Corrosion Control by Organic Coatings

Abstract

The electrochemical behavior of a carbon steel in 3% NaC1 solution has been investigated using a rotating disk electrode. Both steady-state (diffusional current vs. the disk angular velocity plots) and transient (frequency analysis of the electrohydrodynamical impedance) measurements which specifically sample mass transport phenomena, have been carried out. It is shown that oxygen transport takes place not only in the liquid phase but also through a porous layer of corrosion products. From electrochemical impedance measurements, it was found that at the corrosion potential the oxygen reduction reaction is under either diffusional or mixed (activation + diffusion) control depending on both the electrode rotation speed and on the hold time at the free corrosion potential. In addition, it was shown that the oxygen consumption occurs not only by electrochemical reduction but also by chemical oxidation of ferrous to ferric ions. Finally, because of the possible occurrence of mixed corrosion control, it is emphasized that the Use of the polarization resistance in order to evaluate corrosion rates is not always valid. In a previous study (1), steady-state electrochemi-cal techniques (voltage-current curves and polarization resistance measurements) were used to study the corrosion of carbon steel in a stirred and aerated chloride solution (3% NaCI). It was concluded that the corrosion rate is controlled by the reduction of dissolved oxygen, and that the corrosion rate deduced from electrochemical techniques is in fair agreement with that obtained from an absolute measurement such as the determination of the amount of iron species in solution. In the literature, many stud/es have been devoted to corrosion (and inhibition) in acidic media in order to measure the corrosion rate or to identify the elementary processes. However, relatively few studies have been carried out in neutral media (2), probably because of the formation of insoluble corrosion products which adhere to the metal surface (3a, b). In this work, we aimed firstly to be more precise about the quantitative influence of mass transport in the corrosion process. This was achieved by the use of specific analytical methods, such as plotting of the diffusional component vs. the angular velocity Of a disk electrode, and by the frequency analysis of the so-called EHD impedance {4). Secondly, in order to separate the elementary anodic and .cathodic contributions near to the corrosion po-Key words: interface, disk electrode, porous films, activation-diffusion. tential, and, therefore, to get a better insight of the mechanism, we carried out electrochemical impedance measurements for various polarization conditions. Experimental Conditions The steel sample selected for the study is the N 80 type according to the API standards, and has the following composition by percent weight: C _: 0.4; Mn-1.38; P = 0.013; S = 0.024; Si :. 0.19; Cu = 0.04; Ni-0.03; Cr ~ 0.09; Mo-0.19, and Fe to .100. The working electrode consists of the cross section of a cylindrical rod of 1 cm 2 area, a thermoretractable sheath preventing the cylindrical area from making contact with the solution, the electrode surface being only the cross section. In order to avoid possible crevice conditions between the metal rod and the sheath induced by the successive polishings, the sheath was renewed after each run. The surface is polished with a silicon carbide emery cloth (grade 80)r then rinsed with water, and finally dried in pulsed warm air after an ultrasonic washing in ethanol. This electrode is screwed into a conducting rotating shaft. The reference electrode is a saturated calomel electrode (SCE) and the auxiliary electrode is a platinum grid of large area. The corrosive medium is a 3% solution of NaC1 (pro analysis grade) dissolved in bidi~tilled water. The current potential curves are plotted with either a galvanostatic or potentiostatic regulation (Tacussel).