Quality testing of paint and metal coatings

Corrosion protection and therefore quality testing of paint and metal coatings has become essential, says Klaus E. Steingroever

Corrosion protection has become a very important part of product quality in recent years. Manufacturing companies and users are more aware of the damage to their products if they do not provide sufficient corrosion protection. Modern technologies on surface treatment by painting with organic coatings or by protection through metallic coatings ensure longer life and use of the product.

Surface treatment and coatings also serve various other functions, which are partly of a technical nature but also produce an attractive appearance, which helps selling the product. For more than 20 years, a special industry has developed suitable testing instruments for various properties for surfaces and coatings. Basically every component or object has a surface, which can be coated and measured for coating thickness, gloss, colour, roughness and other parameters. Typical industrial fields where corrosion is a major problem are automotive, aircraft, shipbuilding industries as well as machine construction and all metal constructions in general.

Most coatings are applied to metal substrates. One of the most used measuring principles, therefore, is the permanent magnetic principle. It was developed in the mid fifties and is represented by several instruments. Other instruments for measurements on steel use the magnetic induction principle for electronic and digital measurement. For coatings on non-ferrous substrates, electronic instruments use the eddy current principle. Like the ones for measurements on steel, electronic instruments using eddy current principle can be easily handled even by untrained personnel. All the mentioned instruments need direct contact of the measuring probe to the coating surface. However, there are also new developments available, which perform non-contact measurement for the measurement of powder coatings before baking and wet paint coatings before the drying process.

The mentioned measuring principles are described in detail in national and international standards – EN ISO 2361, 2178 for the magnetic attraction and magnetic induction principles for measurements on steel substrates, and EN ISO 2360 for eddy current principle for measurements on non-ferrous substrates. For the user the measuring principle plays a relatively unimportant role if he performs an accurate calibration to avoid false or incorrect readings. The general calibration procedure is done by setting the instrument to a zero value on an uncoated substrate, which must be similar to the one that is to be coated. This calibration sample must be similar in shape, roughness, size and material in order to perform the correct calibration.

Various standards like the ones issued by the American National Standards Institute list the factors which can, and will affect the measuring accuracy like magnetic properties of the substrate including the metal substrate thickness; edge effect, which means that measurements close to edges also need a calibration to zero at the same distance from such an edge; curvature radius; surface roughness; direction of mechanical working of the base material; residual magnetism; stray magnetic fields and foreign particles, which means that the probe needs to be checked before calibration and measurements for small, hardly visible iron particles. Sticky tape is good for removal. Conductivity of coating is also a factor, and especially for measurement on aluminium and other non-ferrous metals. Some instruments work at frequencies between 200-2000 Hz. At these frequencies eddy currents produced in thick, highly conductive coatings may interfere with the reading. Yet another factor is the pressure of probe, which means that there may be a possible indentation into paint coatings produced by the hard metal probe tip if these coatings are not completely cured.

A glossy appearance is a major requirement for products manufactured for non-industrial consumer markets like cars, bicycles and many other products. Easy to use instruments are available from various manufacturers. Often, steel substrates are pre-treated before coating by various blasting procedures, creating more or less surface roughness in order to have a good adhesion of paint to the steel substrate. The measurement of substrate roughness can be performed with small handy instruments in order to check the quality of the blasting process. However, for the coating thickness measurement this roughness creates a problem because the measurement can vary from place to place. Therefore it is necessary in order to take correct readings on previously blasted surfaces to take an average from a sufficient number of readings across the total area. Modern instruments will calculate automatically the average roughness influence, which may be as much as 35 microns. After the coating process, coating thickness measurement must be corrected by this amount established on the uncoated metal surface before coating.

The paint coating process requires an even coating, and also the same colour appearance. Colour measurement can be performed with instruments and is a very difficult field of laboratory research. Most customers, however, need to measure the constant colour factor according to the LAB system. This measurement can be performed easily with a small portable unit capable of absolute colour measurement as well as the definition of colour comparison. For some years many companies have introduced a quality management system according to ISO 9000. In order to fulfil these standards it has become necessary to document the quality control results. A wide range of coating thickness gauges is capable of producing the necessary statistics. Some of these produce, without a separate PC program, a complete statistics evaluation including a histogram. Such a histogram shows the evenness of the finished coating thickness.

Any organic coating can have very small defects called pinholes, which may be as small as a few microns and invisible to the naked eye. Especially products that are coated to withstand a highly corrosive environment like ships, cars, offshore and oil industries, and steel constructions, need to be tested for such pinholes. The measuring principle applies a controlled high voltage to the coating surface and will detect any pinhole by sounding an alarm. This testing method allows instant repair of defects.

(For further information contact: Bombay Tools Supplying Agency: Tel: 022-23426495. Email: info@bombaytools.com)