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Pollution Prevention and Control Technologies for Plating Operations
Section 5 - Substitute Technologies
5.6 CHROMIUM USE REDUCTION/ELIMINATION
Chromium is heavily used in electroplating and aluminum finishing. The most common chromium bearing solutions include: decorative and hard chromium, aluminum conversion coating, bright dipping of copper and copper alloys, chromic acid anodizing, deox/desmut and chromium stripping (see Section 1 for data on the use of each of these processes). Due to its high toxicity and the cost for treatment and disposal of chromium bearing wastes, it has been a target for reduced usage or elimination.
The results of the Users Survey indicate that 18 percent of the respondents have compliance difficulty with chromium. Twenty percent of the respondents indicated that there exists a technology transfer insufficiency with regard to non-chromium metal finishing.
Efforts to reduce chromium usage in the aerospace industry and for military applications have focused on non-chromium aluminum finishing and hard chromium plating substitution (ref. 75, 77, 211, 418). However, among survey respondents, which are mostly job shops, efforts were directed more at substituting trivalent chromium decorative plating and zinc conversion coating solutions for the traditional hexavalent baths. Both of these substitutes offer cost savings in terms of lower wastewater treatment chemical usage since the hexavalent chromium reduction step is eliminated. Also, trivalent chromium plating solutions are formulated with as little as 6 g/l of chromium, significantly less than their hexavalent counterparts (ref. 89). The lower chromium concentration in the baths results in less chromium in the drag-out. The substitution of trivalent plating solutions was generally successful for the survey respondents (15 shops), although some exceptions were identified (see Exhibit 5-6). Sixty percent of these respondents indicated that the process substitution was successful, 20 percent indicated it was partially successful and 20 percent indicated that their substitution efforts failed. The most frequent problem with the process was its acceptability to the shopÕs customers; a problem that was primarily related to the color of the deposit.
Switching from a hexavalent chromium zinc conversion coating (both clear and blue) to a trivalent coating was successful in most cases. Seventy-six percent of the shops attempting this substitution were successful. Another 8 percent were partially successful and only 15 percent resulted in failure. The limited success of one respondent and the failures of others were related to the characteristics of the coating.
In addition to the plating shops that indicated they have substituted a non-chromium, trivalent chromium or low-chromium process for a conventional chromium process, there were four shops that simply eliminated chromium plating and one shop that eliminated chromating of magnesium in order to meet environmental regulations.