Rinsing and disposal of Alodine solution

A. Hi Barry. The first part of your question is easy to answer: yes, immersion rinsing is appropriate.

But the disposal part is laced with ambiguities. First, it would appear that a consumer is not a business governed by the EPA laws 40CFR433 for metal finishing. But is you are selling the processing or the finished parts, you would need to be licensed and permitted. Next, I don't think you can put this down a septic system no matter how you treat it because the accumulation of chromium in the ground could be a problem for you. If you are actually speaking about a public sewer, though, you then have the problem that you are prohibited (in the fine print of your agreement with the sewer authority) from discharging stuff like this.

I think I'd take the parts to a plating shop.

Q. Ted, thanks for your response. This project is a homebuilt airplane for personal use built at my home. I would probably use 3-5 gallons of Alodine 1201 ⇦ this on eBay or Amazon [affil links] for all the Aluminum alloy ribs, spars, and parts. Possibly there would be a hazardous or toxic waste site that would take used Alodine and Alodine rinse water. How does manufacturing deal with this problem?

A. There are two parts to the answer to your question of how manufacturing deals with this problem; the first is technical and the second is procedural.

Technically, it is relatively easy to detoxify chromate conversion coatings and their associated rinsewater. The pH is reduced to about 3.5 (even a little higher if you have plenty of time for slow treatment) with sulfuric acid; then a solution of sodium metabisulfite is introduced to reduce any Cr+6 to Cr+3. Then the pH is raised to 8.5 to 9.0 and the chrome precipitates and is filtered out, and the filtrate is discharged. Sodium metabisulfite is bit dangerous to work with because if the pH goes too low you will generate a lot of toxic, choking, sulfur dioxide gas.

Procedurally speaking, though, manufacturers have to hold permits for the accumulation, treatment, and disposal of these wastes.

A. Hi Siddhartha. When using chromates, two factors may determine what you must do about the waste. First is the nature of the waste ... and the rinse water from this M-CR 600 contains hexavalent chromium and fluorides which require treatment before disposal. Eventually the chromating solution itself will become exhausted and require treatment as well. The second factor though, is what category the EPA considers your business to be in -- because in addition to requirements based on the actual chemistry, there are also 'categorical' standards, i.e., if you are a metal finishing shop you must comply with categorical standards like 40CFR433.

There are "dry in place" chromate conversion chemistries that do not require rinsing, and if the areas of your part which need chromating are basically 'touch up' areas, this may be applicable. Another approach is to constantly recycle your rinse water though ion-exchange cylinders, and to send these cylinders off-site for treatment and regeneration when exhausted.

Regards,

A. Hi Siddhartha. Contact Henkel, the supplier of Alodine, and ask them about dry-in-place chromating. You may wish to take a quick look at letter 10893 first.

Hexavalent chromium is present as the CrO₄^-2 anion so, somewhat strangely, it is removed with anion exchange cylinders rather than the cation exchange cylinders used for most metals. But the thing is, you need to locate a service which takes these cylinders from your premises and recharges them if you want to go the ion exchange route. It does no good to pick the optimum resin and the right cylinder for it if you're still stuck with the hexavalent chromium.

All in all it may be easiest to find an anodizing shop who can also do the chromating. Good luck.

Regards,