What factors primarily determine the corrosion resistance of galvanized sheet?
A: It depends primarily on three factors: first, the thickness and weight of the zinc layer (the thicker the layer, the greater the corrosion resistance); second, the integrity of the coating (the absence of defects such as missing coating, scratches, and inclusions); and third, the operating environment (corrosion resistance in a dry indoor environment is far superior to harsh environments such as coastal areas and chemical plants).
How can one determine whether the zinc coating thickness of galvanized sheet meets the required standards?
A: Two common methods are: a magnetic thickness gauge (non-destructive, measuring the zinc coating thickness through magnetic field induction, suitable for on-site testing); and a gravimetric method (destructive, dissolving the zinc coating on the sample and calculating the zinc coating weight by the difference between the initial and final weights, suitable for precise laboratory testing).
What causes "missing coating" on galvanized sheet? What are the consequences?
A: Missing coating refers to areas of the substrate not being covered with zinc. Common causes include: oily or dirty surfaces, incompletely cleaned oxide scale, ineffective or unevenly applied flux, low zinc bath temperature, or improper immersion speed. The impact is: the iron substrate is directly exposed in the areas where the zinc coating is missing, causing it to rust first, thus reducing its overall service life.
How is the "adhesion" of galvanized sheet tested?
A: The commonly used "cross-cut test" is to use a scribing knife to scratch a cross-shaped grid (with a grid spacing of 1-2mm). Then, tape is applied to the grid and the tape is peeled off firmly. If the zinc coating does not fall off or only falls off at the edges of individual grids, the adhesion is acceptable. If it falls off over a large area, the adhesion is poor (possibly due to a process defect).
Is the "bending performance" of galvanized sheet important? In which applications should it be considered?
A: It is. Bending performance refers to the ability of the galvanized sheet to withstand bending without cracking or falling off the zinc layer. Applications requiring attention include appliance housings (such as bending refrigerator doors), steel structures (such as bending guardrails), and automotive parts (such as bending body frames). Poor bending performance can damage the zinc layer after processing, leading to rust.