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The Non-Destructive Measurement of Metallic Coatings on Magnetic Steels

We are often asked what is a reliable, non-destructive but inexpensive means of measuring the thickness of metallic coatings on a magnetic steel substrate. Keeping the “inexpensive” caveat in mind, there are two well utilized methods for these applications, namely, Magnetic Induction (MI) and Phase Change (Sensitive) Eddy Current (PCEC).


Both of these non- destructive test methods are incorporated into easy to use hand-held or bench top instruments. These units are reasonably priced and, of course, the test methods are documented in ISO and ASTM Standards. So, what’s the difference between the two measuring techniques and which one is best suited to my coating/substrate combinations?

There are multitudes of technical articles discussing the functional theory and operating principles of these measuring methods! Let's keep this simple.

Magnetic Induction

This test method is typically utilized when measuring the thickness of non-conductive coatings (paints, adhesives, etc.) or a non-magnetic metallic coating (just about any metallic plating other than electrolytic Nickel) deposited on a ferro-magnetic substrate material.


MI is sensitive to the magnetic properties of the steel substrate. This means that if you are measuring your platings on several different steel alloys with varying magnetic qualities, you will need to calibrate the instrument to the various alloys or simply make a “Base Correction” or normalization on the unit.


Although metallic plated steel standards can be used to calibrate the MI instrument, most operators simply utilize plastic foils (shims) of known thicknesses. When calibrating the unit, these foils are placed directly over the uncoated steel substrate, thus allowing the instrument to measure the magnetic properties of that particular alloy. If you wish to use plated standards, it is highly recommended that you have known thickness standards made from plated samples of the appropriate metal coating over the actual steel material that coating is plated on. Remember, MI is sensitive to the base material properties! Making thickness standards from your material is a service Delta Sales can provide to you … don’t just resort to buying a standard of Zn/”Steel” for example because the properties of that “Steel” base most likely will not match your actual steel substrate.


Phase Change Eddy Current

Instruments incorporating this measuring principle can determine metallic plating thicknesses on magnetic steels but, as opposed to MI, this method is sensitive to the conductive properties of the metal coating and not so much on the steel substrate.


This means that if you need to measure say, Zn, Cd and Cu on a steel alloy, you will need standards of each of those coatings in order to calibrate the PCEC instrument properly. This is also true if you wish to measure an acid Zn and an alkaline Zn. Separate standards are needed because of the differing properties of those metals.


If you need to measure alloy coatings such as Sn-Pb or Zn-Ni, neither of these two methods works efficiently. This is because of the variations that are normally encountered in controlling the percentages of the alloying metals … thickness measurement results may vary considerably! However, unlike MI, you can measure Electroless Ni coatings/Fe alloys with the PCEC method providing the P% remains fairly constant. When measuring alloy coatings with PCEC, as listed above, you can expect measurement uncertainties of +/- 12% to 15% or greater!

Major Differences

Summary

Selection of which measuring principle is best suited for your particular thickness measuring applications is determined by the metallic platings you need to measure; the properties of the steel alloy substrate those coatings are deposited on and, of course, your budget. Delta will be more than happy to help you make a credible and cost effective decision based on your needs.

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