Technical Paper Corrosion testing
Monitoring rust resistance Eliminating ‘edge effects’ improves accuracy of EIS procedures * Corresponding author: Dr. Adel Husain Kuwait Institute for Scientific Research T +965 24989100
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Adel Husain* Essam Hussain Abdulaaziz Al-Mubarak Electrochemical Impedance Spectroscopy (EIS) is used to measure deterioration in a coating exposed to corrosive conditions. However, the standard test procedures are subjected to errors induced mainly by edge effects. A new test cell avoids this and allows EIS tests to be carried out non-destructively on the actual test panels used in salt spray and accelerated weathering tests.
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“Modern Aspects of Electrochemistry No 35” Antonio Doménech Carbó www.crcpress.com
his paper describes the application of a newly developed test cell for conducting Electrochemical Impedance Spectroscopy (EIS) measurements. It uses graphite as a counter electrode and a fabric layer as a medium to contain and transport the electrolyte solution via capillary action. This modified test compartment avoids the intervention of edge effects and the formation of crevice attack normally faced while using a conventional EIS sample holder and masking material or adhesive tape. In order to assess its viability, a comprehensive set of EIS measurements has been carried out that evaluated the protective properties of multilayer systems of singlecomponent moisture cured polyurethane coatings. The procedure provides an easy test for coated metals and thin foil panels in conjunction with a laboratory cabinet corrosion test or atmospheric field trial test. It also offers a simple way to test the exposed area of almost any coated panel surface of sizes up to 10 or 7.5 x 15 cm rather than a smaller representative area of the conventional test specimens of fixed diameter. The overall results of the EIS spectrum test provided a quick and reliable procedure with repeated measurement for accelerated screening of industrial coated metallic substrates.
Drawbacks of the standard EIS test protocol EIS has proved to be a very useful tool for evaluating many issues, such as coatings degradation and water intake, screening, research etc. The study of electrode interfaces by means of an impedance technique has been the subject of considerable research during the past 20 years, particularly when considering the adverse effect of specimen shape and test cell arrangement on the response of an AC impedance frequency spectrum. In this work, EIS has been used to evaluate four different coating systems, each comprising three coats of singlecomponent moisture cured urethanes. Generally, thin sheet specimens cannot always be mounted properly into the conventional testing cell. [1, 2] This is because the edge effect can dominate the impedance output; in addition, the nature of thin sheet specimens may make their mounting in a conventional flat test cell rather painstaking. Because of the above points, when the thin sheets are dipped into electrolyte, the electrochemical signal that predominates is that with a higher tendency to corrosion, generated from the edges of the specimen. Which may introduce experimental errors and an overestimation of the EIS spectrum results. This can be attributed to two main factors: »»For “Q-panel” specimen sheets, often the cut edges are not effectively coated or may be coated with a paint system having a different surface viscosity that may be thinner at the edges. »»Secondly, any metallic sample or foil with a thin layer coating has a greater tendency to corrode at the edges than in the body of the matrix. Most importantly, high coating impedance signals interact with low impedance signals from the edges. As the coating impedance is very high, the low impedance character of the specimen edges significantly influences the measured impedance values. Therefore, it becomes essential to avoid this edge effect in order to obtain a true representation of the EIS spectrum from the coated specimen.
How the standard EIS test was modified
Figure 1: Two conventional EIS test cells with sealed specimen holder
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Generally, with a conventional EIS test cell a sealed specimen holder is used for thin sheet samples as shown in Figure 1. The test cell arrangement of Figure 1 avoids the edge effect but gives rise to the formation of a new problem of crevices forming near the edge of the sample holder or gasket. The same problem sometimes appears when the edges of sheets are masked with some other types of non-conducting material. Therefore, adjustment of the test specimen area of a conventional cell is a criti-
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Technical Paper Corrosion testing cal experimental parameter and a time-consuming and painstaking process. EIS measurements on coatings should use as large an area as possible. Increasing the area as in the case of the new test design has several beneficial effects. The capacitance of a paint film is directly proportional to the sample area. If one square centimetre of a paint has a non-measureable capacitance (for example, ten pF), 100 cm2 of the same film will have a capacitance of one nF (easily measureable). The main objective of the present work was to develop a testing cell that can accommodate the actual corrosion coupons or a painted “Q panel” of a larger size, 150 x 75 mm, which represents an actual sample size that is normally supplied and used by many specification and test standards for evaluating marine paints or for atmospheric exposure. Emphasis has also been placed on finding a non-destructive test (NDT) sample holder that can provide real-time repetitive measurements using the above-mentioned environments in the newly proposed design of EIS testing cell. Hollaender et al [3] introduced a new EIS cell to facilitate measurement of EIS on lids of polymer coated metal such as aluminium and other packaging material for plastic cups.
Figure 2: Photograph a) and schematic drawing b) of the new EIS test cell compartment
Description of the new EIS test cell The proposed setup of the newly designed test cell is as shown in Figure 2a along with a schematic drawing in Figure 2b. The EIS cell consists of a rectangular mounting stand made up of “Perspex” clear plastic sheet. The metallic or thin flat sheet specimen to be tested is mounted
Results at a glance Electrochemical Impedance Spectroscopy (EIS) provides a valuable means of measuring deterioration in a coating exposed to corrosive conditions. However, the standard test procedures use only a small area of coating and are subject to errors introduced mainly by edge effects. A new test cell has been designed which will carry out measurements on standard paint test panels of about 75 x 150 mm size, and is simple to set up. This allows EIS tests to be carried out nondestructively on the full area of actual test panels that are used in laboratory salt spray and accelerated weathering tests. Four different multi-coat polyurethane systems were evaluated in this way. The EIS test results showed good agreement and furnished good quantitative support when combined with the laboratory accelerated corrosion test (ASTM B117) and “QUV” exposure for all specimens.
on the flat stand, where the testing side or working electrode surface faces upwards. A clean multilayer medical cotton cloth is laid down and stretched over the specimen surface. A graphite plate of the same size as that of the coated specimen is placed on the cloth, to act as a counter electrode (C. E.). Electrical contact between the working electrode and the graphite plate is made with the help of screwed nuts and wires that end with crocodile clips. One end of the wet cloth is immersed in a beaker filled with the electrolyte solution of 5 % NaCl in de-ionised water. The other end of the cloth rests free in an empty beaker. The overall test arrangement is based on a continuous flow of electrolytic solution to be established from one beaker to another by capillary action through the cotton cloth and passing over the specimen while wetting the surface. A small hole is made on the upper cover of the plastic plate to accommodate the reference electrode with an Ag/AgCl tip, which is to be placed in contact with the wetted cloth, or it can be immersed in the beaker of the electrolyte solution.
Paint materials and test methods Single-pack moisture cure urethane coatings used for this study were coated and supplied by the manufacturer. Spray coated “Q-panels” of steel with four different types of protective coating systems were supplied. The application of the coating systems was carried out by the paint supplier and in accordance with the paint application guidelines specified for each type of coat. The coatings systems were applied on steel panels (75 x 150 mm) on a single face, and the other face was coated with a chemically resistant epoxy coating.
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Technical Paper Corrosion testing Table 1: Composition of coating systems System 1 (W1)
Colour Grey
2 (W2)
White
3 (W3)
Blue
4 (W4)
Black
Primer MIO+Zn Moisture cured urethane primer MIO+Zn Moisture cured urethane primer
Intermediate coat 75 µm 75 µm
EIS test results summarised
Topcoat 75 µm
225 µm
75 µm
75 µm
75 µm
225 µm
75 µm
75 µm
75 µm
225 µm
75 µm
125 µm
150 µm
350 µm
Panels were evaluated for corrosion, blistering, rusting, gloss retention and chalking, according to ASTM standards, after exposure to accelerated condition in a cyclic weathering test cabinet for up to 1000 hours. The same coated steel specimens were tested for capacitance and impedance properties during cabinet exposure with an AC impedance technique at sequential measurement periods (after 0, 250, 500 and 1000 h). The duration of the laboratory screening of coating and corrosion test is thus within 2-3 months. Table 1 illustrates the composition of the urethane coating systems that consist of a triple layer, i.e. primer, intermediate and topcoat.
A Solartron instruments’ “model 1287” potentiostat/ galvanostat was used with the “FRA 1260” frequency response analyser for impedance measurements. The measurement is made over a frequency domain ranging from 100 kHz to 10 mHz. A potential amplitude of 10 mV with sinusoidal voltage was applied at the open circuit potential. The experimental results were fitted and analysed with the “Zplot” and “Corr-ware” software of the instrument. The impedance test was carried out in conjunction with the salt spray cycle and QUV according to ASTM B-117 for 1000 h. Results of the AC impedance testing for four coating systems are presented in Figures 3 and 4. The Nyquist, Bode and theta plots, for all coatings in the as received condition (i.e. before exposure to any corrosive environment) showed very high impedance values in the order of hundreds of gigaohm/cm2. The results for coating W3 discussed in detail sets a good example for the visibility of the test cell and explains the gradual effect of exposure to corrosive conditions on the EIS spectrum response of coated panels. The same figures show the Nyquist plot for coating W3 at various exposure conditions. The figures clearly show, as anticipated, that the impedance of the coating was reduced after salt spray exposure. This reduction of coating resistance is usually found when penetrating moisture bearing chloride ions diffuses into and changes the dielectric properties of a coating. In spite of this, the resistance of some coatings is adequately high. Quite unexpectedly, the impedance of some coatings significantly increase after 500 hours of UV exposure. This can perhaps be attributed to a gradual curing effect of the urethane coating over time, due to the UV light energy.
Effects of test exposure cycles summarised
Figure 3: Nyquist, Bode and theta plots obtained by the newly developed EIS test cell for the urethane coatings: (left diagrams) all coating and (right diagrams) for coating W3
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The cumulative effect of a cyclic humidity cabinet test and UV degradation test using the newly developed EIS test cell should permit the ranking of a variety of materials by constituents, characteristic and application in a quick test. However, for better evaluation of corrosion protection properties , impedance data were analysed in depth using “Zplot” software based on equivalent circuit. Figure 4 shows the Nyquist, Bode and phase angle plots respectively, for the four coatings after 1000 hours of salt spray exposure (to ASTM B117). All the coatings showed typical EIS capacitive and active spectrum behaviour, which can be attributed to ingress of moisture into the weaker coatings which in turn reflects changes in the dielectric properties of the polymeric material. Generally, after 1000 hours of salt spray exposure, both W1 and W4 showed lower impedance values than W2 and W3. Also, W1 and W4 showed signs of a typical diffusion tail reflected in the Nyquist plot as a straight line inclined at 45 degrees at the lower frequency domain. This is observed after forming a complete semicircle i.e. presenting diffusion characteristics of the mass transport phenomenon and pores with corrosion product reaction mechanism.
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Technical Paper Corrosion testing It can be seen from the above figures and tables that EIS measurements of these single pack urethane coating systems, using the new cell arrangement have provided quite sensible, quantitative and useful results along with the laboratory weathering test. It can be concluded that among the four coating systems, W3 is the best. The high values of coating resistance provide evidence of good corrosion resistance and give supportive predictions about protective performance for evaluating coatings produced by the same manufacturer during laboratory accelerated corrosion salt cabinet tests. These values of coating resistance are indicative of strong coating which provide excellent barrier properties to protect the steel substrate from any atmospheric or marine corrosive environment.
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REFERENCES [1] Application Note, Evaluation of organic coatings by electrochemical impedance measurements, AC-2, Electrochemical Instruments Group, EG&G Princeton Applied Research, Princeton, New Jersey, 1984. [2] Application Note, Flex cell critical pitting test cell kit, operator’s manual, Gamry Instrument, USA, 1999. [3] Hollaender J., Ludwig E., Hillebrand S., Assessing protective layers on metal packaging material by electrochemical impedance
Figure 4: EIS plots, coating resistance and capacitance values obtained with the new EIS test cell
spectroscopy, Proceedings 5th International Tinplate Conference, London, 1992, pp. 300-315.
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