Holiday Detection in
Applied Coatings
1. Inspection of coatings using spark testing
This is a high voltage technique
that can be used to detect pinholes, bubbles, inclusions and thin areas in
insulating coatings applied to a conductive substrate.
A detector generates a high DC
voltage, which is supplied to a suitable probe, commonly, a long wired brush.
As the probe is passed over the coated surface, a spark and audible and/or
visual alarms in the detector indicate a flaw. The voltage is adjusted to suit
the coating thickness.
The ability of the spark tester,
when correctly adjusted, to detect low thickness in an applied coating makes
this the preferred method of holiday detection. The metal content in Belzona
1321, Belzona 1391 & Belzona 1392 precludes the use of high voltage spark
testers for carrying out final inspection for pinholes and misses.
Inspection using HV
spark testing involves making ONE
pass over the coating at 30 cm/s (1ft/sec). Multiple
passes can “charge” the coating like a capacitor and cause false alarm. (See also NACE SP0188).
1.1
Coatings that can be inspected using a spark tester include:-
Belzona 1341, Belzona 1521, Belzona 2121, Belzona 2131,
Belzona 2141, Belzona 4311, Belzona 4341, Belzona 5811, Belzona 5851, Belzona
5891, Belzona 1391S, Belzona 1391T,
Belzona 5811DW2 and Belzona 1591.
1.2
False Readings
Some of these materials when cured
in a cool or humid environment can form a surface bloom, a greasy film on the
surface, and this can lead to problems during the inspection procedure. When
using a high voltage spark tester it is important to remove any bloom that may
have formed on the surface of the coating which will lead to surface tracking.
If tracking occurs, a circuit will
be formed across the coating surface NOT through the coating to the substrate
and a drop in resistance will occur triggering a "false alarm".
Tracking, caused by a surface bloom or surface condensation, should be
suspected whenever an abnormal number of defects are being signalled in what
appears to be a defect free coating.
This surface tracking can be avoided
by thoroughly washing the surface of the coating using a warm detergent
solution, then thoroughly drying the surface.
Similarly high humidity or
condensation on the coating surface can cause surface tracking of the electric
current generated by the equipment triggering a "false reading".
These problems are particularly to be expected when working in the confines of
a tank or vessel.
In high humidity conditions, it may be
necessary to raise the surface temperature 3 – 5°C above the dew point to avoid
condensation. In addition, it is advisable to maintain adequate ventilation to
prevent any increase in the relative humidity within vessels. Similar problems
can also be experienced when using wet sponge testers.
A bloom or high surface moisture
liable to create false readings can be observed in several ways depending on
the Instrument used. Those with a light diode in the handle will show a glow
when surface tracking is occurring. Similarly a drop in voltage when the probe
is moved over the surface indicates surface tracking. Instruments with the
ability to display the current also see a significant rise over areas of the
coating where no defects are present but have surface contamination capable of
causing tracking and leakage of the applied current to earth.
High voltage holiday detectors with
adjustable sensitivity can be adjusted to overcome some or all of the problems
of "false readings". With the instrument set at the required test
voltage, adjust the sensitivity such that no signal is triggered when the probe
is moved over an area known to be defect free. The instrument should then still
signal a defect when the probe touches the earth lead. On a heavily bloomed
surface a drop in test voltage may be seen and it is acceptable to increase the
test voltage to compensate for this. An excessive voltage should, however, be
avoided.
With careful control, adjustable
sensitivity spark testers may be successfully used on heavily bloomed surfaces
without the need to remove the bloom. Care must,
however, be taken to avoid build up of the bloom on
the probe itself as this may cause discharges and false alarms. Furthermore,
continual use on a conductive surface may lead to rapid drain on the spark
tester's battery. Units with sufficient battery capacity should be used or
facilities made available for recharging.
2. Inspection of applied
coatings using the Wet Sponge Tester
2.1 Why
Wet Sponge Test?
As you are probably aware, the metal
content in Belzona® 1321 & Belzona® 1391 precludes the use of high voltage
spark testers for carrying out final inspection for pinholes and misses. This
leaves visual inspection or low voltage sponge testing as the only option.
2.2 The
principle of the Wet Sponge Test
The principle of the wet sponge
tester is that a sponge, soaked with a wetting agent, is supplied with a low
voltage, typically 90 volts. When the sponge is moved over a pinhole, liquid
flows to the substrate and completes the electrical circuit. The resulting
current flow generates an audible alarm in the detector.
2.3
False readings
When using a wet sponge tester it is important to remove any
bloom that may have formed on the surface which will lead to surface tracking.
If tracking occurs, a circuit will be formed across the coating surface NOT
through the coating to the substrate and a drop in resistance will occur
triggering a "false alarm". Tracking, caused by a surface bloom or
surface condensation, should be suspected whenever an abnormal number of
defects are being signalled in what appears to be a defect free coating.
Remove all surface bloom from the
coating, using a warm detergent solution. Thoroughly dry the surface and then
carefully inspect for any signs of a residual greasy film. Repeat this
procedure as necessary until the surface is totally clean.
In high humidity conditions it may
be necessary to raise the surface temperature 3-5°C above the ambient air
temperature to avoid condensation.
Make sure that the sponge on the wet
sponge tester is uncontaminated, or alternatively fit a new sponge. (Note:
Residual amine bloom on the sponge has been seen to re-contaminate the surface
and cause tracking.)
Moisten the sponge using tap water,
mixed with 1 part Surfactant (typically washing up liquid) to 128 parts tap
water, squeezing out any excess water.
The risk of tracking will be
increased if a film of water is allowed to form over a large area or there is
running water. To avoid this, there must only be sufficient water in the sponge
to wet the immediate contact areas.
Conduct the test over individual
areas of approximately 0.5 sq.m., drying each area after testing before going on to the
next. Two passes should be made at a maximum speed of 30 cm/s (1 ft/s). Any area deemed to require repair should be marked
with a marker that is easily removed. The area around the defect should then be
dried and the test continued in an adjacent area. All discontinuities must be
repaired and those areas re-checked.
Note. Although Amine bloom film is
most commonly formed during the cure of the material it can, under certain
circumstances, also reform after initially being removed. Therefore, following
remedial work it will be necessary to wash the repaired area and the area
immediately surrounding the repair before carrying out the re-test.
2.4
Coating thickness limitations
There has been some confusion as to
the role of low voltage sponge testers in the inspection of Belzona® coatings.
Published data from Manufacturers of these instruments limits their use to
coatings below 500 microns thick. This is in line with various
international standards covering their use. Manufacturers have confirmed our
own tests which have shown that carefully used a sponge tester will pick up a
miss in the coating above 500 microns, subject to the need to wash the coating
as above.
2.5
Inspection during coating
Care during application and visual
inspection, is far more important than a wet sponge test. The latter will only
show a complete miss through to the substrate. Any thin areas, even where the
covering may be no more than 50-100 microns, will not be found or indicated by
the sponge tester. Inspection of the thick, single coat applications,
particularly during application is critical to ensure a continuous defect free
coating is achieved.
The demand for post application
inspection, for new coatings in our range does feature a requirement for them
to be spark testable if possible. However, the ultimate performance of the
coating is paramount and the metal fillers used in Belzona® 1391 and Belzona®
1321 are critical to the erosion resistance and performance in service of these
coatings. For the foreseeable future, therefore, we must continue to inspect
these materials visually, assisted by the sponge test, albeit not ideal, as the
only inspection tool available.
2.6
Equipment Suppliers
England
Elcometer Limited
Edge Lane
Manchester
M43 6BU
Tel: +44 (0)161 3716000
Fax: +44 (0)161 3716010
Buckleys (UVRAL) Ltd
Beta Works
Range Road
Hythe
Kent
CT21 6HG
Tel: (01303) 260127
Fax: (01303) 262115
Paint Test Equipment
3/4 The Courtyard
Greenfield Farm Estate
Congleton
Cheshire
CW 12 4TR
Tel: (01260) 275614
Fax: (01260) 299231
USA
Elcometer Inc
1893 Rochester Industrial Drive
Rochester Hills
Michigan 48309
Tel: +1 248 650 0500
Germany
Elcometer Instruments GmbH
Ulmer Strasse 68
D-73431 Aalen
Tel: +49 (0)7361 528060
Asia & Far East
Elcometer (Asia) Pte Ltd
896 Dunearn Road
Sime Darby Centre #03-09
Singapore 589472
Tel: +65 6462 2822
Modified July/13/2011
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