Delamination of Coatings

The level of dissolved salts of the exposure environment had an effect on the properties of coatings [85]. 

Blistering and delamination are the most common forms of failure found in organic coatings. Factors affecting the performance of a system include surface preparation, coating application, cure regime and film integrity. Soluble salts at the 

Some pathways must exist through the film to allow the sodium ions to the interface in order to produce the alkaline environment. These pathways could be due to pores. However, an alternative theory is proposed by Leidheiser [92] suggests that beyond a given concentration, alkali cations may have a deleterious effect on the coating, which leads to morphological changes and introducing conductive pathways to the interface. 

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. 

Considering a coated steel substrate, immersed in an electrolyte of neutral or near neutral pH, the half-cell reaction responsible for the delamination process is to be oxygen reduction. This reaction generates OH ions at the cathodic site and is responsible for the alkaline environment at the delamination front. The elements 

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Fig. 7 Cathodic delamination of coated iron (a) permeation of water and oxygen ...

Failures due to delamination of the gunite coating have been reported in the USA, but have not been observed to any significant extent in F.urope although some early failures of the anode system have been associated with high local current densities in areas of low concrete cover and high moisture or salt contentThe major application of this anode system is therefore on structures that are relatively dry with a uniform current requirement. 

Electrochemical tests This group includes the various electrochemical tests that have been proposed and used over the last fifty or so years. These tests include a number of techniques ranging from the measurement of potential-time curves, electrical resistance and capacitance to the more complex a.c. impedance methods. The various methods have been reviewed by Walter . As the complexity of the technique increases, i.e. in the above order, the data that are produced will provide more types of information for the metal-paint system. Thus, the impedance techniques can provide information on the water uptake, barrier action, damaged area and delamination of the coating as well as the corrosion rate and corroded area of the metal. However, it must be emphasised that the more comprehensive the technique the greater the difficulties that will arise in interpretation and in reproducibility. In fact, there is a school of thought that holds that d.c. methods are as reliable as a.c. methods. 

The rate of delamination is a function of the type of coating and its thickness. 

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.

Figure 11. SIMS step scan analysis of the interfacial substrate surface between the initial scribe and the edge of the delaminated zone coating and test conditions as in Figure 10.

Cathodic Delamination of Protective Coatings Cause and Control... 

Corrosion-Induced Delamination of Epoxy-Based Coatings. . .. 47... 

Leidheiser 9l) has proposed numerous methods of attack which can be used in an attempt to prevent or minimize the corrosion-induced delamination of polymer coatings from metal substrates. All of these methods involve the control of the chemical and physical properties of the interfacial layer between the coating and the metal substrate. Three of these methods are discussed in Sections 6.3.3-6.3.5. 

The thickness of the coatings on the sidewall is 6 pm, and on the bottom a thickness of 2 pm was observed. The delamination of the catalyst coating is an artefact caused by sample preparation [86],... 

The possible delamination modes are (i) loss of adhesion at paint/phosphate interface, (ii) within phosphate layer due to mechanical fracture (iii) due to dissolution of phosphate (iv) dissolution of coating (v) mechanical failure at coating/steel interface. 

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