Understanding Surface Profile with the Elcometer 224 Digital Surface Profile Gauge
In the industrial and protective coatings industry, metal surfaces are blasted not only to remove corrosion or old coatings, but to generate a surface profile prior to re-painting. In some industries a surface profile is achieved through chemical etching or mechanical abrasion. But, regardless of the method, the surface profile increases the surface area of the metal - and it is this increased surface area which provides a key for the coating to adhere or stick to.
Surface profile should not be confused with roughness. Surface profile is a measurement of the peak-to-valley height. Surface roughness, on the other hand, is the combined measurements of the surface profile and the frequency of the peaks across a linear length (also known as the peak count). Surface roughness is measured using a stylus instrument.
Now that we have defined surface profile as the peak-to-valley height, why is it important for the coatings industry to measure it?
Quite simply, to avoid corrosion. Corrosion is typically caused when three things come together – a ferrous material (in our case the steel substrate – although it is worth noting that non-ferrous materials can also corrode), oxygen (from the air) and moisture. If you can remove just one of the 3 elements, corrosion (or more precisely aerobic corrosion) simply cannot happen. Applying a coating to the steel provides a barrier between the steel and both the air and moisture, preventing corrosion. So if the coating is damaged, or simply rolls off the surface due to poor adhesion, then the steel will rust, corrode, and in time weaken the structure. Not exactly ideal if you are coating a bridge for example.
Typically the surface profile specified is the peak-to-valley height in either microns or mils. The coating is also specified in either microns or mils, as a dry film thickness. The key here is that the required coating thickness is applied to both the valleys and the peaks, otherwise you get rust spots.
So, if the profile is too high, the amount of paint used to cover the peaks is significantly more, otherwise there are thin areas of cover – resulting in poor protective properties, leading to rust rash and early failure.
If the profile is too low, then the coating applied can be too thick, leading to possible cracking of the coating whilst it dries (or cures), resulting in corrosion and pinholes. Not to mention that there may also be an insufficient surface area to provide a suitable key for the coating - leading to poor adhesion and premature coating failures.
So how do we measure surface profile?
There are a number of methods for measuring surface profile, and they all work in different ways.
Surface Comparators are flat, metal plates, which contain a number of reference surface profiles. The comparator is placed on the surface, which is compared with the reference profiles through either sight or touch. The profile is then determined and recorded. While this is a quick assessment of the surface profile, it’s a highly subjective method of measurement, as it can come down to the opinion of one person over another.
A widely used, and less subjective alternative is the Replica Tape method.
A tape with a compressible foam pad is applied to the surface, and rubbed with a burnishing tool. By rubbing the tape into the surface, the foam is pushed down, creating an inverse or mirror image of the profile. The tape is then removed and measured using a thickness gauge, such as the Elcometer 124 gauge. The replica tape method provides a peak-to-valley height measurement of the profile.
Unfortunately this method has a low stated accuracy, and the test itself is quite time consuming – especially for some profile ranges where you need to test the same area with multiple grades of tape and take the average.
A faster, more accurate, and repeatable method for measuring surface profile is the new Elcometer 224 Digital Surface Profile Gauge.
The Elcometer 224 is a needle depth gauge, which is very easy to use. Simply zero the gauge on glass, and, when the gauge is placed on the surface, a spring mounted needle measures the depth of the profile from the peaks of the profile at that particular point. As the measurement point is not necessarily at the very bottom of the valley, and could be somewhere on the slope so to speak, the user takes either 5 or 10 individual measurements, after which the gauge then provides a reading.
The reading displayed is user selectable, and can either be the more common average peak-to-valley height, or the maximum height.
With the ability to measure profiles up to 500 microns (20mils), on flat or curved surfaces, the Elcometer 224 Digital Surface Profile Gauge provides repeatable and reproducible measurements [accurate to ±5%]; can store each reading into memory [150,000 readings in 2,500 batches] – and can instantly transfer them to your PC or mobile device using ElcoMaster; our free, easy-to-use software application, generating professional inspection reports, instantly.
What’s more, the new Elcometer 224’s measurement modes, make sure that you can collect and record the data you need in accordance with your relevant test method, or standard.