Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 36 of 331

Zincated steel surfaces can be damaged by corrosion forming white rust on the sur- face (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 sub- strates 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 con- tent (Galvalume) can be treated very successfully with zinc phosphating process- es, thereby creating a substrate that will show as good corrosion protection as tra- ditionally 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 uni- form crystalline coating on the substrate. It is recommended that zinc phos- phating processes consisting of controlled fluorides be used when aluminum sub- strates 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 ad- dress corrosion resistance and adhesion as well as the demand from the indus- try to handle mixed metal production. The traditionally used, so-called normal zinc processes have, in several cases, been replaced by the use of low zinc or low zinc-manganese-modified processes that create an outstanding performance in all corrosion testing environments (Table III). The differences between the processes used as pretreatment prior to painting 35

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