The Effect of the Dry Film Thickness, Temperature, and Electrolyte on the Cathodic Current of Painted Steel

Abstract

The properties of cathodically polarized painted steel specimens have been examined by determining the change in the net cathodic current and the paint film resistivity as a function of temperature, dry film thickness, and electrolyte. The net (external) cathodic current is found to increase strongly with increasing temperature. This is most probably due to a decrease in resistivity. The activation energy of charge transport through the paint films has been determined and was found to vary between 51 and 227 k J/tool. The resistivity of paint films seems to increase with increasing thickness. The change in resistivity was found to take place mainly below a "critical thickness" of about 100 txm. For short times of exposure, i.e., 10-20 days, the net cathodic current and the paint film resistance are found to be independent of the type of electrolyte for paint films with a dry film thickness above or equal to 100 ~m.The presence of heterogeneities in applied coatings seems to be a general feature of paint films and a likely cause of deterioration of the coating. Local initiation of corrosion is usual. Holidays ~ may be present initially or formed during exposure. Mayne and Mills (1, 2) have concluded that the current between anodic and cathodic areas is limited by the paint film resistance. They also proposed that the rate of corrosion is equal to this galvanic current unless the paint film can limit the rate of corrosion by anodic passivation or cathodic protection (1). However, it has also been proposed that the rate of corrosion is controlled by the polarization resistance rather than by the ionic resistance of the paint film (3, 4).We found, by studying cathodically polarized painted steel specimens exposed to artificial air-saturated sea water, that coated areas usually will act as cathodes vs. areas of exposed steel ("holidays") (5). This is in accordance with results obtained by Craig and Olson (6) and Koehler (7). It was also found by recording polarization curves (5) that the cathodic current most probably is limited entirely by the paint film resistance. For the paints studied, the change in the cathodic current as a function of time and paint composition was found mainly to be due to changes in the paint film resistance. In this article, results from a study of the change in the cathodic current and paint film resistivity with change in dry film thickness, temperature, and electrolyte will be presented. ExperimentalThe steel specimens were prepared from St. 37-2 DIN 17100 steel. The surface was ground to an average roughness of 0.5 t~m, degreased with calcium hydroxide powder, and rinsed first with water and then with ethanol. The paints were applied in two coats to the desired thickness. The first coat was dried for at least 24h at room temperature before application of the second coat.After applying the final coat, the painted specimens were dried for a period of 20 to 60 days at ambient temperature. The dry film thickness was measured with an E1-cometer thickness tester and was found to vary less than...

Effect of Osmotic Pressure by Salt Concentration on Corrosion Resistance of Anti-Corrosive Paint

“…For example, bridges on land and offshore as well as ship and pier cranes, etc. have been mainly protected by painting method, conversely, a cathodic protection method has been widely applied to underwater piles of wharfs [1][2][3][4][5][6][7][8]. Protection cost of the surface coating method accounts for 63% among all protection costs, which was known by japan research report [9].…”

Electrochemical Evaluation on Corrosion Resistance of Anti-corrosive Paints

“…current or sacrificial anode method. [1][2][3][4][5][6][7][8] And by a certain report of Japan, it is reported that cost of coating protection is of more than 63% among total costs for numerous protection methods. [9] It is supposed that in Korea, cost of coating protection would amount to be a higher percentage than that of Japan because for corrosion resistance.…”

Προστασια Χαλυβα Απο Διαβρωση

“…Οι Steinsmo et al(540) μελέτησαν την επίδραση της θερμοκρασίας, του ηλεκτρολύτη, του πάχους της επικάλυψης και του ρεύματος καθοδικής προστασίας σε χαλύβδινες επικα λύψεις προστατευμένες με διάφορα αντιδιαβρωτικά χρώματα (κυρίως εποξυδικά). Αυτοί βρή καν, ότι, όταν η θερμοκρασία αυξάνεται, αυξάνεται πάρα πολύ και το εξωτερικά εφαρμο ζόμενο ρεύμα καθοδικής προστασίας.…”