Metal Finishing Guide Book


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OPERATING CONDITIONS In addition to the chemical make-up of the chromating solution, the following factors also govern film formation. Once established for a given operation, these parameters should be held constant. Treatment Time. Immersion time, or contact time of the metal surface and the solution, can vary from as little as 1 second to as much as 1 hour, depending on the solution being used and metal being treated. If prolonged treatment times are required to obtain desired results, a fault in the system is indicated and should be corrected. Solution Temperature. Chromating temperatures vary from ambient to boiling, depending on the particular solution and metal being processed. For a given system, an increase in the solution temperature will accelerate both the film-forming rate and the rate of attack on the metal surface. This can result in a change in the character of the chromate film. Thus, temperatures should be adequately maintained to ensure consistent results. Solution Agitation. Agitation of the working solution, or movement of the work in the solution, generally speeds the reaction and provides more uniform film formation. Air agitation and spraying have been used for this purpose. There are, however, a few exceptions where excessive agitation will produce unsatisfactory films. Solution Contamination Although the presence of an activator in most treatment solutions is vital, an excessive concentration of this component, or the presence of the wrong activator, can be very detrimental. Most metal-finishing operations include sources of potential activator contamination in the form of cleaners, pickles, deoxidizers, and desmutters. Unless proper precautions are taken, the chromate solution can easily become contaminated through drag-in of inadequately rinsed parts, drippage from racks carried over the solution, etc. A common source of contamination is that resulting from improperly cleaned work. If allowed to go unchecked, soils can build on the surface of the solution to the point at which even clean work becomes resoiled on entering the treatment tank, resulting in blotchy, uneven coatings. Other contaminants to be considered are those produced by the reactions occurring in the treatment solution itself. With very few exceptions, part of the trivalent chromium formed and part of the basis metal dissolved during the coating reaction remain in the solution. Small amounts of these contaminants can be beneficial, and "brokenin" solutions often produce more consistent results. As the concentration of these metal contaminants increases, effective film formation will be inhibited. For a certain period, this effect can be counteracted by adjustments, such as lowered pH and increased hexavalent chromium concentration. Eventually, even these techniques become ineffective, at which point the solution must be discarded or a portion withdrawn and replaced with fresh solution. Rinsing and Drying Once a chromate film has been formed satisfactorily, the surface should be rinsed as soon as possible. Transfer times from the chromating stage to the rinsing stage should be short in order to minimize the continuing reaction that takes place on the part. Although rinsing should be thorough, this step can also affect the final character of the chromate film and should be controlled with respect to time and temperature, for consistent results. Prolonged rinsing or the use of very hot rinsewater can dissolve, or leach, the 431

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