Coating Defects 2-Common Coating Failures and Causes
Blistering is dome- or circular-shaped projections of the coating film held away from the substrate Blisters can have irregular shapes, depending on the cause. They may be filled with pure water, solvent, caustic, gas, oxygen, crystals, or rust. The basic cause is a loss of adhesion in localized areas. They can be any size and distribution. Numerous factors can lead to blistering, but the most common is a contaminant of some kind left on the surface after cleaning.
In atmospheric service, the blisters may be caused by coating over: oil, moisture, grease, dirt, dust, soluble pigments and retained solvents.In immersion or buried service, blistering can also be caused by electro-endosmosis due to: an overactive cathodic protection system, stray currents, osmosis caused by trapped soluble salts
Do not to break blisters unless it is to test them or their contents to determine cause. Remember that blisters may be protecting the surface. The only reasonable repair, if repair is necessary, is to remove the coating system, clean the area, fix whatever caused the blisters and then replace the coating system.
These defects are visible cracks in a coating that may either penetrate down to the substrate, or just penetrate through a single coat in a multi-coat system. The main cause is stress related, either due to movement in the substrate or internal stress in the coating as it ages. Chemically cured coatings that are applied too heavily are prone to cracking. Two other causes are absorption and desorption of moisture. An example is antifouling cracking, which is seen when the ship comes out of the water for inspection or work. Aging also causes cracks in coatings. A good example is sometimes seen in older alkyd coatings. Once a coating has cracked, remove and replace it, preferably with a more flexible coating.
Checking appears as line cracks in the coating surface. Normally, this appears only in the topcoat and the cracks rarely go through to the substrate. Sometimes, magnification is needed to fully see them. The fundamental cause is stress in the coating film. The stress can occur because of incorrect formulation (a manufacturers’ issue), or a poorly written specification which requires a coating that is not meant for the service or is incompatible with the underlying coating.
Alligatoring occurs where a hard, tough coating is applied over a softer extensible coating. Some thickly applied coatings can alligator when exposed to sunlight. Coatings that cure by oxidization-polymerization may alligator if the coating is heated, i.e., the surface cures rapidly relative to the underlying coating.
Adhesion Failures: Flaking, Delamination, Detachment, and Peeling
These defects are due to loss of adhesion between coating layers or the substrate, caused by: coating applied to a contaminated surface, wrong surface preparation specified, failure to inspect surface preparation, insufficient surface profile, exceeding the topcoat window, application of incompatible coatings (e.g., alkyd over IOZ), applying a coating to a glossy surface.
Unless addressed by using a stripe coat, a common source of in-service coating failure is rust that starts at a sharp edge or rough or spattered weld seam The fundamental reason is that coatings pull away from a sharp edge, and industrial application using spray or roller can cause the coating to bridge over small depressions in the substrate.
Weld spatter is a separate issue. Coating does not encapsulate beads of weld spatter, nor does abrasive blasting remove it. Another issue with welds and cut edges of steel is that the heat from certain types of welding can harden the steel surface for a short distance around the weld. The hardened steel may not receive the same profile as surrounding steel during abrasive blasting.
Gouges or Chipped Spots
No matter how careful a contractor is, damage inevitably occurs on coating projects and it must be addressed. Moving equipment around a work site can cause gouges or chipped spots, and unfortunately these gouges normally go through to the substrate. Even a small chip can start an active corrosion cell causing a pit to form.
Cissing (also known as ‘fisheyes or ‘crawling’) is the term for surface breaks in the film that reveals the substrate. It is often a result of grease or oil contamination on the substrate.
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