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PCWI Porosity Holiday Detectors. The instruments and support you need for your industry. High and low voltage testing of coatings, myths and misconceptions.
PCWI Porosity and Holiday Detectors manufactures a vast range of brushware to suit both the DC pulse and the DC units.
These are the 3 units that we have a wet sponge unit a pulse DC and a DC which is a direct current and constant current unit. The wet sponge unit is a low
voltage tester mainly used for thin films.
Can a wet sponge test give a porosity free coating?
No, it cannot. It would only find pinholes through to and exposing the substrate.
A low voltage wet sponge would only find flaw No.1. That would be dependent on the flaws diameter, coating, thickness and the flaws capability of being wetted
out. All the other flaws that a wet sponge unit will not find.
If you are satisfied with that as a result, then the low voltage wet sponge test maybe suitable for your project. That is, providing you can get back to
revisit a few years later to evaluate the coating system, to carry out recoating and repairs. This test does not guarantee a flaw free coating system. Depends
on what you call a flaw??
For years now they have said you cannot test thin films with high voltage Porosity Holiday Detectors. Yes you can. Electronics have changed dramatically over
time. Although do not use a High Voltage pulsed detector. I do not recommend the use of those on thin films. Use the latest DC (Direct Current) Constant
Current high voltage units, test films down as low in thickness as you want.
The high voltage detectors.
Can a stationary probe on the surface cause burn through the coating creating flaws and pinholes?
During testing a stationary probe left long term on the coating surface has no detrimental effect on the coating whatsoever. Can subsequent, multiple passes of
high voltage when testing weaken the coating cause burn through creating pinholes? Multiple passes and retesting with high voltage has no weakening effect on
the coating. Finding new flaws/pinholes when testing repairs in previously tested areas. It does happen and there can be many reasons for this. For a
starter let’s look at some of this.
Heavily corroded substrates only test with a brass wire probe, these fall into the holes, the carbonised rubber strip probes will not fall into the
indentations like a brass strip wire probe will. This should have been the probe that was used here, not a little fan brush. As you can see there is going to
be a terrible amount of missed areas and on retesting you would most likely find all the spots not covered before.
Heavily corroded substrates and rough coatings use overlapping passes as in A and B. This area of porosity as shown in No.4 is a very difficult area to find
and you need enough voltage to virtually break through that to find it. If you are relying on the voltage to find it and not on a visual inspection to find it.
No. 5 is just a low area.
If you use just a carbonised rubber probe you are going to find that the gap is increased to the total thickness but you are not really testing that area with
much more voltage. It sort of skips over the top and if there is any raised sections that actually lifts up, so it is not a good idea to use these rubber strip
The brass wire falls better into the holes especially when a bit of pressure is put on it. This gives increased searching area.
This area here is being tested with a rubber probe. A rubber probe on this sort of thing is not suitable. It skips over areas, it is lifted up and if there is
any undercut with pinholes along the welds it will not find it.
These are the areas it would most likely miss. It could just not be tested with a rubber probe. The 2 oval circles at the top left-hand side of the photo,
nothing is going to salvage those because that is stitch welded. It is not suitable for tank work. It is not suitable for any external work.
Once again these are the flaws that you would be looking at finding. It is easy to not have enough voltage or to use the wrong probe to skip over those.
Generally flaws and lower sections are coated last. This can have coatings applied over dry spray, spray dust, creating underlying porosity.
This is difficult to find as it may not be visible from the surface. This is an area of underlying porosity looking at the backside of the coating. This would
be difficult to find. A further section of this showing the first coat - this is in between 2 coats actually.
Finding new flaws, pinholes when testing repairs in previously high voltage areas. All that can be said is; you either missed them the first time, and you used
the wrong settings or gear. Then maybe the coating has an underlying problem, not visible on the surface to the naked eye.
Porosity under the surface of the skin of the coating is a difficult thing to find.
With the environmental issues today, no one really needs coating failure as you need to repair corroded pitted steel or leaking ruptured pipelines are a
disaster. It is a different world today than what it was a few years ago. May be it is time to do a little rethink on this porosity detector.
PCWI Porosity/Holiday Detector Probes will manufacture anything you require in this area. Earth leads, extensions, adaptors etc. Contact