For the thickness of the coating, we can use micrometers, plug gauges, thread ring gauges, and other methods to check, or we can use the drip method to check.

We can check whether there are defects in the galvanized layer of the steel plate from four aspects, including appearance, bonding strength, porosity, and thickness. We can use the drip method when checking whether the coating thickness is qualified. The drip method refers to using a pipette to suck up the solution and drip it at the measurement point. We need to wipe off each drop of solution with cotton wool after keeping it for 1 minute. Then, drop a second drop of solution until the base metal is exposed. Finally, we calculate the coating thickness based on the total number of drops.

We can check whether there are defects in the galvanized layer of the steel plate from four aspects, including appearance, bonding strength, porosity, and thickness.

In appearance, the galvanized layer must be free of bubbles, peeling, and fingerprints. Light water marks and slight clamp contact points are allowed. The passivation film should be an active rainbow color. Slight scratches on the passivation film could be, but peeling off is not.

For the bonding strength, we can use steel needles or blades to cross-cut the coating and then observe whether there is peeling or falling off at the intersection of the coating to determine whether the galvanized layer is qualified.

Under normal circumstances, the porosity of zinc coating does not need to be checked. We soaked the test solution with filter paper, attached it to the surface of the layer, and kept it for about 5 minutes. Small blue dots appeared on the filter paper, indicating the coating had pores. It is considered qualified if there are no more than three small dots per square centimeter.

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