Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 39 of 331

(replenished) on an ongoing basis during production. New liquid processes offer ad- vantages such as increased lifetime (better process stability). NITRITE-FREE PHOSPHATING The usual practice for most of the zinc phosphate processes to painting is to em- ploy nitrite as a primary accelerator. Ni- trite, however, in the acid phosphating baths, always produces a small quantity of volatile nitrous gases. On the other hand, zinc phosphating processes accelerated with organic nitro-accelerators will not create nitrous gases. In the event the line is shut down for a comparatively long time, steel surfaces may become rusty due to the effect of the at- mosphere surrounding the work. This process is accelerated and reinforced by the presence of nitrous gases. Organic ni- tro-accelerators have less tendency to rust. This type of process can also create coating weights in a very narrow and controlled band, 100–200 mg/ft2 . Coatings in this range are distinguished by especially high flexibility under bending load. Other accelerators have been developed and are increasingly used in production. Hydroxylamine has the advantage of be- ing soluble in the zinc phosphate replen- isher; therefore, no separate accelerator package is needed. Hydrogen peroxide-ac- celerated processes can provide very low coating weights; however, the phosphate crystals tend to be larger in comparison to other accelerators. Since peroxide acceler- ators do not produce any side products other than water, these are the most com- patible where environmental issues are of concern. Recently, nitroguanidine has been introduced into low-zinc phosphate tech- nologies. This is the only accelerator in use that is not self-decomposing. Another ma- jor benefit is its wide working range, which provides a very robust process. NICKEL-FREE PHOSPHATING Nickel has long been known to significantly 38

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