Holidays | coating inspection


Protective coatings are as strong as their weakest point, which is why coating application and measurement techniques are so important, according to Belzona’s Thomas Belli and Ian Wade


Holidays and pinholes: chinks in the armour


A protective coating is as strong as its weakest point. Vulnerabilities – defects - within a coating can lead to the exposure of the substrate beneath, allowing access to the destructive forces of corrosion and erosion and ultimately make the coating redundant. Within the pipeline industry, these defects are referred to as holidays and include a range of anomalies such as missed areas, inadequate coating thicknesses, contami- nant inclusions and pinholes. This article aims to dispel some myths surrounding holidays and pinholes, while also exploring holiday detection and some of the solutions used to fix these defects. A common cause of holidays, and one which is particularly affected by environmental conditions, is coating viscosity. Imperfect viscosity can undermine application and is influenced by application tempera- ture. Outside of the recommended application tempera- ture range, the coating can be either too viscous or too fluid and will become too difficult to apply. As a consequence, it may form sags or “curtains” or may lead to misses or skips within the coating structure. The characteristics of the coating on application can


also increase the likelihood of holidays occurring once it has cured. Insufficient or incorrect mixing of the


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coating’s components may create areas that do not cure correctly, leading to disbondment or a range of other coating defects. Moreover, overmixing the coating prior to application


can introduce too much air into the product, creating an excessive number of air bubbles. When air bubbles rise to the surface of the coating they can burst and, if the coating does not reseal over the exposed surface, create pinholes. These miniscule holes may be limited to the top layer of the coating or they can continue directly through two coats, exposing the substrate. For this reason, when brush-coating a substrate, the applicators must ensure that two layers are applied. It is also important to ensure that, after the initial layer, the second coat is applied perpendicular to the original. This will provide maximum assurance that any potential holidays present in the first layer are covered. Many coating defects cannot be spotted visibly so a number of different technological solutions are used to locate anomalies. Most applied protective coatings are insulating so electrical techniques can be used to locate discontinuities. This type of holiday detector passes a current over the coating and tries to create a continuous circuit with the substrate beneath. Anomalies can be


Main image:


Appearance of pinhole


holidays in a coating


December 2016 | PIPELINE COATING 21


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