Coatings corrosion currents

To avoid galvanic problems, different materials of con-stmction may have to be electrically isolated or at least the electrical resistance between them increased to a level sufficient to reduce the corrosion current to an acceptable value. In some instances it is more practical to paint or otherwise coat the more cathodic of the two parts of the couple. The anodic material should not be coated, since even more rapid penetration would occur at any breaks in the coating. 

A continuous intact film of water-resistant paint forms an effective electrical resistance to the flow of a corrosion current (a resistance of over lO flcm through the film is easily achieved). Underfilm corrosion can then only occur if a channel of electrolyte connecting anode and cathode can be established by local adhesion failure between the coating and the metal substrate. 

The use of the impedance technique in the study of polymer coated steel, has been thoroughly described elsewhere. The present paper compares this technique with that of harmonic analysis, originally proposed by Meszaros ). The authors have presented preliminary data using the latter technique(3) wherein the early stages of polymer breakdown have been studied. The current paper extends this work to polymers which have been immersed for a considerable period of time. The harmonic method gives information not available from the impedance technique in the Tafel slopes and the corrosion current are directly measurable. A brief summary of the harmonic method and the equations used are given below. 

Protection of iron from corrosion by zinc or tin coatings is based on a very small corrosion current density at large areas of these metals so that the whole electrode is brought to potentials where iron (in small areas, for instance pores) is protected and cannot dissolve. It is beyond the scope of the present chapter to discuss corrosion and passivation in detail and the reader is referred to specialized bibliography [142—143]. 

A ballast tank filled with seawater is easily corroded. Corrosion protection by the paint on the metal surface inside the tank, which improves the insulation for the corrosion current, is conducted. The paint has problems with age-related degradation and incipient failure. To protect from the corrosion caused by these problems, plural sacrificial anodes are usually installed in the tank. When seawater is loaded in the tank, the surface of the inside tank becomes cathode and the protective potential works, because of the anode effects. The worse the coating condition becomes, the worse the insulation of the paint becomes and the lower the surface resistance becomes. Therefore, there is the possibility that the coating condition can be evaluated with the monitoring of the surface resistance. 

Corrosion resistant fluoropolymer coatings are currently marketed by W. L. Gore Associates, Inc. (Fluoroshield) and Pfaudler. 

Cabrini M, Cigada A, Rondelli G, Vicentini B (1997) Effect of different surface finishing and of hydroxylapatite coatings on passive and corrosion current of Ti6A14V alloy in simulated physiological solution. Biomaterials 18 783-787... 

The most important result is the significant and reproducible shift of the corrosion potential, mainly together with a decrease in specific corrosion current (or decreased gradient angle of the current curve). Figure 11.57 shows the shifted potential for polyaniline coated metals of which the original corrosion potential/ current curve is to be seen in the same figure. 

Figure 11.58. Corrosion current density/potential for iron passivated with polyaniline with different coat thickness [106].

Recent information suggests that such coatings operate by increasing the electrical resistance between the steel and the concrete both epoxy and polymer-cement-based materials seem to have this effect, reducing local corrosion current exchange [9]. 

EIS is the response of corrosion systems to ac excitations. It has been used in corrosion research to estimate corrosion rates or study the metal passivation, corrosion protection using inhibitors, sacrificial barrier properties, and polymer coating performance on metals. EIS only estimates polarization resistance. As with Hnear polarization, the corrosion current is calculated using the Stem-Geary equation for known values of the anodic and cathodic Tafel slopes. 

Figure 5.15 shows that the corrosion current is one order of magnitude smaller for the pulse current-deposited (pc) Zn-Ni-Cd (7% Cd) sacrificial coatings when compared to the direct current-deposited (dc) Zn-Ni-Cd with 22% Cd in the alloy [36]. The corrosion rates of pulse current-deposited Zn-Ni-Cd sacrificial coatings that contain 7% and 3% Cd were 0.49 and 1.03 mpy, respectively much lower than the 2.87 mpy observed for the direct current-deposited Zn-Ni-Cd alloy with 22% Cd in the alloy. 

Table 6.9 Corrosion Current Values of Selected Coatings Determined in 0.5 M H3BO3 Solution at 25 °C Using the Tafel Extrapolation Method...

Nevertheless, when a conducting polymer in its oxidized state is coated on the metal, there is galvanic coupling between the metal and the polymer. The corrosion current has a kinetic origin and corresponds to a mixed potential resulting from the reduction of the... 

Electrodeposition of PANI derivatives was achieved by Shah and Iroh [108] in the case of Al alloy (2024) and N-ethylaniline in aqueous oxalic acid solution. The coating was electrodeposited by cyclic voltammetry by application of an unusually large potential range (-1V to -I-3V/SCE). Corrosion current was measured in chloride solutions from Tafel plots and was found to be significantly lower (about one order of magnitude) than that of bare Al-2024. Multifunctional coatings (PPy/PANI) were also achieved on aluminium by coelectropolymerization of aniline and pyrrole (in a feed ratio of 3 7) in oxalic acid solution, but protection performances were not given [109,110]. 

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