I. What are the core standards and criteria for judging quality?
Simply put, it's a "scoring sheet" with specific quantifiable indicators. The core criterion is the national standard GB/T 12754 "Color Coated Steel Sheets and Strips" (please refer to the latest standard version for specific chapter numbers), which details various quality indicators. Key indicators include:
Appearance and Color Difference: The surface should be smooth, without significant scratches, bubbles, color spots, or other defects. Compared with the standard color sample, the color difference ΔE value should not exceed a specific range (the national standard requires a specific reasonable range for color difference, but for special products such as those used in purification processes, the color difference requirement can be stricter). For a roll of steel strip, the defective portion should not exceed a specific percentage of the total length of each roll (e.g., the national standard requires it not to exceed a specific percentage of the total length of a roll).
Geometric Dimensions: The allowable deviations in thickness, width, length, and shape (such as camber) must all conform to the standard.
Mechanical Properties: The yield strength, tensile strength, and elongation after fracture of the substrate should meet the requirements to ensure its formability.
Coating performance: For zinc-plated and aluminum-zinc-plated substrates, it is necessary to test the coating mass per unit area (usually expressed in g/m²), which is the basis for corrosion resistance.
II. Coating Thickness and Uniformity: How to Measure Accurately?
Coating thickness is the "firewall" that determines corrosion resistance and performance. Multiple measurements are needed to obtain an average value, with particular attention to the minimum thickness.
Testing Method: Use a non-destructive magnetic thickness gauge (for steel substrates) or an eddy current thickness gauge (for non-ferromagnetic metal substrates). The commonly used reference standard is GB/T 13448.
Uniformity Requirements: Measure the minimum thickness at single points, multiple points, and local areas to ensure it meets design values (e.g., the front coating thickness is typically ≥20μm), and note potential thickness differences between edges and the center. Ideally, the coating thickness error at each point should be controlled within ±5%.
III. Will the paint peel off during sheet processing? How are flexibility and adhesion tested?
This is verified by simulating bending and impact during processing. It mainly includes two key tests:
T-bend performance test: Evaluates the coating's flexibility. Bend the steel strip 180° around itself (e.g., 0T, 1T, etc.) and check if the coating cracks at the bend. Generally, the better the T-bend performance, the better the coating's flexibility (a smaller T-value is better).
Reverse impact test: Tests the coating's impact resistance. Use a heavy hammer to drop freely from a certain height onto the back of the test sheet and check if the coating on the front side cracks or peels off.
IV. Will the coating peel off easily? How is adhesion tested?
The most common testing method is the "cross-cut test," also known as the "X-mark test."
Cross-cut test procedure: Use a special tool to cut a grid pattern (1mm or 2mm spacing) at 60°-90° intervals on the coating surface. Then, use specified adhesive tape to pull the coating through. The coating is graded based on the percentage of area peeling off from the gridded areas.
Quality assessment: Ideal quality is grade 0 (the blade fully cuts into the substrate, the grid edges are completely smooth, and there is no peeling within the grid) or grade 1 (there is very slight peeling in a very small area at the intersection of the cuts).
V. Corrosion Resistance of Color-Coated Steel Sheets: Which Test is Most Critical?
For assessing corrosion resistance, the neutral salt spray test is one of the most critical and essential accelerated testing methods.
Test Principle: The sample is placed in a salt spray test chamber at 35°C, where a 5% neutral sodium chloride (NaCl) solution is continuously sprayed to simulate a marine or high-salt corrosive environment.
Judgment Criteria: During and after the test, several aspects are mainly checked:
Scratches: Observe whether the "corrosion width" extending outward from the scratch is within the specified range.
Surface Condition: Observe whether blistering or rust (red or white rust) appears on the surface.
Test Cycle: Test duration varies from 24 hours to several thousand hours. The longer the cycle, the better the material's corrosion resistance. Different standards have different requirements. For example, relevant standards may require that the longest scratch corrosion width does not exceed a certain range, and the overall rust status rating of the test surface is not lower than a certain level (the maximum allowable rust area ratio does not exceed a specific value).