Dow 20 finishing process on magnesium

A. Dear Mr. Smith,

I would look first at the alloying elements to see if any might be culprits in remaining on the surface after treatment. Also, smut is a typical problem in processing magnesium, and may be formed during almost any step in the cleaning and pickling of magnesium. Your challenge is to whip the process into shape so that smut, if formed, can be removed chemically instead of physically.

A. As mentioned above, this is a very typical challenge for the first time magnesium finisher. I would suggest, if you haven't already obtained it, to contact DOW Chemical and ask for their booklet titled "Operations in Magnesium Finishing". The last number I have for them is 1.800.447.4369. I have found that running several, and I mean several, scrap parts from your customer through trial processing cycles will give a great advantage in diagnosing the problem, and in establishing a production processing cycle. One overlooked item, is that the smut formation coming out of the DOW 20 can be minimized by trying different cycle times. One will notice that both too short and too long times will cause a greater degree of smut formation. Also, try a 50% diluted DOW 20 with various processing times, at different temperatures. One type of casting used to run better if heated up in a hot water rinse after immersion in a 6 oz/gal hot commercial alkaline cleaner and just prior to processing in a 50% dilute solution of DOW 20. Other parts produced less smut when run in an ambient 85% phosphoric acid pickle for ~10-15 seconds, and letting them drip above the tank for ~ 30 seconds and without rinsing, then were dipped in a 10% solution of ambient temp. 6 oz/gal sodium hydroxide. They were then rinsed in a cold water rinse and processed through the DOW 20. Be VERY careful to chart the etch rates in the solutions which cause metal removal as DOW solutions and/or pickles may result in a metal removal rate greater than a mil/minute which creates havoc with dimensional tolerances. This last rate mentioned was charted using alloy AZ91D in a standard, freshly made up solution of DOW 20. A 50% dilute solution at 66 degrees F. removed 4 tenths of a mil in a minute by comparison with the same alloy being tested. Both gave good color. One way to check smut is to use a scotch tape test on the part and then after pulling off, place over white paper next to the process cycle description which produced it. A comparison can then be mad e and used for QA/production purposes. Also, good chemical and temperature control during the charting process to determine the production cycle is critical, as is maintaining the baths in actual production conditions. Each alloy will run differently and magnesium loading in the pickles from high production rates, also will change etch rates/smut formation over time.

A. Dow has exited the magnesium business. I believe the finishing books are no longer available from Dow. They contain a section specifically devoted to smut problems on Dow 20. If you are interested a substitute process, Luke Engineering can offer the hard anodize, Magoxid-Coat.

A. Dow #20 aka Chemical Treatment #20 aka Modified Chrome Pickle, was developed to provide a more uniform colored coating than Dow #1.

It is mainly used for:
- protection during shipping & storage
- good base for paint (more uniform on castings than Dow #1).

A. Dow Chemical Co. developed a number of finishing processes for magnesium, including Dow 20. They published the details in a book entitled, Magnesium Finishing. All of the chemicals and equipment necessary are non-proprietary. However, Dow has exited the magnesium business and I do not know where to send you for a book. Maybe Ted can help.

A. Sir,

Dow stopped their involvement in mag finishing many years ago. The handbook we have on Dow finishing is worth its weight in gold.

A. Hi Andrew, hi Xingwu. If unable to locate a copy of Dow's Magnesium Finishing, Durney's Electroplating Engineering Handbook has a 10-page chapter by H.K. DeLong (of Dow) titled "Anodizing and Surface Conversion Treatments for Magnesium". It includes info on the Mil-M-3171A, Type 1 Chrome Pickle (or Treatment #1 and #20).

Treatment #20 is
- Sodium Acid Fluoride 2 oz/gal
- Sodium Dichromate 1-1/2 lb/gal
- Aluminum Sulphate 1.3 oz/gal
- Conc. Nitric Acid 1 pt/gal

It's operated at 70 - 90°F for 20-30 seconds for diecastings.

Regards,

A. An alternative that would be more expensive but might be well suited for "a part that will be exposed to the elements" is electroless nickel. Atotech offers a process designed specifically for magnesium, which they call Magenta.

A. Many of our customers who have mating parts to their magnesium parts have tried to use Dow 20. It seems that Dow 20 doesn't provide enough protection and our customers always end up coming back to electroless nickel.

adv.
Luke Engineering offers a hard anodic coating for Magnesium, Magoxid-Coat®. It is vastly superior to chromates and other anodizing processes for magnesium in both corrosion and wear resistance. Due to the hazardous nature and regulation of chrome containing processes, they are becoming more difficult and expensive to find. Magoxid-Coat contains no chromium or other heavy metals. The thin version, 8 to 12 um, is very price competitive with Dow processes.

A1. Dow 20 (Modified Chrome Pickle) was developed to provide a more uniform colored coating than Dow 1. Treatment 20 may be used on all magnesium alloys but is particularly adapted for magnesium DIE castings. Sodium Acid Fluoride 2 oz/gal, Sodium Dichromate 24 oz/gal, Aluminum sulphate 1 1/3 oz/gal, Nitric Acid (70% Nitric) 1 pint per gallons. 5 seconds to 2 minutes. I can copy Dow's book which is three pages on this subject and slow mail to you if you come back. Need a more complete address.

A2. It stays not more than 8 hours. That is all.

A. Can it be a plating like electroless nickel, or must it be a surface conversion chemistry?

A. Hi Tim. Maybe Dow 7 or Dow 1 or 20 treatment will do. Please see Tom Pullizzi's excellent article on Finishing of Magnesium. Good luck.