1. Filter Paper Method (Potassium Ferricyanide Method)
Principle: Potassium ferricyanide reacts with the iron substrate to form blue spots, marking the location of pores.
Steps:
Press a filter paper soaked in the test solution (potassium ferricyanide + sodium chloride) tightly against the chrome-plated surface.
After 10 minutes, remove the filter paper and observe the number of blue spots to calculate the porosity per unit area (Porosity = Number of spots / Area).
Applicability: Suitable for single or multi-layer chrome plating on steel substrates.
2. Paste Method
Principle: A paste containing a color indicator is applied to the surface; colored spots are formed at the pores.
Operation: After application, let it stand for 1-2 hours, then clean and count the spot distribution.
3. Metallographic Microscope Method
Steps: Cut the sample → Polish → Observe the cross-section pores under a microscope and directly count the number of pores.
Advantages: Can analyze pore morphology and distribution, high accuracy (±1μm).
4. Scanning Electron Microscope (SEM) Method
Principle: High-resolution imaging of pore morphology, suitable for detecting tiny pores.
Limitations: Expensive equipment, complex operation.
5. Electrochemical Method
Principle: Inferring porosity through changes in current density, suitable for multi-layer coatings such as nickel-chrome.
6. Standard References
National Standards: GB/T 17721-1999 (Ferric Reagent Test), GB/T 18179 (Humid Sulfur Test).
Selection Suggestions
For quick detection: Filter paper method or paste method.
For high-precision requirements: Metallographic microscope or SEM method.
For industrial applications: Prioritize corrosion methods or electrochemical methods that comply with GB/T standards.
