European Coating Journal

84 European Coatings JOURNAL 03 l 2011 www.european-coatings.com

Technical PaperAnticorrosive test

José Javier Gracenea* Maria José GimenoJulio José Suay

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.

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 cor-

rosion and degradation processes in organic-metal sys-tems, the main routines for assessing interactions and materials have been developed experimentally by us-ing different exposure processes and techniques for the measurement of properties.The main types of exposure process are accelerated ageing tests (e.g. 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. Neverthe-less, 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 evalu-ate the protective performance of organic coating/metal systems. The application of electrochemical impedance spectroscopy (EIS) to coated metals has proved to be a

The fast lane to failureCyclic impedance test gives rapid characterisation of coating breakdown

useful technique here [1-8], although long periods are needed (days, weeks and sometimes months) to per-form 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 sub-jected 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 spec-trum can be attributed to coating deterioration (pore for-mation) and a delamination process in the metallic sur-face due to hydrogen and OH- production (if a cathodic reaction takes place).The new ACET (Accelerated Cyclic Electrochemical Tech-nique) 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 de-scribed 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 proceduresTwo epoxy-polyurethane two-coat systems were chosen for the comparative tests. Both systems were solvent-based, 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 50-60 µm.The accelerated salt fog spray test was performed in accordance with ASTM B 117-85/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; blister-ing, corrosion and delamination were measured after

* Corresponding author:José Javier GraceneaMediciones y Corrosión SL+ 34 964 [email protected]

Figure 1: Representation of the ACET test schedule

“Anticorrosive Coatings”

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85www.european-coatings.com 03 l 2011 European Coatings JOURNAL

Results at a glance

Although the neutral salt spray test has been established for many year in evalu-ating anticorrosive behaviour, it gives no information on the underlying corrosion processes, and testing times for high per-formance coatings can be quite long.

An alternative process known as ACET (Accelerated Cyclic Electrochemical Tech-nique) 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 be-haviour is correlated with a strong change in the impedance value at low frequencies during the ACET test cycles.

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 applica-tion 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 imped-ance of the system (using Bode plots and modelled char-acteristic 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 -1.0 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 pro-duction has taken place, the normal electro-chemical corrosion of the system occurs in the

presence of the electrolyte, with production of iron ox-ides 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, produc-ing charge equilibration and reorganisation of the poly-meric 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 forma-tion of corrosion products by electrochemical processes. These processes can be followed by the results obtained in the relaxation process.

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The ACET procedure measures the quality of the coat-ing 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 epoxy-polyurethane 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 varia-tion 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 meth-od. 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 me-tallic substrate. This can produce a concentration of posi-tive 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 po-larisation 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 elec-trolyte 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 per-meability and high ductility) the lower will be the prob-ability 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 de-grading 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 perform-ance and quality of the paint. One possible way of de-tecting the existence of H2 (g) and OH- production (lead-ing to more delamination) at the interface is to study the evolution of the open circuit potential after polarisation

Figure 3: Bode plots for aeronauti-cal epoxy-poly-urethane systems, EP-PU1 (left) and EP-PU2 (right); paramters: ACET: -4 V, 20 min, 25 min EIS, 3 h relax

Figure 2: Processes occurring during the ACET test schedule

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Figure 4: Relaxation plots for aeronauti-cal epoxy-poly-urethane systems EP-PU1 (left) and EP-PU2 (right); pa-rameter: ACET: -4 V, 20 min, 25 min EIS, 3 h relax

Figure 5: Neutral salt spray test results showing good performance of EP-PU1 (left) and severe blistering of EP-PU2 system (right) after 3000 hours

(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 relax-ation, normally around -1 V [27] (with small variations depending on the coating), which corresponds to the termination of the reaction and, afterwards, a second re-laxation 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 corre-sponds to ions and electrolyte leaving the primer or to the reconfiguration of the polymer dipoles. This relaxa-tion will take place over longer times as ions and electro-lyte penetrate deeper into the film, but they will probably need less time than in the first case described above.Figure 4 shows the potential relaxation versus time of the different epoxy-polyurethane systems after five cathodic polarisations. The sample EP-PU1 with the best behav-iour shows only one relaxation at high potential which indicates that during the cathodic polarisation there was no hydrogen production because of its good anticorro-sive properties (low permeability and high adhesion).The sample EP-PU2 shows a relaxation at -1 V that cor-responds 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 relaxa-tion times were used, new relaxation processes could be observed.The 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 C.J., Jnl. Coat. Tech., 1992, Vol 64, pp 77ff.[5] Gwori S., Balakrishnan K., Prog. Org. Coat., 1994, Vol 23, pp 363ff.[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 contami-nants, 1999, Vol 14, No. 6-7, pp 617ff.[11] Hollaender J., Schiller C.A., Strunz W., Proc. EIS 2001, Marilleva-Italy, 2001.[12] Rodriguez M.T. 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 M.T. 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 251-258.[17] García J. et al, Prog. Org. Coat., 2007, Vol 60, pp 303-311.[18] García S. J., Suay J., Prog. Org. Coat., 2006, Vol 57, pp 273–281.[19] Rodriguez M. T. et al, Prog. Org. Coat., 2004, Vol 50, pp 123-131.[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 1891-1897.[22] Amirudin A., Thierry D., Prog. Org. Coat., 1995, Vol 26., pp. 1-28.[23] Walter, G. W. J.nl. Electroanal. Chem., 1981, Vol 118, pp 259-273.[24] Walter, G. W.,Corros. Sci., 1986, Vol 26, No 9, pp 681-703.[25] Gamry Instruments, “Electrochemical Impedance Spectroscopy Primer”, www.gamry.com.[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.

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