Adhesion loss

Fig. 5-8 Total adhesion loss of a 500-/xm-thick coating of EP (liquid lacquer), 0.2 M NaCI, galvanostatic = -1.5 /tA nrr, 5 years at 25"C. Left coating with a pin pore loss of adhesion due to cathodic disbonding. Right pore-free coating loss of adhesion due to electro-osmotic transport of H O. In both cases the loose coating was removed at the end of the experiment.

Etch primer pretreatment This is suitable for overcoating with most coatings but is sensitive to moisture during application. It must not be applied as a thin, transparent coating (typically, 10 pm) or inter-coat adhesion loss can take place. 

The symposium on which this book is based was organized to provide a forum for discussion of recent advances in the use of polymeric materials in corrosion control. Most of the papers presented in the symposium are included in this volume. Several chapters have been added. These include an introductory overview as well as separate review chapters on how organic coating systems protect against corrosion, on mechanisms of adhesion loss of organic coatings, and on the interfacial chemistry of adhesion loss in aggressive environments. 

Only one of the four techniques—the conductivity—showed any correlation with the observed extent of corrosion. The lack of correlation of the tensile adhesion values with corrosion is a result of the fact that the method integrates adhesion loss at the substrate... 

Figure 2. XPS C Is spectra for oligourethane based coatings (a) untested oligomer coating surface (b) interfacial coating surface after mechanically induced adhesion loss (c) interfacial coating surface after humidity induced adhesion loss. Spectral components A, B, C, and D attributed to methyl/methylene, ether, melamine, and urethane carbonyl carbons, respectively. Reproduced from Ref. 19, copyright 1984, American Chemical Society.

Resin (1) Humidity (2) Cathodic Polariz n (3) Mode of Adhesion Loss Ref. 

Time required for general (G) or >4 mm scribe associated (S) adhesion loss. 

The rate of adhesion loss in tests of paint performance can be greatly influenced by coating formulation and resin type (24) this is illustrated qualitatively by the results shown in Table II. To a first approximation, the anodic inhibitor pigments commonly used in... 

TABLE II. Dependence of Paint Adhesion Loss on Test Conditions for Selected Formulations. 

Figure 4. Schematic illustration of corrosion induced paint adhesion loss. In the initial stage (A), iron is oxidized and oxygen reduced at a site of electrolyte penetration as corrosion proceeds (B and C), a rust deposit forms and the anodic and cathodic reaction sites become separated. Cathodically produced hydroxide progressively disrupts paint adhesion. Reproduced from Ref. 25, copyright 1980, American Chemical Society.

The interfacial chemistry of corrosion-induced failure has also been studied for coatings with relatively poor resistance to water (poor wet adhesion) for these materials, the corrosion-induced failure typically involved little chemical change, but appeared to involve the same displacement mechanism observed for hvunidity induced adhesion loss (19). XPS spectra showing this mode of failure are reproduced in Figure 1 for this coating, essentially identical spectra were obtained from all surfaces analyzed, independent of test conditions. 

Polybutadlene-based coatings have been extensively used in model studies of corrosion, and are therefore of special interest. As in the case of humidity induced adhesion loss, the corrosion induced... 

Figure 7. Paint adhesion loss in salt spray exposure (ASTM B117) as a function of ester content for chain-extended epoxy-amine and epoxy-ester resin based coatings. All coatings applied at 20-25 urn film thickness to SAE 1010 steel test panels, baked, scribed and exposed for 24 hours to salt spray conditions.

The interfacial chemistry of corrosion-induced failure on galvanized steel has been investigated (2) adhesion of a polyurethane coating was not found to involve chemical transformations detectable by XPS, but exposure to Kesternich aging caused zinc diffusion into the coating. Similar results were obtained with an alkyd coating. Adhesion loss was proposed to be due to formation of a weak boundary layer of zinc soaps or water-soluble zinc corrosion products at the paint metal Interface. 

Figure 12. Schematic illustration of zone of adhesion loss in cyclic exposure test.

Adhesion Loss of Ultraviolet-Cured Lacquer on Nickel-Plated Steel Sheets... 

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