| The greater use of de-icing salts has led car manufacturers and governments to draw up stringent codes of practice and warranties to improve corrosion protection. To meet these requirements, and to satisfy market expectations of long-term warranties, a range of precoated steel products were developed in collaboration with car manufacturers, pretreatment and paint suppliers to suit the needs of individual car companies. The body in white and closures of current European passenger cars now incorporate a large proportion of precoated steels. Hot Dip Coatings Gain Favour Initially, automotive applications concentrated on electro-coated products but during the 90s, the range of hot dip coatings, particularly galvannealed coatings, have become more popular with car makers. This is partly due to the steel industry investment in research and manufacturing facilities for higher surface and substrate quality and consistency. About 60% of the total UK automotive coated sheet steel usage is now made up of hot-dip coated products. Selection Criteria The selection of the most appropriate coated steel for a particular application requires the user to balance several, sometimes conflicting, factors. Apart from cost and corrosion resistance, manufacturing compatibility is a key factor in choosing the most appropriate type of coated steel. Table 1 summarises the types of precoated steels and their applications in the European automotive Industry. Table 1. Precoated strip steel used in the European Automotive Industry. | | | Hot-dip zinc coated | Standard hot-dip product | Mainly non-visible parts | | Galvannealed/iron zinc alloy | Good weldability and paintability | Body panel and non-visible parts | | Electro-zinc coated | Equivalent range of properties to CR; single sided coatings are available. | Body panels | | Electro-zinc nickel coated | Improved weldability | Body panels | | Electro-zinc nickel coated+organic | Improved corrosion resistance | Body panels | | Aluminium/silicon | Heat resistance + | Exhaust systems | | Aluminium/zinc Tin-lead (terne) + nickel flash; Electro-terne | Corrosion resistance
Resistance to fuel corrosion. | Fuel tanks |
The corrosion resistance provided by a precoated steel is largely a function of the thickness of the coating. Zinc-based metallic coatings provide resistance to atmospheric corrosion through a combination of barrier and sacrificial protection. The corrosion rate is affected by the nature of the corrosive environment and the coating chemistry. In general, electro-coated steels meet individual part requirements by offering a wide range of substrate properties and being easy to paint. A new generation of galvannealed products which have higher strength and are easily formed have and will continue to emerge, giving car makers greater choice and unproved performance in manufacture and service. Effect of Coating Thickness The thicker alloy coatings have a tendency for lower adhesion under severe forming conditions, particularly if in-plane compressive stresses arise during press forming. Thinner hot dip alloy coatings (7-10µm thick) suffer less from these problems. Low coating weights also offer the car maker certain advantages, particularly in resistance spot welding. The welding performance of alloy coatings – electro-zinc-nickel and hot dip galvannealed steels is generally superior to that of pure zinc coatings. Automatic welding electrode tip dressing ensures that production rates with the galvannealed product are close to those achieved with uncoated steels. Duplex Coatings and Pre-Primed Panels Duplex coatings comprising a thin layer of organic primer on a metallic coated base are preferred by some car makers; the organic coating acts as a lubricant in press forming, provides corrosion protection in crevices and clinches, and does not interfere with the welding process. Pre-primed or pre-painted steels have also been investigated for applications in the spare parts market. Although the ability to form pre-primed steels is excellent, welding currently presents difficulties. In practice, car manufacturers rationalise their use of coated steel in the body-in-white to a few different types. The use of a large number of different coated steels can lead to production difficulties and a reduction in flexibility, e.g. more frequent re-setting of manufacturing parameters and the sorting of different scraps. Advantages of Precoated Steels • Assist car manufacturers to meet perforation and cosmetic corrosion warranties as well as retaining steels traditional qualities of stiffness, strength and formability. • Provide corrosion protection in storage, reducing the need for protective oils • Improve lubrication in press forming and reduces costs by saving on their corrosion protection measures such as waxes • From an environmental viewpoint, conserve natural resources through increased product durability Alternatives to Precoated Steels Alternatives to precoated steels include the following: • designing-in corrosion resistance at the initial design stage • using waxes and sealants • employing advanced pretreatment and painting • using other materials, such as plastics and aluminium alloys. To provide long term durability and to protect cars from stone chip damage and subsequent corrosion, car manufacturers have developed their own philosophy based around the above options in combination with the use of precoated steels. In critical areas such as structural members, doors and bonnets, it is difficult to obtain adequate phosphate, paint and wax treatment. The application of the more complex manual post-treatments, such as wax injection and sealing results in extra costs. The use of plastics for those outer body parts which are susceptible to corrosion has its attractions but in practice difficulties can be encountered in assembly and the painting process. Parts produced from alternative materials generally involve higher costs for manufacture and repair. |