The fast lane to failure

Transcription

1 1 The fast lane to failure Two different tests of anticorrosive properties (salt fog spray and a cyclic impedance test called ACET) have been used to evaluate two-layer coating systems for aeronautical application and establish a correlation between them The ACET technique provides good correlation to salt spray, faster evaluation and more details of the mode of coating breakdown Cyclic impedance test gives rapid characterisation of coating breakdown José Javier Gracenea* Maria José Gimeno Julio José Suay Prediction of the anticorrosive properties of paints is one of the most important lines of research in organic coatings Due to the complexity of the corrosion and degradation processes in organic-metal systems, the main routines for assessing interactions and materials have been developed experimentally by using different exposure processes and techniques for the measurement of properties The main types of exposure process are accelerated ageing tests (eg salt fog spray) and natural exposure experiments (which must be planned for long durations and are very expensive to run) Salt fog spray is one of the longest-established evaluation techniques for which different international standards (ISO 9227, ASTM B117) have been developed Nevertheless, this technique is very subjective and does not give quantitative information about the corrosion processes or an interpretation of the overall process itself It is still necessary to develop techniques to measure the anticorrosive properties in a direct sense, thus various electrochemical techniques have been used to evaluate the protective performance of organic coating/metal systems The application of electrochemical impedance spectroscopy (EIS) to coated metals has proved to be a useful technique here [1-8], although long periods are needed (days, weeks and sometimes months) to perform this type of test and obtain good results Combined AC/DC impedance test gives fast results Hollaender et al [9-11] developed a rapid method for testing coated metals in food packaging, consisting of a combination of DC and AC measurements (the AC/DC/AC procedure) which has been successfully adapted and used in liquid paints applied to steel substrates [12-14] After an initial AC measurement, the test sample is subjected for a short time to a constant cathodic voltage (DC) producing a stress on the sample and, following that, an AC spectrum is recorded again The change in the characteristics of the impedance spectrum can be attributed to coating deterioration (pore formation) and a delamination process in the metallic surface due to hydrogen and OH- production (if a cathodic reaction takes place) The new ACET (Accelerated Cyclic Electrochemical Technique) procedure developed by Medco [15-19] is based on the Hollaender method but incorporates two principal innovations First, a long potential relaxation period was introduced after the polarisation, where the potential and intensity are recorded against time, data that is used to evaluate the adhesion of the coatings to the substrate Second, new numerical methods were developed to establish correlations between ACET and traditional salt spray resistance results In this work ACET and salt fog spray were used to study the anticorrosive properties of the paint systems described below The validity of the new technique has also been examined as a useful method for determining the anticorrosive properties of paints in very short times, comparing it with other different evaluation procedures (EIS and salt fog spray) Coating systems and test procedures Two epoxy-polyurethane two-coat systems were chosen for the comparative tests Both systems were solventbased, with a high solids epoxy primer containing Cr(VI) anticorrosive pigments The samples were cured at room temperature for 21 days The total thickness of the paint systems was in the range µm The accelerated salt fog spray test was performed in accordance with ASTM B /ISO 9227 In this test a cross is cut along the coating through to the bare metal Samples are collected after different periods of time in the test and evaluated up to a maximum of 3000 hours exposure After each collection, the samples were dried; blistering, corrosion and delamination were measured after 24 hours of ambient relaxation time Delamination was evaluated after applying 30 mm wide tape to one arm of the cross and peeling off the paint with it The ACET procedure (Figure 1) is based on the application of a stress (cathodic polarisation) to a coated sample, then measuring the impedance (EIS) of the system after it This stress/impedance sequence is repeated several times until the system is substantially damaged In order to obtain more information about the coating itself and the whole system, a depolarisation step can be included for a given period (relaxation time) An initial EIS measurement gives an idea of the impedance of the system (using Bode plots and modelled characteristic parameters and the cycle shown in Figure 1) Underlying corrosion theory summarised A cathodic reaction of water hydrolysis occurs when the potential is more negative than -10 V relative to a saturated calomel electrode [27] The test technique is based on the influence that this has on coating adhesion because of the formation of H2 (gas) and OH- The evolution of H2 will increase local delamination (Figure 2) giving rise to the failure of the coating system (reflected in the variation of the impedance) When the cathodic reaction stops and H2 production has taken place, the normal electrochemical corrosion of the

2 2 system occurs in the presence of the electrolyte, with production of iron oxides and hydroxides On the other hand, the forced polarisation means that the double layer in the interface is disturbed and needs to be reorganised, which is reflected in the variation of the potential at the relaxation process At the same time, the different ions inside the coating will leave it, producing charge equilibration and reorganisation of the polymeric molecule dipoles, also producing a variation in the potential Thus the system is degraded by the loss of adhesion (formation of H2), the pore opening by the incoming of the different species from the electrolyte, and the formation of corrosion products by electrochemical processes These processes can be followed by the results obtained in the relaxation process The ACET procedure measures the quality of the coating and its adhesion through the study of the resistance that the system offers to its degradation by the cathodic polarisations In this study the cathodic polarisation was carried out for 20 minutes at a constant voltage of -4 V Following that, the relaxation time was for 3 hours, and finally an EIS was applied under the stated conditions The test sequence was repeated six times (around 24 hours of testing) and was fully automated in Zahner equipment Results evaluated in terms of cathodic reactions The ACET test was carried out on aeronautical epoxypolyurethane systems Figure 3 shows the Bode plots of these coatings, EP-PU1 (left) and EP-PU2 (right) The worst behaviour is seen on the right-hand side This is characterised by a strong change in the impedance value at low frequencies during the cycles of the ACET technique Systems with better responses are shown on the left, where it can be seen that the impedance variation is smaller than in the plot on the other side In order to understand these figures, it is necessary to look at the theoretical basis underlying this ACET method The cathodic polarisation applied to the coated metal can cause two processes to occur in the paint Firstly, the introduction and passage of different cations (H +, Na+, and so on) from the electrolyte through the paint due to the negative potential imposed in the metallic substrate This can produce a concentration of positive charges in the coating that must be neutralised by a balancing entry of anions (like Cl-) The passage of ions (which can also be hydrated) through the coating can cause its deterioration and the formation of pores Secondly, the cathodic reaction that can take place in the metallic surface depending on the level of negative polarisation and the type of electrolyte [27] is shown in this Equation: 2H2 O (l) + 2e-? H2 (g) + 2 OH (1)The cathodic reaction will take place first if the electrolyte is able to pass through the coating and reaches the interface This depends on the properties of the film (permeability to ions, adhesion to substrate, existence of local film delamination, susceptibility of the coating to form cracks because of its high rigidity, etc) and, of course, the applied cathodic voltage Obviously, the higher the quality of the primer (low permeability and high ductility) the lower will be the probability of the electrolyte reaching the interface, and of the cathodic reaction taking place The deterioration of the coating due to cathodic polarisation can be caused primarily by the film delamination process at the metallic interface produced by the cathodic reaction (evolution of H2), although the passage of ions can also exert a degrading effect Relaxation potential reveals details of film breakdown If it is possible to detect whether the cathodic reactions have taken place during polarisation, this information could be used to learn a little more about the performance and quality of the paint One possible way of detecting the existence of H2 (g) and OH- production (leading to more delamination) at the interface is to study the evolution of the open circuit potential after polarisation (during the relaxation time) When cathodic polarisation is stopped the coated metal potential will relax showing two possible spectra In the first case, if cathodic reactions were taking place (H2 production), the potential would have a quick relaxation, normally around -1 V [27] (with small variations depending on the coating), which corresponds to the termination of the reaction and, afterwards, a second relaxation that corresponds to ions and electrolyte leaving the coating and possibly the formation of a new double layer in the metallic surface In any case, the cathodic reaction will produce the entry of electrolyte through the coating and the production of H2 (g) and OH- at the metal/coating interface The time needed for this electrolyte and the ions to leave the film will therefore be higher because they have to pass through the entire primer film Alternatively, if no cathodic reactions have taken place, there would be a single relaxation process that corresponds to ions and electrolyte leaving the primer or to the reconfiguration of the polymer dipoles This relaxation will take place over longer times as ions and electrolyte penetrate deeper into the film, but they will probably need less time than in the first case described abovefigure 4 shows the potential relaxation versus time of the different epoxy-polyurethane systems after five cathodic polarisations The sample EP-PU1 with the best behaviour shows only one relaxation at high potential which indicates that during the cathodic polarisation there was no hydrogen production because of its good anticorrosive properties (low permeability and high adhesion)the sample EP-PU2 shows a relaxation at -1 V that corresponds to the ending of hydrogen production, because the cathodic reaction of electrolysis could take place due to the presence of water in the interface as a result of the poor quality of the coating Probably if longer relaxation times were used, new relaxation processes could be observedthe relaxation time will give an idea of the evolution with time of the system after the applied stress until it again reaches the steady state (graphics Erelaxation = f(time)) Figure 5 shows salt fog spray test results The system EP-PU1 (left) achieved 3000 h of exposure while EP-PU2 (right) was damaged by blistering Thus, it is clear that a correlation exists in this case between the established salt fog test and the much faster ACET procedure REFERENCES [1] Mansfeld F, Jnl Applied Electrochem, 1995, Vol 25, pp 187ff [2] Bierwagen G P, Jnl Coat Tech, 1992, Vol 64, pp 71ff [3] Liu B et al, Acta Physico-Chimica Sinica, 2001, Vol 17, pp 241ff[4] Skerry B S, Chen C-T, Ray CJ, Jnl Coat Tech, 1992, Vol 64, pp 77ff[5] Gwori S, Balakrishnan K, Prog Org Coat, 1994, Vol 23, pp 363ff

3 3 [6] Selvaraj M, Guruviah S, Prog Org Coat, 1996, Vol 28, pp 271 ff [7] Hernández L S, del Amo B, Romagnoli R, Anti-Corrosion Methods and Materials, 1999, Vol 46, pp 198ff [8] Liu X et al, Materials and Corrosion, 1995, Vol 46, pp 33ff [9] Hollaender J, Ludwig E, Hillebrand S, Proc 5th International Tinplate Conference, London, 1992, pp 300ff [10] Hollaender J, Schiller C A, Strunz W, Food additives and contaminants, 1999, Vol 14, No 6-7, pp 617ff [11] Hollaender J, Schiller CA, Strunz W, Proc EIS 2001, Marilleva-Italy, 2001 [12] Rodriguez MT et al, Prog Org Coat, 2004, Vol 50, pp 68ff [13] Suay J J et al, Prog Org Coat, 2003, Vol 46, pp 121ff [14] Rodriguez MT et al, Prog Org Coat, 2004, Vol 50, pp 123ff [15] García S J, Suay J, Prog Org Coat, XX (2009), Prog Org Coat 66 (2009), p 306[16] García S J, Suay J, Prog Org Coat, 2007, Vol 59, pp [17] García J et al, Prog Org Coat, 2007, Vol 60, pp [18] García S J, Suay J, Prog Org Coat, 2006, Vol 57, pp [19] Rodriguez M T et al, Prog Org Coat, 2004, Vol 50, pp [20] Lee S S et al, Prog Org Coat, 1999, Vol 36, pp 79ff [21] Mansfeld F, Electrochim Acta, 1993, Vol 38, No 14, pp [22] Amirudin A, Thierry D, Prog Org Coat, 1995, Vol 26, pp 1-28 [23] Walter, G W Jnl Electroanal Chem, 1981, Vol 118, pp [24] Walter, G W,Corros Sci, 1986, Vol 26, No 9, pp [25] Gamry Instruments, "Electrochemical Impedance Spectroscopy Primer", wwwgamrycom[26] Tang, N, Ooij W J, Górecki G, Prog Org Coat, 1997, Vol 30, pp 255ff[27] Leidheiser H, Jnl Adhesion Sci Tech, 1987, Vol 1, pp 79ff Results at a glance Although the neutral salt spray test has been established for many year in evaluating anticorrosive behaviour, it gives no information on the underlying corrosion processes, and testing times for high performance coatings can be quite long An alternative process known as ACET (Accelerated Cyclic Electrochemical Technique) has been developed, which produces very rapid breakdown of coatings under cathodic polarisation and also provides some information on different modes of coatings failure A comparison between two polyurethane coating systems showed a good correlation between salt spray and ACET test cycles The time required for the evaluation was 3000 hours in the salt spray but less than 24 hours in the ACET test In the cases studied, poor salt spray behaviour is correlated with a strong change in the impedance value at low frequencies during the ACET test cycles * Corresponding author: José Javier GraceneaMediciones y Corrosión SL josejgracenea@medcoes

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