Feb. 04, 2024
Measurement & Analysis Instruments
Dry film thickness can be measured on either magnetic steel surfaces or non-magnetic metal surfaces such as stainless steel or aluminium using a digital coating thickness gauge. The principle of electromagnetic induction is used for non-magnetic coatings on magnetic substrates such as steel. The eddy current principle is used for non-conductive coatings on non-ferrous metals substrates.
A permanent magnet is mounted on a balanced arm and the force required to pull this magnet from the surface of the coating is a measure of the thickness of the coating. Force is applied through a helical spring attached to the balanced arm at one end and to a scale wheel at the other. As the scale wheel is turned the force is progressively increased until the magnet lifts from the surface. The scale is drawn in thickness units rather than force and the thickness of the coating can be read against a pointer on the case of the instrument.
Electronic coating thickness gauges for measuring on magnetic substrate materials use the electromagnetic induction principle. A three-coil probe system is used where the central coil is powered by the instrument and the other two coils, either side of the central coil, detect the resulting magnetic field. The signal generated by the instrument is sinusoidal and therefore an alternating magnetic field is established round the central coil.
When there is no magnetic materials influencing the probe then the magnetic field cuts through the other two coils equally. As the probe is brought closer to the uncoated substrate the field becomes unbalanced with more field cutting the nearest coil and less cutting the furthest coil. This produces a net voltage between the two coils which is a measure of the distance to the substrate (the coating thickness).
In the case of the eddy current principle, a single coil probe is used with a relatively high frequency signal, several mega-hertz, to generate an alternating field in the non-ferrous metal under the coating. The field causes eddy currents to circulate in the substrate which in turn have associated magnetic fields. These fields influence the coating thickness probe and cause changes to the electrical impedance of the coil. These changes are dependant on the coating thickness.
A key decision on the overall selection of a suitable coating thickness gauge is how accurate do the readings need to be? Within the range of gauge types available, there is a progression from moderately accurate to very accurate gauges, this is mirrored by the prices of the coating thickness gauges, the more accurate, the higher the cost. In addition the coating application process and other factors affect the variability of the coating thickness on a particular surface and the skill and knowledge of the coating thickness gauge operator also influences results.
The basic measure of a coating thickness gauge's performance is the accuracy with which the gauge takes readings. That is the difference between the reading and the true coating thickness.
In order to test the accuracy of a particular gauge it is important to have traceable coating thickness standards. With the gauge adjusted to zero on an uncoated smooth substrate and set to a known thickness standard at or near to the maximum thickness, intermediate thickness standards are measured and the readings compared to the actual thickness of the standard. The errors are the differences between the values of the reading and the value of the standard. These are most conveniently expressed as a percentage of the reading.
(Mechanical)
(OP)
21 May 14 15:58I have a specification that calls for a surface preparation of 1.5-2.0 mils.
It was blasted and measured with Press-o-film testex and measures 4.0 mils.
Is more mils better for adhesion? Is this acceptable? What should I know?
(Geotechnical)
21 May 14 16:32first... check the limitations on your testex film. if you are using "course" film that reading of 4 could actually be more but is not registering because you run out of the white stuff to inject into the crevices.
The product rep for the coating is the best for this question. 3 to 4 mils is pretty typical result from blasting for recoat jobs on bridges and water tanks using epoxies and zinc-primers. Apart from getting better adhesion, this high profile makes it harder for an evenly applied coat to cover all the peaks.
(Mechanical)
(OP)
21 May 14 16:57We were using x-coarse press-o-film so I believe the values are accurate. Up to 4.5. We got an average of 4.1 mils and we took 18 measurements.
It will be coated with Sherwin Williams Zinc Clad II primer.
(Aerospace)
21 May 14 23:51?? Whose specification are you violating? Are you not concerning about not meeting a requirement?
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TomDOT(Materials)
22 May 14 08:17On recoat jobs where someone else has previously blasted, you are pretty well stuck with getting the same (or deeper) profile. It's almost impossible to reduce a profile once you have it.On the flip side, some contractors just blast the hell out of the surface and achieve too deep of a profile all on their own.To determine where the deeper profile is coming from, you would need to clean a small area in a way that will not deepen the profile. Three methods come to mind:1) Blast with a very fine abrasive (maybe 200 mesh or less) and determine DFT on that area.2) UHP water blast determine DFT.3) Chemically strip and determine DFT.Deeper profiles (and sharper profiles) will generally give better adhesion. HOWEVER (and it's a big however) - you have a much higher risk of pinpoint rusting. Blasting almost always results in scattered "rogue peaks" which are much taller (think 2x taller, maybe more) than the typical measurement with Testex tape. Typical solution is to require a thicker primer, with a letter of approval from the owner/specifier and from the paint manufacturer.Epoxy zinc coatings are generally pretty forgiving of extra thickness. IOZ are decidedlyforgiving of over thickness. Zinc Clad II is an IOZ. Over thickness in IOZ generally leads to curing issues and mud cracking.The data sheet (linked below) has a recommended dry film thickness (DFT) of 2-4 mils (50-100 microns), and a(150 microns).
(Geotechnical)
22 May 14 15:26I have a Sherwin Williams letter from one of their NACE3 product reps that should be similar for your condition.... i copy/pasted the text below and removed the reps name and project since this is the internet after all.
"Per our conversation I understand the surface profile on the XXXXXXX County Bridge has averaged
approximately 4.1 mils with a high of 4.3.
Sherwin Williams states a blast profile recommendation on the data sheet at 2 mils. While this is optimal, I
am comfortable with the application of the Zinc Clad II Plus Inorganic Zinc over the higher profile. The
key to the application will be to limit the total dry film thickness of the coating to 6 mils dry. Our primary
concern if applied over this film thickness is possible mud cracking and/or shearing of the inorganic zinc.
Please be advised that cure schedules will also be affected slightly with higher film thicknesses."
(Structural)
23 May 14 09:29The deeper profile can affect the surface of the coating and the little peaks near the surface can form a 'stress point' for the coating surface. Best to minimise rogue peaks. Is this a primer? and do you have a top or intermediate coat over?
Dik
(Materials)
23 May 14 09:45Contractors love that solution "we'll make it up on the next coat!"
Nevermind that the next coat is cheaper per mil, easier to apply and won't offer the same corrosion protection that a proper primer will.
The primer is your primary corrosion protection.
(Structural)
24 May 14 22:48Tom... funny, I've heard the same line up here... must be standard coursework for Contractor 101 classes...
Dik
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