Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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chemical surface preparation THE ART AND SCIENCE OF WATER RINSING BY TED MOONEY FINISHING TECHNOLOGY, BRICK, N.J.; www.finishing.com Rinsing is the removal of a harmful clinging film of process solution from a workpiece by substituting in its place an innocuous film of water. Efficient rins- ing comprises achieving the desired end while expending as little work or effort as possible. The principles and formulas underlying the science of rinsing were devel- oped and widely published decades ago by Joseph B. Kushner, J.B. Mohler, and others. Not expecting the findings from this earlier work to change, why rehash the subject? The answer to this rhetorical question lies in the meaning of the word "effort." It doesn't mean merely an expenditure of brawn, it also implies allocating a portion of a limited resource. Metal finishers must continuously reexamine what "effort" means. A few decades ago, its meaning might have been approximately equivalent to "hours of labor." But the cost of chemicals has become an increasingly important com- ponent of "effort." And as water shortages hit many areas of the U.S. and the world, the amount of water required to operate a process has become a critical part of the definition of "effort." An old joke tells of a bakery not being able to bake any more doughnuts because it had run out of holes. Finishers today don't find the line amusing: the cost of solid-waste disposal imposes strict limits, and the resource that demands the most careful husbanding may be the amount of emptiness left in the dump- ster. Similarly, with tight wastewater effluent standards, each milligram of met- al we allow down the sewer today may equate to one less milligram of entitlement we hold in reserve for tomorrow. The constant pressure to improve quality per- mits fewer and fewer defects; and continuing the doughnut-hole analogy, per- missible-number-of-rejects is becoming a strictly rationed resource against which we must hoard our ration tickets jealously. For these reasons, although the rinsing formulas themselves may be abiding, the shifting sands of changing times demand that we constantly reevaluate the subject with the day's priorities in mind. WHY RINSING IS NECESSARY In general, it is necessary to thoroughly rinse the work between the various treatment stages. For example, ware carrying off an unrinsed film of alkaline clean- ing solution would quickly contaminate a subsequent acid pickle, which, if in its turn went unrinsed, would rapidly contaminate the plating bath. Subjecting the work to a high level of contamination in the rinse tanks can also cause passiva- tion of the work surface or encourage precipitation of reaction products on the work. If the final processing solution is not properly rinsed, salt spotting will occur, which may cause etching or be otherwise harmful, and in any case will be unattractive. Thus, it is necessary to dilute the clinging film of process solution to such an extent that problems such as salt staining and contamination of subsequent processes are limited to manageable levels. 80

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