Metal Finishing Guide Book

2012-2013

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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 after 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 require 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, zinccoated steel sheet. This causes iron from the sheet to diffuse into the zinc coating and form an alloy consisting of 10���15% iron. This substrate has better weldability 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 utilizing 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. Zincated steel surfaces can be damaged by corrosion forming white rust on the surface (zinc carbonate/zinc oxide). Normally, this coating cannot be effectively removed by using mild alkaline cleaners; therefore, the surface will not be properly zinc phosphated and will create a poor paint finish with little corrosion resistance. To avoid the surface corrosion, protection by an oil or passivation by using a chromium treatment can be used. Care should be taken that no passivated substrates are used for parts that should be painted; however, in coil coated operations, the processes are adjusted so that this substrate can be treated with success. Aluminum, aluminum alloys, and zinc-coated steel with high aluminum content (Galvalume) can be treated very successfully with zinc phosphating processes, thereby creating a substrate that will show as good corrosion protection as traditionally used chromate conversion coatings when painted and tested in different corrosion environments. Studies have shown that when the aluminum substrate is ground and the aluminum alloy includes silicon, poor corrosion resistance and paint adhesion will occur, if the zinc phosphating process does not create a uniform crystalline coating on the substrate. It is recommended that zinc phosphating processes consisting of controlled fluorides be used when aluminum substrates will be treated. Table II shows the effects. ZINC PHOSPHATE PROCESS TECHNOLOGIES PRIOR TO PAINTING Over the years different zinc phosphating processes have been developed to address corrosion resistance and adhesion as well as the demand from the indus123

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