Applications - Coating Thickness

The applications for which coating thickness gauges can be used are many and various. Here we outline some of the particular applications and their special requirements and also identifies some of the areas that create difficulties.

In general, applications fit into three main areas of activity.

Although coating thickness is an important parameter in all the above areas the requirements for and the use of a coating thickness gauge varies considerably.

Coatings Applied to Structural Steel

In the structural steel sector, gauges for ferrous substrates predominate and the rough surface calibration techniques are used.

Zinc and aluminium are also used as anti-corrosion coatings with application techniques such as thermal spray zinc and aluminium and hot dipping and electroplating for zinc. If the coating of these non-ferrous metals on the steel is sufficiently thick, > 50 µm, then subsequent paint layers can be measured using an eddy-current gauge calibrated over the zinc or aluminium coating.

The magnetic and eddy current coating thickness gauges are capable of measuring most commonly used anti-corrosion coating formulations but care must be taken with paints containing micacious iron oxide (MIO) pigments. Some, but not all, sources of MIO are magnetic so that when these are added to a paint formulation the resulting film can have magnetic properties depending on the distribution of the pigment in the film.

This magnetic property affects the readings of a magnetic coating thickness gauge as the slightly magnetic coating appears like a substrate to the gauge making readings significantly lower that the real film thickness. As MIO pigments are often used in coatings that are soft when applied, the thickness testing of such coatings has unique difficulties. When measuring paints containing MIO it is important to check that the MIO is not influencing the readings by cross checking a test panel coated with the material using a destructive coating thickness test method such as the Paint Inspection Gauge (P.I.G.) or the Säberg Drill.

If the MIO is from one of the many non-magnetic sources then there will be good correlation between the non-destructive and the destructive gauge readings. Regulations now make the use of thick fireproof coatings increasingly common on the steel used in buildings. Coating thickness gauges with 5 mm and 13 mm range capability are required.

Coatings Applied to Automobiles

In automotive manufacture and refinishing after damage, the highly specialised coatings are applied to smooth surfaces, often with thin substrate. Wet solvent-based sprayed paint applications have been most common. In recent years, development of high solids paints and powder paint techniques have altered the thickness measurement requirements. Like most of the automotive engineering manufacturing systems, statistical process control methods have been used to monitor processes.

The application of coatings can not be treated as a typical engineering process as the degree of control is at least one order of magnitude worse than that which would apply to machining processes.

Gauge R & R, provides information on the variance that applies to measurement systems as they are used for coatings in the automotive industry.

Engineering Coatings.

In this application area there are many considerations that will affect the measurement of coatings. Often the coatings are applied to small components, the coating thickness may be more critical to achieve satisfactory performance and is usually thin, they are often more expensive as materials due too their special properties and the component may be made of a special alloy.

All these considerations have effects on the performance of a coating thickness gauge and the adjustment of the gauge to the conditions of the measurement becomes more critical. In the case of metal coatings, Elcometer gauges can measure non-magnetic coatings on magnetic substrates. Materials such as tin, zinc, chrome, aluminium, lead etc can be measured.

The most notable exceptions are nickel and cobalt as these two metals have magnetic properties. Electro-less nickel, also known as autocatalytic nickel, has only a small magnetic effect which is further reduced by the high phosphorous content of the high wear resistance formulations. Electromagnetic induction gauges have been used successfully to measure electro-less nickel but the accuracy of measurement using a "1 - 3%" instrument is reduced to ± 10% and there are noticeable differences between the readings before and after heat treatment.

Electro-deposited nickel can also be measured but the magnetic properties do vary over the life of the plating bath so the traceability of the reading back to a master standard is poor. These gauges are used to monitor the plating thickness on the shop floor whilst the final bath control is carried out using other methods in the quality control laboratory.

The measurement of metals on non-ferrous metals is not easily achieved using eddy-current gauges unless there is a 3:1 ratio in the conductivity of the coating, relative to the substrate. The calibration of a gauge to measure a metal coating on a non-ferrous metal substrate must be carried out using samples of the materials with known thickness. Such gauges do not use the normal calibration characteristics and would have to be specially made for the specific coating and substrate.

Measuring Thickness of Non-metal Structures.

It is possible to measure materials, such as glass-reinforced plastic (GRP), sheet rubber or leather, by considering them as coatings over a metal base plate. In many cases these materials will be relatively thick compared to paint films and may therefore have a thickness in the order of several mm.

The higher range coating thickness gauges are therefore most appropriate i.e. those with ranges of 5mm or even 13mm. A suitable metal base or zero plate will be required and access to both sides of the material is essential. If the material to be measured is curved then the metal plate should be of a form that does not leave a gap, as this gap will be measured as additional material thickness.