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As regulators of commercial buildings seek to achieve durable and long-lasting structures, many are turning to hot-dip galvanizing (HDG) as a method of protecting structural steel components from atmospheric corrosion for decades. However, when HDG is designated as the primary corrosion protection method for structural steel components requiring fire resistance ratings, passive fire protection specifications may need to be lengthy and detailed to ensure that the entire system can operate as expected by the manufacturer. Although the use of HDG steel for corrosion protection under fire conditions has many benefits, the galvanized coating must be carefully prepared for the system to meet the minimum bond strength requirements and operate as expected.

The term galvanizing is generally used to describe various coating methods used to apply zinc to steel for anti-corrosion purposes. Each of the following application methods has unique characteristics that affect the types of products that can be coated, coating thickness, economics, and environmental performance:

Among them, batch hot-dip galvanizing, also known as ordinary galvanizing, is the most commonly used method for structural steel members in building construction. Therefore, this article reviews specific recommendations for applying passive fire protection to batches of hot-dip galvanized steel.

Batch hot-dip galvanizing involves immersing iron or steel products in a molten zinc bath to produce thick metallurgical bond coatings, including zinc/iron intermetallic alloy layers and pure zinc surface coatings (Figure 1). This process is performed after the manufacture of steel components such as rods, beams, plates, frames and other components. First, before immersing the steel product in a zinc bath (galvanizing pan) containing at least 98% pure zinc and heating it to approximately 443 degrees Celsius (830 degrees Fahrenheit), the steel product is chemically cleaned to remove organic pollutants and oxide scales. And oxide.

When steel is immersed in a galvanized pot, zinc reacts with iron to form a hot-dip galvanized coating, which becomes part of the steel itself, not just the surface/barrier coating. This metallurgical combination forms a barrier coating with excellent wear resistance because the intermetallic layer of the coating is harder than the base steel (please read the "Zinc Coatings" published by the American Galvanizing Association [AGA] in 2011 "). The HDG coating also provides cathodic protection for the steel, ensuring that the steel underneath will not corrode until all nearby zinc is consumed. When minor surface damage occurs to the bare steel, unlike painted steel, the surrounding zinc will still provide cathodic protection.

Another aspect that distinguishes HDG from other building materials is its service life in the atmosphere. The first maintenance time of the hot-dip galvanized coating depends on the thickness of the coating and the corrosiveness of the environment, but depending on the atmospheric conditions (such as the annual average), the service life of the batch hot-dip galvanized products is 50 to 120 years. Rainfall is not uncommon , Temperature, humidity, air salinity and road salt exposure (Figure 2) (see AGA 2014 “Performance of Hot-Dip Galvanized Steel Products in Air, Soil, Water, Concrete, etc.”).

HDG for corrosion protection under passive fire protection

Although HDG has long been recognized for its ability to provide protection against atmospheric corrosion in aggressive environments, there are other benefits when using sprayed fireproof materials (SFRM) and intumescent fireproof materials (IFRM) on galvanized surfaces (see 2014 "Performance of hot-dip galvanized steel products in the atmosphere, soil, water, concrete, etc." published by AGA).

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