Magnesium is usually treated with chromate conversion coatings or an anodize finish prior to the application of organic finishes.
Groshart1 states that magnesium must be passivated before painting. He recommends Dow 17 or HAE anodize (Mil-M-45202 Type I Class A) and Type I Class C, respectively, or Conversion coating (DOW 7 (Mil-M-3171 Type III or VIII) or or DOW 19 (Mil-M-3171 Type VI).
Janssen3 studied thermal reflectance of AZ31B and HK31A magnesium samples and reports the use of the following solutions:
100 ml: ethylene glycol - 900 ml
Below 10 °C
20 amperes/square foot
The chemicals were dissolved in the order given, then the bath was dummied with 1 square foot per gallon of magnesium
20 amperes per square foot ac
For tan coatings the voltage was terminated at 64 VAC, for thick brown coatings the voltage was terminated at 85 VAC.
Dow 17 Process
20 amperes per square foot AC
Sharma4 reports the use of the following solutions for preplating of electroless nickel and gold on AZ31B magnesium alloy (3% aluminum, 1% zinc, 0.2% manganese by weight):
(Water rinse after each step)
To prepare the zincate: zinc sulfate was first dissolved in water, then heated to 65-75 °C. Sodium pyrophosphate was slowly added to it while stirring. While the sodium pyrophosphate was added, a white precipitate formed that later dissolved, giving a clear solution. Sodium fluoride was then added to this clear solution, and finally the pH of the solution was adjusted with the addition of sodium carbonate.
1. Groshart, E.A., Metal Finishing 83 (1), January 1985, "Organic Finishing, Part I - Design Concepts and Surface Preparation".
2. Weed, E.I., 47th Annual Technical Proceedings American Electroplaters' Society, 1960, "Magnesium Corrosion", pp. 66-67.
3. Janssen, J., et al., 49th Annual Technical Proceedings American Electroplaters' Society, 1962, "Thermal Reflectance of Anodized Metals for Spacecraft", pp. 126-132.
4. Sharma, A.K., et al., Metal Finishing, 91(3), March, 1993, "Gold Plating on Magnesium for Space Applications", pp. 34 - 40.
Processing New Castings
Equipment and Materials
Chromic acid pickle
Sulfuric acid pickle
Chemical chromate treatment for magnesium alloys with more than 1% aluminum
Chemical chromate treatment for magnesium alloys with less than 1% aluminum
HF pre treatment (optional)
1. conversion from English inches to metric µm (micrometres):
1 mil (0.001 inches) x 1 inch/1000 mils x 2.54 centimeters / inch x 1 metre/100 centimeters x 1,000,000 µm (micrometers) / metre = 25.4 µm
so 0.001" (inches) = 25.4 µm (micrometres)
2. Proprietary chromate compounds and detergents are available from chemical supply houses serving the finishing industry.
Used for surface inspection (Type I) or to remove casting skin contamination (Type II)
sulfuric acid solution
hydrofluoric acid solution
The pickling of magnesium castings is done under Type I to detect surface flaws, defects, sand holes, and evidences of welding, and under Type II to remove the casting skin and other surface contamination that may affect subsequent chemical treatments.
The removal of casting skin and other contamination removes 25 to 50 µm of material from the casting.
Type I - Treatment for Wrought Material
Type II - Treatment for castings
The treatment of this process will produce a variable colored coating, depending upon the age of the solution, the alloy, and heat treatment condition of the alloy. The best paint base is a matte grey to yellow-red, iridescent color. Bright, brassy coatings are unsatisfactory for a paint base.
Chromic acid pickle
sulfuric acid pickle
Type I Chrome for treatment for wrought material: Pickle solution
Type II Chrome for treatment for castings: Pickle solution
Dow #1 treatment
Type I, agitate the parts in the chrome pickle solution for 30 seconds to 2 minutes, with a 5 second drip time to develop color and reduce dragout. Rinse and dry.
For Type II, immerse the parts in hot (180F) water for 15 - 30 seconds followed immediately by immersion in the chrome pickle for 10 seconds to 2 minutes, with a 5 second drip time. Rinse and dry.
Brush application with approval by the quality control department, is acceptable for repairs or for special parts too large for immersion in tanks.
Maintenance of Dow #1 solutions
30-40 replenishments. See Mil-M 3171C
Trouble shooting summary For Dow#1 processing
Brown, non-adherent, powdery coating - the part was in the air too long before rinsing; ratio of acid concentration to sodium dichromate too high; solution too hot; parts not clean; solution replenished too many times.
Grey, non-adherent, powdery coating on castings - immersed in bath too long; used Type I solution instead of Type II for castings.
Dichromate solution maintenance
A 10 % solution of chromic acid is used to adjust pH. the calcium or magnesium fluoride content is maintained by the continuous immersion of a cloth bag containing one of these compounds: cloth made of vinylidene chloride or acrylonitrile fiber is recommended.
Abnormally heavy and loose powdery deposits - HF acid concentration low; pH of dichromate low; corroded or contaminated surfaces; contact between work and tank;prolonged treatment in the dichromate solution.
Failure to coat or nonuniform coatings - pH of dichromate solution too high; dichromate concentration low; surface contamination because of insufficient cleaning or contaminated after cleaning; temperature of dichromate solution;improper rinsing after HF acid.
Notes on Dow #7
- The dichromate treatment provides an excellent paint base and protective coating on all standard magnesium alloys except EK30A, EK41A, HM31A, HM21A, HK31A, La141A and M1A. No coatings form on these alloys.
- Castings containing bearings, studs, and inserts of brass, bronze, cadmium plate and steel may be treated. Aluminum is rapidly attacked by the HF acid. Do not use the chromic acid pickle on parts containing copper based inserts.
- Use alkaline cleaners containing less than 2 % caustic for ZK60A, ZK60B and some Magnesium Lithium alloys, as dimensional losses due to etching may occur.
MIL-M-45202A, Anodic treatment of Magnesium Alloys
Type I - Dow#17 Process
Type II - HAE Process
Class A - Thin Coating
Class B - Heavy Coating
All castings, before machining, shall be sand or grit blasted and acid pickled with:
Note: Any smut formed after pickling may be removed by dipping in 10-15% HF acid solution.
(Minimum, Maximum, Typical)
Post Treatment for Type II (HAE) coatings
Unless otherwise specified, Class A coatings shall be given a chromate post treatment per Type I, Class A, Grade 2 or MIL_M_45202A and the Class B Coatings shall be given a bifluoride-dichromate post treatment as per Type II, Class A, Grade 3 of MIL-M-45202A.
Notes:- Dissimilar metal inserts must be installed after anodizing.
Stripping of anodic coatings.
20% chromic acid solution used near the boiling point. Since some of the magnesium is used in the formation of the anodic coating, there will be a loss of dimension after stripping.
Repair of coating
Repair of minor scratches, upon approval of the quality control department, may be made by:
MIL-STD-276, Impregnation of Porous Non-Ferrous Metal Castings
MIL-I-6869, Impregnants for Aluminum Alloy and Magnesium Alloy Castings.
Impregnation of castings requires the use of an aqueous detergent tank and hot and cold water rinse tanks for the detergent and uncured impregnating resin. Wastewater treatment of this rinsewater may be complicated by toxic organics present in the uncured resin, if so formulated.
Parts are to be etched prior to inspection.
Applicable Documents - MILITARY SPECIFICATIONS
MIL-I-6866 Inspection, Penetrant Method of
MIL-F-38762 Fluorescent Penetrant, Inspection Units
MIL-I-25135 Inspection Materials, Penetrant
MIL-P-47158 Penetrant Inspection, Soundness Requirements for Materials, Parts and Weldments
MIL-STD-410 Non-Destructive Testing Personnel, Qualification and Certification
Applicable Documents - MILITARY SPECIFICATIONS
MIL-M-6867 Magnetic Inspection Units
MIL-I-6868 Inspection Process, Magnetic Particle
MIL-STD-410 Qualification of Inspection Personnel ( Magnetic Particle and Penetrant)
Following up on Tom Pullizzi's highly detailed and well-researched report from 2000 ...
With the ever increasing consumer demand for small, light, and portable electronic devices like cell phones and stylus-operated computers, magnesium and its surface treatment have become increasingly important.
At the same time, environmental pressures have made many of the older surface treatments undesirable and in some cases have even forbidden their use in some countries and on some applications.
Newer alternatives are available to supplant the older and more toxic treatments based on fluoride processes, silanes, cerium-based processes, micro-arc oxidation, electroplating, and electroless nickel plating. These newer processes are often either retained as trade secrets or covered by active (rather than expired) patents and thus are only available from the patent holders.
The major suppliers of proprietary plating processes offer these proprietary magnesium treatments and should be consulted for these replacement technologies.
Back to finishing.com Home Page