Research and development on zinc coated steel sheets have been intensively carried out in order to achieve so called 3 Rs(Reduce, Reuse and Recycle) toward establishment of recycling-based society and high quality and cost performances. The galvannealing reaction and structure of the coatings have been investigated and lubricative inorganic and organic coatings have been developed, leading to a remarkable improvement of press formability of galvannealed steel sheet for automotive body panels. Pb-free and Cr(Cr6+ and Cr3+)-free coated steel sheets have been developed for both automobile and electric appliance industries to reduce hazardous substances. For building use, corrosion-resistant coated steel sheets with thin organic coatings on 55% Al-Zn alloy and also hot-dipped alloy coatings containing Mg have been developed. This is to meet the needs for long time duration of products and minimizing maintenance. The research and development on zinc coated steel sheets are reviewed in the paper.
Most of can stock is applied to beverage can. The consumption of beverage cans has increased from the mid-1980s to the mid-1990s and the annual consumption has become an enormous amount, about 40 billion cans. During the period, the development of can making technology and can stock such as welded can and DI (Draw and Ironed) can has been focused on productivity and cost reduction. Meanwhile, enormously increased consumption has brought up large effects on environment and more attention was paid for environmental problems. As a countermeasure of environmental problem, the use of laminated material is actively been stepping forward. In this paper, the recent progress of steel can stock in the economical and environmental issues is described.
Recent development in prepainted steel sheet production is reviewed in terms of the control of heterogeneity of resin composition, morphology, and structures of the coatings. The former two are mainly controlled by surface tension or reaction rate differences between resins, and the latter one by design of the resins themselves. Relationship between mechanical properties of the coatings and press formability of prepainted steel sheets is also described. In bending mode, the higher the elongation of films is, the higher the integrity of painted films on steel is, while in drawing mode, the lower the elastic strain energy is, the higher the integrity of panted films is. Development of galvanized steel sheets is also important to improve corrosion resistance of products. 55% Al-Zn alloy-coated and new Zn-Al alloy-coated steel sheets containing Mg have excellent corrosion resistance as prepainted steel sheet substrates. Finally, recent topics on environmentally friendly products without hexa-valent chromium or polyvinyl chloride, are introduced.
Zinc coating by electrodeposition has been utilized in the production of corrosion-resistant steel sheets. In addition, binary Zn alloys such as Zn-iron-group metal, Zn-Mn and Zn-Cr electrodeposited from the sulfate type baths provide excellent corrosion-resistant coatings for the steel sheets used especially as automotive body panels. In the sulfate type baths, single Zn and Zn alloys can be deposited at higher current densities than the critical one. In this paper, based on that hydrogen does not evolve at its equilibrium potential, the electrochemical meaning of the critical current density is discussed. The anomalous deposition behavior of Zn-iron group metal alloys, in which electrochemically less noble Zn is deposited preferentially, is explained according to the hydroxide suppression mechanism. Further, the characteristic deposition behavior of Zn-Mn and Zn-Cr alloys is also referred briefly.
Mechanisms of alloy phases formation of galvannealed steels has been described from transmission electron microscopy observations. Roles of Fe2Al5 intermetallic compound formed prior to Fe-Zn intermetallic phases formation are discussed.
Chemical conversion coatings provide metal surfaces with high functions. The mechanisms of film-formation of conversion coatings on the surfaces of steel, zinc-plated steel, tin-plated steel (that is, tinplate), aluminum alloy, etc. and the properties are reviewed. The following items are systematically explained: i) conventional phosphatings such as zinc, zinc-calcium and iron phosphating systems, ii) recent phosphatings such as nitrous ion- and nickel-free systems, iii) chromatings such as chemical reaction (rinse version)-and coating (no-rinse version)-systems, and iv) non-chromatings such as molybdate/phosphate-, zirconium salt- and silane-coupling agent-systems.
Zinc coated steel sheets have been widely used for automobiles, home electrical appliances, construction materials and so on, because of their high corrosion resistance and other excellent properties. The high corrosion resistance of the zinc coated steel sheets depends on the structures of both the surface coatings and the corrosion products formed under each environment. Characterization of the fine structures of the coatings by micro beam analysis clarifies the mechanism of the corrosion resistance and thus provides the concept for designing the coatings. Firstly, the environment of the used vehicles were estimated by the XRD analysis of the corrosion process and products of the zinc coatings. Secondly, the relationship between the fine structures and the corrosion resistance of the Zn system coatings are described through the characterization of the change of the coatings for the Zn-Ni electrodeposited alloy and electrodeposited Zn-SiO2 coatings during the corrosion process.
Structural changes and anti-corrosion properties of dry-in-place chromate coatings at drying temperatures up to 400 oC have been investigated using DTA, SIMS, XPS and SVET. DTA of the deposits from a chromate bath containing Cr3+, Cr6+ and PO43- species showed two distinct peaks due to endothermic (85∼200 oC) and exothermic (200∼400 oC) reactions. SIMS analysis indicated the presence of high molecular fragments at the final endothermic stage (∼200 oC), suggesting that polymerization occurs; however the polymerized coating was partially decomposed at the subsequent exothermic stage (around 300 oC). XPS analysis disclosed that the deposits consist of Cr3+, Cr6+ (Cr2O72- etc) and PO43- as polynuclear complexes at the endothermic stage. Furthermore, corrosion resistance of chromated electrogalvanized steel and self-healing effect were investigated. The order of corrosion resistance from the highest to the lowest as evaluated using a salt spray test for three drying temperatures was 200 oC, 85 oC and 300 oC; it was found that the corrosion resistance differed with the differences in the final drying temperature. The self-healing effect was remarkable in the chromate dried up to 200 oC, and high corrosion resistance was measured by SVET.