ASTM B504-90 pdf free.Standard Test Method for Measurement of Thickness of Metallic Coatings by the Coulometric Method.
In most， but not all，coulometric thicknessmeasurements，a relatively high anodic-current density isemployed to shorten the test time. It is then necessary to agitatethe electrolyte to maintain a constant anodic-current efficiency.Where agitation is required, insufficient agitation may result inpolarization of the specimen, thereby causing a premature andfalse endpoint.Alloying Between Coatings and Metallic Substrates—The measurement of a coating thickness by the coulometricmethod implicitly assumes that a sharply defined interfaceexists between the coating and the substrate. If an alloy layerexists between the coating and the substrate as, for example, inthe case of coatings applied by hot dipping, the coulometricend-point may occur at some point within the alloy layer, thusgiving a high value of the thickness of the unalloyed coating.5.6 Purity of Coating-—Impurities or additives that code-posit with the coating may change the effective electrochemicalequivalent of the coating and also change the anodic currentefficiency.
Alloy Coating Variations in the composition of alloycoatings will change the effective electrochemical equivalentof the coating.Cleanliness of Test Surface-The surface to be testedmust be clean.Oil, grease, and organic coatings such as lacquermust be removed with suitable solvents.Oxides，conversioncoatings，and corrosion products are preferably removed bycarefully burnishing the test surface with a clean, soft pencileraser. Tin and nickel surfaces,in particular，should be soburnished prior to testing to remove passive oxide films. Density of Coating The coulometric method intrinsi-cally measures coating mass per unit area, the equivalent linearthickness being a function of the density of the coating.If thedensity of the coating tested is different from the value of thedensity used for the calibration,the linear thickness obtainedcoulometrically will be different from the actual linear thick-ness of the coating tested.The equipment, its calibration, and its operation shall besuch that the coating thickness can be determined with anuncertainty of less than 10 %.Instruments suitable for compliance with 8.1 are available commercially. For many coating systems the instrumentsare capable of making measurements with an uncertainty ofless than 4 % (95 % confidence).ASTM B504 pdf download.