Sound technical content, curated with aloha by
Ted Mooney, P.E. RET
Pine Beach, NJ
The authoritative public forum
for Metal Finishing since 1989
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Dow #7 /Dichromate treatment
Q. We continue to have problems with our Dow #7 dichromate process. When using brass racks or processing parts with brass inserts, etching of the magnesium near the brass inserts or near the contact points of the brass racks. Everything seen on this process says that brass is an acceptable rack material.
Dan Bemenderfer- Indianapolis, Indiana
1999
A. Sir, We have been doing aluminum & magnesium finishing for over 40 years. We do Dow #7 (among others) and have never used brass racks. I see no mention of this being acceptable in the >Dow Magnesium Finishing Handbook.
David A. Kraft- Long Island City, New York
How to maintain fluoride concentration in AMS2475 Dichromating Solution for Magnesium
Q. According to AMS2475, the Dichromate solution shall contain sodium dichromate 10 - 15% and 0.25% of magnesium fluoride or calcium fluoride by weight and the solution shall be maintained in a saturated condition with respect to the alkaline earth fluoride by continuous immersion of a cloth bag, or equivalent, containing the compound.
I am confused at the meaning between "0.25 wt%" and saturated". I need information for the solution make up & control of it. How can I make up and control the earth fluoride (I guess this means magnesium fluoride or Calcium fluoride) in dichromate solution ?
Jung, Won- Changwon City, Kyoungnam, Korea
2000
A. Dichromate on magnesium is also known as Dow #7.
Makeup:
- 1-1.5 lbs sodium dichromate
- 1/3 oz calcium or magnesium fluoride
- + water to make 1 gallon
Both the dichromate concentration & the pH of the treatment bath must be controlled.
David A. Kraft- Long Island City, New York
A. See the FAQ for magnesium at www.finishing.com/faqs/magnesium.shtml. If you keep a bag, described in the FAQ, filled, it will keep the solution saturated, and you won't have to analyze for it. The solubility will go up with the temperature, so controlling the temperature will improve the control of the concentration. i.e., if during processing, the bath temperature rises, you will dissolve more chemical from the bag, which may precipitate as the bath cools.
Tom Pullizzi
Falls Township, Pennsylvania
A. Make the solution up as recommended by David, then hang a filter or anode bag full of the fluoride salt in the solution. The salt will dissolve as needed to maintain the saturated state. (Saturation for the fluoride salt in this solution is most likely at approximately 0.25% x wt.)
As for control of the bath chemistry, Dow produced a manual which described in detail the make-up and maintenance of several magnesium treatments. Although Dow no longer is in this line of work, the manual is still a great resource and many finishers have access to them.
Megan Pellenz
- Syracuse, New York
RFQ: I am looking for a company that can do Magnesium Dichromate. We are in the race car business. The castings are black after this process. The coating is very thin. Any help would be greatly appreciated.
Thank you,
Jason
- Livingston, New Jersey USA
2000
Ed. note: Sorry, this RFQ is old & outdated, so contact info is no longer available. However, if you feel that something technical should be said in reply, please post it; no public commercial suggestions please ( huh? why?)
A. Jason,
We do the dichromate treatment process, Dow #7, BUT, it does not produce a black finish. It is normally amber to dark brown.
I think if you want black, it is not the dichromate treatment you want.
David A. Kraft- Long Island City, New York
If Mag Wheels are your specific interest, please see thread 03, "Coating/restoring Halibrand wheels; history of magnesium wheels".
What is Dow 7 processing?
Q. What is DOW 7 - MIL-M-3171C type 3? We have used it in the past -- what makes it a good protective coating for magnesium?
R. Northrop- Chicago, Illinois, USA
2001
Q. Please advise the detail chemical reaction in Dow#7/dichromate treatment of magnesium alloys. What is actually the coating composed?
Thanks,
Angus Chow- Hong Kong
2002
Hi, Mr. Chow, welcome to finishing.com. More people will be likely to spend time on your question, trying to teach you what you want to know, if they can learn something from you at the same time, and if they don't have to supply long pages of "ifs, ands, & buts" covering all sorts of situations that don't apply to you anyway -- so please try your best to take the time to describe your situation & circumstances that motivate the inquiry rather than leaving it abstract. Thanks!
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
Q. Thanks for your prompt advise. We are doing Dow #7 coating for magnesium alloy for our own. I am asking the question to know more about the detail reaction and thus the composition of the coating. I measure the surface resistance of the surface and it was sometimes conductive and sometimes not. I suspect the coating is insulative and it was measured conductive sometimes just because the coating was so thin. But I try to confirm.
Thanks.
Angus Chow [returning]- Hong Kong
2002
by Dow Chemical Co.
is sometimes available on
AbeBooks,
eBay, or
Amazon
... but if not, you can find libraries which have copies on WorldCat.org
(affil links)
A. Thanks, Angus. Now I find your question very interesting! I don't have the experience to answer it, but hopefully another reader will. Meanwhile, try to find a copy of the Dow handbook on Magnesium Finishing. Many of our readers frequently recommend it. .
If you can't find it, Tom Pullizzi's excellent on-line article on "Finishing of Magnesium" includes some good coverage of Dow #7, and may help you in achieving more consistency. Good luck.
Regards,
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
Q. I am trying to find to find out what the magnesium process is that is referred to in the Boeing manuals as Dow 7. I have contacted Dow and they said they no longer deal in magnesium processing. We are in the process of opening an aircraft repair station and I need to know if there is another process available that replaces or is the same as the Dow 7 process.
Thank you,
Doug Havenhill- Aurora, Colorado
2003
A. My company did mag finishing including Dow #7 for 40+ years. We gave it up a few years ago & now only do aluminum finishing. Dow #7 is Dichromate Treatment on Magnesium.
David A. Kraft- Long Island City, New York
A. I still have a copy of "Operations in Magnesium Finishing", 1990 The Dow Company. There are 4 pages on Dow 7, formula, control, etc, Let me know if you want me to copy and send to you.
Robert H Probert
Robert H Probert Technical Services
Garner, North Carolina
Need a Shop to Perform Dow #7
RFQ: I need to clean then want to re-oxidize a magnesium alloy casted part. Want to recreate the black-grey mottled patina. I don't know if a dichromate or oxidizing agent is appropriate. don't know what the exact composition of the material is either or what, if any, original finish was on the part. Thanks!
I want to re-finish it with the dark grey finish that it had originally which I am guessing is a Dow 1 or Dow 7 finish originally. The part is from a 1982 porsche.
Who can apply a Dow #7 or Dow #1 finish to a magnesium alloy part from a car? If so, how much does it cost?
Paul G [last name deleted for privacy by Editor]- Hillsborough New Jersey
2004
Ed. note: Sorry, this RFQ is old & outdated, so contact info is no longer available. However, if you feel that something technical should be said in reply, please post it; no public commercial suggestions please ( huh? why?)
RFQ: Dow #7 on larger magnesium parts, 50" X 20"?
I'm looking for a shop that can process Dow #7 on larger pieces of magnesium. We are a government facility & require certification for the processing. Preferably a shop in the New England area? Massachusetts or a surrounding state would be helpful.
Mechanical Planner - Lexington, Massachusetts
2005
Ed. note: Sorry, this RFQ is old & outdated, so contact info is no longer available. However, if you feel that something technical should be said in reply, please post it; no public commercial suggestions please ( huh? why?)
Q. Help!
I have a new Volkswagen magnesium alloy engine case where the factory "greenish" protective coating/treatment appears to have been inadvertently removed (partially) during pre-assembly chemical cleaning (oops). I am trying to identify what this coating/treatment is and how I can have the engine case "reprocessed". I have the following questions:
1) I have been told that this may be a coating similar to "Dow #7". Does this sound correct?
2) Is there a better process/coating than Dow #7?
3) This engine case has had some machine work done to it. Can this processing dimensionally alter the case in any way?
By the way, while my particular engine case had a greenish hue, I have seen many others with more of a "gold-ish" hue. Hope that helps with the identification.
RFQ: Is there someone in southern California that can do this reprocessing for me? If not, I am open to sending the case out of state.
Regards,
Hobbyist - Lakewood, California, USA
2005
Ed. note: Sorry, this RFQ is old & outdated, so contact info is no longer available. However, if you feel that something technical should be said in reply, please post it; no public commercial suggestions please ( huh? why?)
Outgassing properties of dichromate treatment on magnesium
Q. I have been considering using DOW No.7 dichromate treatment on my AZ31B parts in order to maintain good electrical conductivity as well as prevent oxidation on the surface of my parts. As my parts are intended for use in space, I also need all my parts (and their surface treatments) to be low outgassing (<1% TML, <0.1% CVCM).
I was wondering if anyone knows what the outgassing properties of DOW No. 7 dichromate treatment are? Any info would be appreciated.
engineer - Toronto, Ontario, Canada
August 14, 2008
A. Two quotes from "Operations in Magnesium Finishing", Dow 1990, may or may not be of interest to your inquiry.
1. Page 4, "At temperatures below 400 F the chromate type coatings (Chemical Treatment numbers 1,4,7,9,19,20,22) are unaffected and the corrosion rate of the magnesium remains constant. After heating higher than 400 °F some of the chromate coating apparently is reduced to free chromium, which increases the corrosion rate of magnesium" End Quote. That sounds like dehydration to me.
2. Page 20, Table 15, "Failure to coat or non-uniform coatings" "Part is made of HK31A . . . HM31A" End Quote. That sort of contradicts Dow
7 listed above. I could not find AZ31B anywhere in their booklet.
Robert H Probert
Robert H Probert Technical Services
Garner, North Carolina
Q. I need to know if Magnesium has DOW 7 applied to it will the DOW 7 be conductive?
Thomas Galyean- McKinney, Texas, USA
December 7, 2010
A. Hi Thomas. These chromate treatments are intended as paint bases and, perhaps for that reason, there seems to be very little information on their conductivity in the literature. But there is a Dow #23 "Stannate Immersion Treatment" which, according to H K DeLong's article "Anodizing and Surface Conversion Treatments for Magnesium" in the Electroplating Engineering Handbook.
"... is comparable to treatments No. 7 and No. 17. It is also used as a paint base, especially (because of its low electrical resistance) where RF grounding is required".
Regards,
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
Chromate and phosphate conversion coating on AZ63 magnesium alloy
Q. I would like to create phosphate and chromate conversion coatings on the AZ63 magnesium alloy. I'm not sure if it's possible, since this alloy is used as sacrificial anode, and therefore is very active. I've tried to reproduce the Alodine solution from recipies found online, but I get only a black surface, which turned out to be more active. Corrosive media is 1 mmol NaCl, maybe that's the reason the coating fails? Does the Alodine coating prevent corrosion in salt water? To sum up my questions:
Is it possible to apply chromate conversion coating on AZ63 with Alodine?
If not, can someone recommend a corrosion protection method of AZ63 in salt water?
Does Alodine coating prevent corrosion in salt water?
If so, can someone recommend a recipe?
Thank you in advance,
PhD student - Pecs, Hungary
March 19, 2014
A. Hi Andrais. "Alodine" is actually a Henkel tradename for proprietary chromate conversion processes, rather than a generic name. While I encourage your efforts on this research, researchers like yourself "stand on the shoulders of giants". So we appended your inquiry to a thread which discusses the fact that Dow wrote a significant treatise on this subject with 23 different approaches to surface treatment of magnesium, and that many of these methods have been in practice for decades.
I'm sure you realize that no process "prevents" corrosion, they only "deter" corrosion; so I point that out not for your benefit but for the benefit of more casual readers.
I believe that Dow 17 and HAE anodizing processes offer significantly greater corrosion protection than the chromate dip processes. New "spark anodizing" processes may be applicable as well, but I am not familiar enough with them to know if they are doable in a small lab. Please see threads 32564, "Dow #17 and other finishes on magnesium", letter 6238, "Magnesium anodizing processes & coatings, HAE, Dow 17", and thread 685, "Microarc Oxidation". Best of luck.
Regards,
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
Chemistry of DOW 7 Conversion Coating
Q. Hello,
Can anyone explain chemically why calcium or magnesium fluoride is required for DOW 7 to properly convert magnesium? I was recently in a pinch where I had to make up a bath of DOW 7 solution for a batch process, but I had run out of calcium fluoride. In thinking that the F- ion was the critical component, I made up the solution with sodium fluoride, which I had on hand. To my surprise, no conversion reaction took place.
From what I have read, it appears that an alkaline earth metal is required for this solution to function properly. Does anyone know why this is the case?
Thank you,
Adam
Process Engineer - Dayton, Ohio, United States
July 18, 2016
RoHS Chromate Conversion of Mg Alloys?
Q. Hello,
I am considering having some magnesium alloy (alloys are AZ31B-H24 and ZK60B-T5) parts treated with a chromate conversion. Typically, I have my parts plated with electroless nickel so this process is new to me.
A shop I am considering offers ROHS zinc trivalent chromate plating, but my only experience with chromate conversion is with aluminum, where I normally specify MIL-5541,Type II Clear, Class 3. I’m not sure if there is a difference, or if the zinc trivalent chromate plating is simply the process to achieve the MIL spec above. I know that this MIL spec is for aluminum alloys, so is there an equivalent spec for magnesium alloys?
Regards,
Stephen
- Toronto, Ontario, Canada
October 7, 2016
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