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:
|
Runs and sags
|
The wet coating moves under gravity leaving a thin dry film.
|
Cissing
|
Occurs when a coating does not re-flow to cover the voids generated by air bubbles being released from the surface of a coating.
|
Cratering
|
Occurs when the substrate is wet or if the coating has poor flow characteristics, thus creating voids in the coating.
|
Pinholes
|
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.
|
Over coating
|
If too much coating is applied to a substrate, as the coating cures it can crack from internal stresses of the coating.
|
Under coating
|
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.
|
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:
|
|
Wet Sponge Technique
|
Suitable for measuring insulating coatings on metal less than 500µm on conductive substrates. The wet sponge technique is ideal for powder coatings and any thin coating where the User does not wish damage to occur to the coating.
A low voltage is applied to a sponge, moistened with a wetting agent. When the sponge moves over a coating flaw, liquid penetrates to the substrate and completes an electrical circuit, setting off the alarm.
This technique will identify coating flaws where the substrate is uncovered, i.e. cissing, cratering, pinholes and some forms of over and under coating flaws.
|
