TESTING & MEASURMENT
Measuring surface profile with the Elcometer 224
A major factor affecting the performance of a coating system , primarily on steel , is surface profile . Steel surfaces are blasted not only to remove mill scale , corrosion or previous coatings , but to generate a surface profile . A surface profile can also be achieved through chemical etching or mechanical abrasion . Regardless of the method , the surface profile increases the surface area of the metal - and it is this increased surface area which provides a better key for the coating to adhere to the substrate .
Fig 1 . The surface profile increases the surface area
There may also be an insufficient surface area to provide a suitable key for the coating - leading to poor adhesion and hence premature coating failures .
A fast , accurate and repeatable method for measuring surface profile is by using a digital surface profile gauge such as the Elcometer 224 Digital Surface Profile Gauge .
The Elcometer 224 is a needle depth gauge , which is very easy to use . The gauge is easily zeroed on glass , ensuring the probe is 90 ° to the substrate . Then , to take a measurement , the gauge is placed perpendicularly on to the blasted surface . The spring mounted needle will measure the depth of the profile from the peaks of the profile at that particular point .
Fig . 2 Elcometer 224 Digital Surface Profile Gauge
The coating manufacturer or the coating specification agreed to by all parties specifies the required surface profile . The surface profile specified is directly related to the required dry film thickness of the coating being applied .
The surface profile specified is usually the peak-to-valley height in either microns or mils and the dry film thickness is also specified in either microns or mils . It is important that the specified coating thickness should be applied to both the valleys and the peaks .
If the profile were too high , the amount of paint required to cover the peaks would significantly increase . Extra paint would be needed to ensure the high peaks remain coated . Peaks that are left uncoated or too thinly coated are at risk of pinpoint rust or rust spots ( rust rash ).
If the profile is too low , then the coating thickness applied will be too high for the profile . The coating thickness is calculated for a specified profile and if the resulting coating is too thick , possible cracking of the coating whilst it cures can occur , resulting in corrosion .
30 MAY 2021
As the measurement point is not necessarily at the very bottom of the valley , and could be somewhere on the slope , either five or ten individual measurements should be taken within a 100mm ( 4 ”) diameter area . The gauge will then provide a reading .
Recently , there have been changes to one of the most used profile standards , ASTM D 4417 . Having previously stated that a profile measurement using a needle profile gauge should be the average of ten readings , this was changed to the maximum of ten readings .
Within the industry , some concerns were raised that this was not a true characterisation of the surface and following some revision , the Standard has recently been further revised . The Standard now allows either the maximum of ten readings or the average of ten readings , which some think is a more realistic and statistically correct way of assessing the readings and the profile .
CONTINUED ON PAGE 32
read online : www . surfaceworld . com