Delamination
Delamination or peeling creates a rough coating on the steel where the zinc has peeled off. There are a number of causes for zinc peeling. Many large galvanized parts take a long time to cool in the air and form zinc-iron layers after they have been removed from the galvanizing kettle. This continued coating formation leaves behind a void between the top two layers of the galvanized coating. If there are many voids formed, the top layer of zinc can separate from the rest of the coating and peel off the part. If the remaining coating still meets the minimum specification requirements, then the part is still acceptable. If the coating does not meet the minimum specification requirements then the part must be rejected and regalvanized. If delamination, as seen in Figure 29, occurs as a result of fabrication after galvanizing, such as blasting before painting, then the galvanizer is not responsible for the defect.
Distortion
Distortion, as seen in Figure 30, is defined as the buckling of a thin, flat steel plate or other flat material such as wire mesh. The cause of this is differential thermal expansion and contraction rates for the thin, flat plate and mesh than the thicker steel of the surrounding frame. In order to avoid distortion, use a thicker plate, ribs, or corrugations to stiffen flat sections or make the entire assembly out of the same thickness steel. Distortion is acceptable, unless distortion changes the part so that it is no longer suitable for its intended use.
Drainage Spikes
Drainage spikes or drips are spikes or tear drops of zinc along the bottom edges of the product. These result when the surfaces of the product are processed horizontal to the galvanizing kettle, preventing proper drainage of the zinc from the surface as the product is withdrawn from the kettle. Drainage spikes, as seen in Figure 31, are typically removed during the inspection stage by a buffing or grinding process. Drainage spikes or drips are excess zinc and will not affect corrosion protection, but are potentially dangerous for anyone who handles the parts. These defects must be removed before the part can be accepted.
Dross Inclusions
Dross inclusions are a distinct zinc-iron intermetallic alloy that becomes entrapped or entrained in the zinc coating. This is caused by picking up zinc-iron particles from the bottom of the kettle. Dross, as seen in Figure 32, may be avoided by changing the lifting orientation or redesigning the product to allow for proper drainage. If the dross particles are small and completely covered by zinc metal, they will not affect the corrosion protection and are acceptable. If the dross particles are large, then the dross must be removed and the area repaired.
Excess Aluminum in Galvanizing Bath
Another type of surface defect, shown in Figure 33, is caused by an excess amount of aluminum in the galvanizing bath. This creates bare spots and black marks on the surface of the steel. The excess aluminum can be avoided by ensuring proper control of the aluminum level in the galvanizing bath by means of regular sampling and analysis, and by adjusting the levels in a regular and controlled manner. For small areas of bare spots, the part may be repaired as detailed in the specification. If this condition occurs over the entire part, then it must be rejected and regalvanized.
