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Causes And Control Of White Rust On Hot Galvanized Steel Coil
May 14, 2018

Causes and Control of White Rust on Hot Galvanized Steel Coil

The galvanized sheet is stored under humid conditions and rust is called white rust. Its mechanism is: when the galvanized coil is exposed to moisture, the surface adheres to water, the formed water film, the surface of the zinc layer between the water film and the edge of the water film. The surface of the zinc layer has a different concentration of oxygen, so that the central area of the water film becomes an anode, and the edge area becomes a cathode, ie, an "oxygen concentration battery", which causes rust to occur.

The white rust of the galvanized sheet is mainly affected by the passivation quality, coiling temperature, coil and ambient temperature during packaging, transportation, storage environment, storage time and other factors.

Control measures:

1. Improve the passivation process

1) Increase the frequency of passive film detection. The X-fluorescence spectrometer was used to measure the passivation film thickness, increase the frequency of passivation film thickness detection, and reduce the risk of missed detection. When the product is replaced or the process is adjusted, a passivation film thickness test is required.

2) Increase the film thickness of passivation film, increase the concentration of passivation solution, increase the chromium content of passivation solution, and increase the resistance to white rust.

3) Improve the coating roller and pick-up roller speed ratio.

4) To reduce the uneven passivation film thickness caused by abnormal wear of the coating roller, the passivated coating roller is replaced in pairs and synchronized with the sink roll replacement cycle. Coating machine passivation coating roller replacement cycle shortened.

5) In the production of galvanized goods rolls, the production operator wipes the upper and lower surfaces of the steel belt on the driving side and the operating side of the steel strip on the outlet side of the coating head with a paper towel after the lacquer coating head is put into use. It is normal.

6) Adjustment after passivation. Increase the pick-up roller, coating roller speed ratio. Reducing the pressure of the coating roller and the pick-up roller and increasing the pressure of the passivation film on the upper and lower coating head application roller pressures, on the contrary, the film weight decreases. The chrome point increases, the film thickness increases, the chrome point decreases, and the film thickness decreases.

(7) The passivation circulation device such as trays, pipes, and circulation tanks shall be thoroughly cleaned by the production staff every 3 months.

2, control the coil packaging temperature

The steel coils must be stored indoors for a long enough period of time to keep the temperature close to that of the room, preventing condensation on the surface of the coil due to temperature differences. At the same time, the surface of the coil must be kept dry and non-condensing water before packaging.

3, save the environment and time

The storage environment is very important for galvanized products. The storage environment must be dry, especially in the open air. Nor can it be put together with chemical media. For non-galvanized galvanized products, generally used within 2 months, for passivated, oiled galvanized products, generally used within 3 months

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