Galvanized Coatings - Delamination

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Hot-dip galvanized coatings are famous for their toughness. They are among a handful of industrial coatings with the strength to withstand the adversities of transport and handling without being damaged. However, problems can arise in specific fabrication processes in the form of delamination of the galvanized coating, sometimes without even applying any mechanical stresses.

The galvanized coating can be peeled, delaminated, or blistered only if a particular set of metallurgical conditions occurs, which includes the following:

• Steel chemistry that reacts with molten zinc and results in coatings that are thicker than normal
• A cooling cycle that induces mechanical stress into the coating at the steel/coating interface
• Mechanical stresses on unprotected galvanized coatings

Hot-Dip Galvanizing

Hot-dip galvanized coatings are formed by immersing steel in molten zinc at 455 °C. The developed coating relies on the steel’s chemistry and surface condition, and the immersion time in zinc.

The thickness of the hot-dip galvanized coatings is dependent on the thickness of the zinc-iron alloy layers that develop when the steel reacts with zinc. Typically, these alloy layers are made up of 95% zinc and 5% iron. They are hard and inflexible.

Thicker galvanized coatings provide improved durability, but once the coating thickness exceeds about 200 μm, the thick alloy layers become more vulnerable to delamination.

Delamination

Delamination of galvanized coatings is mainly caused by “heat peeling.” This occurs when the steel, which typically has a high section thickness, is cooled slowly or insufficiently during the quenching process, resulting in reheating of the coating because of the residual heat in the steel mass.

The thermal stresses generated by this differential heating or cooling will form high shear forces at the steel/coating interface, thus resulting in localized delamination of the coating. This will result in blister formation or, in the worst situation, flaking of the coating from the surface.

Generally, mechanical delamination happens on edges or areas where the galvanized coating suffers higher pressure or localized impact.

Thanks to the enhanced mechanical keying of the coating to the steel surface, the possibilities of delamination of galvanized coating on rough steel surfaces are less. Exceptionally smooth surfaces on ERW pipe, RHS, or other cold-rolled sections are more susceptible to delamination on reactive steel. Furthermore, the presence of residual alloying elements, like phosphorus, increases the possibility of delamination.

Minimizing Delamination Problems

The first step is to use steels with appropriate chemistry. However, this is beyond the control of the galvanizer. Most of the Australian-made steels are acceptable in this regard and have steel chemistry better matched for galvanizing.

At present, nearly 35% of structural and merchant section steel is imported, which makes the steel chemistry and surface more diverse. The key steps in minimizing the delamination problems are given below:

• Immersion time should be decreased to curtail the growth of zinc-iron alloy layer. This is not only a design factor but also a processing challenge in the galvanizing plant. An efficient design will allow the fabrication to be smoothly handled via the galvanizing process.
• The product should be cooled at an ideal consistency after galvanization. The product must be fully cooled with care during the quenching process, and re-heating of the coating by the residual heat in the thick sections must be prevented.
• In areas where it is essential for the fabrications to be air-cooled, care must be taken to ensure they are left in areas that are properly ventilated, so that radiant heat from adjacent products does not have any effect on the individual products.

Galvanized coating on plate products usually gets delaminated when the plates are stacked on top of one another while they are still hot. This can result in delamination of even steels that are usually not vulnerable.

A thin zinc-iron alloy layer (the gamma layer) is usually left behind on the surface when the galvanized coatings get delaminated. Thus, corrosion is likely to happen within a short period. The delaminated area can be restored by using an approved epoxy zinc-rich paint repair system.