1.Why isn't color difference a core indicator?
Salt spray tests (such as the Neutral Salt Spray Test (NSS) and the Copper Accelerated Acetic Acid Salt Spray Test (CASS)) primarily assess the corrosion resistance of materials. Evaluation criteria typically include:
Percentage of corroded area: The percentage of rusted area relative to the total area (rating standards such as ISO 10289, ASTM D610, and GB/T 6461).
Type of corrosion: Whether it is red rust (corrosion of the base iron) or white rust (corrosion products of the zinc plating).
Time of corrosion occurrence: The time it takes for white rust to appear, and for red rust to appear.
"Color difference" here refers more to the visual presentation of the corrosion result than to a quantifiable performance parameter.
2.What are the manifestations of color difference in galvanized cold-rolled coils (GI) or galvanized nickel alloys?
White rust: Corrosion products of the coating (basic zinc chloride, zinc oxide, etc.) appear as white or grayish-white spots or powder. This creates a significant color difference compared to the original metallic luster (silver, bright gray).
Red rust: If the testing time is long enough, the coating is completely corroded, and the base iron begins to rust, showing yellowish-brown to red rust spots. This is a more serious color difference and performance failure.
Color difference value estimation: The color difference value ΔE between areas with white or red rust and uncorroded areas can be very large, often exceeding 5.0 or even 10.0, representing a significant difference clearly visible to the naked eye. However, this value is usually not measured; instead, the grade of white/red rust is recorded (e.g., Grade 4: >50% area of white rust).
3.What are the signs of color difference in bare cold-rolled coils (without coating)?
It will directly produce red rust. The color change is also very drastic. This material itself has extremely poor salt spray resistance (it may start to rust within hours), and is generally not used in applications requiring corrosion resistance.
4.What are the signs of color difference in cold-rolled coils that have undergone oiling or passivation treatment?
The treatment layer will delay the occurrence of corrosion. Initial color difference may manifest as decreased gloss, darkening, water stains, or slight discoloration, at which point ΔE may be between 1.5 and 3.0 (a slight, perceptible change), before developing into white or red rust.
5.In what specific circumstances is it necessary to quantify "color difference"?
Evaluate the color retention of decorative coatings (such as colored zinc and black zinc).
Test the weather resistance of coated cold-rolled steel sheets (such as painted sheets).
Customers have extremely high requirements for product appearance.