Rough zinc coating is usually thicker than smooth zinc coating, so the corrosion resistance life may be longer, but the appearance is poor, and this kind of defect is generally acceptable. Therefore, it should not be used as a quality reason for rejection.
The surface roughness of zinc coating refers to the micro geometric characteristics of small spacing and micro peak valley unevenness of zinc coating surface. At present, optical method, comparison method, needle tracing method and impression method are used.
The main reasons for surface roughness of galvanized coating of angle steel are as follows:
1、 The effect of "over pickling" on the surface roughness of zinc coating of diagonal steel is produced in the process of pretreatment.
"Over pickling" makes the steel surface rough or produce bean marks, which is one of the reasons for the surface roughness of the workpiece after hot galvanizing.
2、 The effect of too high temperature or too long time of zinc dipping on the surface roughness of zinc coating on diagonal steel is studied.
When the temperature of liquid zinc is too high, the time of zinc dipping is too long, or because of the influence of angle steel composition and Sandrine effect, too much or uneven phase of Zn Fe alloy will be formed, and the surface roughness will also be caused. The thicker the zinc coating, the rougher the surface is.
The surface roughness of hot-dip galvanized angle steel may also appear due to slow cooling after being taken out of the liquid zinc, or the steel section is too thick, or the angle steel is immediately stacked without isolation after hot-dip galvanizing.
3、 The influence of composition or surface condition of steel used in angle steel manufacturing on the surface roughness of zinc coating of diagonal steel.
The smoothness and roughness of zinc coating surface are relative. Different from "continuous hot galvanizing" of steel plate, steel pipe or steel wire, hot galvanizing of steel angle steel belongs to the category of batch hot galvanizing process, and the zinc coating is not blown and wiped by air knife. If the surface of the steel used for hot-dip galvanized angle steel is rough and uneven, with rust pits or obvious rolling stripes, the surface of hot-dip galvanized angle steel will not be smoother than before.
The composition of the steel will also affect the formation of Zn Fe alloy phase layer due to the Sandrine effect, resulting in the surface roughness.