Galvanization, or galvanisation, is the process of applying a protective zinc coating to steel or iron, to prevent rusting. The most common method is hot-dip galvanization, in which parts are submerged in a bath of molten zinc. Galvanizing protects in two ways:
it forms a coating of corrosion-resistant zinc which prevents corrosive substances from reaching the more delicate part of the metal
the zinc serves as a sacrificial anode so that even if the coating is scratched, the exposed steel will still be protected by the remaining zinc.
Hot-dip galvanizing deposits a thick robust layer that may be excessive. In the case of automobile bodies, where additional rust proofing paint will be applied, a thinner form of galvanizing is applied by electrogalvanization. The hot-dip process does generally not reduce strength on a measurable scale, with the exception of high-strength steels (>1100 MPa) where hydrogen embrittlement can become a problem. This is a consideration for the manufacture of wire rope and other highly stressed products. The protection provided by hot dip galvanizing is insufficient for products that will be constantly exposed to corrosive materials such as salt water. For these applications, more expensive stainless steel is preferred. Some nails made today are electro-galvanized. Nonetheless, electroplating is used on its own for many outdoor applications because it is cheaper than hot dip zinc coating and looks good when new. Another reason not to use hot dip zinc coating is that for bolts and nuts size M10 (US 3/8″) or smaller, the thick hot-dipped coating fills in too much of the threads, which reduces strength (because the dimension of the steel prior to coating must be reduced for the fasteners to fit together). This means that for cars, bicycles and many other light mechanical products, the alternative to electroplating bolts and nuts is not hot dip zinc coating but making the bolts and nuts from stainless steel.
Galvanized surface with visible spangle
The size of crystallites in galvanized coatings is a visible and aesthetic feature, known as “spangle”. By varying the number of particles added for heterogeneous nucleation and the rate of cooling in a hot-dip process, the spangle can be adjusted from an apparently uniform surface (crystallites too small to see with the naked eye) to grains several centimetres wide. Visible crystallites are rare in other engineering materials.
Thermal diffusion galvanizing, or Sherardizing, provides a zinc diffusion coating on iron or copper-based materials. Parts and zinc powder are tumbled in a sealed rotating drum. At about 300 °C zinc will evaporate and diffuse into the substrate to form a zinc alloy. The preparation of the goods can be carried out by shot blasting. The process is also known as dry galvanizing, because no liquids are involved, there will be no danger of hydrogen embrittlement of the goods. The dull-grey crystal structure of the zinc diffusion coating has a good adhesion to paint, powder coatings, or rubber. It is a preferred method for coating small, complex-shaped metals, and for smoothing rough surfaces on items formed with powder metal.