Overview
Anodized film is widely used in the field of metal surface protection and decoration, and its sealing quality directly affects the corrosion resistance, wear resistance and insulation properties of the film. The hole sealing process aims to seal the porous structure of the oxide film and reduce porosity, thereby improving its overall performance. Accurate assessment of sealing quality is critical for process control and product quality assurance. As a precision measuring instrument based on electrochemical principles, the Coulomb thickness gauge provides a reliable and reproducible test method for the evaluation of sealing quality by quantitatively determining the dissolution characteristics of oxide films. This method does not rely on subjective judgment, can provide objective numerical results, and has important application value in industrial quality control and laboratory research.
Measurement principle
The core principle of coulomb thickness gauge to measure the quality of the sealing hole is the anodic dissolution method. During the test, the specimen is used as a cathode in a specific electrolyte solution (usually an acidic or neutral solution) to electrolytically dissolve the aluminum matrix at the bottom of the pores of the anodized film that is not effectively sealed or where the sealing hole is weak by applying a constant DC voltage or current. As the dissolution progresses, the oxide film is peeled off layer by layer, and when the aluminum matrix is fully exposed, the electrode potential will mutate, and the instrument will determine the end point of the test by detecting this potential mutation point.
There is a clear quantitative relationship between the amount of electricity consumed (Q) and the mass of dissolved aluminum (m) during the entire dissolution process, following Faraday's law of electrolysis:
m = (Q * M) / (n * F)
where m represents the mass of dissolved aluminum, Q represents the total charge, M is the molar mass of aluminum, n is the valence state of aluminum ions (usually 3), and F is the Faraday constant. Once the density of the oxide film (ρ) and the test area (A) are known, the average thickness of the dissolved oxide film (d) can be calculated:
d = m / (ρ * A)
The worse the sealing quality, the more pores or defects in the layer, the easier it is for the electrolyte to penetrate into the aluminum matrix, resulting in less electricity required to dissolve the complete film layer per unit area, and the calculated "apparent thickness" or dissolution time will be significantly lower than the theoretical film thickness of the reference sample that is not sealed or poorly sealed. The sealing effect can be quantitatively evaluated by comparing the dissolved electricity of the specimen before and after sealing or the relevant parameters calculated from it.
Test methodology
To ensure the accuracy and comparability of measurement results, the test follows a standardized operating procedure. At present, the internationally widely referenced standards include ISO 3210 and ISO 2931, and China has also formulated corresponding national standards. The following is an overview of the key steps:
1. Sample preparation: Select representative, clean surface, and free of scratches and oil stains anodized specimens. Wash and dry with appropriate solvents.
2. Instrument Calibration: Calibrate the Coulomb thickness gauge using a standard film thickness sheet of known thickness to ensure the fuel measurement system is accurate.
3. Test Cell Assembly: Mount the specimen as the cathode under the sealed rubber ring of the test cell, ensuring that the exposed test area is constant and free from electrolyte leakage. An auxiliary electrode, usually platinum or stainless steel, is immersed in the electrolyte as an anode.
4. Parameter setting: Set the constant test voltage (or current), electrolyte concentration and temperature according to relevant standards or material specifications. The commonly used electrolyte is phosphoric acid-chromic acid solution or neutral salt solution.
5. Test Execution: Start the test, and the instrument automatically records the charge-time curve during the dissolution process. When a potential sudden change is detected, the test is automatically terminated.
6. Data processing: The instrument automatically calculates and displays the thickness of the oxide film based on the total amount of electricity consumed. The quality of the sealing hole is evaluated by comparing the test results with the test results of the unsealed specimen of the same process. Common evaluation indicators include sealing degree or mass loss value.
Sealing quality assessment
After measuring with a Coulomb thickness gauge, the following key indicators can be used to quantify the sealing quality. These metrics are usually calculated by comparing the test results of a sealed specimen with an unsealed reference specimen.
| Evaluate the indicators | Meaning and calculation |
| Dissolved electricity difference | The difference between the dissolution of the sealed and unsealed specimens to the end point. The larger the difference, the better the sealing quality. |
| Apparent thickness loss | The measured thickness of the sealed specimen is lost as a percentage of the original average thickness of the unsealed specimen. The smaller the loss value, the higher the sealing quality. |
| Sealed hole degree | Based on the calculation of mass loss, the specific formula refers to the relevant standards. The higher the value, the more complete the sealing hole. |
| Dissolution time | Time from the start of the test to the point of potential mutation. Samples with well-sealed wells dissolve longer. |
Influencing factors
The measurement results of coulomb thickness gauges are affected by a variety of factors and need to be strictly controlled in actual operation to ensure the accuracy of the evaluation.
1. Electrolyte solution: The composition, concentration, pH value, and temperature of the solution must strictly comply with the standard regulations. Contamination or aging of the solution can cause test results to drift.
2. Specimen status: The cleanliness and flatness of the specimen surface and the selection of the test area directly affect the sealing effect and current distribution, thereby affecting the results.
3. Test Parameters: The applied voltage or current density must be constant and appropriate. Too high a voltage can lead to local breakdown, while too low can lead to long test times or insensitive endpoint judgments.
4. Characteristics of the oxide film itself: The initial thickness, porosity, composition and structural uniformity of the oxide film are all background factors. Samples from the same substrate and oxidation process should be used as controls to evaluate the sealing quality.
5. Instrument status: Electrode cleaning, rubber sealing ring, and the accuracy of the electrical measurement system need to be regularly maintained and verified.
Scope of application:
The coulomb thickness measurement method is suitable for evaluating the sealing quality of aluminum and aluminum alloy anodized films, especially for comparing and monitoring the effects of various processes such as chromate sealing, hot water sealing, medium temperature sealing and nickel-free sealing. The method is fast, quantitative, and repeatable, and is suitable for sampling on the production line and in-depth research in the laboratory.
However, this method also has certain limitations. It mainly evaluates the electrolyte penetration resistance of the oxide film, and the correlation with certain specific performance (e.g., alkali corrosion resistance, dye color retention) may need to be verified in conjunction with other test methods. In addition, for very thick layers or those containing special sealants such as organic matter, the judgment of the test endpoint may require adjustments to the criteria.
Conclusion
Based on Faraday's law of electrolysis, the Coulomb thickness gauge provides an objective and quantitative electrochemical test method for the evaluation of the sealing quality of anodized films. By strictly following standardized testing processes and controlling key factors such as electrolyte, specimen status, and instrument parameters, repeatable and reliable dissolution charge or thickness data can be obtained. By comparing the changes of relevant indicators before and after hole sealing, the adequacy and uniformity of the hole sealing process can be effectively judged, which has clear guiding significance for optimizing the production process and ensuring product quality. In practical applications, it is recommended to combine the coulomb thickness measurement method with other physical and chemical testing methods to conduct a more comprehensive evaluation of sealing hole quality.
References
ISO 3210: Anodizing of aluminium and its alloys — Assessment of quality of sealed anodic oxidation coatings by measurement of the loss of mass after immersion in acid solution(s).
ISO 2931: Anodizing of aluminium and its alloys — Assessment of quality of sealed anodic oxidation coatings by measurement of admittance or impedance.
GB/T 12967.3: Detection methods for anodized films of aluminum and aluminum alloys - Part 3: Copper-accelerated acetate spray test (CASS test).
"Aluminum Surface Treatment Technology and Application". The relevant chapters discuss in detail the sealing principle and test method of anodized film.