What are some recommended practices for hot-dip galvanizing of thick steel?

The hot-dip galvanizing of very thick steels (beyond 2-3 inches thick) presents process challenges to galvanizers such as a high zinc consumption rate and extra processing time.  Often times, such articles are left in the galvanizing kettle for very long periods of time to allow the part to reach the galvanizing temperature and allow the coating to develop.

The requirement for an excessive immersion time (beyond an hour) is often the result of a cocoon effect which slows the heating of the steel.  When the steel first enters the bath, there is a very large difference between the steel and the molten zinc temperatures.  Then, the molten zinc begins to transfer heat to the steel and bring the steel to the galvanizing temperature.  The molten zinc nearest the steel transfers the most heat and reaches a temporary equilibrium, leaving an unmoving cocoon of frozen zinc directly surrounding the steel article. As a result, the heat within the bath becomes unevenly distributed, and the time required to heat the steel increases significantly.

With excessive immersion times in the kettle, there is concern over the development of excessively thick and brittle coatings.  The use of an excessive immersion time is especially problematic for reactive steels with high silicon/phosphorus content.  For reactive steels, the alloy layers of the coating will continue to grow at a rapid rate during the immersion, while steel grades of optimal chemistry will experience a diminishing growth rate that eventually tapers off (see Figure 1).  

Although a thick coating is associated with a longer time to first maintenance, coatings developed beyond 8-10mils tend to be very brittle and are susceptible to flaking both at the galvanizing plant or after delivery to the job site upon exposure to impact forces during handling, transportation, or installation by the customer (see Figure 2).  Such delamination issues often lead to misguided customer complaints about poor quality galvanizing.

The occurrence of flaking and repair are not the responsibility of the galvanizer after the articles have been accepted by the customer, but flaking is still undesirable.  Although there are many variables beyond the galvanizers control which can make hot-dip galvanizing thick articles a challenge, there can be ways to minimize the coating overgrowth and therefore reduce the susceptibility for excessive zinc consumption and handling damage.

The most effective practice when galvanizing thick steel articles is to move the article to the molten zinc bath in such a way to lessen the cocoon effect.  The article can be agitated and slowly moved around the bath utilizing overhead cranes where possible. These actions promote a more even distribution of heat within the bath, allows the formation of a uniform coating thickness, and greatly minimizes the required immersion time in the molten zinc.

If the steel is of reactive steel chemistry, then coating thickness can also be controlled by blast cleaning the steel prior to galvanizing. Blast cleaning the steel causes the zeta layer of the coating to become very rough and interferes with the zinc coating growth.  This interference during the hot-dip galvanizing process keeps the zinc coating from growing at an accelerated rate and produces a coating thickness much lower than would be without blasting.

Lastly, it is also known the kettle temperature will fluctuate after the introduction of articles into the zinc bath.  Where available, a deeper kettle (at or beyond 8ft. in depth) will retain heat within the galvanizing kettle more effectively than a shallow kettle, resulting in a higher operating temperature during the reaction and a reduced immersion time.