Cathodic delamination 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. 

The cathodic delamination mechanism is discussed in detail in Sect. 5.4.3.1.2 while the FFC on iron is discussed together with FFC on aluminum in Sect. S.4.3.3 because of the similar mechanisms. 

The major unknown in the cathodic delamination process is the mechanism by which the interfacial bond is broken. Alkaline attack of the polymer, surface energy considerations, and attack of the oxide at the interface have all been proposed, but none of the available evidence allows an unequivocal answer. 

The relationship between performance reliability and adhesive formulation is not simple. The key step in improving the reliability of adhesives on cathodically protected substrates is fully understanding the cathodic delamination process. Various mechanisms have been proposed in the literature. A large number of investigators have focused attention on the damage hydroxide ion does to coating adhesion. 

Other explanations of the nature of the polymer to metal bond include mechanical adhesion due to microscopic physical interlocking of the two faces, chemical bonding due to acid/base reactions occuring at the interface, hydrogen bonding at the interface, and electrostatic forces built up between the metal face and the dielectric polymer. It is reasonable to assume that all of these kinds of interactions, to one degree or another, are needed to explain the failure of adhesion in the cathodic delamination process. 

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. 

After the initiation step, blisters can grow by different mechanisms, known as cathodic delamination, anodic undermining or filiform corrosion (FFC). The first two shall be briefly described, whereas FFC, due to its specific characteristics, will be left for Sect. 5.4.3.3. 

Figures 32(a and b) show typical microscopic pictures of FFC on polymer-coated iron, and aluminum. FFC develops in the presence of pores, mechanical defects, unprotected cut edges, or residual salt crystals underneath the organic coating. The corrosion filaments start growing perpendicular from a defect into the polymer-coated area. FFC occurs only at moderate humidity (60-95%) and therefore, not under full immersion conditions. FFC has been found to be triggered by anions such as chloride, bromide, and sulfate. The filament growth rate increases with temperature. Like for cathodic delamination on iron and zinc the corrosion kinetics depend strongly on the surface pretreatment and coating composition.

For aluminum and magnesium, FFC and blistering are the predominant coating failure mechanisms. For iron, FFC is observed only under special conditions and cathodic delamination is the primary failure mechanism see Ref. [168] and references therein. 

The situation changes when the defect is prepared just down to zinc and the kinehcs of zinc dissolution are rather slow. In this case, the cathodic delamination determines the kinetics of undermining. The delaminated area of the phosphated sample is now smaller than for the defect down to steel, whereas the just alkaline cleaned sample shows delamination that is much faster than in the case of the defect down to steel. This example shows how complex the corrosion mechanisms are and that no generally accepted mechanism can be found in the literature. 

A number of explanations have been put forward for delamination mechanism whereby the alkaline environment under the film affects the integrity of the metal-polymer interface, or perhaps more properly the interface between the oxide and the polymer. Koehler [94] showed that this form of failure only occurs when there are alkali metal cations available in the environment to act as counter ions to the cathodically generated OH ions. 

Leidheiser, Jr.H., Wang, W., Igetoft, L. the mechanism for cathodic delamination of organic coatings from a metal surface. Prog. Org. Coat. 11 19-41 (1983)... 

Figure 12.21 Mechanisms of the degradation of paint fihns (a) degradation of a polymer due to ultraviolet radiation or chemical attack (b) the formation of blisters by osmosis and (c) cathodic delamination due to the formation of corrosion cells.

Alkali can be generated by the cathodic half of a corrosion reaction or the cathodic reaction may be driven by means of an electrical potential. When the cathodic reaction occurs between the rubber and metal surface the pH of the solution under the rubber may be as high as 14. Many factors (summarised by Leidheiser [3]) concerned with cathodic delamination are detailed. No definitive mechanism for this type of delamination has been determined although a number of suggestions have been put forward [3]. These include alkaline attack on the polymer, surface energy considerations and attack of the oxide at the interface. 

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). 

Figure 7 Schematic representation of the mechanism of cathodic delamination starting...

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