Galvanising

The most common hot dipping process for industrial applications is galvanizing, which refers to the coating of zinc over iron or steels for rust proofing. It is an extremely versatile and easy means of providing corrosion protection for construction materials exposed to atmospheric conditions. This protection is imparted in two ways:

• As a continuous barrier by protecting the underlying substrate from contact with the corrosive electrolyte
• As a galvanic protector by sacrificing itself slowly in a corrosive aqueous environment

Hot dip galvanizing is essentially a two-step process. First, the steel must be properly prepared to accept the zinc coating. This is normally accomplished by pickling the component in an alkaline solution to remove any rust or grease. A clean base metal is essential in achieving a good, galvanized coating. The second step involves immersing the component in a bath of molten zinc, long enough for the zinc to form a series of alloy layers with the base steel of sufficient thickness to meet certain required weight specifications. The thickness is primarily a function of the duration of immersion, the speed of withdrawal from the bath, and the bath temperature (zinc melts at 420°C, but a typical bath temperature is around 450°C). It should be noted that under certain conditions, the ductility of steels can be affected by the galvanizing process. While this phenomenon has been termed 'galvanizing embrittlement', the loss of ductility is not related to the presence of the zinc. In fact, it has been attributed to a form of strain age embrittlement, which relates to a change in properties of cold worked steels over time at elevated temperature. The effect of heating the steel during the galvanizing process can serve to accelerate the changes in the properties of cold worked steel. Research has shown that the amount of cold working has a strong influence on the development of strain age embrittlement.

The chemical composition (primarily carbon) of irons and steels determines the suitability of these metals for galvanizing. Although steels containing up to 0.4% carbon have been successfully galvanized on a production basis, low carbon steels having a maximum carbon content of 0.15% are considered the most suitable. To avoid brittleness of the iron-zinc alloy layer, cast iron should be low in phosphorus and silicon; a preferred composition may contain about 0.1% P. and about 1.2% Si.

A galvanized coating can also be applied without the use of a bath - which is called 'dry' or mechanical galvanizing. While not nearly as common as hot dip galvanizing, mechanical galvanizing is a room temperature process in which the metal components to be coated are placed in a large tumbler together with zinc powder, glass beads and an aqueous phase. The glass beads act to mechanically alloy the surface of the component by 'hammering' the zinc into the surface of the part. This impact galvanizing process is credited with producing adherent coatings that provide uniform protection in recesses as well as on the crests of threaded parts. The most common application of mechanically galvanized parts is for fasteners.