Pollution Prevention and Control Technologies for Plating
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.
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