In addition to coating measurements and a visual inspection of the appearance and finish, there are a few other tests that may be conducted on hot-dip galvanized steel. These tests are typically only done when there is a question or concern about a specific part.
Testing of the zinc coating adherence to the steel is achieved using a stout knife (Figure 52). The coating shall be deemed not adherent if it flakes off and exposes the base metal in advance of the knifepoint. The test is not an attempt to pare or whittle the zinc coating. If the coating is adherent the knife should put a slight mark in the zinc metal surface, but should not cause any delamination of the coating layers.
Adhesion Test with a Stout Knife
- Push down point of stout knife
- Coating must not flake off exposing the base metal
- Do not perform at edges or corners of the product
- No paring or whittling with knife is acceptable
Peeling and flaking that occurs to the coating when bending rebar after galvanizing is not a cause for rejection and may be repaired. Bend tests on steel fabrications are strictly to test for embrittlement. The recommended bend radius is 3x the section thickness. There are various tests used to assess the ductility of steel when subjected to bending. One test may include the determination of the minimum radius or diameter required to make a satisfactory bend. Another test may include the number of repeated bends the material can withstand without failure when it is bent through a given angle and over a definite radius.
Rebar is commonly bent prior to the hot-dip galvanizing process. Steel reinforcing bars bent cold prior to hot-dip galvanizing should be fabricated to a bend diameter equal to or greater than the specified value in ASTM A767/A767M. However, steel reinforcing bars can be bent to diameters tighter than the specified values if they are stress relieved at a temperature of 900 to 1050 F (480 to 560 C) for one hour per inch (25 mm) of diameter.
The specification to determine the presence of chromate on zinc surfaces is ASTM B201. This test involves placing drops of a lead acetate solution on the surface of the product, waiting five seconds, and then blotting it gently. If this solution creates a dark deposit or black stain, then there is unpassivated zinc present. A clear result indicates the presence of a chromate passivation coating.
When there is suspicion of potential embrittlement of a product, it may be necessary to test a small group of the products to measure the ductility. These tests are usually destructive to the zinc coating and possibly to the product as well. Products suspected of embrittlement shall be tested according to the specification ASTM A143. Depending on the service conditions the product will be exposed to, one of three embrittlement tests: similar bend radius test, sharp blow test, or steel angle test may need to be performed. The embrittlement test uses a known force to provide a stress that should be lower than the yield stress of the part. If there is a fracture or permanent damage created during the testing process, the parts must be rejected.