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Cathodic delamination of polyurethane films on oxide covered steel – Combined adhesion and interface electrochemical studies
Corrosion Science 51 (2009) 1664–1670
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Corrosion Science journal homepage: www.elsevier.com/locate/corsci
Cathodic delamination of polyurethane films on oxide covered steel – Combined adhesion and interface electrochemical studies J. Wielant a,*, R. Posner b, R. Hausbrand c, G. Grundmeier b,d, H. Terryn a a
Research Group Electrochemical and Surface Engineering, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium Christian-Doppler Laboratory for Polymer/Metal Interfaces at the Max-Planck-Institut für Eisenforschung GmbH, Max-Planck-Str. 1, 40237 Düsseldorf, Germany c ArcelorMittal R&D Industry Gent – OCAS NV, Pres. J.F. Kennedylaan 3, 9060 Zelzate, Belgium d University of Paderborn, Technical and Macromolecular Chemistry, Warburger Str. 100, 33098 Paderborn, Germany b
a r t i c l e
i n f o
Article history: Received 19 March 2009 Accepted 16 April 2009 Available online 3 May 2009 Keywords: A. Steel A. Organic coatings B. Scanning kelvin probe C. Interfaces C. Cathodic delamination
a b s t r a c t The stability of polyurethane/iron oxide/steel interfaces was evaluated for different chemically modified passive films in humid and corrosive atmosphere. Iron oxide layers were formed on polished steel by thermal annealing in oxygen rich atmosphere, water plasma modification, immersion in alkaline solution and by anodic polarisation in borate buffer solution. Scanning Kelvin Probe studies of the interface stability were performed under the conditions of cathodic delamination. The corresponding progress rates specifically depended on the iron oxide. Complementary measured peel-off forces of the respective coating/substrate systems showed the same trend as detected for the corrosive delamination process. Additional contact angle measurements and investigation of the interfacial ion mobility on the uncoated oxide surfaces confirmed a correlation between high peel forces, increasing oxide surface energy and decelerated cathodic delamination kinetics. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction If a metal passive film is electron conducting, cathodic delamination is often responsible for a proceeding polymer/metal interface deterioration starting from an actively corroding coating defect. It is driven by a galvanic cell with metal dissolution at the local anode in the defect area and a dominating oxygen reduction process at the local cathode within the delaminated zone. Thereby generated hydroxide species require a transport of cations from the defect electrolyte to the front of delamination for reasons of charge compensation [1–9]. Especially the Scanning Kelvin Probe (SKP) has been established as method for non-destructive measurements of electrode potentials at buried polymer/oxide/substrate interfaces and for the investigation of interfacial ion transport processes [10,11]. As long as oxygen reduction is not rate-determined by electron transfer kinetics, the progress of cathodic delamination depends on the square root of time and is reflected by the interfacial cation mobility and transport rate [4]. Nevertheless parameters determinant for the cation mobility itself were not comprehensively investigated up to now. Therefore a detailed study of the dependency of interfacial ion mobility on the substrate properties will be a prerequisite for further predictions of polymer/metal oxide interface * Corresponding author. Tel.: +32 26293533; fax: +32 26293200. E-mail address: [email protected] (J. Wielant). 0010-938X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2009.04.014
stabilities. Such investigations have to focus on the small interface section commonly described as electrolyte front position [12]. This region is especially sensitive to changes of the interfacial water activity, which will affect the chemical and structural environment between polymer film and oxide substrate. A semi-quantitative connection could be drawn between interfacial water activity, relative polymer/substrate peel-force differences and velocity changes of a subsequently initiated cathodic delamination mechanism [12]. Processes at the electrolyte front position can be correlated to the initial steps of corrosive delamination and preliminary interface degradation: (a) interfacial enrichment of liquid water due to atmospheric H2O diffusing through the polymer bulk [13], (b) replacement of secondary polymer–oxide interactions and partly saturation of oxide adsorption sites [14,15], (c) increase of interfacial free volumes by polymer swelling [16–18], (d) acceleration of the oxygen reduction kinetics, hydroxide formation, compaction of the electric double layer at the oxide/liquid interface, attraction of cations from the already delaminated area [3–8] and (e) adjustment of the oxide morphology due to passive layer formation at increasing interfacial pH [3]. Induced by the destruction of covalent bonds and by reactive radicals evolving during oxygen reduction [5], the polymer will be macroscopically lifted-off. As for steel substrates no corrosion products precipitate in the delaminated area during cathodic delamination, the interfacial ion transport processes are only affected by the physicochemical
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condition of the interface. This enables the investigation of the influence of iron oxide surfaces with different structure and composition on the progress of cathodic delamination [19,20]. For the present publication steel/iron oxide substrates were covered by a pigment free polyurethane film. SKP studies of cathodic delamination processes are correlated to the results of polymer/oxide peel-off tests and oxide surface energies. Reactive wetting experiments along the uncoated substrates offer a further insight into the rate determining parameters for corrosive delamination at polymer/iron oxide interfaces. 2. Experimental 2.1. Sample preparation DC06 steel according to the EN10130(98) norm was applied as substrate. The steel surfaces were ground with SiC-paper (500– 800–1200–4000 grid) and subsequently polished with 1 lm grade diamond paste to obtain mirror-like appearance. For specific formation of iron oxide layers, samples were treated by one of the four procedures described below [19–21]. (1) ‘Thermal’ oxide films were formed at 250 °C during 8 min in air. (2) To receive ‘Gardoclean’ oxides, polished steel substrates were immersed into a NaOH containing 80 g/L Gardoclean 390 solution (obtained from Chemetall/Germany) for 8 min at 70 °C. (3) Electrochemically passivated steel samples (‘borate buffer’ oxide) were obtained in a 0.075 M Na2B4O710H2O + 0.3 M H3BO3 solution (pH = 8.2) after 1 h polarisation at 1.04 VSHE applying a Pt-counter and calomel reference electrode. Prior to passivation, the steel sheets were cathodically polarised at 0.76 VSHE for 30 min to remove the native oxide film. (4) Plasma oxides were formed first by a H2/Ar plasma to remove residual organic contaminations, followed by a H2O plasma treatment. For procedure details and parameters see [22]. Prior to coating application the four iron oxide sample types were ultrasonically cleaned for 5 min in acetone and chloroform. As polymer film, a commercial two component GlasuritÒ lacquer from BASF Coatings AG, Germany, was used. A polyurethane lacquer was formed by mixing a polyacryl based resin (MS-Clear 923-155) with an isocyanate based hardener (HS-Topcoat Hardener 929-93) at 2:1 ratio. The coating thickness was around 60 lm and was set by a barcoater. Curing of the coating took place in ambient atmosphere for 4 days. 2.2. Applied methods Advancing contact angles on freshly prepared oxide films were obtained in inert atmosphere with 5 ll droplets at 0.2 ll/s advancing rate, applying a dataphysics Contact Angle System OCA 20. Surface energies were calculated with the Owens–Wendt–Kaeble equation [23,24]
ð1 þ cos hi ÞcL;i qffiffiffiffiffiP qffiffiffiffiffiffiffi ¼ cS 2 cDL;i
sffiffiffiffiffiffiffi
cPL;i qffiffiffiffiffiDffi þ cS cDL;i
with hi as contact angle, cL,i as total surface tension, cDL;i and cPL;i as dispersive and polar surface tension component of the liquid i and cDS and cPS as dispersive and polar surface tension component of the solid substrate S. Purified water with pH = 6.9 (cL = 72.8, cDL ¼ 21:8, cPL ¼ 51:0), diiodemethane (cL = 50.8, cDL ¼ 50:4, cPL ¼ 0:4) and ethylene glycol (cL = 48.0, cDL ¼ 29:0, cPL ¼ 19:0) were used as liquids (surface energy values listed as mJ/mm2) [24,25]. Peel tests were performed in humid atmosphere of >95% rh with custom made equipment [3,6] at constant angle perpendicular to the sample surface, constant rate of 3.4 mm/min and with strips of 5 mm in width. Coated samples were measured immedi-
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ately after preparation or were exposed 24 h to humid atmosphere prior to the peel-off test. SKP measurements were carried out with a custom made height regulated Scanning Kelvin Probe [10] in an atmosphere of high humidity (>95% rh). Detected interface potentials could be correlated with respect to the standard hydrogen electrode after calibration against a Cu/CuSO4 electrode. Prior to the experiment, the coated samples were exposed in humid atmosphere to remove electrostatic charging from the polymer film. Cathodic delamination was initiated by filling an artificial defect with 0.5 M NaCl or KBr solution. Time-of-Flight Secondary Ion Mass Spectroscopy (ToF-SIMS) measurements have been executed by means of a PHI TRIFT CE, Physical Electronics, USA, applying a gallium ion gun at an acceleration voltage of 15 kV and a spot size of 100 100 lm.
3. Results and discussions 3.1. Cathodic delamination along different iron oxide/polymer interfaces Scanning Kelvin Probe (SKP) studies have been performed to reveal the influence of the iron oxide preparation procedure on the cathodic delamination kinetics at the polyurethane/substrate interfaces. The comparability of the observed delamination progress could be ensured by parallel investigation of sample sets exposed to the same environmental conditions. Thereby any interfering variations of oxygen partial pressure or relative atmospheric humidity could be avoided. Fig. 1 illustrates the detected potential profiles for the four studied oxide covered substrates. In general two potential levels characterise the SKP profiles: the defect potential and potential of the intact coated area. The interface potential of the non-delaminated, intact area varies between 0 and +200 mVSHE, depending on the oxide structure [3]. Once an electrolytic connection to the defect is established, the interface potential in the respective section is cathodically shifted towards the defect potential. For corroding polymer covered iron the defect potential is about 300 to 500 mVSHE. This can be ascribed to the open circuit potential of iron in chloride containing aqueous solutions. The progress of cathodic delamination is reflected by the lateral displacement of the turning points within the potential profiles of Fig. 1a–d [4]. Fig. 2 displays the delamination front position vs. the square root of time. The linear progress with square root of time suggests that the process is obviously rate-determined by the interfacial ion mobility [4]. The averaged delamination rate varies between approximately 2120 and 430 lm/h0.5. It was carefully checked that it reproducibly decreases in the order ‘borate buffer’ oxide > ‘thermal’ oxide ‘plasma’ oxide > ‘Gardoclean’ oxide. The disproportionate deceleration of cathodic delamination on plasma oxide at longer timescales (see Figs. 1b and 2) can be attributed to an ongoing passivation within the defect area. This phenomenon does not represent a typical property of the plasma substrate, as repeated experiments underlined. A higher linear potential slope between defect area and electrolyte front position can be correlated with a decelerated cathodic delamination process (see Table 1 and compare to Fig. 2). Such potential slope (DEdistance normalised) is reflected by the IR drop between the local electrodes of the galvanic element, determined by Ohm’s law [5]:
DEdistance normalised ¼ Ic Rc where Ic is interfacial cation current density, and Rc is distance normalised resistance towards interfacial cation transport. Quantification of Rc and Ic in fact is difficult. But it can be assumed that a large IR drop for the ‘Gardoclean’ oxide is attributed to a larger resistance
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Fig. 1. SKP corrosive de-adhesion study for the four polyurethane/iron oxide interfaces in humid air, applying 0.5 M NaCl solution as defect electrolyte. Illustrated are the potential profiles recorded on (a) ‘borate buffer’ oxide, (b) ‘plasma’ oxide, (c) ‘thermal’ oxide, (d) ‘Gardoclean‘ oxide.
3.2. Polymer/substrate adhesion strength as function of the iron oxide type
Fig. 2. Diagram of the delamination front position vs. square root of time, calculated from the potential profiles of Fig. 1.
Table 1 Typical values of the averaged distance normalised potential (IR) drop for the four iron oxides detected at the delaminated interface. Oxide type
IR drop (mV/mm)
‘Borate buffer’ oxide (fast) ‘Thermal’ oxide ‘Plasma’ oxide ‘Gardoclean’ oxide (slow)
7 53 60 73
Rc and a decreased interfacial Na+ mobility compared to the ‘borate buffer’ oxide. Alternatively, an increased interfacial cation current Ic, is less plausible, as it is connected with accelerated oxygen reduction kinetics and consequently with an acceleration of the cathodic delamination process itself.
In previous paragraph, the different interfacial cation mobility seemed to determine the observed de-adhesion rate for the four iron oxides. To further evaluate if the higher ion mobility is correlated to the interfacial adhesion strength, polymer/iron oxide 90° peel-off tests were performed. A first approach concentrated on the remaining adhesion of the polyurethane based lacquer within the delaminated area. As expected the polymer could be easily lifted off, but the applied force was nearby the detection limit of the applied equipment (not shown here) [4]. A rough qualitative estimation nevertheless seemed to confirm that a large IR drop detected before by SKP was connected to a higher peel-off force in the delaminated area. But as a distinct tendency for the complete set of the four oxides could not be given following this approach, the polymer adhesion strength in the intact areas was investigated accordingly. Fig. 3a illustrates the resulting peel-off forces detected immediately after film application and curing. They varied between 0.02 and 0.11 N/mm and increased in the order ‘borate buffer’ oxide < ‘thermal’ oxide < ‘plasma’ oxide < ‘Gardoclean’ oxide. The tendency detected for corrosive delamination is remarkably well reflected. As the SKP experiments were performed in highly humid atmosphere, the peel-off experiments were repeated after 24 h of sample exposure to these conditions. An even better correlation was obtained, because the relative adhesion strength differences between ‘plasma’ and ‘thermal’ oxide are distinctly decreased and rather similar to the small delamination velocity differences for these two substrates (compare Fig. 3b with Fig. 2). In general all peel-off forces are reduced after exposure in humid air due to water replacing secondary polymer/iron oxide interactions [14]. It can be concluded that a gradual decrease of the polymer/iron oxide adhesion strength also directly results in gradually
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Fig. 3. Investigation of the polyurethane/iron oxide adhesion force by peel-off measurements, (a) immediately after preparation in ambient atmosphere, (b) after 24 h exposure to humid air.
accelerated cathodic delamination kinetics. Initial adaptations of the interface structure after water uptake obviously will be maintained during the subsequent stages of electrochemical interface degradation caused by cathodic delamination. As always the same polymer was applied on the iron oxides, comparable bulk polymer swelling in humid air has to be supposed. But oxide morphology change during passivation and an expected increase of interfacial free volumes will occur differently, dependent on the substrate surface. Adhesion forces detected for the intact polymer/iron oxide interface at least qualitatively reflect such substrate properties.
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lowest for ‘borate buffer’ oxide. For ‘plasma’ oxide the polar component turned out to be slightly increased compared to that one of ‘thermal’ oxide. This remarkably exactly corresponds to the respective sequences illustrated in Figs. 2 and 3. It can be confirmed that an increase of the polar surface energy component has a positive effect on the adhesion strength for polymer films with polar functional groups [26,27]. Some studies attributed these findings to an increased amount of interactions between surface hydroxyls and organic functionalities of the polymer [28,29]. In previous publications it was shown for the same set of oxides that the surface hydroxyl density alters the number of oxide/adsorbant interactions and the adsorption mechanism of organic molecules [19,21]. Less strong interactions take place when molecules are adsorbed on poorly hydroxylated surfaces (‘thermal’ and ‘Gardoclean’ oxide) compared to hydroxyl-rich surfaces (‘borate buffer and ‘plasma’ oxide) [19,21]. For the polyurethane film, a higher hydroxyl density does not seem to result in a higher peel strength of the coating. Anyhow the peel forces of the polyurethane films indicate that the oxide surface energy is the crucial parameter towards adhesion (compare Figs. 3 and 5) and that the differences in adhesion strength can be attributed to the oxide surface chemistry and/or to the very first nanometers of the polymer/substrate interphase. The hydrophilic properties of the oxide surfaces determine structural interface adjustments due to water ingress, which are directly reflected by different polymer/substrate adhesion forces. 3.4. Electrolyte spreading along the uncoated iron oxide surfaces
Contact angle measurements have been performed on the four substrate types to gain a better understanding of the observed differences within the polyurethane/iron oxide adhesion forces. Fig. 4 illustrates the values detected for water. The surface wettability decreases within the order ‘borate buffer’ oxide > ‘plasma’ oxide ‘thermal’ oxide > ‘Gardoclean’ oxide and generally reflects the observed tendency for the cathodic delamination progress as well as that one detected for the peel-off forces. The calculated oxide surface energies could be apportioned into polar and dispersive contributions (see Fig. 5). Obviously the polar component is an adequate parameter for the prediction of peel-off force variations and cathodic de-adhesion rates for the applied coating. The highest polar surface energy was determined for ‘Gardoclean’ oxide, the
It could be proven that the interfacial cathodic delamination progress indirectly depends on the hydrophilic surface properties via the polymer/substrate adhesion strength. Nevertheless a direct correlation of cation mobility and surface energy may be effective as well. To further analyse how a polyurethane layer determines the interfacial ion transport, reactive electrolyte wetting experiments on the uncoated oxide surfaces were performed. Fig. 6 presents SKP potential profiles recorded for 0.5 M KBr liquid spreading on the four bare substrates in humid air. By addition of 3% agar, the defect electrolyte viscosity was increased to prevent it from immediate leaking [30]. Nevertheless a thin liquid film began to spread along the oxides starting from the solid droplets at the defect area. The wetted surface areas remained shiny, whereas the defect area was red coloured by precipitation of corrosion products. The SKP potential profiles recorded during this process vary concerning absolute potential levels of wetted and dry surface (see Fig. 6), but in shape are similar to those obtained for cathodic delamination at the polyurethane/iron oxide interfaces (see Fig. 1). The spreading velocity lies between approximately 2150 and
Fig. 4. Averaged water contact angles measured on the four iron oxide surfaces.
Fig. 5. Total surface energy of the four iron oxide substrates. Polar and dispersive components are indicated.
3.3. Surface energy of the different iron oxides
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Fig. 6. Reactive electrolyte wetting along uncoated iron oxide surfaces in humid air applying 0.5 M KBr solution of high viscosity after addition of agar. Illustrated are the potential profiles for a SKP study on (a) ‘plasma’ oxide, (b) ‘Gardoclean’ oxide, (c) ‘thermal’ oxide, (d) ‘borate buffer’ oxide.
550 lm/h0.5 and reproducibly decreases within the order ‘plasma’ oxide > ‘Gardoclean’ oxide ‘thermal’ oxide > ‘borate buffer’ oxide (see Fig. 7). It does not seem to be correlated with the tendency observed before for cathodic delamination at the coated iron oxides (compare Fig. 2 with Fig. 7). Furthermore surface energies and water contact angles, which were also determined on the bare oxides, do not allow the prediction of any electrolyte wetting tendency. Therefore it had to be ensured that liquid spreading along the uncoated iron oxide surfaces is dominated by electrostatically determined migration of defect electrolyte cations. This was already proven for cathodic delamination along coated substrates [5,30]. Fig. 8 presents ToF-SIMS profiles recorded on the ‘Gardoclean’ and ‘borate buffer’ oxide surfaces after termination of the
Fig. 7. Diagram of the spreading front position vs. square root of time, calculated from the potential profiles of Fig. 6.
electrolyte wetting experiments. The latter showed slowest electrolyte spreading (see Fig. 7). The process therefore should be most susceptible for any contribution of diffusive ion transport. For the ‘Gardoclean’ oxide, unusual potential profiles have been recorded (see Fig. 6b) which may point at a changed process mechanism. Although a quantitative comparison of K+ and Br amount is not possible, the distribution of bromide and potassium ions at the surfaces reflects the different sample areas. A varying amount of KBr could be detected within the defect section. At the transition to the spreaded area, the Br signal diminishes, but K+ still is definitively detectable. In agreement with [30], a basic contamination level for both ions is reached within the unaffected surface beyond the electrolyte front position. Obviously solely cations are transported into the electrolyte wetted sections. This is consistent with the assumption of an oxygen reduction induced K+ transport process necessary for charge compensation after OH formation at the substrate surface [31]. In this context, both ion transport mechanisms at uncoated ‘Gardoclean’ and ‘borate buffer’ oxide are comparable. Deviations due to application of NaCl solution for cathodic delamination studies at the polymer coated substrates and KBr solution for the spreading experiments along the uncoated iron oxides are not to be expected [30,31]. As also a square root of time dependency is observed (see Fig. 7), the basic mechanisms of electrolyte spreading and cathodic delamination are comparable. So obviously ion mobility at the uncoated iron oxides is not dependent on the substrate surface energy, whereas it plays a crucial role for the progress of cathodic delamination at the polyurethane/iron oxide interfaces. For the latter case, surface energies even seem to allow semi-quantitative predictions of the interface stability for sets of similar samples. Based on the detected correlation with to the respective coating/oxide adhesion forces, the
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Fig. 8. ToF-SIMS ion profiles recorded on iron oxide surfaces after reactive electrolyte spreading in humid air. Highly viscous 0.5 M KBr solution, stabilised with agar, was applied as electrolyte. (a) K+ distribution on ‘Gardoclean’ oxide, (b) correlated Br distribution on ‘Gardoclean’ oxide, (c) K+ distribution on ‘borate buffer’ oxide, (d) correlated Br distribution on ‘borate buffer’ oxide.
presented results may help to introduce conventional polymer/ substrate peel-off tests as cost-efficient method for such an evaluation.
4. Conclusions The present study offers a further insight into the dependency of interfacial ion mobility on the rate determining parameters for transport controlled cathodic delamination processes at polymer/ iron oxide interfaces. Moreover, parameters influencing the cation mobility itself were investigated in detail. A direct connection could be detected between oxide surface energies, adhesion of the polymer film after sample exposure in humid atmosphere and the delamination rate of the polyurethane coating on the steel substrates with different interfacial iron oxide structure and morphology. An increasing polar oxide surface energy component was correlated with increasing polymer/iron oxide peel-off forces and deceleration of delamination within the order ‘borate buffer’ oxide > ‘thermal’ oxide ‘plasma’ oxide > ‘Gardoclean’ oxide. In contrast, reactive electrolyte spreading along the uncoated oxide substrates was not dependent on the oxide surface energies, although both spreading and delamination are dominated by migration of cations from the defect electrolyte to the cathodic delamination front. It can be concluded that hydrophilic oxide surface properties determine structural changes of the polyurethane/ substrate interface after ingress of atmospheric water. They are directly reflected by differences of the polymer/substrate adhesion forces and will be maintained during the subsequent stages of cathodic delamination.
Acknowledgments Jan Wielant is financed by the Flemish Institute for the promotion of scientific and technological research in industry (IWT). His work is realised with the financial support of ArcelorMittal Research and Development Industry Gent-OCAS N.V. The authors thank the Christian Doppler Research Association, Vienna, for providing the experimental facilities at the Max-Planck-Institut für Eisenforschung GmbH. References [1] M. Stratmann, A. Leng, W. Fürbeth, H. Streckel, H. Gehmecker, K.H. GroßeBrinkhaus, The scanning Kelvin probe: a new technique for the in situ analysis of the delamination of organic coatings, Prog. Org. Coat. 27 (1996) 261. [2] W. Fürbeth, M. Stratmann, Scanning Kelvin probe investigations on the delamination of polymeric coatings from metallic surfaces, Prog. Org. Coat. 39 (2000) 23. [3] A. Leng, H. Streckel, M. Stratmann, The delamination of polymeric coatings from steel. Part 1: Calibration of the Kelvin probe and basic delamination mechanism, Corr. Sci. 41 (1999) 547. [4] A. Leng, H. Streckel, M. Stratmann, The delamination of polymeric coatings from steel. Part 2: First stages of delamination, effect of type and concentration of cations on delamination, chemical analysis of the interface, Corr. Sci. 41 (1999) 579. [5] A. Leng, H. Streckel, K. Hofmann, M. Stratmann, The delamination of polymeric coatings from steel. Part 3: Effect of oxygen partial pressure on the delamination reaction and current distribution at the metal/polymer interface, Corr. Sci. 41 (1999) 599. [6] W. Fürbeth, M. Stratmann, The delamination of polymeric coatings from electrogalvanised steel - a mechanistic approach. Part 1: Delamination from a defect with intact zinc layer, Corr. Sci. 43 (2001) 207. [7] W. Fürbeth, M. Stratmann, The delamination of polymeric coatings from electrogalvanised steel - a mechanistic approach. Part 2: Delamination from a defect down to steel, Corr. Sci. 43 (2001) 229.
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Corrosion Science journal homepage: www.elsevier.com/locate/corsci
Cathodic delamination of polyurethane films on oxide covered steel – Combined adhesion and interface electrochemical studies J. Wielant a,*, R. Posner b, R. Hausbrand c, G. Grundmeier b,d, H. Terryn a a
Research Group Electrochemical and Surface Engineering, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium Christian-Doppler Laboratory for Polymer/Metal Interfaces at the Max-Planck-Institut für Eisenforschung GmbH, Max-Planck-Str. 1, 40237 Düsseldorf, Germany c ArcelorMittal R&D Industry Gent – OCAS NV, Pres. J.F. Kennedylaan 3, 9060 Zelzate, Belgium d University of Paderborn, Technical and Macromolecular Chemistry, Warburger Str. 100, 33098 Paderborn, Germany b
a r t i c l e
i n f o
Article history: Received 19 March 2009 Accepted 16 April 2009 Available online 3 May 2009 Keywords: A. Steel A. Organic coatings B. Scanning kelvin probe C. Interfaces C. Cathodic delamination
a b s t r a c t The stability of polyurethane/iron oxide/steel interfaces was evaluated for different chemically modified passive films in humid and corrosive atmosphere. Iron oxide layers were formed on polished steel by thermal annealing in oxygen rich atmosphere, water plasma modification, immersion in alkaline solution and by anodic polarisation in borate buffer solution. Scanning Kelvin Probe studies of the interface stability were performed under the conditions of cathodic delamination. The corresponding progress rates specifically depended on the iron oxide. Complementary measured peel-off forces of the respective coating/substrate systems showed the same trend as detected for the corrosive delamination process. Additional contact angle measurements and investigation of the interfacial ion mobility on the uncoated oxide surfaces confirmed a correlation between high peel forces, increasing oxide surface energy and decelerated cathodic delamination kinetics. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction If a metal passive film is electron conducting, cathodic delamination is often responsible for a proceeding polymer/metal interface deterioration starting from an actively corroding coating defect. It is driven by a galvanic cell with metal dissolution at the local anode in the defect area and a dominating oxygen reduction process at the local cathode within the delaminated zone. Thereby generated hydroxide species require a transport of cations from the defect electrolyte to the front of delamination for reasons of charge compensation [1–9]. Especially the Scanning Kelvin Probe (SKP) has been established as method for non-destructive measurements of electrode potentials at buried polymer/oxide/substrate interfaces and for the investigation of interfacial ion transport processes [10,11]. As long as oxygen reduction is not rate-determined by electron transfer kinetics, the progress of cathodic delamination depends on the square root of time and is reflected by the interfacial cation mobility and transport rate [4]. Nevertheless parameters determinant for the cation mobility itself were not comprehensively investigated up to now. Therefore a detailed study of the dependency of interfacial ion mobility on the substrate properties will be a prerequisite for further predictions of polymer/metal oxide interface * Corresponding author. Tel.: +32 26293533; fax: +32 26293200. E-mail address: [email protected] (J. Wielant). 0010-938X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2009.04.014
stabilities. Such investigations have to focus on the small interface section commonly described as electrolyte front position [12]. This region is especially sensitive to changes of the interfacial water activity, which will affect the chemical and structural environment between polymer film and oxide substrate. A semi-quantitative connection could be drawn between interfacial water activity, relative polymer/substrate peel-force differences and velocity changes of a subsequently initiated cathodic delamination mechanism [12]. Processes at the electrolyte front position can be correlated to the initial steps of corrosive delamination and preliminary interface degradation: (a) interfacial enrichment of liquid water due to atmospheric H2O diffusing through the polymer bulk [13], (b) replacement of secondary polymer–oxide interactions and partly saturation of oxide adsorption sites [14,15], (c) increase of interfacial free volumes by polymer swelling [16–18], (d) acceleration of the oxygen reduction kinetics, hydroxide formation, compaction of the electric double layer at the oxide/liquid interface, attraction of cations from the already delaminated area [3–8] and (e) adjustment of the oxide morphology due to passive layer formation at increasing interfacial pH [3]. Induced by the destruction of covalent bonds and by reactive radicals evolving during oxygen reduction [5], the polymer will be macroscopically lifted-off. As for steel substrates no corrosion products precipitate in the delaminated area during cathodic delamination, the interfacial ion transport processes are only affected by the physicochemical
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condition of the interface. This enables the investigation of the influence of iron oxide surfaces with different structure and composition on the progress of cathodic delamination [19,20]. For the present publication steel/iron oxide substrates were covered by a pigment free polyurethane film. SKP studies of cathodic delamination processes are correlated to the results of polymer/oxide peel-off tests and oxide surface energies. Reactive wetting experiments along the uncoated substrates offer a further insight into the rate determining parameters for corrosive delamination at polymer/iron oxide interfaces. 2. Experimental 2.1. Sample preparation DC06 steel according to the EN10130(98) norm was applied as substrate. The steel surfaces were ground with SiC-paper (500– 800–1200–4000 grid) and subsequently polished with 1 lm grade diamond paste to obtain mirror-like appearance. For specific formation of iron oxide layers, samples were treated by one of the four procedures described below [19–21]. (1) ‘Thermal’ oxide films were formed at 250 °C during 8 min in air. (2) To receive ‘Gardoclean’ oxides, polished steel substrates were immersed into a NaOH containing 80 g/L Gardoclean 390 solution (obtained from Chemetall/Germany) for 8 min at 70 °C. (3) Electrochemically passivated steel samples (‘borate buffer’ oxide) were obtained in a 0.075 M Na2B4O710H2O + 0.3 M H3BO3 solution (pH = 8.2) after 1 h polarisation at 1.04 VSHE applying a Pt-counter and calomel reference electrode. Prior to passivation, the steel sheets were cathodically polarised at 0.76 VSHE for 30 min to remove the native oxide film. (4) Plasma oxides were formed first by a H2/Ar plasma to remove residual organic contaminations, followed by a H2O plasma treatment. For procedure details and parameters see [22]. Prior to coating application the four iron oxide sample types were ultrasonically cleaned for 5 min in acetone and chloroform. As polymer film, a commercial two component GlasuritÒ lacquer from BASF Coatings AG, Germany, was used. A polyurethane lacquer was formed by mixing a polyacryl based resin (MS-Clear 923-155) with an isocyanate based hardener (HS-Topcoat Hardener 929-93) at 2:1 ratio. The coating thickness was around 60 lm and was set by a barcoater. Curing of the coating took place in ambient atmosphere for 4 days. 2.2. Applied methods Advancing contact angles on freshly prepared oxide films were obtained in inert atmosphere with 5 ll droplets at 0.2 ll/s advancing rate, applying a dataphysics Contact Angle System OCA 20. Surface energies were calculated with the Owens–Wendt–Kaeble equation [23,24]
ð1 þ cos hi ÞcL;i qffiffiffiffiffiP qffiffiffiffiffiffiffi ¼ cS 2 cDL;i
sffiffiffiffiffiffiffi
cPL;i qffiffiffiffiffiDffi þ cS cDL;i
with hi as contact angle, cL,i as total surface tension, cDL;i and cPL;i as dispersive and polar surface tension component of the liquid i and cDS and cPS as dispersive and polar surface tension component of the solid substrate S. Purified water with pH = 6.9 (cL = 72.8, cDL ¼ 21:8, cPL ¼ 51:0), diiodemethane (cL = 50.8, cDL ¼ 50:4, cPL ¼ 0:4) and ethylene glycol (cL = 48.0, cDL ¼ 29:0, cPL ¼ 19:0) were used as liquids (surface energy values listed as mJ/mm2) [24,25]. Peel tests were performed in humid atmosphere of >95% rh with custom made equipment [3,6] at constant angle perpendicular to the sample surface, constant rate of 3.4 mm/min and with strips of 5 mm in width. Coated samples were measured immedi-
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ately after preparation or were exposed 24 h to humid atmosphere prior to the peel-off test. SKP measurements were carried out with a custom made height regulated Scanning Kelvin Probe [10] in an atmosphere of high humidity (>95% rh). Detected interface potentials could be correlated with respect to the standard hydrogen electrode after calibration against a Cu/CuSO4 electrode. Prior to the experiment, the coated samples were exposed in humid atmosphere to remove electrostatic charging from the polymer film. Cathodic delamination was initiated by filling an artificial defect with 0.5 M NaCl or KBr solution. Time-of-Flight Secondary Ion Mass Spectroscopy (ToF-SIMS) measurements have been executed by means of a PHI TRIFT CE, Physical Electronics, USA, applying a gallium ion gun at an acceleration voltage of 15 kV and a spot size of 100 100 lm.
3. Results and discussions 3.1. Cathodic delamination along different iron oxide/polymer interfaces Scanning Kelvin Probe (SKP) studies have been performed to reveal the influence of the iron oxide preparation procedure on the cathodic delamination kinetics at the polyurethane/substrate interfaces. The comparability of the observed delamination progress could be ensured by parallel investigation of sample sets exposed to the same environmental conditions. Thereby any interfering variations of oxygen partial pressure or relative atmospheric humidity could be avoided. Fig. 1 illustrates the detected potential profiles for the four studied oxide covered substrates. In general two potential levels characterise the SKP profiles: the defect potential and potential of the intact coated area. The interface potential of the non-delaminated, intact area varies between 0 and +200 mVSHE, depending on the oxide structure [3]. Once an electrolytic connection to the defect is established, the interface potential in the respective section is cathodically shifted towards the defect potential. For corroding polymer covered iron the defect potential is about 300 to 500 mVSHE. This can be ascribed to the open circuit potential of iron in chloride containing aqueous solutions. The progress of cathodic delamination is reflected by the lateral displacement of the turning points within the potential profiles of Fig. 1a–d [4]. Fig. 2 displays the delamination front position vs. the square root of time. The linear progress with square root of time suggests that the process is obviously rate-determined by the interfacial ion mobility [4]. The averaged delamination rate varies between approximately 2120 and 430 lm/h0.5. It was carefully checked that it reproducibly decreases in the order ‘borate buffer’ oxide > ‘thermal’ oxide ‘plasma’ oxide > ‘Gardoclean’ oxide. The disproportionate deceleration of cathodic delamination on plasma oxide at longer timescales (see Figs. 1b and 2) can be attributed to an ongoing passivation within the defect area. This phenomenon does not represent a typical property of the plasma substrate, as repeated experiments underlined. A higher linear potential slope between defect area and electrolyte front position can be correlated with a decelerated cathodic delamination process (see Table 1 and compare to Fig. 2). Such potential slope (DEdistance normalised) is reflected by the IR drop between the local electrodes of the galvanic element, determined by Ohm’s law [5]:
DEdistance normalised ¼ Ic Rc where Ic is interfacial cation current density, and Rc is distance normalised resistance towards interfacial cation transport. Quantification of Rc and Ic in fact is difficult. But it can be assumed that a large IR drop for the ‘Gardoclean’ oxide is attributed to a larger resistance
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Fig. 1. SKP corrosive de-adhesion study for the four polyurethane/iron oxide interfaces in humid air, applying 0.5 M NaCl solution as defect electrolyte. Illustrated are the potential profiles recorded on (a) ‘borate buffer’ oxide, (b) ‘plasma’ oxide, (c) ‘thermal’ oxide, (d) ‘Gardoclean‘ oxide.
3.2. Polymer/substrate adhesion strength as function of the iron oxide type
Fig. 2. Diagram of the delamination front position vs. square root of time, calculated from the potential profiles of Fig. 1.
Table 1 Typical values of the averaged distance normalised potential (IR) drop for the four iron oxides detected at the delaminated interface. Oxide type
IR drop (mV/mm)
‘Borate buffer’ oxide (fast) ‘Thermal’ oxide ‘Plasma’ oxide ‘Gardoclean’ oxide (slow)
7 53 60 73
Rc and a decreased interfacial Na+ mobility compared to the ‘borate buffer’ oxide. Alternatively, an increased interfacial cation current Ic, is less plausible, as it is connected with accelerated oxygen reduction kinetics and consequently with an acceleration of the cathodic delamination process itself.
In previous paragraph, the different interfacial cation mobility seemed to determine the observed de-adhesion rate for the four iron oxides. To further evaluate if the higher ion mobility is correlated to the interfacial adhesion strength, polymer/iron oxide 90° peel-off tests were performed. A first approach concentrated on the remaining adhesion of the polyurethane based lacquer within the delaminated area. As expected the polymer could be easily lifted off, but the applied force was nearby the detection limit of the applied equipment (not shown here) [4]. A rough qualitative estimation nevertheless seemed to confirm that a large IR drop detected before by SKP was connected to a higher peel-off force in the delaminated area. But as a distinct tendency for the complete set of the four oxides could not be given following this approach, the polymer adhesion strength in the intact areas was investigated accordingly. Fig. 3a illustrates the resulting peel-off forces detected immediately after film application and curing. They varied between 0.02 and 0.11 N/mm and increased in the order ‘borate buffer’ oxide < ‘thermal’ oxide < ‘plasma’ oxide < ‘Gardoclean’ oxide. The tendency detected for corrosive delamination is remarkably well reflected. As the SKP experiments were performed in highly humid atmosphere, the peel-off experiments were repeated after 24 h of sample exposure to these conditions. An even better correlation was obtained, because the relative adhesion strength differences between ‘plasma’ and ‘thermal’ oxide are distinctly decreased and rather similar to the small delamination velocity differences for these two substrates (compare Fig. 3b with Fig. 2). In general all peel-off forces are reduced after exposure in humid air due to water replacing secondary polymer/iron oxide interactions [14]. It can be concluded that a gradual decrease of the polymer/iron oxide adhesion strength also directly results in gradually
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Fig. 3. Investigation of the polyurethane/iron oxide adhesion force by peel-off measurements, (a) immediately after preparation in ambient atmosphere, (b) after 24 h exposure to humid air.
accelerated cathodic delamination kinetics. Initial adaptations of the interface structure after water uptake obviously will be maintained during the subsequent stages of electrochemical interface degradation caused by cathodic delamination. As always the same polymer was applied on the iron oxides, comparable bulk polymer swelling in humid air has to be supposed. But oxide morphology change during passivation and an expected increase of interfacial free volumes will occur differently, dependent on the substrate surface. Adhesion forces detected for the intact polymer/iron oxide interface at least qualitatively reflect such substrate properties.
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lowest for ‘borate buffer’ oxide. For ‘plasma’ oxide the polar component turned out to be slightly increased compared to that one of ‘thermal’ oxide. This remarkably exactly corresponds to the respective sequences illustrated in Figs. 2 and 3. It can be confirmed that an increase of the polar surface energy component has a positive effect on the adhesion strength for polymer films with polar functional groups [26,27]. Some studies attributed these findings to an increased amount of interactions between surface hydroxyls and organic functionalities of the polymer [28,29]. In previous publications it was shown for the same set of oxides that the surface hydroxyl density alters the number of oxide/adsorbant interactions and the adsorption mechanism of organic molecules [19,21]. Less strong interactions take place when molecules are adsorbed on poorly hydroxylated surfaces (‘thermal’ and ‘Gardoclean’ oxide) compared to hydroxyl-rich surfaces (‘borate buffer and ‘plasma’ oxide) [19,21]. For the polyurethane film, a higher hydroxyl density does not seem to result in a higher peel strength of the coating. Anyhow the peel forces of the polyurethane films indicate that the oxide surface energy is the crucial parameter towards adhesion (compare Figs. 3 and 5) and that the differences in adhesion strength can be attributed to the oxide surface chemistry and/or to the very first nanometers of the polymer/substrate interphase. The hydrophilic properties of the oxide surfaces determine structural interface adjustments due to water ingress, which are directly reflected by different polymer/substrate adhesion forces. 3.4. Electrolyte spreading along the uncoated iron oxide surfaces
Contact angle measurements have been performed on the four substrate types to gain a better understanding of the observed differences within the polyurethane/iron oxide adhesion forces. Fig. 4 illustrates the values detected for water. The surface wettability decreases within the order ‘borate buffer’ oxide > ‘plasma’ oxide ‘thermal’ oxide > ‘Gardoclean’ oxide and generally reflects the observed tendency for the cathodic delamination progress as well as that one detected for the peel-off forces. The calculated oxide surface energies could be apportioned into polar and dispersive contributions (see Fig. 5). Obviously the polar component is an adequate parameter for the prediction of peel-off force variations and cathodic de-adhesion rates for the applied coating. The highest polar surface energy was determined for ‘Gardoclean’ oxide, the
It could be proven that the interfacial cathodic delamination progress indirectly depends on the hydrophilic surface properties via the polymer/substrate adhesion strength. Nevertheless a direct correlation of cation mobility and surface energy may be effective as well. To further analyse how a polyurethane layer determines the interfacial ion transport, reactive electrolyte wetting experiments on the uncoated oxide surfaces were performed. Fig. 6 presents SKP potential profiles recorded for 0.5 M KBr liquid spreading on the four bare substrates in humid air. By addition of 3% agar, the defect electrolyte viscosity was increased to prevent it from immediate leaking [30]. Nevertheless a thin liquid film began to spread along the oxides starting from the solid droplets at the defect area. The wetted surface areas remained shiny, whereas the defect area was red coloured by precipitation of corrosion products. The SKP potential profiles recorded during this process vary concerning absolute potential levels of wetted and dry surface (see Fig. 6), but in shape are similar to those obtained for cathodic delamination at the polyurethane/iron oxide interfaces (see Fig. 1). The spreading velocity lies between approximately 2150 and
Fig. 4. Averaged water contact angles measured on the four iron oxide surfaces.
Fig. 5. Total surface energy of the four iron oxide substrates. Polar and dispersive components are indicated.
3.3. Surface energy of the different iron oxides
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Fig. 6. Reactive electrolyte wetting along uncoated iron oxide surfaces in humid air applying 0.5 M KBr solution of high viscosity after addition of agar. Illustrated are the potential profiles for a SKP study on (a) ‘plasma’ oxide, (b) ‘Gardoclean’ oxide, (c) ‘thermal’ oxide, (d) ‘borate buffer’ oxide.
550 lm/h0.5 and reproducibly decreases within the order ‘plasma’ oxide > ‘Gardoclean’ oxide ‘thermal’ oxide > ‘borate buffer’ oxide (see Fig. 7). It does not seem to be correlated with the tendency observed before for cathodic delamination at the coated iron oxides (compare Fig. 2 with Fig. 7). Furthermore surface energies and water contact angles, which were also determined on the bare oxides, do not allow the prediction of any electrolyte wetting tendency. Therefore it had to be ensured that liquid spreading along the uncoated iron oxide surfaces is dominated by electrostatically determined migration of defect electrolyte cations. This was already proven for cathodic delamination along coated substrates [5,30]. Fig. 8 presents ToF-SIMS profiles recorded on the ‘Gardoclean’ and ‘borate buffer’ oxide surfaces after termination of the
Fig. 7. Diagram of the spreading front position vs. square root of time, calculated from the potential profiles of Fig. 6.
electrolyte wetting experiments. The latter showed slowest electrolyte spreading (see Fig. 7). The process therefore should be most susceptible for any contribution of diffusive ion transport. For the ‘Gardoclean’ oxide, unusual potential profiles have been recorded (see Fig. 6b) which may point at a changed process mechanism. Although a quantitative comparison of K+ and Br amount is not possible, the distribution of bromide and potassium ions at the surfaces reflects the different sample areas. A varying amount of KBr could be detected within the defect section. At the transition to the spreaded area, the Br signal diminishes, but K+ still is definitively detectable. In agreement with [30], a basic contamination level for both ions is reached within the unaffected surface beyond the electrolyte front position. Obviously solely cations are transported into the electrolyte wetted sections. This is consistent with the assumption of an oxygen reduction induced K+ transport process necessary for charge compensation after OH formation at the substrate surface [31]. In this context, both ion transport mechanisms at uncoated ‘Gardoclean’ and ‘borate buffer’ oxide are comparable. Deviations due to application of NaCl solution for cathodic delamination studies at the polymer coated substrates and KBr solution for the spreading experiments along the uncoated iron oxides are not to be expected [30,31]. As also a square root of time dependency is observed (see Fig. 7), the basic mechanisms of electrolyte spreading and cathodic delamination are comparable. So obviously ion mobility at the uncoated iron oxides is not dependent on the substrate surface energy, whereas it plays a crucial role for the progress of cathodic delamination at the polyurethane/iron oxide interfaces. For the latter case, surface energies even seem to allow semi-quantitative predictions of the interface stability for sets of similar samples. Based on the detected correlation with to the respective coating/oxide adhesion forces, the
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Fig. 8. ToF-SIMS ion profiles recorded on iron oxide surfaces after reactive electrolyte spreading in humid air. Highly viscous 0.5 M KBr solution, stabilised with agar, was applied as electrolyte. (a) K+ distribution on ‘Gardoclean’ oxide, (b) correlated Br distribution on ‘Gardoclean’ oxide, (c) K+ distribution on ‘borate buffer’ oxide, (d) correlated Br distribution on ‘borate buffer’ oxide.
presented results may help to introduce conventional polymer/ substrate peel-off tests as cost-efficient method for such an evaluation.
4. Conclusions The present study offers a further insight into the dependency of interfacial ion mobility on the rate determining parameters for transport controlled cathodic delamination processes at polymer/ iron oxide interfaces. Moreover, parameters influencing the cation mobility itself were investigated in detail. A direct connection could be detected between oxide surface energies, adhesion of the polymer film after sample exposure in humid atmosphere and the delamination rate of the polyurethane coating on the steel substrates with different interfacial iron oxide structure and morphology. An increasing polar oxide surface energy component was correlated with increasing polymer/iron oxide peel-off forces and deceleration of delamination within the order ‘borate buffer’ oxide > ‘thermal’ oxide ‘plasma’ oxide > ‘Gardoclean’ oxide. In contrast, reactive electrolyte spreading along the uncoated oxide substrates was not dependent on the oxide surface energies, although both spreading and delamination are dominated by migration of cations from the defect electrolyte to the cathodic delamination front. It can be concluded that hydrophilic oxide surface properties determine structural changes of the polyurethane/ substrate interface after ingress of atmospheric water. They are directly reflected by differences of the polymer/substrate adhesion forces and will be maintained during the subsequent stages of cathodic delamination.
Acknowledgments Jan Wielant is financed by the Flemish Institute for the promotion of scientific and technological research in industry (IWT). His work is realised with the financial support of ArcelorMittal Research and Development Industry Gent-OCAS N.V. The authors thank the Christian Doppler Research Association, Vienna, for providing the experimental facilities at the Max-Planck-Institut für Eisenforschung GmbH. References [1] M. Stratmann, A. Leng, W. Fürbeth, H. Streckel, H. Gehmecker, K.H. GroßeBrinkhaus, The scanning Kelvin probe: a new technique for the in situ analysis of the delamination of organic coatings, Prog. Org. Coat. 27 (1996) 261. [2] W. Fürbeth, M. Stratmann, Scanning Kelvin probe investigations on the delamination of polymeric coatings from metallic surfaces, Prog. Org. Coat. 39 (2000) 23. [3] A. Leng, H. Streckel, M. Stratmann, The delamination of polymeric coatings from steel. Part 1: Calibration of the Kelvin probe and basic delamination mechanism, Corr. Sci. 41 (1999) 547. [4] A. Leng, H. Streckel, M. Stratmann, The delamination of polymeric coatings from steel. Part 2: First stages of delamination, effect of type and concentration of cations on delamination, chemical analysis of the interface, Corr. Sci. 41 (1999) 579. [5] A. Leng, H. Streckel, K. Hofmann, M. Stratmann, The delamination of polymeric coatings from steel. Part 3: Effect of oxygen partial pressure on the delamination reaction and current distribution at the metal/polymer interface, Corr. Sci. 41 (1999) 599. [6] W. Fürbeth, M. Stratmann, The delamination of polymeric coatings from electrogalvanised steel - a mechanistic approach. Part 1: Delamination from a defect with intact zinc layer, Corr. Sci. 43 (2001) 207. [7] W. Fürbeth, M. Stratmann, The delamination of polymeric coatings from electrogalvanised steel - a mechanistic approach. Part 2: Delamination from a defect down to steel, Corr. Sci. 43 (2001) 229.
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