Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 34 of 331

in manufacturing. With bare steel, the particular alloy, soft or high strength, does not affect zinc phosphating as much as the condition of the surface. Excess residual carbon left on the surface after the annealing process where recrystallization takes place, or af- ter the steel has been cold rolled, will cause poor corrosion resistance (see Fig. 2). This residue cannot be removed effectively by the most mild alkaline cleaners on the market and action should be taken so that the steel used for production has a controlled low level of residual carbon. Hot-dipped galvanized steel strip is produced by applying molten zinc on the strip. The metal mix in the bath specifies the alloy that will be applied on the strip and metals, such as lead, tin, antimony, and aluminum, can affect phosphatability. In particular, aluminum will form on aluminum oxide film that has a thickness less than 5 nm on the surface, which negatively influences the surface reactivity during zinc phosphating. Zinc-aluminum alloy coatings, such as Galvalume, have similar but more severe problems with aluminum oxide surface coatings. These re- quire zinc phosphate processes that are specially designed to treat larger amounts of aluminum surfaces. Most of those processes add controlled fluoride to the bath. Galvanneal is a substrate that is produced by heat treating the hot-dipped, zinc- coated steel sheet. This causes iron from the sheet to diffuse into the zinc coat- ing and form an alloy consisting of 10–15% iron. This substrate has better weld- ability and is not as reactive in the zinc phosphating process to form "white spotting" as do pure zinc coatings. Electrogalvanized steel sheets are made by plating zinc onto the steel strip uti- lizing electroplating technology. Different electrolytes will create different zinc coatings and alloys, which can vary the physical properties as well as the corrosion resistance of the substrate itself. Most of those alloys are easily zinc phosphated. 33

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