Questions to pose to customer for incorrectly galvanized steel
What questions should I pose to my customer when his steel doesn't galvanize quite the way either of us expected?
This is a great question, and the technical staff of the AGA regularly goes through a series of interrogatories with fabricators and galvanizer members calling the hotline (1.800.HOTSPEC). The actual questions are slightly different if the issue is appearance or structurally related (cracks, broken welds, warpage/distortion, etc.), but in general, focus on factors not under the galvanizers control. This is not to say galvanizers dont make mistakes in the process. For the particular anomalies of uncoated areas, different appearance across the same piece of steel, excessively thick coatings peeling/flaking, and cracking, the respective list of questions is as follows:
Does the steel have a lacquer or sealer applied to it or was there some adhesive on the surface for a sticker?
Caustic and acid cleaning does not remove these materials from steel prior to hot-dip galvanizing.
Is the steel a high carbon, such as C-1045, that has been heat-treated?
Heat-treating hardens the surface with carbon and there is no iron available at the surface to react with the molten zinc.
Is there weld spatter or blow out from an incorrectly welded area?
Weld pinholes cause gases and liquids to coat the surface during the immersion into the bath, leaving a bare area.
Is the matte gray on an otherwise shiny surface in a pattern?
A spiral pattern is caused by mechanical stresses induced during the rolling process of pipe/tube and the stressed area reacts differently with zinc than the rest of the pipe/tube.
Is the matte gray on the surface random?
Steel does not always have a homogenous chemistry and so the different level of iron, silicon, or phosphorous in a particular area may cause a complete consumption of the free zinc Eta layer, resulting in the matte gray appearance.
Is the wet-storage stain or white rust only in a defined area?
Wood dunnage may trap moisture and cause staining or white rust, water dripping from a low point on the parts above may be concentrated in the white rust area; strapping/chains left on for a period of time may also cause staining or rust in an unrecognizable pattern.
Is the center of the part dull gray and the outside edge bright and shiny?
The part can still continue to grow intermetallics after it is out of the kettle due to slow cooldown.
If the galvanized steel is exhibiting iron oxide (rust) after just a few years of exposure, what is the coating thickness measurement at the rust spot?
Called brown staining, this usually occurs when the zinc coating is fully alloyed, i.e. there is no Eta layer of zinc, and the zinc-iron alloy layers contain unbound iron oxide. The zinc coating thickness will likely meet ASTM specification and because the iron oxide is non-magnetic, a coating measurement is possible.
Excessively Thick Coatings (peeling/cracking)
What is the steel chemistry, and is the silicon content <0.04% or between 0.15% and 0.25%? Is the phosphorous content <0.04%? What is the value of the silicon content plus 2.5 times the phosphorous content?
A percentage outside the Sandelin curve may be the cause. Blasting of high silicon and high phosphorous steels may actually minimize the coating growth, as the crystalline alloy layer formation tends to be parallel to the steel surface and not perpendicular.
Was the steel blasted prior to galvanizing?
When carbon steel with the proper silicon and phosphorous content is blasted, the rough surface causes heavier coating development.
Is the steel weathering steel (ASTM A588)?
Weathering steel is very reactive with zinc.
Has this product been galvanized before without a problem, and if so what changes were made by the fabricator?
If the galvanizing process hasnt changed, then the only other variables are within the control of the fabricator.
Is a different steel instead of mild carbon steel being used?
Abrasion-resistant (e.g. AR400) and high yield strength alloy (e.g. T-1) will galvanize, but they are very hard and their high strength (tensile >150 ksi) promotes strain aging in the galvanizing bath, causing cracking of the steel.
If a fabrication containing two or more different steel thickness and/or shapes (flat bar, HSS, plate), is the crack near the weld connecting the dissimilar steel thicknesses?
The heating/cooling gradient is different for each thickness of steel, and the galvanizing temperature induces stresses that may result in cracks
Summarizing, following the line of questioning above is an efficient way to get to the heart of most un-met expectations. It is a professional and even scholarly way to team with your customer, convincing him/her you are serious about understanding and ultimately resolving the problem. Additionally, if you have answers to these questions prior to calling the AGA for consultation, we will be more able to give you an immediate opinion or even definitive answer.
© 2023 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.