Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D11/00—Electrolytic coating by surface reaction, i.e. forming conversion layers
  • C25D11/02—Anodisation
  • C25D11/30—Anodisation of magnesium or alloys based thereon

Definitions

• This invention is a method for the anodisation of magnesium and its alloys which, at least in the preferred form, produces an even and corrosion resistant film and which, at least in the preferred form, is suitable as a pre-treatment for other processes or as a final treatment for magnesium articles.
• Magnesium is a very light, yet strong metal and is finding increasing acceptance for metal die castings, particularly where weight savings are desired.
• its property of shielding electromagnetic radiation is causing it to be of interest as a replacement for plastics in applications such as computers and mobile telephones.
• it is a reactive metal and corrosion, whether general or by galvanic effects, is a major problem.
• the anodisation of aluminium and its alloys is often conducted in sulphuric acid in which the oxide layer formed is slightly soluble.
• the rate of build decreases, so ultimately there is a point at which the rate of dissolution is equal to that of further film growth.
• the dissolution of the film causes the formation of pores through which the ionic migration necessary to the electrochemical oxidation of the metal takes place. Without these pores only very thin films would be possible. After the electrochemical oxidation process is complete, the pores are sealed.
• a further method of anodising magnesium or alloys of magnesium relies on this property to create a rough, very porous layer which forms an excellent base for paint or other surface coatings to be applied afterwards.
• an anodic film may be formed in an electrolyte of very high pH, containing alkali hydroxides. The process proceeds by means of sparking which sparking forms a sintered ceramic oxide film as the metal substrate is coated.
• the object of the invention is to provide as an alternative to or as a refinement of the process of WO 96/28591 a process which also can produce an even film, or which at least provides the public with a useful choice.
• the present invention consists in a method of anodising magnesium or magnesium alloys comprising or including: immersing said magnesium containing material in an electrolyte as an anode; providing a cathode in said electrolyte; and passing a current through said electrolyte; and wherein the electrolyte, possessing a pH greater than 7, comprises or includes in water
• phosphoric acid and said phosphoric acid is provided in the range of 0.05 to 0.2 molar.
• said electrolyte contains a foaming agent.
• said electrolyte contains a water soluble peroxide.
• an amine is used alone for step (i) or as partial replacement for ammonia in step (i) and said amine is a water-soluble primary, secondary or tertiary amine having a pKa greater than 5.
• said amine has a pKa greater than 9.
• the magnesium or magnesium alloy article is cleaned by a pretreatment step prior to anodisation.
• the pre-treatment step includes at least one of the following:
• an AC voltage usually between zero to 40 volts, but under some circumstances more; and/or (ii) a pulsed (square wave form) voltage usually between zero and 40 volts, but sometimes greater; and (II) a current density from 50 to 1000 amps per square metre.
• the current density is from 200 to 350 amps per square metre.
• the invention consists in a method of anodising magnesium or a magnesium alloy (hereafter "the magnesium material”) comprising or including; providing an electrolytic solution; providing a cathode in or for said solution; placing the magnesium based material as an anode in said solution and, passing a current between the anode and cathode through said solution so that an anodised surface results, and wherein the electrolyte, possessing a pH greater than 7, comprises or includes in water (a) (i) ammonia and an amine or (ii) an amine and
• (A) (i) if no hydrogen peroxide and/or a soluble peroxide is present in the electrolyte solution, greater than 220 Volts, and (ii) if hydrogen peroxide and/or a soluble peroxide is present in the electrolyte solution , greater than 210 Volts, and (B) below that which provides any substantial degree of spark formation on the magnesium material or its anodising surface as anode and/or plasma discharges yet is higher than would otherwise be possible without any substantial degree of spark formation on the magnesium material or its anodising surface and/or plasma discharges were it not for the ammonia and/or amine presence in the electrolyte solution.
• said amine is capable in alkaline solution of expressing ammonia gas or a volatile amine moiety.
• said electrolyte solution includes at least one source of phosphate ions.
• the anodisation is carried out whilst the electrolyte solution is below
• the voltage limit is in the case of (A) (i) greater than 300 Volts and preferably less than 600 Volts, and in the case of (A) (ii) greater than 280 Volts and preferably less than 550 Volts.
• the aqueous electrolyte solution contains at least 3% w/v ammonia
• the aqueous electrolyte solution contains 5% w/v ammonia or above (when expressed as ammonia gas).
• At least one source of phosphate ions is selected from the group of phosphoric acid, soluble phosphate salt(s) and soluble ammonium phosphate(s).
• the phosphate ions have been obtained by adding phosphoric acid to the bath thereby forming various phosphate anions by hydrolysis.
• a source of phosphate ions in the range of from 0.01 to 0.2 molar is present.
• the source of phosphate ions are present at about 0.05 to about 0.15 molar.
• the electrolyte solution contains in addition at least one of the group of aluminates, silicates, borates, fluorides, phosphates, citrates and phenols.
• the electrolyte solution is free of any substantial presence of chromium (III) and chromium (VI).
• the electrolyte solution contains no alkali salt yielding hydroxide ions upon hydrolysis.
• the invention consists in a method of anodising magnesium based material (ie; magnesium or magnesium alloys) comprising or including providing an electrolytic solution and wherein the electrolyte, possessing a pH greater than 7, comprises or includes in water
• a magnesium or magnesium alloy article anodised by a method of any one of the preceding claims.
• the invention consists in a method of anodising magnesium or magnesium alloys comprising or including operating an anodising system of an electrolyte solution, a cathode and the magnesium or magnesium alloy material as the or an anode, wherein the electrolyte solution contains (i) an amine or amines or (ii) (a) an amine or amines and (b) ammonia in solution, in electrical input conditions which but for the inclusion of the amine(s) and any ammonia in the solution would not provide a coherent anodised coating (whether owing to so-called "sparking" or otherwise).
• Figure 1 shows a diagrammatic view of an anodisation bath in accordance with one embodiment of this invention.
• magnesium containing material such as magnesium itself or its alloys
• the process has been found to be useful on substantially pure magnesium samples as well as magnesium alloys such as AZ91 and AM60 which are common magnesium alloys used in casting.
• Suitable connections such as cables 5 and 6 are provided from the electrodes 3 and 4 to a power supply 7.
• the solution 2 is provided to include ammonia to a suitable concentration.
• concentration of the ammonia in the electrolytic solution 2 may vary. However, a preferred range of between 1% and 33% w/v, ammonia (when expressed as the gas) is desirable.
• ammonia concentration to work suitably in the region of 5 to
• sparks can occur for a number of reasons.
• the ammonia acts to repress sparks generally, but the concentration of salts in the bath also has an effect. If the ammonia gets too low, sparks may form. If the concentration of phosphate is increased greatly, sparks may occur at higher voltages, though the coating may form completely before the voltages are increased to such a voltage.
• peroxide may be added to the electrolytic solution.
• the addition of peroxide has been observed to decrease the voltage at which the coating forms without spark formation.
• a solution of 5% w/v ammonia (expressed as the gas), 0.05M sodium ammonium hydrogen phosphate and 0.1M sodium peroxide or hydrogen peroxide produces a coating at 210 V DC very similar to a 300 V DC coating formed in the absence of the peroxide. This may be advantageous in circumstances where a lower operating voltage is desired.
• a coating forms on the material 3 forming the anode on that portion 8 of the material 3 which is immersed within the solution 2.
• the process itself is, to a large degree, self terminating with the current drawn by the anodising bath 1 falling off as the depth of coating on the portion 8 increases.
• the placement of an article 3 as an anode within the anodising bath 1 tends to draw current until the coating is formed and when sufficient coating exists to substantially isolate the magnesium in the material 3 from the electrolytic solution 2, the current drawn falls and can act as an indicator that the coating has been applied.
• a number of additives may be provided in the solution 2 to alter the final coating and its appearance.
• phosphate compounds may be used to provide a finish similar to anodised aluminium and it has been found that phosphate compounds provided in the range of 0.01 to 0.2 molar can be suitable. Generally a concentration less than 0.01 molar tends to provide a finish which is somewhat transparent. Concentrations greater than 0.2 molar lead to an opaque finish which again alters the appearance of finished product.
• a preferred range of 0.05 to 0.15 molar of a phosphate compound such as ammonium sodium hydrogen phosphate has been found to be suitable if it is desired to provide a finish similar in appearance to anodised aluminium. The ammonium phosphate has been found particularly useful and other ammonium phosphate compounds could act as direct substitutes.
• ammonium phosphate compounds give significant corrosion resistance to the coating. Also the coating is particularly suited to further coating with paint or other organic sealers.
• the electrolytic solution 2 may contain compounds such as ammonium dihydrogen phosphate, or alternatively or additionally, diammonium hydrogen phosphate. Both of these compounds may be more readily available in commercial quantities for the anodisation process compared with compounds such as ammonium sodium hydrogen phosphate.
• An alternative additive to provide a finish similar to anodised aluminium has been found to be the use of fluoride and aluminate in similar concentrations to the phosphate compounds.
• Typical concentrations of compounds such as sodium aluminate and sodium fluoride are 0.05 molar of each of these compounds.
• the finish changes to a pearl coloured finish.
• this may be aesthetically pleasing in itself, it is not directly comparable with the anodised aluminium finish and, therefore, may be less suitable if it is desired to manufacture components for the same product from the different materials and be able to provide matching finishes on both aluminium and magnesium products.
• the process itself is conducted at relatively low currents compared with the previous anodisation of magnesium processes.
• the current drawn is in the order of 100 amps per square metre of magnesium surface.
• the low current and lack of spark formation lead to a decrease in the temperature rise within the bath 1 to form an equivalent depth of coating compared with the alkaline hydroxide baths used previously. This reduction in the temperature rise of the bath leads to a significant decrease in the cooling equipment necessary to conduct the process.
• additives includes a phosphate additive and/or a fluoride additive. If the fluoride additive is used in substitution for the phosphate additive, this leads to greater problems with the disposal of the solution. Fluoride compounds are environmentally costly owing to stringent environmental regulation of their effluent and disposal. By comparison, the phosphate compounds are less damaging to the environment and may be preferred for this reason alone.
• the additives may also include sealants or other compounds and many of the additives used in the previous anodisation processes such as aluminates, silicates, borates, fluorides, phosphates, citrates and phenol may be used.
• the coating formed on the magnesium may be a mixed coating of magnesium oxide and magnesium hydroxide with further constituents according to any particular additives used in the process.
• the embodiment in which sodium ammonium hydrogen phosphate is provided leads to a magnesium phosphate component in the coating.
• the embodiment in which fluoride and aluminate compounds are provided may lead to the presence of magnesium fluoride and magnesium aluminate in the finished coating.
• ammonia in the solution may necessitate the use of ventilation in the area about the anodisation bath 1.
• a most preferred electrolyte composition where ammonia alone is used is: ammonia - 3.0-3.3 molar* (usually made up from 25% aqueous solution); phosphoric acid - 0.1-0.2 molar (alternatively a phosphate s ⁇ Jt may be used); and a foaming agent - 0.1ml per litre of a non-ionic foaming agent.
• This bath has a pH of approximately 11.6. *The ammonia concentration is 3.0 to 3.3 molar after the addition of the phosphoric acid, hence the ammonia added initially to the bath is slightly more than this.
• the foaming agent ideally has the effect of reducing ammonia loss to the atmosphere.
• the most preferred electrochemical conditions for anodisation with such a composition comprise:
• the temperature is in the range from 0°C to 35°C (most preferably 10-30°C).
• the present invention recognises that partial or complete substitution of the ammonia by an amine may be made whilst otherwise operating a process as disclosed in WO 96/28591 but under the conditions previously disclosed for the present invention.
• Simple amines such as methyl or ethyl amine are volatile so it is recommended that any substitution involve a longer chain or more complex amine.
• Suitable amines must be water soluble at least to a level of 3.0 molar and should feature basicity similar to that of ammonia (ability to form hydroxyl, OH- ions in solution).
• Some examples of amines that may be used are diethylene triamine and ethanolamine.
• anodising voltage may be used, and this will most preferably be from 250V DC upwards, with AC voltage imposed additionally as may be required.
• Example 1 An AZ91D magnesium plate was pre-cleaned in a solution containing 0.2 molar sodium tetraborate and 0.07 molar sodium pyrophosphate. This was then anodised in an electrolyte comprising 4.9% ammonia (expressed as w/v NH 3 ) and 0.2 molar diammonium hydrogen phosphate at a voltage that peaked at 400V DC at a bulk current density of 200 amps per square metre. After attainment of 400V, which took just over seven minutes, the power supply was cut off and an anodic film of 9 microns was observed on the sample. Total cycle time was 7 minutes.
• An AM50 magnesium component was anodised at 100 amps per square metre, up to an endpoint voltage of 350V DC.
• the electrolyte composition was 3% ammonia (expressed as w/v ammonia gas) and 0.2 molar diammonium hydrogen phosphate.
• the component received a rinse prior to anodisation but no other pretreatment.
• the power was maintained to the sample and held at 350V DC for approximately ten minutes.
• the sample Upon rinsing the sample was found to have an anodic film of approximately 17 microns.
• the cycle time was approximately 30 minutes.
• An AZ91D magnesium plate was anodised in an electrolyte comprising ammonia at
• An AZ91D magnesium plate was anodised in an electrolyte comprising ammonia at 5.0% (expressed w/v as ammonia gas), 0.1 molar phosphoric acid and 0.03 molar hydrogen peroxide.
• the plate was pre-cleaned as per example #3 above and activated as per example #3 above. It was then anodised using a power supply comprising a DC voltage that reached 385V, and an AC voltage which reached 52V.
• the DC current density was 280 amps per square metre while the AC current density peaked at 90 amps per square metre.
• the DC endpoint voltage was held for five minutes, then the sample was post-treated for two minutes in a bath containing 1.0 molar sodium dihydrogen phosphate at 60 ° C .
• the sample was found to have an anodic coating of 19.7 microns .
• the anodising cycle required a total time of 15 minutes.
• Example 5 An AZ91D test plate was pre-cleaned in a bath comprising 0.2 molar sodium tetraborate and 0.07 molar sodium pyrophosphate as in example #3 above. It was then anodised in an electrolyte comprising 2.5% ammonia (expressed as ammonia gas) and 0.5 molar diethylene triamine (DETA), together with phosphoric acid at 0.1 molar, at a DC voltage that attained 360V which was held for five minutes. The current density was 200 amps per square metre. The plate was found to have an anodic coating of 28.2 microns. The total cycle time was 21 minutes for the anodising process.
• an electrolyte comprising 2.5% ammonia (expressed as ammonia gas) and 0.5 molar diethylene triamine (DETA), together with phosphoric acid at 0.1 molar, at a DC voltage that attained 360V which was held for five minutes.
• the current density was 200 amps per square metre.
• the plate
• Example 6 An AZ91 D test plate was precleaned in the mixture described in example #3 (but not activated). It was then anodised in a solution comprising 19.8% monoethanolamine (w/v) and 0.2 molar sodium dihydrogen phosphate at a DC voltage that attained 350V which was held for five minutes. The current density was 200 amps per square metres. The sample was found to have an anodic coating of 20.2 microns. The total anodising cycle time was 16 minutes 30 seconds.
• process times quoted represent anodising times, not including pre-cleaning or activation where these are specified, nor any post-anodisation treatments.

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