I did everything I could to make sure the steel part came out of the galvanizing kettle with a bright and shiny coating, and then I checked underneath the top flange of the beam that I was galvanizing and all of these black skimmings were on the coating right below the flange. What do I do with these black skimmings?
The exposure of molten zinc metal to air results in a thin layer of zinc oxide formation on the surface of the molten metal. This layer can also contain small amounts of zinc chloride or ammonium chloride from the flux. This material forms a black powdery substance when trapped on the surface of the galvanized coating, as Figure 1 evidences. According to ASTM A123 section 6.4 on appearance, this substance is not prohibited from the coating surface.
The reason that this material is allowed to reside on the surface of the coating is that there is good galvanized coating underneath the black powdery substance. The micrograph in Figure 2 shows the black zinc skimmings at the top and the intact coating underneath the black powder.
Since there is coating underneath the powder, the corrosion protection is not affected by the black substance. In spite of the specification, most galvanizing customers will not accept a part with deposits of black powdery zinc skimmings on their newly galvanized parts. This powdery material can be removed with a nylon bristle brush or with a hand grinder, just remember not to take too much zinc away when removing the skimmings. In this case, it is best to give the customer what he wants and not a piece with black powder on the coating.
© 2021 American Galvanizers Association. The material provided herein has been developed to provide accurate and authoritative information about after-fabrication hot-dip galvanized steel. This material provides general information only and is not intended as a substitute for competent professional examination and verification as to suitability and applicability. The information provided herein is not intended as a representation or warranty on the part of the AGA. Anyone making use of this information assumes all liability arising from such use.