Electrochemical Studies of Vinyl Ester Coatings for Fuel Tanks

Chapter 23

Electrochemical Studies of Vinyl Ester Coatings for Fuel Tanks

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V. N. Balbyshev , Gordon P. Bierwagen , and R. L. Berg

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Downloaded by CORNELL UNIV on July 2, 2012 | http://pubs.acs.org Publication Date: March 30, 1998 | doi: 10.1021/bk-1998-0689.ch023

Department of Polymers and Coatings, North Dakota State University, Fargo, ND 58105 Xerxes Corporation, 7901 Xerxes Avenue, Minneapolis, MN 55431 2

Several types of vinyl ester coatings, known for their outstanding corrosion resistance properties, have been subjected to immersion and evaluated for corrosion resistance by electrochemical noise methods (ENM) and electrochemical impedance spectroscopy (EIS). The test solution was chosen to emulate the water bottoms found in underground fuel storage tanks. Electrochemical noise Impedance, calculated from electrochemical noise data, has been used to supplement the ENM and EIS techniques and for a better understanding of low-frequency behavior of these coating systems. The data forfirstthree months of exposure shows good agreement between the ENM test results and electrochemical noise impedance. The DC resistance and EIS values have been found to deviate from equivalent parameters obtainedfromthe above two techniques. The results from four different electrochemical methods are interpreted, and the correlation among them is discussed. Corrosion in fuel tanks of crude oil and refined products is closely associated with the presence of water. Petroleum and refined products themselves do not enter in the corrosion reaction but may pick up water and other corrodents such as oxygen, chlorides, sulfates, and others. Therefore, reducing the availability of one or several of these species would also tend to reduce corrosion (7). The family of vinyl ester resins is earning increasing commercial use in the fabrication of industrial equipment. Vinyl esters provide a number of advantages over those made with conventional metal and polyester materials. Their outstanding resistance to corrosion by many different chemicals makes them well suited for industrial corrosion resistant applications. Chemical attack of polyester resins occurs through hydrolysis of the ester groups or the splitting of unreacted carbon-to-carbon double bonds through oxidation

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©1998 American Chemical Society

In Organic Coatings for Corrosion Control; Bierwagen, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.


293 or halogenation. In vinyl ester resins, the double bonds are at the ends of the molecular chain (unlike the conventional polyesters where the double bonds occur throughout the molecular chain). These react completely on polymerization, giving a more chemically resistant structure. Also because the unsaturation in vinyl ester resins is terminal, the double bonds are extremely active, and as a result, the resins cure rapidly and consistently. Cured vinyl ester resins contain only terminal cross-linking, and the entire length of the molecular chain is available to elongate under stress and thus absorb mechanical or thermal shocks. We investigated several type of vinyl ester coatings using electrochemical methods (2,3,4) such as electrochemical noise measurement (ENM) (5) and electrochemical impedance spectroscopy (EIS) (6,7). Both methods were used to evaluate protective properties of the coatings. The test electrolyte solution we used for this electrochemical testing was rather unique. We designed an immersion electrolyte based on the informationfromgasoline suppliers and reference literature as a worst-case scenario for water condensate in the bottoms of tanks storing ethanol/methanol modified gasolines (1,8,9). This is described in detail in the experimental section. The electrochemical noise method is a powerful technique for assessing corrosion resistance of steel alloys, and possibly for determining mechanisms of corrosion. One of the major advantages of the ENM is that it is non-intrusive, i.e. it does not perturb in any way the system being investigated. The EIS is somewhat intrusive: a small amplitude sinusoidal signal is applied to the system. A resistance noise parameter, R obtained from the ENM is used to give rapid ranking of the coating performance (10,11). The R„ value can be correlated with the low-frequency limit impedance modulus obtained directly from the EIS measurements. However high impedance coatings such as the vinyl esters investigated in this study show a lot of scatter in the low-frequency region of the EIS spectrum. Using FFT and Maximum Entropy Spectral Analysis (MESA) algorithms, it is possible to derive the power spectral density functions (PSD) of current and potential noisefromthe ENM time series and calculated a noise impedance value, Z„. Using this approach it is possible to eliminate the low-frequency scatter and get a better extrapolation of the low-frequency impedance modulus (12,13). nj

Experimental The individual specimen preparation for the Electrochemical Noise (ENM) studies was identical to the one for the Electrochemical Impedance Spectroscopy (EIS) tests. All measurement were made at room temperature. Cell Preparation. The 6"x 6" coated carbon steel panels were supplied by Xerxes Corporation (Minneapolis, MN). The panels had been coated with the following vinyl ester resins: 470 (470-2000) - one, two, and three coat systems; 8084 with 470, and Dion 367. Very little technical information could be obtained about the resins. For ENM measurements each of the original panels was cut half to produce two 3" χ 6" specimens. Thefilmthickness was measured on each specimen with Elcometer® 345 Digital Coating Thickness Gauge (Table I).


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Table L Film Thickness of Investigated Systems Coating System Film Thickness frm) 200 (±30 μηι) 470-2000, 1 Coat 470-2000, 2 Coat 400 (±30 μηι) 600 (±50 μηι) 470-2000, 3 Coat 300 (±40 μηι) 8084 with 470 75 (±10μιη) Dion 367 'averaged over several measurements

A piece of 1-1/2" PVC pipe was affixed to each panel with Marine Goop™ adhesive to form a tested area of approximately 11 cm . The position of the PVC pipe on the panel was chosen such as to achieve similar film thickness for the two specimens comprising a pair for the ENM experiment. Thefilmthickness variation between the two panels in an ENM pair was 50 μπι on the average. The test solution, duplicating the aqueous bottoms found in tanks, was prepared based on the information found in the literature and obtainedfromthe major gasoline suppliers (8,9). It contained 2% NaCl, 2% Na C0 , and 1.5% (NH^SO^


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Electrochemical Tests. The ENM tests were performed on a semi-continuous basis using a three-electrode setup with two 3"x 6" coated carbon steel panels (working electrodes) and an SCE reference electrode (Figure 1). The two working electrodes were electrically connected via the agar/KCl salt bridge. The potential noise was measured between the pair of nominally identical working electrodes and the reference electrode. The current noise was monitored between the two working electrodes. The measurements were made using Gamry Instruments potentiostat and CMS 100/120 software package. The current and potential noise were sampled at 2 second intervals for 512 seconds producing 256 raw data points per cell. From each individual record of 256 points, a potential and current noise standard deviation were calculated. The resistance noise value, R„, was derived using Ohm's law as the ratio of the potential to current noise standard deviation (14):

The R„ value is an important ENM parameter that allows one to rapidly rank the system's protective properties. Data was typically acquired over 12 hours to produce one R value per hour per cell. The ENM experiment was repeated every few days. The EIS measurements were made on each working electrode separately using an SCE reference electrode and a Pt counter electrode (Figure 2). The EIS spectra were obtained in the regionfrom65 kHz to 100 mHz (10 mV applied AC potential) using Gamry Instruments potentiostat, Schlumberger 1250 Frequency Response Analyzer (FRA), and CMS 100/300 software from Gamry Instruments. The lowfrequency impedance modulus values, obtainedfromthe two working electrodes being part of an ENM pair, were averaged using the geometric mean to obtain a single n



Figure 1. ENM Cell Setup.

Figure 2. EIS Cell Setup.


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value, Hm|Z(/)|, that could be compared to the R value for the particular system n

(75):

ΙΜΖΟΟΙ^ΙΖ,Ι-ΙΖ,Ι ι

/->0

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ι

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where |Z/| and \Z \ are low-frequency limit impedance moduli for individual specimens. For noise impedance (Z ) calculation, the DC and drift components of the signal was subtracted before performing Fourier transform and MESA. The noise impedance spectrum was obtained as the ratio of the power spectrum density functions of potential and current (16): ^PSD (f) \Z (f)\ = PSD,(f) 2

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The low-frequency limit of noise impedance, Z„(0), was compared to the values of R and low-frequency limit of impedance modulus, Hm|Z(/)| obtainedfromdirect EIS n

measurement. All calculations were performed using Mathcad™ PLUS 6.0. The DC resistance measurements were made after each ENM run, not before, to ensure that the applied during DC resistance measurement potential does not perturb the system by polarizing the electrodes. All the specimens were visually assessed at the end of each ENM test.

Results and Discussion The composition of the test electrolyte solution was one of the most important modifications of our standard test protocol that we introduced into this electrochemical study of underground storage tank coatings. Most tests of this kind utilize the 3% NaCl solution which emulates the sea water environment. However, as it has been shown in the literature (1,8,9) the content of other salts in the water bottoms of fuel tanks is rather significant and comparable to that of sea water (Table Π).

Table Π. Corrosion Products Found in Fuel Tanks Product Iron, total Sulfur, total Sulfide Sulfate Chloride Carbonate Carbon, total

%wt. 39.0 0.9

Electrochemical Studies of Vinyl Ester Coatings for Fuel Tanks

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