Zinc coated steels

The automotive industry,in seeking to provide extended warranties,is turning increasingly to the use of zinc coated steels. Modern automobiles must be not only of high quality but also durable and economical,as perceived by their purchasers. These vehicles are expected to exceed an average of seven years without structural or cosmetic deterioration due to corrosion. Increasingly aggressive environmental influences tend to shorten the life of the car,whereas ever more specialized steel sheets are being incorporated into vehicle construction in the battle against corrosion.
The use of zinc coated steels has dramatically increased to meet these challenges. Different areas of a vehicle require different zinc coatings and coating weights to meet appearance and performance criteria. These are available in both hotdipped (BS 2989:1982) and electrolytically deposited (BS 6687:1986) versions in a range of coating weights or thickness. Both types offer barrier and sacrificial corrosion protection,and the choice of product depends on the particular application and requirements. The hot dip product (available as plain zinc,or iron-zinc alloy) is generally used for underbody parts. The electrolytic product is used for exposed body panels,where a full-finish surface quality is available to ensure that a showroom paint finish is achieved. The electrolytic product is available in single-sided and double-sided coating. (See Figure 4.1.)

Single-sided zinc coated steel Free zinc is applied to one side of a steel sheet by either the hot dip or the electrolytic process for this material. Its uncoated side provides a good surface for paint appearance,so it is used mainly for outer body panels. Since free zinc is towards the inside of the car, it protects against perforation corrosion.
One-and-a-half-sided zinc coated steel In this case,one side of the sheet is coated with free zinc, and a thin layer of zinc-iron alloy is formed on the other side. This product is produced mainly by the hot dip process. It is primarily used for exposed panels,where the zinc-iron layer is on the outside for cosmetic protection and the free zinc side provides perforation protection.
Double-sided zinc coated steel This product is manufactured by applying free zinc to both sidesofthe sheet with equal or differential coating weight. All types are readily paintable and weldable. However,care should be taken to ensure that welding conditions are comparable with thematerial used:for example,higher weld current ratings may be necessary on the heavier coatings.