Coating Defects
1.0 GENERAL
Defects present in applied coatings/linings can lead to premature failure. Therefore, it is essential to conduct a proper inspection to avoid placing a coating/lining into service when such defects are present.
2.0 DESCRIPTIONS OF COMMON DEFECTS
2.1 Alligatoring
Appearance — coating appears to be cracked, it has an appearance similar to alligator skin, the defects usually do not penetrate to the steel substrate
Causes — insufficient drying of coating before application of the second coat, coating applied and cured at too high a temperature, excessive coating thickness, or coating cures too quickly
2.2 Blistering
Appearance — area of localized coating swelling, sometimes resembling bubbles
Causes — coating applied over too hot a surface, condensation forms at the coating/metal interface, trapped solvent, oil, grease or rust under the coating. Coatings and linings placed into service may blister due to degradative processes, e.g., those due to osmotic forces, cathodic disbondment or trapped solvent
2.3 Blushing
Appearance — coating finish has a dull, milky appearance
Causes — moisture condensation in a high-humidity environment, condensation on a cold surface
2.4 Chalking
Appearance — coating loses its gloss and usually develops a white, powdery surface
Causes — general weathering of the coating in atmospheric environments (i.e., ultraviolet degradation of the coating)
2.5 Cracking
Appearance — discontinuities/breaks, usually sharp in nature, in the coating that exposes the substrate
Causes — insufficient drying of coating before application of the second coat, coating applied and cured at too high a temperature, excessive coating thickness, coating cures too quickly, especially a thick coating/lining that is rapidly heated
2.6 Cratering
Appearance — small, uniform indentations in the coating/lining.
Causes — oil or moisture are present in the air lines during spraying (conventional), air pockets trapped in wet coating during spraying
2.7 Disbondment
Appearance — loss of adhesion to the substrate or between coats
Causes — early disbondment of the topcoat or the complete coating system is usually due to poor surface preparation; e.g., substrate or intercoat contamination, excessive cure time between coats. For heat-cured MarineLINE, excessive heating can sometimes lead to coating disbondment due to high coating thickness, surface contaminants or excessive flexing of the substrate.
2.8 Dry Spray
Appearance — surface of coating has a rough texture and porous appearance
Causes — poor atomization of the spray, spray guy is too far from the surface during application, high air temperature and low humidity can lead to rapid solvent evaporation thus producing partially dry spray particles
2.9 Foreign Inclusions
Appearance — foreign particles are embedded or partially embedded in the coating/lining
Causes — dust, dirt or abrasives that settle into coating while it is still wet, the sources for solid particle contamination can be airborne, from adjacent structures, equipment or dirty scaffolding, tracked in by personnel, etc.
2.10 Fish Eyes
Appearance — separation of the coating/lining film that resembles holes or deep depressions, can also be separation of the wet film just after application
Causes — application of the coating/lining over oil, dirt, moisture, silicone residue or other contaminants that reduce the wetting of the coating film over the substrate, coating over incompatible coatings
2.11 Missed Areas
Appearance — presence of localized bare areas on an otherwise coated item
Causes — missed regions are usually associated with difficult to coat areas
2.12 Orange Peel
Appearance — finely dimpled surface texture that resembles the skin of an orange
Causes — usually due to poor atomization of spray of low pressure, sometimes associated with fast solvent evaporation or spray gun positioned too close to the surface
2.13 Overspray
Appearance — a deposit of wet or dry coating particles on areas other than those intended to be coated
Causes — coating material that drifts onto other areas coated areas of the structure due to the configuration of the structure, e.g., “I” beams, or material that is carried by the wind or falls along walls of a vessel from elevated sections to the floor or lower sides.
2.14 Pinholing
Appearance — small diameter, deep holes that penetrate either through the coating/lining to the substrate or through the topcoat to the basecoat, often referred to as “holidays.”
Causes — coating applied over pits in the substrate, dry spray, embedded abrasives and dirt, solvent entrapment, hot substrates and incorrect viscosity of the coating material
2.15 Pinhole Rusting
Appearance — rusting that occurs at defects through coating/lining
Causes — defects present in coating/lining, e.g., pinholes, cracks, embedded particles, metal slivers in substrate that protrude into coating/lining, weld splatter or insufficient coating coverage over high points in the surface profile
2.16 Runs and Sags
Appearance — excess coating material that runs down vertical surfaces
Causes — coating applied at too high of thickness, excessive thinning of the material, application made to cold surfaces or poor air flow across coating in high relative humidity environments (i.e., retards solvent evaporation)
2.17 Softness
Appearance — coating dries on the surface but remains soft in the interior of the coating, one can indent the coating with a fingernail or leave a fingerprint when pressing on the coating
Causes — excessive coating/lining thickness, insufficient drying time, drying hindered by low temperatures and high humidity, poor ventilation which slows solvent evaporation
2.18 Tackiness
Appearance — coating/lining has only partially set and has a sticky feel but is not wet
Causes — excessive thinner, insufficient drying, cold temperatures, poor ventilation
2.19 Wrinkling
Appearance — coating/lining has a rough, crinkled texture
Causes — excessive coating thickness and temperature is too warm or cold
3.0 INSPECTION PROCEDURES
3.1 Examination of Coating Defects: Most defects are visible to the unaided eye. However, a low power pocket microscope can enhance the details of suspected defects. Flashlights or spotlights are required for inspections inside tanks, vessels, pipes, etc. They are also useful in other areas where natural lighting is poor. Utility, pocket or putty knives can be useful for examining coating adhesion loss due to flaking or peeling. Be careful not to damage the coating/lining. Dry film thickness readings should be made regularly over the coated structure according to the appropriate application procedures.
3.2 When to Inspect: The coating shall be inspected after each spray and stripe coat and before the next step in the coating operation. Also, all repairs and touch-ups must be inspected during and after completion.
3.3 Unacceptable Defects: The inspector is to use his/her discretion in determining which defects are unacceptable and those that are acceptable, however, the acceptance of a defect(s) shall in no way compromise the integrity of the coating/lining. This implies that the applicable application procedures shall be followed, otherwise, a non-conformance document must be prepared and authorized by the owner’s representative and the Technical Services Department at Advanced Polymer Coatings.
3.4 Marking Defects: Defects such as pinholes, low areas of dry film thickness, runs, sags, etc., can be identified using grease-free chalk, stone chalk or a suitable marking pen.
4.0 CORRECTIVE ACTIONS
All unacceptable defects shall be repaired according to the procedures provided in the appropriate applications procedure.
5.0 CONCERNS
5.1 Pinholes: Does spraying additional coats of material over pinholes adequately correct this defect? The answer to this question is no. Pinholes must be either removed or enlarged, and coating brushed into them and then overcoated by spray, brush or roller. See appropriate application procedures. Pinholes can reappear if material is not pushed down into the pinhole.
5.2 Excessive Thickness. Excessive dry film thickness or high thickness associated with runs and sags can eventually crack through organic movement or during heating curing.
