Cathodic delamination

Water Aggregation. An interesting question arises at the outset as to what constitutes an aqueous phase. How many water molecules are required before an electrochemical process can be activated Conversations with many well-known electrochemists have led us to use a IM solution as a reference. Another basis for using IM is the observation that the pH at the active front under a cathodically delaminating coating approaches a value of slightly under 14, i.e., approximately IM in hydroxyl ions. A IM solution is 55M with respect... 

Some of the important facts about cathodic delamination are summarized in the following itemized statements ... 

No significant delamination is observed in the absence of metal cation. No cathodic delamination occurs in pure acid solutions. 

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. 

Cathodic Blistering. In the absence of a purposely-imposed defect in the coating, the cathodic delamination phenomenon is known as cathodic blistering. An example of cathodic blistering as a function of time is shown in Figure 1. 

Figure 4. Cathodic delamination of pigmented epoxy coatings on steel. A defect was placed in the coating and the coated metal was maintained at a potential of - 0.8 v vs. SCE while immersed in NaCl solutions of different concentrations.

Another example is the very slight delamination that occurs when a thin copper layer is overcoated with an organic coating such as a photoresist and the system is made anodic. The rate of disbonding is a function of the applied potential and hence the rate of dissolution of the copper beneath the coating. Anodic delamination occurs very slowly relative to cathodic delamination at equal potential differences from the corrosion potential. 

Anodic undermining has not been studied as extensively as cathodic delamination because there do not appear to be any mysteries. Galvanic effects and principles which apply to crevice corrosion provide a suitable explanation for observed cases of anodic undermining. 

In all cases studied to date, the rate of cathodic delamination is greater in CsCl solutions than in NaCl solutions of the same molarity. The increased rate has been attributed to the greater rate... 

Cathodic Delamination of Protective Coatings Cause and Control... 

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. 

During the cathodic delamination process there are two important reactions which can occur at the cathode and which are catalyzed on the thin layer of metal oxide which covers the cathode surface. These reactions are ... 

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. 

NaOH solution and in 3.5%(Wt.) solution of NaCl. Thus, the upper curve, showing the volume change in the presence of hydroxide ion, will be typical of the behavior of the primer at the debond during cathodic delamination. 

Fig. 17. Cathodic delamination rates of galvanized steel/epoxy-polyamide coating systems when a 0.1 M CoCl2 dipping pretreatment was applied to the metal substrate prior to the application of the coating 91K (Reprinted with permission from Ref. 91Copyright (1981) American Chemical Society)...

The corrosion phenomena commonly observed on painted metals include cathodic delamination, anodic undercutting, and filiform corrosion. Cathodic delamlnatlon results when the alkali produced by the cathodic corrosion reaction disrupts the paint-metal interface. This phenomenon has long been observed on cathodically protected painted steel (18) and has also been demonstrated to be responsible for the loss of paint adhesion that often occurs adjacent to corrosion sites on painted steel (19). The localization and separation of anodic and cathodic sites associated with corrosion at a break in a paint film on steel are schematically illustrated in Figure 7. 

Nevertheless, the initial disruption of paint adhesion is most likely a cathodic delamination (21). Perhaps the most interesting question—the origin of the filamentary form—has not been answered with certainty. 

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. 31.3 Cathodic delamination potential profiles of an unmodified epoxy adhesive layer (adhesive as in Fig. 31.2, about 50 pm thick) on an iron substrate (purity 99.99%) with 0.5 M NaCI as electrolyte measured (a) 20 h after the addition of the electrolyte (b) 50 h after the addition, x increases positively with distance from the electrolyte-filled defect.

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.

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. 

Alkaline resistance An important chemical property of the phosphate layers is their ability to resist attack by hydroxide ion when exposed to alkaline electrolytes [36, 39]. Such an exposure may occur during (1) the cationic deposition of paint (by imposition of a cathodic current) (2) during subsequent alkaline degreasing operations (in particular for prephosphated steel products) and (3) under the paint layer during cathodic delamination. The destruction of the phosphate layer by the alkaline... 

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. 

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. 

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