Premature corrosion of a substrate is usually due to the failure of the coating. A major cause of failure is the presence of flaws in the finished coating. Collectively referred to as a coating’s porosity the main types of flaw are described below:
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Runs and sags
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The wet coating moves under gravity leaving a thin dry film.
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Cissing
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Occurs when a coating does not re-flow to cover the voids generated by air bubbles being released from the surface of a coating.
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Cratering
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Occurs when the substrate is wet or if the coating has poor flow characteristics, thus creating voids in the coating.
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Pinholes
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Caused either by air entrapment which is then released from the surface, or by the entrapment of particulates (dust, sand, etc.) which do not stay in place.
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Over coating
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If too much coating is applied to a substrate, as the coating cures it can crack from internal stresses of the coating.
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Under coating
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Areas are not coated or the coating flows away from particular edges, corners of a substrate and welds. Furthermore, over a rough surface profile, insufficient coating may leave the profile’s peaks exposed.
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The consequent cost of repairs and subsequent loss of production can be considerable. Early inspection for coating flaws will prevent the expense and inconvenience of a coating failure. Instruments used to detect coating flaws are referred to by many different names, these include spark or jeep testers, porosity or holiday detectors and pinhole testers.
There are two methods of testing:
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High Voltage Technique
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Locates all flaws in insulating coatings on conductive substrates, the high voltage technique can be used to test coatings up to more than 7mm thick. This method is ideal for inspecting pipelines and other protective coatings. Coatings on concrete can also be tested using this method.
A power supply generates a high DC voltage which is connected to a suitable probe with an earth return connected to the substrate. As the probe is passed over
the coated substrate, a flaw is indicated by a spark at the contact point which sets off the alarm.
This technique is suitable for identifying all of the flaws described above, however care is required on thin coatings.
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