A Current Affair
Eddy current measurement of plating thickness on steel offers advantages over other methods.
There are multiple choices available for platers who need to accurately measure deposit thickness. These include X-Ray technology, coulometric measurement and magnetic induction. All three have advantages and limitations.
A fourth alternative is eddy current measurement. According to Michael Justice, president of UPA Technology Inc. (West Chester, OH), there are two types of eddy current instruments available: phase-sensitive and amplitude. "Amplitude eddy current systems are commonly used for measuring anodizing or paint on aluminum, but will not work on ferrous substrates," he explains. "Phase-sensitive eddy current instruments can cover a wide variety of applications, such as plating on steel, anodizing on aluminum, and a variety of coatings on other conductive substrates."
Phase-sensitive eddy current systems work by measuring the conductivity difference of the plating and substrate. The measuring range depends on both the frequency of the probe used and the conductivity of the plating deposit. "For measuring thickness of zinc over a steel substrate, a probe frequency for measurement from 0-1.0 mil (0-25 µm) would be chosen," Justice explains. "Choosing a probe with the correct frequency ensures accuracy and adequate resolution throughout the entire zinc plating range, so measuring thickness at only 0.1-0.2 mils is not an issue."
Eddy current probes are available with tips of different sizes, with each tip size having a different measuring area. Smaller parts are measured using probes with smaller tips, allowing for fast, precise measurements on small components without the need for fixturing.
Eddy current measurements are based on specific conductivity of the plating deposit, so they need to be calibrated using plated standards, Justice says. "For example, zinc-plated thickness standards are used for calibrating to measure galvanized parts. Different plating baths will have different conductivities. Amounts of brighteners, specific chemistries and other factors affect the conductivity of the plating deposit. Thus, it is important to use calibration standards that have been made using the same plating bath."
Most eddy current systems include a "conductivity correction" feature to account for variations in deposit conductivity. However, Justice says, it's important to understand that instruments and standards are typically calibrated by an outside laboratory and certified with traceability to a National Institute of Standards and Technology (NIST) reference standard. "If a correction factor is implemented to alter the original calibration, then all NIST traceability is lost and the certifications are no longer valid," he explains. "Using a ‘fudge factor' also prevents the ability to reconfirm a system's accuracy, since readings taken on the calibration standards will no longer correlate to marked values."
According to Justice, magnetic systems measure based on the specific magnetic properties of the ferrous substrate, and magnetic properties of steel will differ significantly due to changes in alloy or heat treatment and processes such as cold rolling. Such differences require magnetic systems to be calibrated using thickness shims and bare parts. "For job shops that commonly plate hundreds of different parts the additional measurement time and effort is simply is not practical," he says.
"Eddy current systems derive plating deposit thickness based on the conductivity of the plated deposit," Justice continues. "Minor differences associated with commonly used steel substrates will have little effect on accuracy. A single calibration with plated standards can then be used for measuring virtually all parts. "
Another advantage of eddy current versus magnetic: In eddy current systems, the probe doesn't have to be in direct contact with the part—an effect called probe lift-off. "In fact, eddy current systems can measure plating thickness accurately even under a layer of paint," Justice says. "Probe lift-off allows for accurate measurement on even rough or machined surfaces, which would render magnetic systems imprecise."