Galvanizing Difficult Steels

Sarin Kunthong / Shutterstock

Sometimes galvanizers receive questions about galvanizing unique steels or uncommon steel sections that are outside the standard range processed by the company’s galvanizing operations.

Although such steels can seem like a challenge for galvanizing, these issues can be solved through the design of the components or through alterations to the hot-dip galvanizing process.

Galvanizing Alloy Steels

Although high-strength alloy steels are not frequently used for structural applications, they are sometimes used as performance-critical parts in assemblies or as individually manufactured products. Three factors affect the ability of steel to be galvanized. These include:

• The chemical composition of the steel
• The thickness of the steel section*
• The strength rating of the steel (the yield strength in MPa)

*This is a factor as it establishes the immersion time of the steel in the molten zinc.

When a request for galvanizing a special type of steel is received, the chemical composition is first analyzed. The majority of the special steels contain chromium (Cr), manganese (Mn), carbon (C), silicon (Si), sulfur (S), phosphorus (P), and nickel (Ni). They may also include copper (Cu), vanadium (Va), and other elements that are used to impart specific performance characteristics to the steel.

What is in Alloy Steels?

There are hundreds of diverse kinds of unique steels available, but there are also generic chemistries that match particular applications and help in establishing the potential results when these steels are hot-dip galvanized.

The following list is a guide to the most significant alloying elements that can be found in unique steels. These steels also have an effect on the capability of these alloying elements to be galvanized:

• Tough steels: High levels of manganese which may be over 1.0%
• Hard steels: High levels of carbon which may be over 1.0%
• Electrical steels: High levels of phosphorous
• Spring steels: High levels of silicon which is up to 2.0%
• Stainless steels: High levels of nickel and chrome
• Free machining steels: High levels of sulfur

High Silicon Steels

High silicon steels will give rise to thick galvanized coatings that may be brittle because the steel reacts very rapidly with zinc. The effects of high silicon content can be minimized by keeping the immersion time in zinc as short as possible. This becomes progressively difficult as the thickness of the section expands.

High Manganese Steels

High manganese steels will develop brownish-colored coatings that may be fragile and easily damaged while handling when compared to galvanized coatings on traditional steels.

High Carbon Steels

If the yield strength of high carbon steels is within a satisfactory range, they can be effectively galvanized.

High Sulfur Steels

High sulfur steels are used in high-speed machined-components (threaded fasteners, sockets, etc.) and must not be galvanized. High sulfur steel wears out significantly in the galvanizing process, rendering threaded items unserviceable.

High Phosphorous Steels

High phosphorous steels are rarely found in galvanizing processes because they react rapidly with the zinc to form thick, dark coatings that are easily damaged and may even delaminate from the surface.

High Nickel and Chrome Steels

Stainless steels can be galvanized but are susceptible to liquid metal embrittlement and can break under load when immersed in molten zinc. Stainless steels can be galvanized only if they are fixed to mild steel assemblies.

Steel Strength and Galvanizing

High-strength steels (around 1000 MPa yield strength and more) are susceptible to hydrogen embrittlement that results from the pickling process in galvanizing. It is essential to avoid pickling for steels that fall under this strength range.

Steel Size and Galvanizing

Although all standard steel structural sections can be galvanized from time to time, unconventional sections also emerge that may cause difficulties. It may not be easy to appropriately galvanize sections with a thickness of over 100 mm in a traditional galvanizing bath.

These items have a very high mass per unit of volume, and since the zinc in the galvanizing bath is just about 35º above its freezing point, it freezes around the item when immersed and may form a layer of frozen zinc containing a thickness of 50 mm or more.

This zinc must be re-melted and then the item itself should be heated up by the zinc bath to bath temperature, to facilitate the development of the galvanized coating. These steps may affect the performance of the flux on the surface of the item and result in uncoated regions on the surface.

These flaws can be decreased by pre-heating the item or working at higher galvanizing bath temperatures that need an exclusive galvanizing bath design.

Summary

Tough steels, especially spring steels, are the types that mostly need to be galvanized, and can be effectively galvanized as well. If the thickness of the section is satisfactory and the springs can be abrasively blasted instead of being pickled, a galvanized coating can be formed on certain types of coil and leaf springs that will not affect their performance.