Thick Coating Requests
What do I tell a customer who wants a coating much thicker than specified by ASTM A123?
Most people understand a greater zinc coating thickness will provide better corrosion protection performance to the base steel. However, they may not understand the limitations of the coating thickness galvanizers do. Somebody may ask you for a coating thickness over 10 mils thick by double dipping the steel, without being aware of some important issues. The chemistry of the steel, a brittle coating, the issues with double dipping, and the level of existing corrosion protection already present from a standard thickness, are all subjects that may be confusing to someone unfamiliar with the process. As with many topics in the hot-dip galvanizing industry, opening up the lines of communication and educating your customer is the best way to ensure satisfaction. Here are some simple answers you can provide your customer when asked for an excessively thick coating.
The chemistry of the steel can make it impractical.
Some customers may want you to leave the steel in the zinc bath for a longer amount of time in order to produce an extra thick coating. This may be an option for reactive steels, where the thickness of the coating grows linearly with time. However, if the steels silicon and phosphorous levels are within the limits recommended by ASTM A385, the rate of the galvanized coating thickness growth will be parabolic with time, that is the growth will peak and then decline. After a short span of a quick growth rate, the thickness will grow much slower, making a much thicker coating impractical to achieve.
The coating may become embrittled.
Hot-dip galvanized coatings that are too thick raise concerns about the brittleness of the coating. When an excessively thick (about 10 mils or greater) coating cools after galvanizing, the stress between the steel and the zinc becomes great enough to strain the coating and induce flaking. The coating may flake off upon the application of an outside force such as rough handling of the material, or the coating may even begin to flake before it leaves the plant. A coating thickness closer to those recommended in ASTM A123 will not be susceptible to flaking. This may be the most important reason a thicker coating is not desirable for your customer.
Double dipping the steel does not work.
There seems to be a perception that a galvanized coating is similar to a paint coating: you can apply multiple coats to achieve a greater thickness. However, a hot-dip galvanized coating actually relies on a reaction between the steel and zinc, forming intermetallic layers that grow perpendicular to the base steel. Once the coating has cooled, redipping the material into molten zinc may only melt away the free zinc (Eta) layer of the coating and replace it with a new one upon removing it from the bath. The intermetallic layers will only grow once the steel is heated back to the galvanizing temperature. A longer initial immersion time will have the same effect as a second immersion and is a much more efficient option. The term 'double dipping' should not be confused, or used interchangeably with the term progressive dipping. Progressive dipping is done when a piece is too large to be fully submerged in the galvanizing kettle, so each end of the piece is dipped sequentially to coat the entire item.
A coating thickness near standards already provides adequate corrosion protection.
Often times, the corrosion protection of hot-dip galvanized steel exceeds the design life of the structure itself. Even coatings near the specified thicknesses of ASTM A123 can provide upwards of 70 years of corrosion protection with zero maintenance in atmospheric exposure. The AGAs Time to First Maintenance chart is a great tool for convincing a customer their steel will be adequately protected by hot-dip galvanizing and an excessively thick coating is not necessary. The Service Life of Galvanized Steel Articles in Soil Applications chart, along with several case studies shown on the website can also help ease your customers' mind.