Polymer cathodic delamination

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. 

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. 

As will be shown later, during cathodic delamination of a polymer from a metal surface due to ingress of an electrolyte into the metal/polymer interface, an additional liquid phase could be formed between the substrate and the organic layer. In this case, the metal/electrolyte interface can be treated as a conventional electrochemical interface, but an additional Galvani potential difference ADonnan potential or membrane potential [24—26]) has to be taken into account at the electrolyte/polymer interface. The latter is directly correlated with the incorporation of ions into the polymer membrane according to Eq. (14). 

Fig. 31.4 Schematic illustration of the electrochemical processes that lead to cathodic delamination of polymer films from iron substrates (current /, galvanic potential difference A ). Middle overview of the polarization curves at the defect (left), the intact interface (right). Bottom the situation after galvanic coupling of these two parts.

The construction of a cell permitting both FTIR measurements and electrochemical impedance measurements at buried polymer/metal interfaces has been described [266]. Ingress of water and electrolyte, oxidation (corrosion) of the aluminum metal layer, swelling of the polymer and delamination of the polymer were observed. A cell suitable for ATR measurements up to 80°C has been described [267]. The combination of a cell for ATR measurements with DBMS (see Sect. 5.8.1) has been developed [268]. It permits simultaneous detection of stable adsorbed species and relatively stable adsorbed reaction intermediates (via FTIR spectroscopy), quantitative determination of volatile species with DBMS and elucidation of overall reaction kinetics. An arrangement with a gas-fed electrode attached to the ATR element and operated at T = 60°C has been reported [269]. In this study, the establishment of mixed potentials at an oxygen consuming direct methanol fuel cell in the presence of methanol at the cathode was investigated. With infrared spec-... 

Cathodic Delamination on Polymer-coated Iron Certain areas at the metal-polymer interface may become sufficiently cathodic to promote a cathodic reaction underneath the coating. This cathodic polarization might be the result of a purposely induced polarization, for example, cathodic protection of pipelines... 

Fig. 21 Microscopic pictures of (a) cathodic delamination and (b) FFC on polymer-coated iron.

Cathodic Delamination on Poly-mer[Pg.537]

Fig. 24 Principal corrosion model explaining the formation of a galvanic element in case of cathodic delamination on polymer-coated iron, (a) Cross section through a metal-polymer interface with a defect in the polymer coating (b) overview of the polarization curves at the defect (i), the intact interface fii) and the situation after galvanic coupling of the parts (c).

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... 

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.

Recently, with a view to a better understanding of the protection of iron by conducting polymers, the kinetics of the cathodic delamination of PPy doped with molybdate (Mo04 ) and phosphopolymolybdates (PMoi2O40 ) were considered by Paliwoda-Porebska et al. [72,73]. From a suspension of doped PPy nanoparticles (diameters ranging between 80 and... 

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. 

Plasma polymers with a special surface structure suitable to bond to an epoxy amine primer were used as interfecial coupling layers on iron and galvanized steel and led to even better results [122]. However, in all cases the system always showed cathodic delamination at the polymer/metal oxide interface, indicating the importance of oxygen reduction on the oxide surface. For verification, after the delamination of the sample, the delaminated polymer was pulled off and the underside of the polymer and the iron surface were investigated by XPS to reveal whether the system delaminated at the plasma polymer/metal interface or at the... 

Figure 6. Cr 2p XPS spectra from the chromated steel/epoxy system, (a) Metal surface following chromate treatment (b) interfacial metal surface following cathodic delamination. Note the reduction in the Cr component when compared with the as-received substrate prior to coating. The underside of the polymer coating, (c), shows a very low concentration of Cr . (From Ref. 42.)...

The self-healing effect provided by dopant ions has been observed in the woric of Dominis etal (2003) for emeraldine salt primers loaded with different eorrosion inhibitors. Also, in the work of Kinlen et al (2002), PANi formulations with phosphonic acid derivatives as dopant anions showed better corrosion protection performances than formulations with sulfonic acids and derivatives. Kendig et al (2003) also reported the development of smart coatings based on conducting polymers doped with different corrosion inhibitors, showing that the release of inhibitors was triggered by the electrochemical activity at defects. In the work of Paliwoda-Porebska et al (2005, 2006) the cathodic delamination rate in iron coated with PPy doped with phosphomolybdates was fonnd to decrease in comparison with PPy doped with molybdates. 

Such thin films have to act as a barrier between corrosive ions and the metal and have to inhibit electron as well as ion transfer reactions. Moreover, they should be stable over a quite wide pH range since corrosive reactions especially under polymer films change the local pH at the front of deadhesion ranging from acidic pH values (e.g., during filiform corrosion) to very alkaline ones (e.g., cathodic delamination on steel) [140]. 

SKP line scans of polymer-coated eleclrogalvanized steel during the cathodic delamination process (a) alkaline cleaned zinc surface without conversion layer and (b) conversion layer coated zinc surface. (Prom KKmow, G. et al., Electrochim. Acta, 53,1290,2007)... 

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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