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Am I (customer) being charged for thick coatings? Why do I keep getting thick coatings from the galvanizer– are they trying to inflate the total selling price by putting more zinc on the steel?

This is a question the AGA receives from time to time when your customers have concerns over the thickness of the hot-dip galvanized coating. Although sometimes a thick coating can be desired to achieve greater longevity of the galvanized coating, other times an excessive coating thickness potentially results in issues related to fit-up between parts, appearance concerns, or flaking of the coating. This can lead to a soured relationship with the customer if they believe galvanizers are providing thick coatings in an attempt to charge more for their services on a per-pound basis.

However, galvanizers know this is certainly not the case. Zinc is the largest material cost factor in the hot-dip galvanizing process, and a low zinc consumption rate lowers the cost of production. If we look at the cost of zinc used in the hot-dip galvanizing process, it is possible to see the impact zinc prices and thick coatings have on the galvanizer’s bottom line.

Prices of zinc over the last year (USD):

  • 52-week low: $1.04/lb
  • 52-week high: $1.64/lb
  • Current: $1.19/lb

Galvanizers charge for their services by weighing the part after it has been galvanized, and this results in pricing anywhere from $0.20 to $0.50 (USD) per pound of finished weight. This pricing information is general to the industry in North America and based on survey data obtained from AGA galvanizing members. The customer may realize an increase in coating thickness adds extra weight (and therefore cost) to the final product. However, if you consider the price of zinc referenced above, this means the galvanizer actually loses $0.69 to $0.99 for each additional pound of zinc on a part which is beyond the minimum needed to meet the galvanizing specification requirements.

With this information, you can explain it is not in the galvanizer’s best interest (financial or otherwise) to provide excessively thick coatings to customers. Instead, the galvanizer’s profit margins increase when the minimum average coating thickness is met while overall zinc consumption is minimized.

Then why does the galvanizer occasionally provide thick coatings when it is not desired by either the galvanizer or the customer? The primary challenge occurs when the customer provides the following types of steels or steel assemblies that make limiting the coating thickness very difficult. The list below contains links to Dr. Galv articles explaining why these conditions may result in thick coatings:

Should you experience any of these steels or assemblies at your plant, there are some methods the galvanizer can employ to try and limit the coating thickness. It is important to note not all of these methods may be applicable to a project, and a thinner coating is never guaranteed:

  • Reduce immersion time
  • Request the customer blast clean the parts before hot-dip galvanizing
  • Move/agitate the part as much as possible while immersed in the bath
  • Increase immersion rate and decrease withdrawal rate from the zinc bath (effectiveness limited by poor venting/drainage design)
  • Add elements to the zinc bath known to reduce the surface tension of the zinc and/or reduce reactivity for Sandelin Steels with 0.04 – 0.15 % Si content (impractical to change existing bath chemistry for one customer)
  • Lower the bath temperature (often impractical)

In order for these methods to be used successfully, you must be informed of the steel chemistry and/or assembly make-up ahead of time. If you can obtain mill test reports (MTRs) and drawings from the customer in advance, it is possible to evaluate the steel chemistry, review the overall design, and then determine which measures are appropriate to limit the coating thickness at your plant. Unfortunately, many galvanizers report to the AGA this information is often very difficult to obtain from the customer.

To avoid concerns from the customer about thick coatings and pricing in the future, inform your customers that direct communication between the galvanizer and the customer is critical to ensure a successful combination of design, fabrication, and galvanizing practices are employed. These practices help ensure the customer’s expectations are met regarding cost, fit-up, appearance, and overall quality. 


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Comments

Albert Gebeshuber

Dear Alana, cleaning the steel by blasting to reduce the coating is not recommended at all, becuase you increase the roughness of the surface and therefore you generally get higher coatings. Also, when reducing the time of withdrawel, do not increase the time of inmersion generally. When you have a reactive steel, you have to reduce the dipping time to the least. When you have silicon contents below 0.3% all this is not any issue, on the opposite, becuase you have problems to get the coating acc. to the code. And then you can blast etc etc. Any comment is very appreciated, Best regards from ZinkPower, ALbert Gebeshuber

Alana Hochstein (AGA)

Hi Albert, Although it is true blasting will increase coating thickness for steels of normal chemistry (by providing increased surface area), blasting significantly decreases coating thickness for steels of reactive chemistry. The peaks and valleys of the blasted surface actually interfere with the zinc coating growth for reactive steels -- the coating crystals grow into one another (as the coating grows perpendicular from each peak/valley) which keeps the zinc coating from growing at an accelerated rate. The final coating thickness after blasting is then much lower than the coating thickness would be without blasting. When I mention "increase the immersion rate" I do not mean increase time the parts are in the bath. Instead, "increase immersion rate" means lower the steel underneath the zinc as quickly as possible instead of slowly lowering it into the bath. I hope this clarifies, but please let us know if you have further comments!

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