Hot-dip galvanizing deposits a thick, robust layer of zinc iron alloys on the surface of a steel item. In the case of automobile bodies, where additional decorative coatings of paint will be applied, a thinner form of galvanizing is applied by electrogalvanizing. The hot-dip process generally does not reduce strength on a measurable scale, with the exception of high-strength steels (>1100 MPa) wherehydrogen embrittlement can become a problem. This deficiency is a consideration affecting 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 acids, including acid rain in outdoor uses. For these applications, more expensive stainless steelis preferred. Some nails made today are 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 of 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 practical alternative to electroplating bolts and nuts is not hot-dip zinc coating, but making the fasteners 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, even though they are usually present.
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. Around 300 °C (572 °F), zinc will diffuse into the substrate to form a zinc alloy. The advance surface preparation of the goods can be carried out by shot blasting. The process is also known as "dry galvanizing", because no liquids are involved; this can avoid possible problems caused by hydrogen embrittlement. 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 sintered metal.