Galvanized steel is carbon steel coated with a layer of zinc to prevent corrosion. This protective zinc coating distinguishes it from standard steel, making it a popular choice for outdoor and exposed environments. The strength of galvanized steel is determined by its mechanical properties and not its surface treatment. The true strength comes from its base material, not the thin zinc layer.
Separating Mechanical Strength from Corrosion Protection
The fundamental strength characteristics of galvanized steel, including tensile strength, yield strength, and hardness, are determined entirely by the grade of the underlying steel. Galvanization is a surface process intended to provide corrosion resistance, not to increase the material’s load-bearing capacity. The zinc coating is typically only a few thousandths of an inch thick and does not contribute structurally to the material’s integrity.
Mechanical properties like yield strength measure the stress at which the material begins to deform permanently. For common structural steel grades, this range is approximately 250 to 355 megapascals (MPa). The zinc coating is too thin to influence these bulk properties. The protective function of the zinc is to act as a sacrificial anode, corroding before the underlying steel is exposed.
In most standard structural applications, the galvanizing process has a negligible effect on the steel’s mechanical performance. However, the heat involved in the coating process can slightly reduce the yield strength of extremely high-strength steel alloys by up to 18% and the tensile strength by up to 13%. For the mild and medium-strength steel used in most construction, the strength remains the same as the ungalvanized base metal.
The Hot-Dip Galvanization Process
The most common method for applying the protective layer is the hot-dip galvanization (HDG) process, which creates a metallurgical bond between the zinc and the steel. This process begins with thorough surface preparation, including cleaning and pickling the steel in an acid bath to remove scale and rust. The prepared steel is then immersed in a flux solution, which helps the molten zinc wet the surface.
The material is then fully submerged into a kettle of molten zinc, maintained at approximately 450 degrees Celsius (842 degrees Fahrenheit). While submerged, a diffusion reaction occurs between the iron in the steel and the molten zinc. This reaction forms a series of distinct zinc-iron intermetallic alloy layers bonded to the steel, topped by an outer layer of pure zinc. This multi-layered coating provides long-term corrosion protection.
Selecting Galvanized Steel for Structural Use
When choosing galvanized steel for high-strength applications, the grade of the base steel must be the primary consideration, such as selecting an S355 grade over a standard S275 grade. Designers must also consider the potential for hydrogen embrittlement, which mainly affects very high-strength steel alloys with tensile strengths exceeding 1,000 MPa (150 ksi). This occurs when atomic hydrogen is absorbed into the steel during the pickling stage of the galvanization process.
The high temperature of the zinc bath usually drives off most absorbed hydrogen. However, specialized precautions are necessary for extremely high-strength components to prevent brittle failure. Fabrication methods also require attention; welding galvanized steel demands the removal of the zinc coating from the weld zone to prevent porosity and protect the welder from zinc fumes. After welding or drilling, the exposed steel must be re-coated, or “touched-up,” to maintain continuous corrosion protection.