Ask the Expert Question-and-Answer Archive
(Hard Chrome Plating)
by Larry Zitko, ChromeTech, Inc.
California's South Coast Air Quality Management District Amended Rule 1469, Hexavalent Chromium Emissions from Chrome Plating and Chromic Acid Anodizing Operations
Q. California's South Coast Air Quality Management District recently issued
its amended Rule 1469, Hexavalent Chromium Emissions from Chrome Plating
and Chromic Acid Anodizing Operations. I would like to know how the emission
limits compare to EPA's NESHAP since the limits are expressed differently
in the two rules. The NESHAP limits are expressed as mg per dry standard
cubic meter while the limits in AQMD's rule 1469 are expressed in mg per ampere
hour. Example: the NESHAP for large new hard chrome operators is 0.015
mg/dscm. The limit for certain hard chrome facilities in the AQMD's rule
is 0.0015 mg/AH. Is it possible to say which is more stringent?
A. I will limit my discussion to hard chromium electroplating operations, the
topic of this forum.
In its chromium NESHAP, EPA has set concentration-based discharge limits
for the controlled emissions of chromium from hard chromium electroplating
operations. As you have stated, the mass of chromium (in milligrams)
divided by the volume of exhaust air (in dry standard cubic meters) is limited to
0.015 for new sources.
Since the exhaust air volume for a hard chrome tank is typically
proportional to the surface area of its liquid-to-air interface (the top
of the plating bath), tanks with larger widths and lengths (the tank depth
and gallonage don't matter) will generally be fitted with exhaust hoods that
have higher volumetric flow rates than smaller tanks. Since this factor
appears in the denominator of the mg/dscm fraction, these tanks with
larger surface area, and higher exhaust rates, will find it easier to meet EPA's
It should be noted that many legitimate and effective designs for the
exhaust ventilation systems for hard chrome plating tanks feature a
reduced volumetric flow rate. An example would be a "push-pull" exhaust system
that utilizes a small push-air manifold on one side of the tank to direct
process fumes toward the capture slots on the pull hood. In a push-pull system,
the reduced volumetric flow rate for the pull hood is independent of the
surface area of plating tank or bath. It is apparent that any exhaust system
design that incorporates a reduced air volume will be disadvantaged by EPA's
It is also important to note that, unlike an hourly discharge rate (like
pounds per hour for example), the concentration based limitation does
nothing to discourage facilities from installing more plating tanks. A
facility could have a hundred chrome plating tanks, instead of one, and
still easily meet the EPA limitation, even though the mass of chromium
discharged from the stacks may increase a hundredfold for a given time
The mass of chromium appears in the numerator of the EPA limitation, so it
is advantageous to a) generate less chromium mist at the surface, and b)
have a higher removal efficiency for the air pollution control device(s).
It is my opinion that, during the development phase of the standards, EPA
struggled with the task of finding a suitable emission factor for chromium
emissions from hard chrome plating tanks. As an example, a previous
uncontrolled emission factor related the mass of chromium released to the
atmosphere to the mass of chromium added to the plating tank as a makeup
chemical. EPA later tried to relate uncontrolled chromium emissions to
ampere-hours consumed during the plating process. I have performed
numerous Initial Performance Tests (stack tests) on hard chrome plating tanks,
where I have measured both uncontrolled (before control devices) and controlled
(after control devices) emissions. I don't see any strong relationship
between ampere-hours and emissions. In other words, if you double the
plating amperes, the uncontrolled emission does not double. Many factors
not considered by EPA influence the quantity and particle size of chromium
mist released to atmosphere during plating. On the controlled side, the
meshpad-based air pollution control devices act more like constant-output
devices than constant-efficiency devices, and typically show much higher
removal efficiencies when the input loading of chromium is high than when
the input loading is low.
Nearly all modern air pollution control devices for chromium incorporate
meshpad technology to removed entrained chromium from the exhaust air
stream. These devices can easily meet EPA's discharge limit of 0.015 mg
Cr/dscm. In fact, chromium concentration measured during a performance
test may be an order of magnitude lower or better.
The California limitation of 0.0015 mg/AH is harder to meet. This is not a
concentration-based limit, but rather, it relates the mass of chromium
discharged to the ampere hours from the chromium electroplating power
supplies. A favorable plating and pollution control situation would be to
plate large parts that generate a small amount of mist (for their size)
and a high efficiency air pollution control device. In explanation, larger
parts typically are plated with higher amperage, which increases the
ampere-hours in the denominator of the fraction. Conversely, an unfavorable situation
would be to plate parts that generate a proportionally higher amount of
mist for their surface areas and current levels. As an example, many cylinders
for the printing industry are plated horizontally in large plating tanks.
They are rotated continuously during plating, but only a portion of their
surface area is actually submerged. This lowers the plating amperage, yet
the evolved mist is very high due to the constant rotation which results
in a wetted chromic acid film on the un-submerged portions. This type of
plating will have difficulty meeting the mg/AH limitation.
It is possible to calculate the mg/AH value when the mg/dscm is known. For
simplicity, I have not addressed the differences between acfm (actual
cubic feet per minute) or conversions to "standard" conditions. Assume a
volumetric flow rate of 6,000 dscfm (dry standard cubic feet per minute,
i.e - "6,000 cfm"), a controlled emission rate of 0.015 mg/dscm and 2,000
ampere-hours/hr (i.e. - 2,000 amps delivered to plating parts for an
(0.015 mg/1 m3)(1 m3/35.31467 ft3)(6,000 ft3/ 1 min)(60 min/1 hr)(1
hr/2,000 ampere-hour) = 0.07646 mg/AH.
In other words, the scenario above met the EPA limitation, but with 6,000
cfm's and 2,000 AH, resulted in a discharge that was about 50 times higher
than the 0.0015 mg/AH limitation. Even if the 6,000 cfm tank was plating a
large part at 10,000 amperes, and the controlled emission of chromium was
still 0.015 mg/dscm), the resultant 0.01529 mg/AH would still be more than
ten times higher than required.
In all fairness, most meshpad mist eliminators do much better than the
0.015 mg/dscm EPA limitation. Let's take another scenario whereby a 5,000 cfm
tank is plating at 10,000 amps and discharging 0.0017 mg/dscm. This situation
could actually occur if a large roll was plated vertically in a tank with
modest width and length, and a very-high-efficiency air pollution control
device was performing properly.
(0.0017 mg/1 m3)(1 m3/35.31467 ft3)(5,000 ft3/ 1 min)(60 min/1 hr)(1
hr/10,000 ampere-hour) = 0.00144 mg/AH.
In this case, both the EPA and California discharge limitations would have
been met. However, it takes a very good control device to discharge only
0.0017 mg/dscm with a 10,000 amp plating scenario.
In conclusion, the 0.0015 mg/AH limitation is much more difficult to meet
than EPA's 0.015 mg/dscm.