Air Permits and Dichromate Seals

Question: Are there any air regulations dealing with dichromate seal baths used in the anodize process?


Are there any air regulations dealing with dichromate seal baths used in the anodize process? We plan to use a 5% sodium dichromate seal that runs at 200°F. Is there any data available for chrome emissions from this bath? J.B.


Since you asked two questions, I will respond in two parts.

While there are no USEPA regulations that specifically deal with dichromate seal baths for anodized aluminum (there are USEPA air pollution regulations for chromic acid anodizing), your state EPA air regulations may or may not cover this bath, as well as your entire anodizing process. State air pollution regulations typically have specific process exemptions as well as general process exemptions based upon emissions below a threshold. You will need to contact your state air pollution control agency in order to determine if your source is regulated and requires permitting or registration. Even if it is regulated, that does not necessarily mean that an air pollution control device is mandated. However, since your anodizing process does generate sulfuric acid mists, we would strongly recommend, at a minimum, venting that tank through a chevron or mesh pad mist eliminator with periodic washdown in order to protect your building, surrounding structures and vehicles from possible fallout. Anyway, air pollution regulations require that all new sources use Best Available Technology (BAT) to control emissions and your local air pollution control agency may already mandate a control device based upon its previous experience with metal finishers.

In the limited time available, the only source of emission data from this type of bath is a USEPA paper presented at the 2000 AESF/EPA Conference for Environmental Excellence, “Characterizing Site-specific Source Emissions for EPA’s Risk Assessment Tool for the Metal Finishing Industry.” However, this paper has been heavily critiqued by the metal finishing industry and found to be lacking. Estimated emissions using its data may not be representative of your operation. That being said, let’s look at what level of emissions could be generated.

Since sodium dichromate is not volatile, its only likely generation as an air pollutant is as a mist from rising steam bubbles in the bath. The USEPA study only looked at nickel acetate sealing at a concentration of 2.5 grams nickel/liter or 0.25% nickel. Therefore, your chromium concentration in your bath is about seven times greater than nickel in the nickel acetate bath; nickel acetate sealing typically runs at about the same temperature as the dichromate seal. Assuming a 40-sq-ft tank and a tank ventilation rate of 225 cu ft per minute/sq ft of surface area or 9,000 cfm, the USEPA study estimated a nickel emission rate of 0.0099mg/cu meter or 0.0005 lbs/8-hour shift. Assuming that your chromium emission rate is seven times higher due to higher concentration, this yields an estimated chromium emission rate of 0.0035 lbs/8-hour shift, or 0.01 lbs/day, or about4 lbs/year if you were running around the clock. At this extremely low emission rate, almost every state—except maybe California—would very likely not mandate controls for these emissions.

The most accurate method for estimating emissions is to perform stack testing on an existing source that is performing close to what you propose. It this is not possible and your local air pollution control agency has concerns, you may be able to negotiate a condition in your permit-to-install or permit-to-construct where you agree to perform stack testing after the source is operating, report results to the agency, and negotiate your final permit-to-operate conditions.