HAVACOAT MG 15 CHROMATE PROCESS FOR MAGNESIUM

Transcription

1 CHROMATE PROCESS FOR MAGNESIUM is a simple room temperature, short dip, chemical process for producing a corrosion protective coating on magnesium and magnesium alloys. can be applied by dip or brush and forms an excellent bond with paints and lacquers. OPERATING INSTRUCTIONS: Quick Glance Flow Chart: 1. Alkali Clean Cold Running Rinse 6. Cold Running Rinse 3. Acid Pickle Optional 7. Cold Running Rinse 4. Cold Running Rinse 8. Dry Note: Because of possible contamination of the treated surface, for best results, thorough rinsing is recommended after. EQUIPMENT DATA: Tank linings to hold working solution: Koroseal or equal / Polyethylene Heating Coils: Karbate / Tantalum/ Quartz Electric Immersion Heaters. Page 1 of 6 Revised February 13, 2006 Approved: Graham M. Torr Technical Service Mgr. Haviland Products Co. 421 Ann St. N.W. Grand Rapids, MI (616) Fax (616)

2 Page 2 of 6 SOLUTON MAKEUP AND OPERATING CONTROLS: I. Low Aluminum Alloys-Wrought, Cast and Extrusions such as AZ 31 a. Hot Alkaline Clean b. Rinse thoroughly c. Acid Pickle (chromic-nitrate) optional if used. 1. On mill pickled stock use 15 to 30 seconds immersion time. 2. On un-pickled stock use 3 minute immersion time. d. Rinse thoroughly e. 7-10% by Volume Hydrochloric Acid 7.5 fl oz/ gallon DESCALER FEX PLUS 1ml/ gallon Temperature Range Immersion Time ph Range 0.2 to to F If pickle has been used sec If pickle is not used - 2 min f. Rinse Thoroughly (see Rinsing Procedures) g. Dry Air blast or hot circulating air up to F. II. High Aluminum Alloys Sand and Die Cast such as AZ 61, AZ 81, AZ 91 a. Hot alkaline clean b. Rinse thoroughly c. Acid pickle (Bright Pickle) 1. On mill pickled stock use 15 to 30 seconds immersion time. 2. On un-pickled stock or before machining use 2 minutes immersion time. d. Rinse thoroughly e. 7-10% Hydrochloric Acid 7.5 fl oz/ gallon DESCALER FEX PLUS 1ml / gallon Temperature Range F Immersion Time 10 to 30 Seconds ph Range f. Rinse thoroughly g. Dry- Air blast or hot circulating air up to F.

3 Page 3 of 6 III. Alloy without Aluminum such as: ZK60, HK31, EZ33, HZ32, ZK51, ZH62, M1A a. Hot Alkaline Clean b. Rinse thoroughly c. Acid Pickle (chrome-nitrate) optional if used. 1. On mill pickled stock use 15 to 30 seconds immersion time. 2. On un-pickled stock use 3 minutes immersion time. d. Rinse thoroughly e. Dry-Air blast or hot circulating air up to F. When a magnesium surface is thoroughly prepared, by mechanical cleaning or by alkali and acid cleaning, the solution acts uniformly and forms it s coating in 15 to 30 seconds. Extending the immersion in MG 15 beyond this point does not increase the film thickness; however, when the hydrofluoric acid pickle is used, it may be necessary to extend the immersion time in the MG 15 to 45 to 90 seconds to get complete coverage. Magnesium die and sand castings which have bee previously pickled should not exceed ½ to 1 minute immersion in or an aluminum smut will develop. MAINTENANCE: 0 solution is maintained by adding MG 15 and Hydrochloric acid in the original proportions to keep within the ph range given or to maintain the color standard desired. DESCALER FEX PLUS should be maintained by adding 1ml of FEX PLUS for every 10 liters of when solution is in constant use. When solution has been allowed to stand for a week or more, and a thin foam layer does not form during processing, add 1 ml of FEX PLUS for each 10-liter solution. It is very important that the FEX PLUS provides a thin blanket of foam during the processing. CLEANING PROCEDURES: PRE-PICKLING: Acid cleaning on magnesium removes oxides, but it also performs a far more important function. Acids are used to remove surface impurities such as sand and iron in castings, and embedded mill scale in sheet stock. These inclusions form nuclei for galvanic corrosion cells and if not removed, will lower the overall corrosion resistance. Maximum uniformity results only w hen sheet stock is pre-pickled in a solution of acetic acid and sodium nitrate long enough to remove.0005 of metal per surface. Castings should be pickled, after sand blasting, in a dilute sulfuric acid solution to remove.002 of metal per surface. These pre-pickling operations are done most economically at the mill or foundry, and before any forming or machining are performed. Note that the Chrome pickle process (specification MIL-M- 3171a, Type I, which is used as a temporary protective treatment) is not an adequate substitute for the pre-pickling operation described above. The Chrome Pickle does not remove enough metal for complete removal of surface inclusions. The appropriate pre-pickling should be specified in ordering the metal from the mill or foundry.

4 Page 4 of 6 PRE-CLEANING: Heavy oils and greases are frequently removed by vapor degreasing. It is important to remember that a magnesium part, being lighter in weight than steel or aluminum, condenses far less solvent in a vapor degreaser. Because of this, it is frequently necessary to allow the work to cool and then repeat the degreasing operation to do a thorough job. Pigmented drawing compounds and buffing compounds are best removed with an emulsion cleaner used in a soak tank or power washer. When graphite is used as a lubricant in hot drawing these pre-cleaning methods do not remove it. This is done by a 20-minute soak in a strong caustic soda solution followed by immersion in the chromic acid sodium nitrate pickle described below. This operation should follow drawing as soon as possible. If the graphite is allowed to remain on the magnesium for over 24 hours it is very difficult to remove. ALKALI CLEANING: CLEANER ASC@ % F for 3 to 5 minutes of the types normally used on steel are recommended and result in no measurable metal loss. Mild inhibited alkalis such as those used for non-etch cleaning of aluminum are not generally applicable to magnesium since they may cause etching. The ph of the cleaning solution used must be 12 or higher. The alkali cleaning solution also removes old chrome pickle films, which can interfere with the process. PICKLING: Work in process is pickled to remove oxides and residues from alkali cleaning. The following solutions have been found effective in preparing work for. Only short immersions are necessary if the work has been pre pickled adequately or on machined surfaces. The choice of pickle depends on the alloy being processed. For alloys with 3.5% aluminum or less Chromic Nitrate Pickle CHROMIC ACID 24 OZ/ GALLON SODIUM NITRATE 4 OZ/ GALLON TEMPERATURE 65 TO 90 0 G IMMERSION TIME SEE ABOVE ph RANGE 0.5 TO 0.7 Maintain within ph range by additions of Chromic Acid

5 For alloy with aluminum over 3.5% Bright Pickle Page 5 of 6 CHROMIC ACID CONC. NITRIC ACID HYDROFLUORIC ACID TEMPERATURE IMMERSION TIME 37 ½ OZ / GALLON 3 ½ FL OZ/ GALLON 1 FL OZ/ GALLON F SEE ABOVE Maintain by adjustment to original ph by addition of Chromic Acid. FLUORIDE PICKLES: When dimensions are extremely critical, a hydrofluoric acid or an ammonium bifluoride pickle may be substituted for the pickles above. It must be remembered, however, that a fluoride pickle is a compromise. The only impurities it can remove are those at the metal surface. Furthermore, if the time is not limited, it may passivate the metal to the point where MG 15 or other chromate film will not form. For maximum protection when this type of pickle is used, the following conditions must be met. 1. Sheet stock must be given the acetic acid- sodium nitrate mill pickle either before forming, or during the cleaning cycle. Castings must be pre-pickled at the mill or before machining to remove at least.002 per surface. 2. The immersion time in a fluoride pickle must be limited to the period of initial vigorous gassing. When the gassing stops, or is markedly reduced, the work must be removed from the pickle and rinse thoroughly. The following formulations are those conventionally used for this type of solution. HYDROFLUORIC ACID (Hydrofluoric Acid 60%) AMMONIUM BLFLUORIDE (Ammonium Acid Fluoride) 20% (24 fl oz/ gallon) 6 2/3 oz/ gallon Either solution is used at room temperature, for only as long as it takes the initial vigorous gassing to subside. Another use for the fluoride pickle occurs when die castings (AZ91) and thin (1/8 thick or less) sand castings (AZ63 or AZ92) are being treated. These metals have irregular surface layer of A1 2 Mg 3 or A1 3 Mg 4 present, which will not take a dark coating. Even when the final color is uneven, the lighter surface is passivated by the sufficiently enough to resist corrosion as well as the adjacent darker surface. However, if an even brown finish is necessary, extending the time in the Hydrofluoric Acid pickle to 1 or 2 minutes will usually remove the surface layer of aluminum magnesium compound. In the subsequent treatment with MG 15, it may be necessary to extend the immersion time.

6 Page 6 of 6 RINSING AND DRYING PROCEDURES: RINSING: With all surface treatments of magnesium, rinsing is an important step of the process. The time of all cold rinses should be 15 to 20 seconds. This includes the rinses following MG 15 as well as those following cleaning and pickling. Two rinses are recommended following the MG 15 operation to insure a contamination free surface. On castings, the final rinse may be hot ( F) using an in and out dip to facilitate drying. This temperature on sheet stock results in softening of the coating, which may be detrimental in some applications. DRYING: Drying may be accomplished by air blast, centrifuge, or hot circulating air. Drying temperatures in excess of F will product a dark brown coating of increased hardness and reduced solubility. Temperatures up to F are satisfactory. HANDLING AND STORAGE: CAUTION: is a strong acidic liquid containing chromates and fluorides. Handle with care. Wear protective glasses or face shield, rubber gloves and apron. If spilled on skin, flush with water. Consult a physician if pain or irritation develops. For eye contact, flush with cool water for 15 minutes and obtain immediate medical attention. STORAGE: contains oxidizers. Contact with wood, paper, rags, organic materials (especially solvents) may cause spontaneous combustion or render these materials dangerously flammable. NON-WARRANTY: The data contained in this bulletin is believed by Haviland Products Company to be accurate, true, and complete. Since, however, final methods of use of this product are in the hands of the customer and beyond our control, we cannot guarantee that the customer will obtain the results described in this bulletin, nor can we assume any responsibility of the use of this product by the customer in any process which may infringe the patents of third parties.