Premature corrosion of a coated substrate is usually due to the failure of the coating.
The coatings are applied to a substrate to prevent corrosion, reduce abrasion and reduce product contamination.
The degree of coating continuity required is dictated by the service conditions.
To achieve a long-term life with a coating, it is important to ensure a continuous coating is applied and tested.
Discontinuities in a coating are frequently very minute and not readily visible so a holiday detection test is conducted.
Flaws
A major cause of corrosion in the presence of flaws in the coating system.
Collectively reference to a coatings porosity, the main types of flaws are:
Runs and sags – the wet coating moves under gravity leaving a thin dry film.
Cissing – Breaks in the coating where it recedes from surface contamination such as oil and grease.
Pinholes – caused by entrapment of air or solvent where the paint does not flow to seal the resultant hole in the film.
Over thickness – cracking due to internal stress. Under thickness and misses – insufficient coating leaving a substrate exposed.
General misses and poor application.
Low Voltage Holiday Detector
The low voltage holiday detector is a simple and effective process for assessing the integrity of a dried coating.
The equipment is used on low film thickness type coatings to ensure the coating is free from flaws, pinholes and general misses and areas of damage to the coating.
Low voltage holiday detection is generally specified for coatings that are immersed in a product or liquid and used for protection such as tank and vessel linings.
There are a number of suppliers of such specialist equipment around the globe.
Low Voltage Holiday Detectors
The low voltage holiday detectors are available as assembled kits from the suppliers.
Minimal setting up is required for the equipment to function correctly.
Ensure you purchase or use equipment that is supplied by a reputable organisation and all parts and spares are from the same manufacturer.
A certificate of conformity may be a requirement with new or refurbished equipment.
Some modern equipment is self-calibrating and this test can be conducted on every use.
How it Works
The low voltage holiday detector is a simple, accurate and versatile piece of equipment, however, the operator should be fully conversant with the set-up and use.
The equipment has a sponge attached to the handle. The sponge is saturated with a conductive liquid and the sponge is moved over the coated structure to be tested.
The sponge is connected to one pole of a low voltage source, the other pole is connected to an exposed area of the coated substrate.
If the liquid makes contact with the substrate via a fault in the coating, then a current will flow.
The current is used to operate a visual and audible alarm.
How it Works
The sponge has to be slightly moistened with tap water and the use of a wetting agent also will assist in reducing the surface tension, allowing the water to penetrate a fault.
The wetting agent can be supplied by the equipment manufacturers.
The sponge should not be too wet or too dry or the equipment may not function correctly.
Always test the equipment by contacting the sponge and the earth cable which should be attached to an exposed piece of the structure
Voltage Settings
It is important to have the correct voltage setting on the low voltage holiday detector.
This equipment is generally suitable for coatings with thickness up to 500 microns (20 mils) by agreement between parties.
Typically, the low voltage holiday detector will come with a number of settings:
9v
67.5v
90v
Check the coating specification or paint supplier for recommended voltage settings before using the equipment.
Coating Inspection
Once the correct settings have been established, the sponge has to be moistened (including wetting agent) and the structure suitably earthed before the equipment can be used.
Test the equipment by connecting the earth cable which should be attached to the structure, with the sponge.
The warning light should display and a noise or buzzer should be clearly heard.
The sponge should then be pushed over the coated surfaces as a steady back and forth pace of between 0.1m/s and 0.3m/s.
Continuously check to ensure the sponge is moist and periodically check the earth by re-connecting the sponge and earth clip.
Grounding
The ground cable (earth) is attached directly to the conductive substrate for positive electrical contact.
When testing a coated steel surface ensure you attach the cable to a section of bare steel.
If you are testing concrete then attach to the concrete rebar.
If the concrete has no rebar you may make a connection by placing the wire on the concrete and placing a weight (wet sandbag) over.
Slight moistening of the concrete may also be required.
Always test that the equipment is working correctly before use.
Coatings Cure and Testing
The coating to be tested should be sufficiently dry to enable the inspector or operator to walk over the coating.
The coating shall be dried/cured in accordance with the manufacturer's instructions prior to testing.
The surface of the coating shall be dry and free from oil, dirt and other contaminants.
The coating should not, if possible, be fully cured before conducting the test as repairs may be necessary and this could be difficult on cured coatings.
All holidays should be clearly 'marked-up' for repairs to be conducted.
Further holiday detection will then be necessary for the repairs.
Inspection of the Coatings
This video demonstrates the use of a low voltage holiday detector to a coated section of pipework.
The instrument is earthed to a bare section of the coated pipe and the operator moves the instrument all over the coating looking and listening for defects and detected defects or misses in the coating.
The sponge can fit around most items of coated steelwork, however, the process will be relatively slow on large areas.
Always re-check to ensure the instrument is working especially when no flaws are detected.
Inspection of the Coatings
This animation demonstrates the use of a low voltage holiday detector on a coated surface.
The instrument is earthed to a bare section of the coated structure and the operator moves the instrument all over the coating looking and listening for defects and detected defects or misses in the coating.
The sponge should be dampened and checked regularly for damage and dirt that may be picked up.
Set-up
To summarise use and set-up.
Check the instrument for wear, tear and damage.
Ensure the equipment matches the requirements of the coating specification including voltage etc.
Check the batteries are inserted and the lights and alarm work correctly.
Establish the correct voltage (if necessary).
Connect earth cable to structure to be tested with the use of a crocodile clip.
Moisten the sponge and use a wetting agent as required.
Use the low voltage holiday detector on the coated structure.
Mark up flaws for repair.
Standards
There are various standards available for low voltage holiday detection, including NACE Standard SP0188 and ISO 2960:2011-Assessment of porosity in a dry film.
“Discontinuity (holiday) testing of new protective coatings on a conductive substrate”
This standard practice provides a procedure for electrical detection of minute discontinuities in new coating systems that are applied to a conductive substrate.
Other standards are available and utilised in the coatings industry and should be detailed in the coating specification.
Summary
Within this training unit, we have reviewed the use of a low voltage holiday (sponge) detector and discussed the coating flaws, the set up procedure, manufacturer's instructions, standards, coating thickness and use of the equipment.
