Corrosion is an extremely costly and disruptive worldwide problem. The oxidation of metal (and concrete) surfaces, exposed by the breakdown or inadequacy of the applied protective coatings call for early detection to correct serious extension.
Surfaces requiring testing are generally expected to exhibit long life without maintenance, and are often difficult and very expensive to assess. Under these circumstances, it is essential that regular inspection, good measurement methods, and competent instruments are used, and accurate test results obtained, both before and after installation.
Increasingly, Porosity Detectors are employed in the non-destructive detection and location of pinholes (holidays), bare spots or thin points in protective coatings applied for corrosion protection over metal and concrete (conductive) surfaces.
PCWI manufactures both high and low voltage (wet sponge) instruments with a complete range of relevant accessories for such inspection and detection.
All instruments are battery-operated, passing the voltage output via a probe to an electrode. As the electrode moves over the coating surface, if it encounters a pinhole or bare spot, a small current flows activating a visible (non hazardous) spark at the point of contact and an audible and visible alarm in the unit.
For more than 30 years PCWI has been active in application, inspection and instrumentation for the coatings industry.
Realising that Porosity (Holiday) Detectors are a “working tool”, and as such must be operator friendly over extended inspection periods, PCWI several years ago commenced manufacturing lightweight technically advanced units to supersede the bulky and cumbersome units on offer. The PCWI® Compact Detector is the culmination of lengthy field experience and laboratory testing.
Benefits of the Latest Compact Range
It is hoped that the following facts may assist users when testing.
With the surge ahead within the electronics industry, High Voltage Porosity Detection (holiday detection / spark testing) is changing rapidly with the latest PCWI® Compact Detectors:
LCD's now display the actual voltage applied to the coating during testing;
internal voltage feedback loops monitor and hold steady that applied voltage;
fine voltage tuning allows operating from low volts to maximum 60kV within one unit;
sensitivity controls on the alarm circuit allows false results to be calibrated out; and
the latest refinement: - constant test current over the whole voltage range, allows the unit to test low film builds.
This new testing capability has revolutionised the porosity testing industry and the superior test method has seen the virtual demise of wet sponge testing in the industrial coatings industry in Australia. Not only is high voltage testing faster, larger probes can be used to cover more area in the same time, even at very low voltage settings. Touch-up can be carried out immediately after testing as water is not used, and drying time is not required.
Limitations of Wet Sponge Testing
Whilst we manufacture low voltage instruments for the inspection of very thin (less than 150μm) coatings, you should remember the limitations of wet sponge testing:
The test is operator dependent, wetness of sponge will vary results, and working in an overhead situation presents particular challenges.
You should be fully aware that wet sponge testing only locates holes that actually expose the naked substrate. Flaws in the coating that cannot be wetted out to the substrate cannot be found.
A wet sponge holiday test would NOT locate nor indicate the following flaws:
A thin area of coating - this could be as low as 2 microns for example the side of a weld or the centre of a crater or pinhole.
An air bubble in the coating, where the coating has sagged and left washed out areas or pinholes exposing the primer.
A steel sliver, lamination or hackle on the surface of the substrate.
Abrasive grit or airborne insects that have landed in the coating.
In the longer term these may well be areas of coating failure, yet would have been passed as sound with the wet sponge method of testing.
A Practical Example
During a field inspection to determine coating suitability, we used a high voltage holiday detector to test a three coat coating system on the external surfaces of two tall steel exhaust stacks. The stacks had external stiffening rings and wind deflectors and, due to the nature of the plant, it was a highly corrosive environment.
Results: we found defects around the welds and on the edges of the stiffeners and wind deflectors. This was to be expected, however on the upper half on one side of one stack we found a great variety of pinholes not visible to the naked eye but flaws to the HV detector. These pinholes were in the second and third coats, he primer being pinhole free.
Why: the stacks were close together and the spray painters started at the top of the stacks at the same time. The painter downwind had problems coping with over-spray on the wind side that day, and caused pinholes to develop in the second coat that were not fully covered by the final coat. These pinholes were re-coated and nearly ten years later the coating is slightly chalky but the stacks are corrosion free.
A “Wet Sponge” test would not have found these defects and the maintenance cost of rectification would have been substantial.
Old Tales that are No Longer True
Leaving the brush stationary on the surface will burn a hole in the coating? I believe that this has never been the case with constant voltage DC detectors, and tests carried out have shown no harm to the coating, providing that the set test voltage is not higher than the dielectric strength of the coating.
Only high voltage test the coating once, do not retest the surface over and over? A cured, sound coating that is up to its specified thickness tested at the correct voltage cannot be made to fail by retesting many times.
High Voltage Testing
Some points that may be of assistance:
When testing in vessels - clear all solvents.
At all times monitor the applied voltage.
Check all earthing, not only to the instrument, but also the substrate to ground.
Positive earths and ground spikes are the best; in dry areas pour water onto the ground spikes.
Trailing earth leads should not be used in dry sandy areas.
Use a PCWI® Compact Crest Meter to prove voltage at the probe tip, this will also prove earths, connections and leads.
Wash all salt deposits from the surface and make sure surfaces are sufficiently dry (DC testing).
Do not run the probe too fast over the surface
Do not test lower than specified film builds, thickness test all areas prior to HV testing, a HV detector should not be used in place of a coating thickness gauge.
Do not test uncured coatings, as these may contain solvents, these coatings may not have reached their full dielectric strength, in which case burn-through would be easier.
With dry sprayed coatings, generally pinholes can be found with every pass of the brush, especially on retreating.
A flaw is found easier the second time than it is the first time, likely due to a carbon track being produced by the arc of the first spark.
Coating Capability Trials and Self-Tests
Manufacture a coated test plate under controlled conditions.
Drill a number of very small holes (0.6mm) down to the substrate.
Earth the plate to ground as well as to the detector.
Pass the electrode over the flaw, increasing the voltage with every pass, this will indicate the minimum voltage required to locate a clean flaw down to the substrate. Once a flaw or spark has been generated do not use that area for a second test. Go to a new flaw.
Place the probe on a sound area of coating and keep increasing the voltage until the coating fails, this would be the absolute maximum breakdown voltage under ideal conditions, you would not test at this voltage, but it will give you an indication of the breakdown voltage and where the level of test voltage should not be.
It may surprise you to find out exactly how much voltage it actually needs to break down a sound coating.
There are formulae available to determine test voltages that may vary with differing coating types (refer AS3894.1-2002). Keep in mind that they may be minimum test voltages designed for use only with soft fine wire bristle brushes.
Should test voltages be set at the low end of the scale, calculated from the minimum or the specified thickness, flaws at a higher film build may not be found.
Fine brass wire brushes are the best probes. These lie flat on the surface and make better contact, especially on irregular shapes. The voltage (spark) leaves the end of the fine sharp bristle easier so has better searching capability than a flat or round object such as a coil.
Voltages may need to be increased when pipe coils or rubber strip probes are used.
Rubber strip probes should not be used on welds, pitted steel, in corners, or on irregular uneven shapes.
Probes need to be in contact with the coating surface at all times, any increase in the gap would mean a need for increased test voltage, Pipe Coils with large gaps between segments are not a good idea.
Longitudinal welds are difficult to test as coils run up on the high spots leaving gaps at the welds edge.
Slightly higher test voltages make for easier more precise faster testing.
Higher test voltages could be applied when coatings are introduced into severe environments.
Test Voltages in many instances, depend on, to what degree do you want to prove your coating system, the type of service the coating is used in and can it be readily repaired should it fail.
Fabricators and Applicators:
Grind all sharp edges and keep welds smooth.
Brush an additional coat (stripe coat) on all edges and welds.
A little extra care in the final stages can be a major cost saving exercise in the long term.
It is easier to prevent flaws than to try to repair them afterwards.
How long does it take to Pososity Test?
If you are painting an item with a multi-coat system, allow the same time to High Voltage test it as it took to apply the final coat - if you took that helpful hint and used a little extra care then it should be porosity (holiday) free.
Inspection & Detection with PCWI Detectors is Cost Effective
For newly constructed tanks, pipelines and other installations, where corrosion prevention coatings have had to be applied, specifications will normally call for a specific coating to a specific thickness. By using a PCWI Detector to verify the competency of the sealing coatings, the contractor can be assured that his coatings responsibility has been met. In ongoing maintenance programmes, regular inspection and detection can prevent serious corrosion and consequential loss.