Reduction of Pb has become an urgent topic in discussions of ways to reduce the burden on the environment. The authors have been developing substitutive materials made of Pb-8 wt% Sn coated steel sheets (which are known as Terne sheets) for automotive fuel tanks and chassis for home electrical appliances.Results have confirmed that Sn-Zn hot-dip coated steel sheets produced by a flux method can substitute for Terne sheets.The Sn-Zn hot-dip coated layer has a lower potential than Fe due to the content of a small amount of Zn. The layer also displays sacrificial corrosion protection.Zn crystal in the coated layer becomes coarse and Zn dissolves excessively when the Sn-Zn coated layer is over 11 wt % Zn.Accordingly, Sn-8 wt% Zn is the most balanced composition for sheets used for inner and outer corrosion resistance in fuel tanks.Results confirmed that Sn-Zn hot-dip coated steel sheets treated with special chemically treated film can confirm to various characteristics for fuel tanks. Subsequent use of lead for automotive parts may be reduced sharply.

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To improve red rust resistance at the initial stage of corrosion in exposed steel parts of Zn-11 mass%Al-3 mass%Mg-0.2 mass%Si coated steel sheets, we investigated the mechanism and effects of applying a chemical conversion coating including a corrosion inhibitor to the zinc alloy coating. A pure-water-spray cyclic corrosion test revealed effects of combining a zinc alloy coating and chemical conversion coating. Red rust resistance in the exposed part of the steel substrate at the initial stage of corrosion is improved by application of a phosphoric-acid-containing chemical conversion coating. When the phosphoric-acid-containing chemical conversion coating was applied, Mg, Zn, and P of the coating components moved farther from the coating layersteel substrate interface. Because the Mg 2＋ , Zn 2＋ , PO 4 3-, CO 3 2-, and OHcompound protected the steel substrate, the steel substrate corrosion was suppressed even when the water film became thin or island-shaped and when the sacrificial anticorrosive action of the coating worked only with difficulty. As the electrical conductivity of the water film increased, the increase in the sacrificial protection distance to the steel substrate by the coating component probably also contributed to suppression of the steel substrate corrosion when the water film became thick.