Common testing standards for metal surface treatment (electroplating / baking paint / spraying) inspection and quality control certification

May 11, 2026

In the inspection of metal products, the anti-rust effects and appearance quality of surface treatment processes such as electroplating, painting, and spraying are of great significance. Inspectors need to conduct reliability evaluations based on unified inspection standards. The following are the commonly used testing methods and judgment criteria during the inspection, factory verification, and certification processes, which are applicable to the quality control of coating layers on metal workpieces.
I. Process Overview (Inspection Factory Audit Points)
Painting: Three coats of primer and four coats of topcoat are sprayed on the substrate. After each layer, it is sent to a dust-free and constant-temperature drying oven for drying. During the inspection factory audit, the uniformity and cleanliness of the drying oven should be verified.
Spraying: The coating is evenly applied to the surface of the workpiece using a spray gun or an atomizing device. During inspection, the uniformity and thickness of the coating should be checked.
Electroplating: This process involves depositing metal coatings (such as chromium, zinc, and nickel) on the surface of workpieces using the principle of electrolysis. It serves functions such as rust prevention, wear resistance, and decoration. During inspection and certification, attention should be paid to the adhesion strength of the coating and its corrosion resistance.
II. Common Testing Standards (Inspection and Testing Items)
1. Color Fade Test (Light Resistance Test)
Method: Cover half of the test surface with opaque cardboard. Place it under a UV lamp with a wavelength of 2800-3000 Å and a power of 15W for 72 hours. The lamp tube is 25 cm away from the sample.
Judgment: No fading is allowed.
→ This applies to the certification of products used in outdoor or illuminated environments.
2. Adhesion Test (Coating/Dip Coating Adhesion Inspection)
Method: Select a 10mm × 10mm area. Use a grid knife to make 1mm² squares 100 times. Stick a 3M 600 tape (20mm wide) and quickly pull it at a 90-degree angle.
Judgment: No more than 10% shedding is allowed, and no more than 5% shedding is permitted.
→ This is a routine destructive test in the inspection process. During factory audits, the sharpness of the test knife and the calibration of the pulling force must be verified.
3. Abrasion Resistance Test (Surface Abrasion Resistance Inspection)
Method: Use 1mm thick and 1cm long cotton cloth, apply a force of 1000g, with a stroke of 30cm, and perform 500 reciprocating friction cycles.
Judgment: No fading, peeling or blistering allowed.
→ Simulates daily wear and tear, suitable for products that are frequently used.
4. High temperature and high humidity test (environmental aging test)
Method: Temperature 40 ± 3℃, relative humidity 90 - 95%, for 48 hours.
Judgment: No fading, peeling or blistering is allowed.
→ Evaluating the durability of the coating in a humid and hot environment is a common climate aging test in the certification process.
5. Pencil hardness test (surface hardness inspection)
Method: Use a 2H hardness pencil, apply a force of 1 kgf, and draw along a 45° angle for 6 to 7 mm.
Judgment: No scratch marks are allowed.
→ Rapid assessment of the coating's scratch resistance capability. It can be operated on-site during inspection.
6. Alcohol test (resistance to solvents test)
Method: Dip a piece of cloth in a 96% alcohol solution, apply a pressure of 500g/cm², and rub it back and forth for 30 times.
Judgment: Does not allow discoloration or peeling.
→ Simulated contact with daily cleaning agents, suitable for electronic, furniture, etc. products.
7. Salt Spray Test (Corrosion Resistance Test - Certification Criterion)
Method: 5% NaCl solution, 100% relative humidity, 35 ± 1℃, continuous spraying for 24 hours.
Judgment: No corrosion, oxidation or detachment is allowed.
→ This is a mandatory test in the certification process (such as the Neutral Salt Spray Test NSS). During the factory inspection, the calibration records of the salt spray box need to be checked.
8. Film thickness test (coating/dissolution layer thickness inspection)
Method: Measure using a film thickness gauge (magnetic or eddy current type).
Decision: Wet coating thickness 30-60 μm, powder coating thickness 40-100 μm (as per customer requirements or standards).
→ During inspection, a sample check will be conducted for each batch. The inspection factory needs to verify the calibration certificate of the thickness measuring instrument.
III. Summary
The inspection and certification of electroplating, painting, and spraying must cover key performance indicators such as adhesion, hardness, wear resistance, weather resistance, corrosion resistance, and thickness. Enterprises should establish a complete factory inspection system (including equipment calibration, operation norms, incoming material control), and regularly entrust third parties to conduct certification tests. The above testing methods can serve as the basis for quick on-site judgment and laboratory testing during inspection, ensuring the reliable and long-lasting surface treatment quality of metal products. For more professional content, please continue to follow this column.