Rationale
The Coulomb coating thickness gauge is based on the principle of electrolytic dissolution and calculates the thickness by measuring the amount of electricity required to dissolve the coating in a known area. According to Faraday's law of electrolysis, the mass of the dissolved metal is directly proportional to the amount of electricity that passes through. The instrument uses a specific electrolyte, and the plating acts as an anode to electrolyze through a constant current. When the coating is dissolved, the matrix is exposed, and the voltage jumps to record the power consumed at this time. The coating thickness can be calculated by the formula:
d = (Q × M) / (F × ρ × A × z)
where d is the thickness of the coating (microns), Q is the power consumption (coulombs), M is the molar mass of the metal (g/mole), F is the Faraday constant (96485 coulombs/mole), ρ is the density of the metal (g/cubic centimeter), A is the electrolytic area (square centimeters), and z is the valence state of the metal ion. This method is suitable for metal plating measurements on conductive substrates.
Precious metal plating measurement
Precious metal coatings such as gold, silver, platinum, palladium, etc., due to their high value and performance requirements, thickness measurement needs to take into account both accuracy and non-destructiveness. The Coulomb method has no macroscopic damage to the sample as a whole through local micro-electrolysis, and only small spots are formed after measurement, which does not affect the function of the components. The electrolyte should be selected according to the coating composition to ensure that the coating is selectively dissolved without eroding the matrix. Instruments are usually equipped with microprocessors that automatically control the electrolytic endpoint determination and reduce human error. The measurement range is typically between 0.1 and 50 microns, with a resolution of 0.01 microns, meeting the requirements of most industrial standards.
Operational points
The surface of the sample should be cleaned to remove oil and oxides before measurement. Choose the appropriate electrolytic cell and sealing ring to ensure that the electrolytic area is closed. Inject an appropriate amount of electrolyte and set the appropriate current density. After starting the measurement, the instrument automatically electrolyzes and monitors voltage changes. When the coating is dissolved, the voltage jumps, and the instrument stops and calculates the thickness. During operation, attention should be paid to the validity period, temperature stability and sealing of the electrolyte to avoid measurement errors caused by electrolyte leakage. For multi-layer coatings, layered measurements can be achieved by changing the electrolyte.
Influencing factors
Measurement accuracy is influenced by various factors. The composition of the electrolyte affects the dissolution efficiency and selectivity. Current stability is directly related to the accuracy of electricity measurement. temperature fluctuations may change the electrolysis rate; The coating alloy composition or porosity can lead to uneven dissolution. Calibration is typically verified using a standard thickness sheet or a reference sample of known thickness. Regularly calibrate instrument circuits and electrolytic cells to ensure accurate Faraday constant and area parameters. It is recommended to verify on the reference sample before each measurement, and if the deviation is outside the allowable range, check the electrolyte or instrument status.
Applications:
This method is widely used in the detection of precious metal coatings in electronic connectors, jewelry, precision instruments, and other fields. Relevant international standards such as ISO 2177 and ASTM B764 provide methodological guidance; Domestic standards such as GB/T 4955 also cover the requirements of the Coulomb method. These standards specify electrolyte formulations, current density ranges, calibration procedures, and more, ensuring comparability of measurement results. Users need to select the appropriate standard according to the product specification, and indicate the measurement conditions and standard basis in the report.
Notes:
The coulomb method is not suitable for non-conductive substrates or insulating coatings. For coatings with thicknesses greater than 50 microns, extended electrolysis time may affect accuracy. If the coating contains multiple metals or has a diffusion layer, the endpoint determination may be ambiguous. After measurement, small electrolytic areas may need to be treated with anti-rust. Protective equipment should be worn during operation to avoid electrolyte contact with the skin. The waste liquid shall be treated in accordance with the regulations of chemical waste.
Data logging
The measurement results usually include information such as thickness value, measurement position, electrolysis parameters, etc. It is recommended to take an average of multiple measurements and calculate the standard deviation to assess uniformity. If the result is abnormal, check whether the coating has porosity, inclusions or uneven thickness. The report should clearly indicate the measurement conditions for easy traceability. The following are examples of common precious metal plating measurement parameters:
| Plating type: | Typical electrolyte |
| Gold plating | Potassium iodide solution |
| Silver plating | Sodium nitrate solution |
| Platinum plating | Hydrochloric acid-based solution |
| Palladium plating | Ammonium sulfate solution |
| Rhodium plating | Phosphate-based solution |
References
ISO 2177:2016, Metallic coatings — Measurement of coating thickness — Coulometric method
ASTM B764-04(2020), Standard Test Method for Simultaneous Thickness and Electrode Potential Determination of Individual Layers in Multilayer Nickel Deposit
GB/T 4955-2005, Metal overlay - Thickness measurement - Anode dissolution coulomb method
W. Blum, G. B. Hogaboom, Principles of Electroplating and Electroforming, McGraw-Hill, 1949