Elemental Additions to the Kettle
I seem to be having problems producing coatings of a proper thickness. What are some things that can be done to make a thicker or thinner coating?
This is a problem that many galvanizers seem to face on a regular basis. The thickness of a hot-dip galvanized coating is very important as it affects the mechanical properties of the zinc coating, the appearance of the coating, and corrosion protection performance of the coating. If the coating is left too thin, it will not meet the specifications of ASTM A123 and will not provide adequate corrosion protection performance. If the coating is too thick, it may become brittle and subject to flaking which will leave spots of bare steel exposed to the environment.
A common problem galvanizers face is galvanizing reactive steels and struggling to keep the coating below an acceptable thickness. Another problem may be galvanizing steel that just wont seem to take the required amount of zinc; such as aluminum-killed steel. Elemental additions affect the final coating properties in a complex manner and can be used to either increase or decrease the thickness of a coating. We can associate certain coating properties and problems with the levels of elemental additions in the zinc bath. ASTM A123 requires the bath of molten zinc in the hot-dip galvanizing process to be at least 98.0% zinc. This leaves a certain amount of wiggle room for a galvanizer to make additions to his or her bath in accordance with specific needs.
The addition of nickel controls the growth of the galvanized coatings intermetallic layers. This is found to be especially useful in the peak region of the Sandelin Curve where the silicon content of the steel causes excessively thick coatings. Promoting the stability of the alloy layer growth through the addition of nickel creates strongly adherent coatings with a bright layer of free zinc on the surface. A nickel level of 0.04% to 0.09% has been shown to cause this positive effect. However, the presence of nickel in the zinc bath can also cause the coatings on some non-reactive steels to fall below the minimum requirements.
Although there has been a shift in the industry to using smaller amounts of lead in the zinc bath, lead can have a beneficial effect on the properties of the galvanized coating. While the addition of lead does not seem to have a large effect on the growth mechanism of the coating, the decreased surface tension of zinc from this elemental addition can lend to a more desirable coating thickness. A 1% addition of lead will reduce the surface tension by more than 30%. This leads to better drainage of molten zinc from steel parts, preventing bulges in the coating and accumulation of zinc within interior angles. The reduced surface tension also allows a galvanizer to run the kettle at a lower temperature and reduce the risk of creating an excessively thick coating.
If dealing with reactive, silicon killed steels, an elemental addition of aluminum decreases the growth of intermetallic layers. This will allow a galvanizer to better control the thickness of the coating. An aluminum level of 0.01% can brighten the coating and control the growth of intermetallic layers. However, the introduction of too much aluminum in the zinc bath will result in killed flux, leaving bare spots on the steel.
Temperature vs Silicon Content
As is well known, the silicon content of the steel can have an effect on the thickness of a hot-dip galvanized coating based on the Sandelin Curve. However, there is a lesser-known relationship between the temperature of the zinc bath and the silicon content of steel that dictates the formation mechanism of the galvanized coating. Two types of formation mechanisms exist in the hot-dip galvanizing process, each with its own benefit. A coating whose growth is linear with time may be beneficial when a galvanizer is working with a non-reactive steel and needs to add more zinc to get to the required minimum thickness. This region is denoted by the shaded region on the chart to the right (taken from Frank C. Porters book) which shows the space the two mechanisms take place based on silicon content and bath temperature. The non-shaded areas, still within the dashed lines, are regions where the coating growth rate will slow over time. These regions may be targeted when working with reactive steels.
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