Paints failure

Product Utilization. The principal appHcation for chromium phosphate coatings is as a paint base for painted aluminum extmsions and aluminum beverage can stock. In these appHcations, extremely demanding performance criteria are met by the chromium phosphate conversion coatings. As an example, the Architectural Aluminum Manufacturer s Association Voluntary Specification 605.2-92 requires humidity and salt spray testing for 3000 hours and allows only minimal incidence of paint failure after testing (26). 

Flame Cleaning Now little used as a preparatory method, flame cleaning is a process whereby an intensely hot oxyacetylene flame is played on the surface of the steel. In theory, differential expansion causes millscale to detach. In practice, there is evidence that the treatment may not remove thin, tightly adhering millscale. Also, steel less them 5 mm thick can buckle. Finally, the process can burn in chemicals deposited on the surface, causing premature paint failure. 

The processes are dealt with fully in Chapters 11, 14 and 15. Because many paint systems include an initial surface pretreatment, e.g. chromated aluminium or phosphated steel, BS4479 1990, Part 3 deals with conversion coatings and should be consulted by designers. Whatever the method of treatment, liquids must be able to drain quickly and freely from the surfaces. Crevices where liquids can become entrapped are best avoided. The surface configuration needs to be such that active solutions can be washed away, leaving the surface to be painted completely free from unreacted pretreatment solution. Failure to achieve the requisite level of freedom from the surplus chemicals causes paint failure, e.g. osmotic blistering. 

In view of the wide scope of the subject, paint failure can be treated here only in general terms detailed accounts will be found in the literature. ... 

A consideration of the most important causes of paint failure must include the following inadequate surface preparation, application of the paint... 

Premature failure can also occur as a result of lack of attention to design. Facilities should, therefore, be provided for ventilatory drainage of water (rain, condensation, etc.), and all structures should be designed so as to permit ready access for repainting. Due consideration by architects and structural engineers at the design stage can indeed help to obviate certain of the causes of paint failure mentioned in this section (see also Sections 9.3 and 11.5). 

Paint failure is related to climatic conditions, and the weather prevailing during application of the paint and during subsequent exposure will determine the life of the paint system. This applies, of course, particularly to outdoor work. In unfavourable weather conditions, cracking and blistering can be promoted as a consequence of the expansion of the products of corrosion, and in the case of iron and steel this can lead to under-rusting. 

Factors Which Cause Paint Failure in Industrial Applications... 

Good supervision, careful working, and common sense can contribute a great deal to reduce paint failures and the wasteful work which is necessary to put a job right. 

When corrosion develops on painted steel the question is often raised as to whether corrosion was a result of paint failure or the paint failure was caused by corrosion. Several studies have shown that adhesion forces are reduced greatly after water soaking or even at very high humidity -and it has been argued that film detachment by water usually precedes underfilm corrosion . Against this view others have claimed that those paints known to have reduced wet adhesion, e.g. those based on alkyd resins, are not uniquely, or even especially, subject to underfilm corrosion Several factors should be considered in this discussion ... 

Paints are considered in detail in Chapter 14, paint failures being discussed in Section 14.4. 

Chalking. Paint failure that is characterized by a layer of loose pigment powder on the surface of a weathered film. Chalking is often a desirable failure because of its self-cleaning action. 

The following alphabetical listing of defects within the above four groups covers most of the causes for paint failure. There are still more reasons that can cause coatings to fail which are not included here [4]. 

Figure 3.--Paint failure over latewood of flat-grained siding (W. (M147 211-12)...

Such tests can identify incorrect component dimensions, lack of adhesive, lack of filleting, edge voids and porosity at the manufacturing level, throughout the cross section of a laminate after the part has been cured [29]. Other in-service damages detectable include paint failure, lighting strikes, and peeled surfaces. 

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