Cathodic delamination blisters

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. 

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. 

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. 

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.

Figure 12.8 Blister initiation and propagation under a defective coating (cathodic delamination).

In cathodic delamination the adhesion of the coating fails and causes lateral blister growth as a result of a high pH at the delamination front [6,55,56]. The loss of... 

From the preceding discussions it is concluded that blisters imder intact coatings grow due to cathodic delamination. 

Figure 5. XPS survey spectra from polybutadiene/mild steel failure interfaces, (a) Blistered region representative of cathodic delamination and lateral diffusion of hydroxyl ions (b) wet adhesion failure resulting from the downward diffusion of water molecules. (From Ref. 30.)...

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... 

Schematic representation of the mechanism of cathodic delamination starting from a defect and from an osmotic blister.

As brine concentration decreases, osmotic water transport through the membrane increases. The sulfonic anolyte layer of a composite membrane carries this increased flow easily, but the carboxylic cathode layer is more resistant. Osmotic pressure therefore builds up between the layers. This causes the formation of blisters or, in the extreme, complete delamination of the membrane. A lower limit of about 170 gpl NaCl on the brine concentration protects against these problems and incidentally prevents any loss in current efficiency due to low salt concentration. The value of 200 gpl mentioned above allows for some variation between electrolyzers and their individual cells. 

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