Considerations for Selecting Steel to be Galvanized
Galvanized coatings are formed by a chemical process during which steel and zinc metallurgically bond, forming a series of corrosion-inhibiting, highly abrasion-resistant zinc/iron alloy layers.
The chemistry of the steel being galvanized influences the galvanized coating’s appearance. Galvanized coatings on steels with common chemistries are typically shinier than are coatings on reactive steels, which tend to produce matte gray coatings.
Additionally, high-strength steel chemistries for steels with ultimate tensile strength of 150 ksi (1100 MPa) or more may cause hydrogen embrittlement of the steel after galvanizing.
Galvanized coatings are specified for their corrosion resistance. While a gray or matte appearance may occur, the corosion protection afforded is no different than that provided by shinier galvanized coatings. The corrosion resistance of matte coatings, mil for mil, is equal to the more typical bright, shiny galvanized coating.
When possible, the galvanizer should be advised of the grade of steel selected in order to determine whether to make accommodations in the galvanizing process.
Though most ferrous materials can be hot-dip galvanized, the chemical composition of the material affects the characteristics of the galvanized coating.
Steel compositions vary depending on strength and service requirements. Trace elements in the steel, such as silicon and phosphorus, affect the galvanizing process as well as the structure and appearance of the galvanized coating. For example, certain elements present in the steel may result in a coating composed almost entirely of zinc-iron alloy layers with little or no free zinc layer.
Uniform, shinier coatings are produced by steels with the following chemistries:
- Levels of carbon less than 0.25%
- Levels of phosphorus less than 0.04%
- Levels of manganese less than 1.35%
Silicon (which is present in many steels commonly galvanized even though it is not a part of the controlled composition of the steel) levels less than 0.03% or between 0.15% and 0.25% have slower coating growth rates and produce more controlled coatings.
Phosphorus acts as a catalyst during galvanizing, resulting in rapid growth of the zinc-iron alloy layers. This growth is difficult to control during the galvanizing process.
The characteristics of a coating made up primarily of zinc-iron alloy layers differ from those of a typical galvanized coating. The zinc-iron alloy coating may have a matte gray appearance because it does not have an outermost free zinc layer. A primarily zinc-iron alloy coating also tends to be thicker than a typical galvanized coating. In extreme situations, as the thickness of this type of coating increases, the coating may experience adhesion problems under external stress.
Hydrogen embrittlement is of concern for steels of ultimate tensile strength of 150 ksi (1100 MPa). During the chemical reaction between steel, rust and the chemicals used in the pickling stages of the galvanizing process, hydrogen is produced and absorbed by the steel. The very small grain structure of high-strength steel prevents the hydrogen from being expelled during immersion in the molten zinc. When put under stress in use, the steel may become brittle and fracture.