A rough substrate surface - Galvanizing

A rough substrate surface - Galvanizing - Coating defects

Figure 1: A rough substrate surface results in Zn-Fe alloy outbursts in the coating.

Defect name: Rough substrate surface
Record No.: 1992
Type of defect (Internal/Surface): Internal, surface
Defect classification: Coating defects
Steel name: Steel
Steel composition in weight %: No data.
Note: Although the science and technology of continuous galvanizing have improved significantly over the years, it is still a challenge to produce defect-free coatings. Studies indicate that poor substrate surface quality, insufficient strip cleaning, mechanical damage to the substrate surface due to handling, and inadequate process control are the main causes of coating defects. The recent introduction of high strength steels makes the production of galvanized coatings for applications such as exposed autobody panels even more challenging. To produce high quality zinc-coated strip, galvanizers need to improve quality control of the incoming strip as well as the continuous galvanizing process.

A rough substrate surface containing iron fines and slivers is prone to the development of outbursts in the coating. The resultant coating is rough with numerous areas of localized thick coatings. After being flattened in temper rolling, these areas reflect light differently, appearing as either dark or bright spots depending on the lighting condition. A typical sample is shown in Figure 1. Studies indicate that the density of this kind of defect decreases with increasing coating weight, suggesting that such a defect can be masked by a thick coating.
Reference: Not shown in this demonstration version.