plications, the use of sodium metabisulfte (MBS) is best.
Sodium metabisulfte (Na2S2O5), when added to water, forms sodium bisulfte (NaHSO3), which reacts with
the hexavalent chromium (Cr+6) to form trivalent chromium (Cr+3) according to the following reaction:
3NaHSO3 + 2H2CrO4 >>> Cr2(SO4)3 + 3NaHSO4 + 5H2O.
Approximately 2.8 lbs of MBS and 1.5 lbs of sulfuric
acid are needed to reduce 1 lb of hexavalent chrome. Because the speed of the reaction is pH-dependent (speed
decreases as pH increases), we typically design for a retention time of 15���20 min and a pH of 2���2.5.
To feed the MBS solution into the rapidly mixed
chrome reduction tank, an oxidation-reduction potential (ORP) sensor and controller is used. A set point of
-300 millivolts is adequate for most applications. As you
become more familiar with the system, you may want to
increase (become more negative) the set point in 25-milli-volt increments and check the validity of the setting.
There are several suppliers of colorimetric testing kits
for hexavalent chrome (sample turns purple if hexavalent chrome is present) that can be used to determine
the best ORP set point; -400 millivolts is the maximum
set point that most have found to be acceptable, but each
wastestream needs to be evaluated.
Alternative technologies for chrome reduction include
sulfur dioxide gas, sodium hydrosulfte, ferrous sulfate,
scrap iron/steel/steel wool and electrochemical. n
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Coating Thickness
Material Analysis
Microhardness
Material Testing
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