Polymer anodic delamination

It is obvious that in case of a defect down to steel, which leads to the enhanced anodic dissolution of zinc, the delamination of the purely alkaline cleaned galvanized steel surface is not faster than that of a phosphated surface. Such a behavior can be explained by an anodic delamination process. If the corrosion conditions are such that no formation of a cathode is possible in front of the anode, then just the kinetics of zinc dissolution determine the degradation of the polymer-metal composite. 

Oxygen reduction takes place in the defect with a rate that is controlled by the transport of oxygen through the electrolyte layer (a, i). Thus, a galvanic current is established between the anodic site (zinc within the delaminated zone) and the defect (cathode). In the area between the two potential steps (b, ii), no equilibrium potential surface is observed but the potential rises continuously from the borderline of the local anode to the potential jump, which indicates the intact metal-polymer interface. It can be assumed that the closer the zinc to the cathodic delamination front the smaller is the local anodic current while the... 

It is significantly accelerated by the presence of hydrolyzable ionic contaminants (for example, hahdes and acids from flux residues or extracted from polymers). Delaminations or voids that promote the accumulation of moisture or contaminants can promote dendritic growth. Conductive anodic filament growth (discussed later) is a special case of dendritic growth. Time to failure is inversely proportional to spacing squared and voltage. The failure mechanisms in accelerated tests have been reviewed. ... 

Materials which built up blocking layers on the surface of the anode, e.g., hydraulic oil or polymer films resulting from delamination of membranes. The irreversible poisoning of coating is caused by ultrathin aluminum silicate layers. 

For primers with self-healing and corrosion inhibitive properties, it is desirable to have one component to inhibit corrosion of the underlying metal and a second component to repair the polymer coating. These individual components can be multifunctional or multistep. For the protection of metals, many paint systems consist of an inhibitive primer and a topcoat. In case the coating is mechanically damaged or local delaminations are present, the repair function is usually achieved via release of the inhibitors from the inhibitive primer, whereby the corrosion performance of the underlying metal surface benefits from autonomous surface recovery processes by the precipitation of stable inhibitor compounds at cathodic and/or anodic sites. 

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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