We are having trouble measuring electrocoat film thickness on galvanized steel. How is this done best? How do we calibrate our film thickness meter? M.B.
Since most dry film thickness gages used to measure organic coatings, including electrocoat, powder and liquids are designed to measure over ferrous (steel and iron) parts, pre-coated stock interferes with thickness measurements. Pre-coated stock includes all types of galvanized (zinc coating), including hot-dipped, electro-galvanized, one-sided galvanized and “paint lock” type coatings. Inorganic electroplated parts including zinc, chrome, copper, brass, cadmium, etc. also effect the measurement of the electrocoat film thickness.
Since these coatings are conductive, the electrocoat deposits in the same way it does on steel or iron parts. However, the pre-coat has a film thickness of its own, which is included when the total coating thickness is measured after electrocoating. The thickness of the pre-coat needs to be subtracted from the total film thickness to get the electrocoat film thickness. The biggest concern in using this subtraction method is how consistent and uniform is the thickness of the pre-coat. These coatings can vary from 0.01 mil to more than 1 mil depending on the coating and how it is applied.
If the pre-coat film thickness is uniform within 0.05–0.1 mil (or less), the average film thickness can be subtracted from the total film thickness to get the electrocoat film thickness, since this is the normal tolerance of most meters. If the non-uniformity of the pre-coat thickness is greater than 0.1 mil or your electrocoat measurement requirements are very tight, then the actual pre-coat film thickness needs to be determined prior to electrocoating. The most accurate method of getting electrocoat film thickness measurements on pre-coated stock is to measure the pre-coating on the part before electrocoating and then measure the same location after electrocoating. This can be done on a sample basis using actual parts and noting the pre-coat thickness on a sketch of the part and then re-measuring after electrocoat. Instead of using parts, test panels of the same pre-coated substrate(s) can be used. Another alternative is to run sample non-pre-coated steel test panels of the same material thickness as the parts that are run and measure the electrocoat thickness directly.
Electrocoat film thickness is mostly a function of bath and process parameters rather than substrate. Bath solids, P/B ratio and solvent level can effect film thickness. Operating parameters including voltage, tank loading (sq ft/ft or loadbar) and coating time also effect coating thickness. The more uniform the bath and operating parameters are maintained, the more uniform the electrocoating thickness will be.
As far as calibrating your meter is concerned, follow the manufacturer’s recommendation. Calibration should be based on the approximate anticipated coating film thickness. That is, if your galvanized coating is suppose to be about 0.6 mil, you should calibrate with a shim of 0.4-0.8 mil when measuring the pre-coat. Likewise, if you normally