Measurement on
Rough Surfaces

Shot or Grit Blasted Surfaces

In the field of anti-corrosion coatings applied to steel structures, it is common for the steel to be prepared for coating by blast cleaning. Angular grit and rounded shot are the most commonly used media but there are developments in the area of blast cleaning which are introducing new media such as abrasive loaded sponge and even hydroblasting using high pressure water jets. Each of these blasting methods leaves a characteristic profiled surface.

This profile has an effect on the accuracy of a coating thickness gauge. If a gauge is adjusted on a smooth surface and then used on a blasted surface significant reading errors as high as 40 - 50 µm (1.6 - 2.0 mil/thou) on a very coarse grit blast will be seen. The simplest way to account for this effect is to adjust the gauge to read correctly on a smooth surface and then correct the reading by subtracting a value which is dependant on the profile.

This effect imposes a limit on the measurement capability of magnetic and electromagnetic gauges such that coatings with a thickness at or less than the peak-to-valley height of the profile can not be determined using direct measurements. Typically this means that coatings with a thickness of 50 µm or less can not be measured on a profiled surface.

A technique recommended for these circumstances is to use a flat, smooth test panel placed over the blasted surface and coated during the application. This coating can then be measured with a coating thickness gauge when it is cured and the area covered by the test plate can be coated when the plate is removed. This method has the benefit of providing a permanent record of the coating process in that area of the job.

Smooth Surface Calibration Adjustment

The simplest and most consistent way to take measurements on a blasted or otherwise roughened surface is to adjust the gauge on a smooth surface and to correct the reading for the effect of the roughened surface.

The CEN method for measuring coating thickness on steel structures, currently at the draft stage but expected to be published in the year 2000, gives a default correction value of 25 µm when the profile is unknown and values between 10 and 40 µm for fine and very coarse profiles as defined by ISO 8503. This is applicable to all coating thickness gauges from all manufacturers but does not take account of the different probe designs.

Experiments carried out at Elcometer show that different probe designs have different characteristics on roughened surfaces and that the effect is thickness and profile dependant. Rough Surface (or Two Point) Calibration Adjustment An alternative method is described in an annex to the CEN document where the gauge is adjusted over a sample profile surface that is typical of the profile for the work in hand.

By using a thin foil value, below the expected coating thickness, to adjust the lower adjustment point and a thicker foil, above the expected coating thickness, to adjust the higher adjustment point, a gauge is set to read the thickness of the coating above the peaks of the profile.

This technique has been shown to have good agreement with the smooth surface - correction value method but it is more difficult to adjust the gauge correctly and not all instruments have the capability to be adjusted in this way. It has been shown that a statistical approach to the two-point adjustment technique is also beneficial as the gauge can be adjusted at a point that is typical of the whole surface.

Again not all gauges have this feature and mechanical gauges and the simpler electronic gauges do not have any form of reading memory. The CEN method has the rough surface adjustment technique described above included as an annex to comply with existing national standards, most notable the Swedish standard for the measurement of coatings on structural steel.