Metal Finishing Guide Book

2012-2013

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dustry is the calcium-modified normal zinc technology. This technology has an advantage in that the process does not need a grain refiner or activation compound prior to phosphating, but because of its disadvantages with high process temperature as well as heavy sludge development, the process has been replaced by the use of the manganese-modified, low-zinc phosphate processes. ACTIVATION OF PHOSPHATE COATING FORMATION The phosphating rate, thickness, and crystal size of the zinc phosphate coating depend not only on the composition and form of the phosphating bath, but also on the pretreatment of the metal surface prior to phosphating. Strong alkaline cleaner and acid pickling processes frequently produce a coarsening of the coating texture and prolong the time required for the formation of a phosphate coating with uniform coverage. Mechanical treatments, for example, grinding or polishing, brushing, and even simply wiping, produce marked refinement of the subsequent zinc phosphate coating. The reason for this phenomenon lies in the different number of nuclei on the metal surface at which crystallization of the phosphate coatings can begin. Special prerinses applied to the metal surface prior to phosphating provide a considerable increase in the number of nuclei for phosphate crystallization. This is termed activation of the phosphate coating formation. It is manifested in the subsequent phosphating process by an increase in the number of phosphate crystals per unit surface area; a decrease in the dimensions of the individual crystals of the coating; a reduction in coating weight; and a reduction in the minimum phosphating time. Titanium phosphate produced according to a special process has been found to be a highly effective nucleating agent. In industrial practice, the activating agent is added either directly to the rinse bath prior to phosphating or to preliminary alkaline cleaning baths. Since the lifetime of the activation agents is affected by several factors, such as pH, water hardness, and temperature, they are normally added (replenished) on an ongoing basis during production. New liquid processes offer advantages such as increased lifetime (better process stability). NITRITE-FREE PHOSPHATING The usual practice for most of the zinc phosphate processes to painting is to employ nitrite as a primary accelerator. Nitrite, 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 atmosphere surrounding the work. This process is accelerated and reinforced by the presence of nitrous gases. Organic nitro-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 being soluble in the zinc phosphate replenisher; therefore, no separate accelerator package is needed. Hydrogen peroxide-accelerated processes can provide very low coating weights; however, the 127

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