involves blocking of active pores and defects by the Cr(III) film, which Lytle et al. [19] suggested that dynamic repair of newly created breaks or defects in the CCC film is an important mechanism. In summary, the mechanism of CCC formation could be a two step process. The first step occurs in CCC treatment solutions where a porous corrosion product layer forms on the metal surface. This explains the fact that a freshly treated electrode in 0.1 M NaCl solution exhibits high corrosion rates comparable to that of untreated electrodes and it also explains the noise resistance shown in Figs. 1 and 2. The cross section of a porous CCC film is proposed in Fig. 6. The second step occurs in corrosion environment where the pore areas of the CCC film were filled with inhibiting substances and/or corrosion products. When the treated electrode is exposed to the air for ageing treatment, a water layer can form on the bottom of the fine pores due to capillary effect; water can condense in capillary pipe even under low humidity. Corrosion inhibitors in CCC can dissolve in the water layer forming an inhibiting environment or forming an inhibitor film on these pore sites. This
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Fig. 6. Possible porous CCC film structure.
Fig. 7. A model showing the mechanism of corrosion protection by a porous CCC film.
mechanism predicts that ageing treatment would not work if a CCC treated electrode were placed in a very low humidity environment. Similarly, when electrode is exposed to 0.1 M NaCl brine, a micro-environment can also form on the bottom of pores. This inhibiting micro-environment can be maintained because capillary effects can hold liquid in the pores. Corrosion product formation in 0.1 M NaCl solution may also contributes to block the pores in the CCC film. A surface model is proposed in Fig. 7 to explain the mechanism of corrosion protection by CCCs. Suggestions can thus be made on the development of chromate-free CCC treatment by mimicking chromate-containing CCC treatment. The CCC films should have a capillary pipe structure (Fig. 6). CNRC could be used as an empirical indicator for monitoring the formation of such porous film. The CCC film should contain sufficient inhibiting substance to maintain an effective inhibiting concentration in the capillary pipe solution as shown in Fig. 7.
5. Conclusions The continuous noise resistance calculation technique was used to detect electrochemical kinetic information from the formation process of chromate conversion coatings (CCC) on aluminium electrodes. It was found that the noise resistance (Rn ) of aluminium electrodes remained almost unchanged during its immersion in a chromate containing acidic solution where the CCC form rapidly. This result indicates that the formation of CCC was associated with continued corrosion of the
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aluminium electrode and that the CCC films formed on aluminium surface were not intact barrier films, but most likely porous layers. The CCC films became protective only after they were aged in the environment.
References [1] J.K. Hawkins, H.S. Isaacs, S.M. Heald, J. Tanquada, G.E. Thompson, G.C. Wood, Corros. Sci. 27 (1987) 391. [2] J.S. Wainwright, O.J. Murphy, M.R. Antonio, Corros. Sci. 33 (1992) 281. [3] M.W. Kendig, A.J. Davenport, H.S. Isaacs, Corros. Sci. 34 (1993) 41. [4] J. Zhao, G.S. Frankel, R.L. McCreery, J. Electrochem. Soc. 145 (1998) 2258. [5] G.O. Ilevbare, O. Schneider, R.G. Kelly, J.R. Scully, In situ Confocal Scanning Laser Microscopy Studies of the Effects of Surface Treatments on the Corrosion of AA 2024-T3, The Electrochemical Society 198th Meeting––E1––Corrosion and Corrosion Prevention of Low Density Metals and Alloys, Arizona, 2000. [6] Y.J. Tan, S. Bailey, B. Kinsella, Br. Corros. J. 32 (1997) 212–216. [7] Y.J. Tan, S. Bailey, B. Kinsella, The evaluation of corrosion inhibitor film persistency using electrochemical impedance spectroscopy and electrochemical noise analysis, Corrosion, vol. 96, paper 352, NACE, Denver, 1996. [8] Y.J. Tan, S. Bailey, B. Kinsella, Corros. Sci. 38 (1996) 1681–1695. [9] Y.J. Tan, S. Bailey, B. Kinsella, Corros. Sci. 40 (1998) 513–517. [10] D.A. Eden, K. Hladky, D.G. John, J.L. Dawson, Electrochemical noise resistance, Corrosion, vol. 86, NACE, Houston, 1986, Paper 274. [11] F. Mansfeld, H. Xiao, J. Electrochem. Soc. 140 (1993) 2205. [12] F. Mansfeld, C.C. Lee, Corros. Sci. 39 (1997) 1141. [13] U. Bertocci, C. Gabrielli, F. Huet, M. Keddam, J. Electrochem. Soc. 144 (1997) 31–43. [14] Y. Tan, S. Bailey, B. Kinsella, Corros. Sci. 40 (1998) 513. [15] Y. Tan, S. Bailey, B. Kinsella, Corrosion (NACE) 55 (1999) 469. [16] Y.J. Tan, J. Corros. Sci. Engng. (WWW) 1 (1999) paper 11. [17] Y.J. Tan, S. Bailey, B. Kinsella, A. Lowe, J. Electrochem. Soc. 147 (2000) 530–540. [18] H.A. Katzman, G.M. Malouf, R. Bauer, G.W. Stupian, Appl. Surf. Sci. 2 (1979) 416. [19] F.W. Lytle, R.B. Greegor, G.L. Bibbins, K.Y. Blohowiak, R.E. Smith, G.D. Tuss, Corros. Sci. 37 (1995) 349.
Studying the formation process of chromate conversion coatings on aluminium using continuous electrochemical noise resistance measurements Yong-Jun Tan a,*, Stuart Bailey b, Brian Kinsella b a
b
Division of Materials Science, School of Materials Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798, Singapore Department of Applied Chemistry, School of Applied Chemistry and Environmental Biology, Curtin University of Technology, P.O. Box U 1987, Perth, Western Australia 6845, Australia Received 16 January 2001; accepted 27 July 2001
Abstract A unique electrochemical technique, namely continuous noise resistance calculation (CNRC), was used to obtain electrochemical kinetic information from the formation process of chromate conversion coatings (CCC) on aluminium electrodes. It was found that the noise resistance (Rn ) of aluminium electrodes remained almost unchanged during electrodes’ immersion in a chromate containing acidic solution where the CCC films were supposed to form rapidly. This result indicates that the formation of CCC was associated with continuous corrosion of the aluminium electrodes and that the CCC films formed on aluminium surface were not intact barrier films, but most likely porous layers. The CCCs became protective only after they were aged in the environment. Based on these findings, the formation and inhibition mechanisms of CCC have been discussed. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Electrochemical methods; Electrochemical noise; Electrochemical noise resistance; Chromate conversion coatings; Surface treatment; Aluminium
*
Corresponding author. Fax: +65-790-9081. E-mail address: [email protected] (Y.-J. Tan).
0010-938X/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 0 - 9 3 8 X ( 0 1 ) 0 0 1 3 1 - 7
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1. Introduction Chromate conversion coatings (CCC) have been used in industry such as the aircraft manufacture and maintenance industries for a long time to inhibit aluminium corrosion. Traditionally CCCs are prepared by immersing aluminium in acidic solutions of chromate and fluoride for a short period of time, usually from a few seconds to a few minutes. Such coatings have been shown to contain both Cr(III) and Cr(VI) [1,2], which provide excellent corrosion inhibition for aluminium and its alloys. Unfortunately, hexavalent chromium is toxic and is environmentally unacceptable, and thus environmentally friendly replacements of hexavalent chromium have to be found. A practical approach of developing chromium-free CCC treatment is to mimic chromium-containing CCC treatment. A prerequisite for doing so is a good understanding on the process and mechanism of CCC formation. The CCC formation is generally believed to involve destabilisation of the passive oxide film by fluoride, followed by reduction of chromate by the exposed aluminium [3]; however, details about the process have not yet been clearly understood. During recent years significant research has been carried out in order to understand the CCC using various surface analytical techniques such as Raman spectroscopy [4] and scanning laser microscopy [5]. A major unanswered question that is frequently asked by researchers and engineers is that ‘what electrochemical processes have really happened during the short immersion of aluminium electrodes in chromate-containing solutions’? To address such question, electrochemical information regarding the rapid electrochemical process occurring at the interface of aluminium electrodes and CCC treatment solution has to be obtained. One technique that could provide such information is corrosion potential monitoring, i.e. to continuously measure corrosion potential vs. time. However corrosion potential measurement can only obtain electrochemical thermodynamic information and it cannot provide electrochemical kinetic information, which is the key to understand the CCC formation process. This present work is a new approach of studying the CCC formation using a unique electrochemical technique, namely continuous noise resistance calculation (CNRC) [6–9], to study the electrochemical kinetics of the CCC formation process. The CNRC technique [6–8] is based on the measurement and calculation of noise resistance (Rn ) [10] under continuous bases. Previously this technique has been used in the continuous monitoring of batch treatment inhibitor performance [6,7] and the monitoring of the formation and destruction of corrosion inhibitor films [8]. It has been shown to be uniquely capable of monitoring rapidly changing electrochemical processes. It appears to be exactly the technique that could be used to detect valuable data from the rapid electrochemical process of CCC formation.
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Fig. 1. The experimental arrangement for electrochemical noise recording and for linear polarisation measurements.
2. Experimental 2.1. Experimental set-up The electrochemical cell and the experimental arrangement for electrochemical noise (EN) recording are schematically shown in Fig. 1. A dual cylinder electrode with two identical cylindrical electrodes (pure aluminium) of the same surface area (3.02 cm2 ) was used for all EN recording and also for linear polarisation (LP) measurements. When carrying out LP measurements, the two identical electrodes (top and bottom electrode) were measured individually. In this way, noise resistance and linear polarisation resistance can be compared directly. The dual cylinder electrode was rotated at 2700 rpm (equivalent to 100 m/min) during the experiment and air sparging was continued in order to simulate the industrial CCC treatment process that solutions are sprayed onto the aluminium surface. A platinum electrode with large surface area (>10 cm2 ) was used as a counter electrode and a 3 M Ag/ AgCl electrode was used as a reference electrode. An automatic zero resistance ammeter (AutoZRA, ACM Instruments, England) was used for EN recording. The AutoZRA enables simultaneous measurements of current noise (325 mA to 10 pA) and potential noise to be taken and displayed automatically. Its data logging and analysis software run in the Microsoft Windows environment complete with a real time Excel link. Thus the analysis of noise data can
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be performed with Microsoft Excel. Potential noise was recorded by measuring the free corrosion potential of the two identical electrodes (short circuited via the AutoZRA) against an Ag/AgCl reference electrode, using an AutoZRA connected to a personal computer. The current flow between the two short-circuited identical electrodes was monitored simultaneously to obtain current noise using the same Auto-ZRA. Very fast sampling rate was applied to both potential and current noise measurements (sampling intervals were 0.02 s, i.e. 50 data points per second). A potentiostat model 273A with software M398A (EG&G Princeton Applied Research) was used for LP measurements. 2.2. Experimental procedures The surfaces of the aluminium electrodes were polished with 400, 800 and 1200 grit silicon carbide paper and cleaned with ethanol and isopropanol. Electrodes were exposed to 600 ml chromium-containing CCC treatment solutions within 2 min of being polished and cleaned. Two commercial CCC treatment solutions were studied in this work. The CCC solutions are used as received. CCC solution A is a commercial chromic acid solution namely IRIDITE 9L6 (Gibson Chemicals Limited, Cheltenham, Victoria, Australia). Its ingredients are: sodium dichromate: 10–30%, chromic acid: <10%, fluoride salt: <10%, water: balance, recommended treatment temperature: 66 °C. CCC solution B is a commercial chromate passivation chemical namely Oakite Okemcoat F1 (Tak Pty Ltd., Bayswater North, Victoria, Australia). It is designed for zinc coated steel with the following ingredients: phosphoric acid: 20%, chromium trioxide: 18%, chromium phosphate: 10–20%, water: balance, recommended treatment temperature: 25 °C. All other chemicals were reagent grade. Electrochemical potential and current noise measurements were performed immediately after electrode immersion in CCC solutions in order to continuously monitor the rapid electrochemical changes during first 3 min of immersion. Following the 3 min electrochemical noise recording, LP measurements on both the dual electrodes were carried out. Electrodes were then withdrawn from CCC solution for air drying at 55 °C for 2 min. The electrodes were used for further ageing treatment test or immersion test in corrosive media. In the case of immersion test, electrodes were immediate immersed in 0.1 M NaCl solution (600 ml, 25 °C under
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steady conditions) to perform regular linear polarisation measurements for evaluating the corrosion prevention performance of the CCC against that of untreated electrodes. In case of ageing treatment test, electrodes were exposed to the air at room temperature (about 25 °C) for three days and then they were immersed in 0.1 M NaCl solution (600 ml, under steady conditions) for immersion test.
3. Data analysis Rn is defined as the ratio of the standard deviation of the potential noise to that of the current noise between two identical working electrodes which are linked by a zero resistance ammeter (Rn ¼ rv =ri ). Rn has been confirmed to be equivalent to polarisation resistance (Rp ) experimentally and also theoretically [10–17]. This similarity allows Rn to be used to determine corrosion rates quantitatively by means of the Stern–Geary equation. The CNRC method uses very small and moving time windows for Rn calculation. This is briefly explained below. If a series of voltage–time records (k data points), fV1 ; V2 ; V3 ; . . . Vi1 ; Vi ; Viþ1 ; . . . Vk g, and corresponding current–time records, fI1 ; I2 ; I3 ; . . . Ii1 ; Ii ; Iiþ1 ; . . . Ik g, are experimentally recorded. For a datum point of voltage and current noise, Vi and Ii , a small data series of 2m þ 1 neighbour points (in time window Dt), can be used to calculate noise resistance Vn;i . Rn;i ¼
rV fVim ; Vimþ1 ; . . . Vi ; Viþ1 ; . . . Viþm g rI fVim ; Vimþ1 ; . . . Vi ; Viþ1 ; . . . Viþm g
ð1Þ
where rV fVim ; Vimþ1 ; Vi ; . . . Viþm g and rI fIim ; Iimþ1 ; Ii ; . . . Iiþm g are the standard deviations of voltage and current noise data (2m þ 1 neighbour points of i). A Rn value can thus be calculated for each data point of voltage and current noise, forming a series of Rn values: fRn;m . . . Rn;i1 ; Rn;i ; Rn;iþ1 ; . . . Rn;km g. In this way, a corrosion process can be continuously monitored by plotting the Rn value series. This technique can thus be used to analyse corrosion systems where parameters are subject to rapid change (i.e. processes before the steady state has been achieved, or where there is a change in the steady-state condition such as during inhibitor film breakdown). In this work, each noise measurement file contains 30,000 data points (i.e. k ¼ 30,000) and the sliding time window for Rn calculation was 0.4 seconds (20 data points, i.e. m ¼ 20).
4. Results and discussion A common belief on the CCC is that a protective and intact surface passive film forms during treatment in CCC solutions. If this is true, the treatment process would lead to a rapid increase in Rn since a protective passive film would significantly increase polarisation resistance and thus a dramatic decrease in corrosion rates. This expected rapid increase in Rn , however, did not occur in experiments. As shown in
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Fig. 2. Electrochemical noise resistance calculated from aluminium electrodes immersed in CCC solution A at 66 °C.
Fig. 2, Rn of aluminium electrodes kept low and remained almost unchanged during 3 min of immersion in CCC solution A at 66 °C. This result is in contrast with previously studied processes that rapid Rn increases were recorded when inhibitor films formed [6–9]. This result was confirmed by linear polarisation measurements carried out immediately after the 3 min noise recording. Linear polarisation resistances of 5.3 X cm2 for the top electrode and 12.5 X cm2 for the bottom electrode, i.e. average 9 X cm2 , were obtained. These polarisation resistance values are reasonably close to the average Rn value shown in Fig. 2 (approximately 15 X cm2 ), which corresponded to a corrosion rate of about 30 mm/year (estimated using the Stern–Geary equation) and indicated rapid corrosion of aluminium electrodes. A logical explanation to this result is that a protective film did not form on aluminium surfaces. Similar characteristics were also observed when aluminium electrodes were immersed in CCC solution B at 25 °C. As shown in Fig. 3, during the first 3 min of immersion Rn of aluminium electrodes did not show significant increases although
Fig. 3. Electrochemical noise resistance calculated from aluminium electrodes immersed in CCC solution B at 25 °C.
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indeed an increase in Rn was observed during the first 30–40 s of immersion. However, the Rn decreased with time to approximately 750 X cm2 after reaching a maximum. This Rn value is in agreement with linear polarisation measurements immediately after the 3 min noise recording. Polarisation resistances measured were 1129 X cm2 for top electrode and 727 X cm2 for bottom electrode, i.e. average 928 X cm2 . The fact that Rn values shown in Fig. 3 are much higher than that in CCC solution A (Fig. 2) may indicate different CCC treatment mechanisms and requires further investigation. These experimental findings suggest that passivation films did not form on aluminium surfaces in the CCC solutions. The films formed on aluminium electrodes were most likely porous layers. To further understand such layers, untreated, treated, treated-aged aluminium electrodes were immersed in 0.1 M NaCl brine solutions in order to monitor and evaluate their corrosion behaviour and their protection ability using linear polarisation (LP) measurements. Fig. 4 summarises the results obtained from LP measurements. At the very beginning of the exposure tests, as shown in Fig. 4, the corrosion rates of CCC treated specimens were similar to those of untreated metal specimens. This result suggests that freshly prepared CCC films did not provide extra protection to the aluminium electrodes and that the CCC layer must be porous. The corrosion rates of the treated specimens dropped significantly with the extension of exposure to 0.1 M NaCl (Fig. 4). This suggests that some form of surface change occurred and corrosion protection was enhanced during exposure to the corrosive environment by some unknown mechanism. Ageing treatment of CCC treated electrode also significantly improved corrosion protective ability. As shown in Fig. 5, after being aged in the air for three days aluminium electrodes exhibited much lower corrosion rates at the beginning of exposure to 0.1 M NaCl and also better protection during
Fig. 4. Comparison of corrosion behaviour of treated and untreated electrodes in 0.1 M NaCl solution. (a) Treated with CCC solution A; (b) treated with CCC solution B.
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Fig. 5. Exposure tests of untreated, freshly treated and aged aluminium electrodes in 0.1 M NaCl solution.
extended exposure. These results confirmed that ageing of treated electrodes can improve the protective ability of CCC and this again supports that CCC formation is not simply a solution process, but more likely involve enhancement during exposure to environment. One explanation to these results is that during the exposure or ageing process, pores in the CCC became blocked with corrosion products since these pore areas could be the anodic sites of corrosion. Another possible explanation is that these anodic pore sites became covered by corrosion inhibiting substance during ageing period due to migration of a Cr(VI) species to pore sites, and subsequent blocking of corrosion. These explanations are in line with the mechanism of corrosion inhibition by CCC films proposed by other researchers [18,19]. Katzman et al. [18] proposed that the inhibition mechanism of CCC involves blocking of active pores and defects by the Cr(III) film, which Lytle et al. [19] suggested that dynamic repair of newly created breaks or defects in the CCC film is an important mechanism. In summary, the mechanism of CCC formation could be a two step process. The first step occurs in CCC treatment solutions where a porous corrosion product layer forms on the metal surface. This explains the fact that a freshly treated electrode in 0.1 M NaCl solution exhibits high corrosion rates comparable to that of untreated electrodes and it also explains the noise resistance shown in Figs. 1 and 2. The cross section of a porous CCC film is proposed in Fig. 6. The second step occurs in corrosion environment where the pore areas of the CCC film were filled with inhibiting substances and/or corrosion products. When the treated electrode is exposed to the air for ageing treatment, a water layer can form on the bottom of the fine pores due to capillary effect; water can condense in capillary pipe even under low humidity. Corrosion inhibitors in CCC can dissolve in the water layer forming an inhibiting environment or forming an inhibitor film on these pore sites. This
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Fig. 6. Possible porous CCC film structure.
Fig. 7. A model showing the mechanism of corrosion protection by a porous CCC film.
mechanism predicts that ageing treatment would not work if a CCC treated electrode were placed in a very low humidity environment. Similarly, when electrode is exposed to 0.1 M NaCl brine, a micro-environment can also form on the bottom of pores. This inhibiting micro-environment can be maintained because capillary effects can hold liquid in the pores. Corrosion product formation in 0.1 M NaCl solution may also contributes to block the pores in the CCC film. A surface model is proposed in Fig. 7 to explain the mechanism of corrosion protection by CCCs. Suggestions can thus be made on the development of chromate-free CCC treatment by mimicking chromate-containing CCC treatment. The CCC films should have a capillary pipe structure (Fig. 6). CNRC could be used as an empirical indicator for monitoring the formation of such porous film. The CCC film should contain sufficient inhibiting substance to maintain an effective inhibiting concentration in the capillary pipe solution as shown in Fig. 7.
5. Conclusions The continuous noise resistance calculation technique was used to detect electrochemical kinetic information from the formation process of chromate conversion coatings (CCC) on aluminium electrodes. It was found that the noise resistance (Rn ) of aluminium electrodes remained almost unchanged during its immersion in a chromate containing acidic solution where the CCC form rapidly. This result indicates that the formation of CCC was associated with continued corrosion of the
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aluminium electrode and that the CCC films formed on aluminium surface were not intact barrier films, but most likely porous layers. The CCC films became protective only after they were aged in the environment.
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