Cathodic delamination corrosion products

Differences in detected Volta potentials between pristine and corroded Al-Mg alloy surfaces could be related to the factors influencing thickness and conductivity of the corrosion product layers [219]. Corrosion layers developed in the presence of ion-containing solutions yielded lower Volta potentials and showed higher conductivity. Cathodic delamination of poly aniline-based organic coatings on iron have been studied with SKP [220]. The role of dioxygen reduction and of the poly aniline fraction in the coating were included in a proposed corrosion mechanism. 

For nongalvanized steel, cosmetic corrosion generally involves a cathodic delamination mechanism the surface under the paint becomes cathodic and the surface exposed in the hole becomes anodic. To slow down or prevent atmospheric corrosion, it is therefore important that the siuface treatment be a good cathodic inhibitor in the finished product. The phosphate layer increases corrosion resistance by limiting the available free surface for the cathodic reaction. In general, the activity of the free surface is further reduced by passivating posttreatments or by the deposition of amorphous phosphate films between the crystals. 

J2 Corrosion of steel reinforcing ban (reban) in a concrete bridge head, (a) The corrosion cell which arises due to water penetration and chloride pick-up from the concrete or from deicing salts, (b) The expansive corrosion products may cause cracking of the concrete. Tn severe cases, delamination results, (c) Impressed current, cathodic protection may be applied, (d) A flexible anode mesh may be installed (which is then covered with a concrete overlay)i Power usage is < 10 Wm" of the concrete surface. (Courtesy Raychem Ltd.)... 

The electrochemical mechanism of filiform corrosion is described by a differential aeration cell between the front (low oxygen concentration) and the back (open to air through the cracked/porous tail of dry corrosion products) of the filament s active head . Therefore, the head is the local anode and the tail the local cathode, the opposite to the cathodic delamination described earlier. This is shown in Fig. 7-67 where an optical photograph of the filiform head is compared to the potential distribution in air (center) and in nitrogen (right) (Schmidt and Stratmann, 1998). 

In that case the phosphate layer provides hardly any protection. The zinc layer is consumed locally by the anodic reaction, and the steel substrate is exposed. The corrosion products are not taken up in the paint film. The eathodie delamination takes place along the zinc-paint interface. The corrosion process continues where the damaged site reaches the steel substrate as shown in Figure 4. All eontaminants reach the paint-phosphate-metal interface. Zine dissolves anodieally atthe tip while oxygen reduction takes place at the exposed steel surface. At cathodic sites the pH... 

Anodic undermining is mainly reported on aluminum, which forms only Al + ions and porous corrosion products. Especially in the presence of chlorides, the natural oxide is easily attacked. Similarly to cathodic delamination, in this case also the anions and cations are separated leading to the formation of an HCl solution at the delamination front. 

In more detail, the model of cathodic delamination can be described and understood as follows [3,57] the delamination starts with randomly distributed anodes and cathodes in a defect or at a dalaminated area. When iron is taken as an example, the dissolved Fe " at the anode is further oxidized to Fe + by oxygen and forms insoluble corrosion products in the defect that often adhere to the polymer (in the case of a blister) and at the edges of the defect (where oxygen enters the defect). In what way the defect is blocked and a cap of corrosion products is formed on the top of the blister see Figure 20.7. In this cap oxygen... 

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