Fish Boning
Fish boning is an irregular pattern over the entire surface of the steel part. This is caused by differences in the surface chemistry of a large diameter steel piece and variations in the reaction rate between the steel and zinc. These reaction differences cause the thickness of the galvanized coating to vary in sharply defined zones across the surface. Fish boning, as seen in Figure 34, has no effect on the corrosion protection provided by the zinc coating and is not cause for rejection of the hot-dip galvanized part.
Flaking
Flaking results when heavy coatings develop in the galvanizing process, usually 12 mils or greater. This generates high stresses at the interface of the steel and the galvanized coating and causes the zinc to become flaky and separate from the surface of the steel. Flaking can be avoided by minimizing the immersion time in the galvanizing kettle and cooling of the galvanized steel parts as quickly as possible. Figure 35 shows a micrograph of flaking. In addition, using a different steel grade, if possible, may also help avoid flaking. If the area of flaking is small, it can be repaired and the part can be accepted; however, if the area of flaking is larger than allowed by the specifications, the part must be rejected and regalvanized.
Flux Inclusions
Flux inclusion can be created by the failure of the flux to release during the hot-dip galvanizing process. If this occurs, the galvanized coating will not form under this flux spot. If the area is small enough, it must be cleaned and repaired; otherwise, the part must be rejected. Flux spots can increase if the flux is applied using the wet galvanizing method, which is when the flux floats on the zinc bath surface. Flux deposits on the interior of a hollow part, such as a pipe or tube, as seen in Figure 36, cannot be repaired, thus the part must be rejected. Any flux spots or deposits,picked up during withdrawal from the galvanizing kettle do not warrant rejection if the underlying coating is not harmed, and the flux is properly removed.
Oxide Lines
Oxide lines are light colored oxide film lines on the galvanized steel surface. Oxide lines are caused when the product is not removed from the galvanizing kettle at a constant rate. This may be due to the shape of the product or the drainage conditions. Oxide lines, as seen in Figure 37, will fade over time as the entire zinc surface oxidizes. They will have no effect on the corrosion performance; only the initial appearance will be affected. This condition is not a cause for rejection of the hot-dip galvanized parts.
