US20110120595A1 - Method for forming conversion coating on outer surface of magnesium or magnesium alloy - Google Patents
Method for forming conversion coating on outer surface of magnesium or magnesium alloy Download PDFInfo
- Publication number
- US20110120595A1 US20110120595A1 US12/785,235 US78523510A US2011120595A1 US 20110120595 A1 US20110120595 A1 US 20110120595A1 US 78523510 A US78523510 A US 78523510A US 2011120595 A1 US2011120595 A1 US 2011120595A1
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- United States
- Prior art keywords
- magnesium
- magnesium alloy
- aqueous solution
- conversion coating
- water washing
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Abandoned
Links
- 229910000861 Mg alloy Inorganic materials 0.000 title claims abstract description 53
- FYYHWMGAXLPEAU-UHFFFAOYSA-N Magnesium Chemical compound [Mg] FYYHWMGAXLPEAU-UHFFFAOYSA-N 0.000 title claims abstract description 45
- 239000011777 magnesium Substances 0.000 title claims abstract description 45
- 229910052749 magnesium Inorganic materials 0.000 title claims abstract description 45
- 238000007739 conversion coating Methods 0.000 title claims abstract description 38
- 238000000034 method Methods 0.000 title claims abstract description 38
- 239000007864 aqueous solution Substances 0.000 claims description 41
- 229910052751 metal Inorganic materials 0.000 claims description 30
- 239000002184 metal Substances 0.000 claims description 30
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 claims description 29
- 238000005406 washing Methods 0.000 claims description 23
- 239000011248 coating agent Substances 0.000 claims description 19
- 238000000576 coating method Methods 0.000 claims description 18
- KWYUFKZDYYNOTN-UHFFFAOYSA-M Potassium hydroxide Chemical compound [OH-].[K+] KWYUFKZDYYNOTN-UHFFFAOYSA-M 0.000 claims description 16
- HEMHJVSKTPXQMS-UHFFFAOYSA-M Sodium hydroxide Chemical compound [OH-].[Na+] HEMHJVSKTPXQMS-UHFFFAOYSA-M 0.000 claims description 15
- NBIIXXVUZAFLBC-UHFFFAOYSA-N Phosphoric acid Chemical compound OP(O)(O)=O NBIIXXVUZAFLBC-UHFFFAOYSA-N 0.000 claims description 14
- 239000000080 wetting agent Substances 0.000 claims description 11
- GRYLNZFGIOXLOG-UHFFFAOYSA-N Nitric acid Chemical compound O[N+]([O-])=O GRYLNZFGIOXLOG-UHFFFAOYSA-N 0.000 claims description 10
- 229910017604 nitric acid Inorganic materials 0.000 claims description 10
- 238000005530 etching Methods 0.000 claims description 9
- 238000005238 degreasing Methods 0.000 claims description 8
- KRHYYFGTRYWZRS-UHFFFAOYSA-N Fluorane Chemical compound F KRHYYFGTRYWZRS-UHFFFAOYSA-N 0.000 claims description 7
- 229910000147 aluminium phosphate Inorganic materials 0.000 claims description 4
- 239000012153 distilled water Substances 0.000 claims description 4
- 150000001412 amines Chemical class 0.000 claims description 3
- QPJSUIGXIBEQAC-UHFFFAOYSA-N n-(2,4-dichloro-5-propan-2-yloxyphenyl)acetamide Chemical compound CC(C)OC1=CC(NC(C)=O)=C(Cl)C=C1Cl QPJSUIGXIBEQAC-UHFFFAOYSA-N 0.000 claims description 3
- AEQDJSLRWYMAQI-UHFFFAOYSA-N 2,3,9,10-tetramethoxy-6,8,13,13a-tetrahydro-5H-isoquinolino[2,1-b]isoquinoline Chemical compound C1CN2CC(C(=C(OC)C=C3)OC)=C3CC2C2=C1C=C(OC)C(OC)=C2 AEQDJSLRWYMAQI-UHFFFAOYSA-N 0.000 claims description 2
- 239000000176 sodium gluconate Substances 0.000 claims description 2
- 235000012207 sodium gluconate Nutrition 0.000 claims description 2
- 229940005574 sodium gluconate Drugs 0.000 claims description 2
- 230000007797 corrosion Effects 0.000 abstract description 12
- 238000005260 corrosion Methods 0.000 abstract description 12
- 238000004381 surface treatment Methods 0.000 abstract description 6
- 239000003929 acidic solution Substances 0.000 abstract description 5
- 230000002378 acidificating effect Effects 0.000 abstract 1
- 239000012670 alkaline solution Substances 0.000 abstract 1
- 239000000203 mixture Substances 0.000 description 12
- 238000007744 chromate conversion coating Methods 0.000 description 9
- 239000000654 additive Substances 0.000 description 6
- 230000000996 additive effect Effects 0.000 description 6
- 230000015572 biosynthetic process Effects 0.000 description 4
- 238000002845 discoloration Methods 0.000 description 4
- 150000003839 salts Chemical class 0.000 description 4
- 239000000126 substance Substances 0.000 description 4
- 238000006243 chemical reaction Methods 0.000 description 3
- 239000007769 metal material Substances 0.000 description 3
- KRVSOGSZCMJSLX-UHFFFAOYSA-L chromic acid Substances O[Cr](O)(=O)=O KRVSOGSZCMJSLX-UHFFFAOYSA-L 0.000 description 2
- 239000012895 dilution Substances 0.000 description 2
- 238000010790 dilution Methods 0.000 description 2
- 230000005611 electricity Effects 0.000 description 2
- 239000003925 fat Substances 0.000 description 2
- AWJWCTOOIBYHON-UHFFFAOYSA-N furo[3,4-b]pyrazine-5,7-dione Chemical compound C1=CN=C2C(=O)OC(=O)C2=N1 AWJWCTOOIBYHON-UHFFFAOYSA-N 0.000 description 2
- XLYOFNOQVPJJNP-UHFFFAOYSA-M hydroxide Chemical compound [OH-] XLYOFNOQVPJJNP-UHFFFAOYSA-M 0.000 description 2
- 239000012535 impurity Substances 0.000 description 2
- 239000000463 material Substances 0.000 description 2
- 239000003921 oil Substances 0.000 description 2
- 239000012466 permeate Substances 0.000 description 2
- 239000000243 solution Substances 0.000 description 2
- 239000007921 spray Substances 0.000 description 2
- 239000004094 surface-active agent Substances 0.000 description 2
- 231100000331 toxic Toxicity 0.000 description 2
- 230000002588 toxic effect Effects 0.000 description 2
- RGHNJXZEOKUKBD-SQOUGZDYSA-M D-gluconate Chemical compound OC[C@@H](O)[C@@H](O)[C@H](O)[C@@H](O)C([O-])=O RGHNJXZEOKUKBD-SQOUGZDYSA-M 0.000 description 1
- DGAQECJNVWCQMB-PUAWFVPOSA-M Ilexoside XXIX Chemical compound C[C@@H]1CC[C@@]2(CC[C@@]3(C(=CC[C@H]4[C@]3(CC[C@@H]5[C@@]4(CC[C@@H](C5(C)C)OS(=O)(=O)[O-])C)C)[C@@H]2[C@]1(C)O)C)C(=O)O[C@H]6[C@@H]([C@H]([C@@H]([C@H](O6)CO)O)O)O.[Na+] DGAQECJNVWCQMB-PUAWFVPOSA-M 0.000 description 1
- PWHULOQIROXLJO-UHFFFAOYSA-N Manganese Chemical compound [Mn] PWHULOQIROXLJO-UHFFFAOYSA-N 0.000 description 1
- 239000002253 acid Substances 0.000 description 1
- 239000003513 alkali Substances 0.000 description 1
- 239000003795 chemical substances by application Substances 0.000 description 1
- ZCDOYSPFYFSLEW-UHFFFAOYSA-N chromate(2-) Chemical compound [O-][Cr]([O-])(=O)=O ZCDOYSPFYFSLEW-UHFFFAOYSA-N 0.000 description 1
- JOPOVCBBYLSVDA-UHFFFAOYSA-N chromium(6+) Chemical compound [Cr+6] JOPOVCBBYLSVDA-UHFFFAOYSA-N 0.000 description 1
- 230000000694 effects Effects 0.000 description 1
- 238000009713 electroplating Methods 0.000 description 1
- 239000005002 finish coating Substances 0.000 description 1
- 229940050410 gluconate Drugs 0.000 description 1
- 239000000395 magnesium oxide Substances 0.000 description 1
- CPLXHLVBOLITMK-UHFFFAOYSA-N magnesium oxide Inorganic materials [Mg]=O CPLXHLVBOLITMK-UHFFFAOYSA-N 0.000 description 1
- AXZKOIWUVFPNLO-UHFFFAOYSA-N magnesium;oxygen(2-) Chemical compound [O-2].[Mg+2] AXZKOIWUVFPNLO-UHFFFAOYSA-N 0.000 description 1
- 229910052748 manganese Inorganic materials 0.000 description 1
- 239000011572 manganese Substances 0.000 description 1
- 238000012986 modification Methods 0.000 description 1
- 230000004048 modification Effects 0.000 description 1
- 229910052761 rare earth metal Inorganic materials 0.000 description 1
- -1 rare earth salts Chemical class 0.000 description 1
- 229910052708 sodium Inorganic materials 0.000 description 1
- 239000011734 sodium Substances 0.000 description 1
- KIEOKOFEPABQKJ-UHFFFAOYSA-N sodium dichromate Chemical compound [Na+].[Na+].[O-][Cr](=O)(=O)O[Cr]([O-])(=O)=O KIEOKOFEPABQKJ-UHFFFAOYSA-N 0.000 description 1
- 150000003754 zirconium Chemical class 0.000 description 1
Images
Classifications
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- C—CHEMISTRY; METALLURGY
- C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C22/00—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
- C23C22/05—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions
- C23C22/06—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions using aqueous acidic solutions with pH less than 6
- C23C22/34—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions using aqueous acidic solutions with pH less than 6 containing fluorides or complex fluorides
- C23C22/36—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions using aqueous acidic solutions with pH less than 6 containing fluorides or complex fluorides containing also phosphates
-
- C—CHEMISTRY; METALLURGY
- C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C22/00—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
- C23C22/78—Pretreatment of the material to be coated
-
- C—CHEMISTRY; METALLURGY
- C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
- C23F—NON-MECHANICAL REMOVAL OF METALLIC MATERIAL FROM SURFACE; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL; MULTI-STEP PROCESSES FOR SURFACE TREATMENT OF METALLIC MATERIAL INVOLVING AT LEAST ONE PROCESS PROVIDED FOR IN CLASS C23 AND AT LEAST ONE PROCESS COVERED BY SUBCLASS C21D OR C22F OR CLASS C25
- C23F1/00—Etching metallic material by chemical means
- C23F1/10—Etching compositions
- C23F1/14—Aqueous compositions
- C23F1/16—Acidic compositions
- C23F1/22—Acidic compositions for etching magnesium or alloys thereof
-
- C—CHEMISTRY; METALLURGY
- C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
- C23G—CLEANING OR DE-GREASING OF METALLIC MATERIAL BY CHEMICAL METHODS OTHER THAN ELECTROLYSIS
- C23G1/00—Cleaning or pickling metallic material with solutions or molten salts
- C23G1/02—Cleaning or pickling metallic material with solutions or molten salts with acid solutions
- C23G1/12—Light metals
-
- C—CHEMISTRY; METALLURGY
- C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
- C23G—CLEANING OR DE-GREASING OF METALLIC MATERIAL BY CHEMICAL METHODS OTHER THAN ELECTROLYSIS
- C23G1/00—Cleaning or pickling metallic material with solutions or molten salts
- C23G1/14—Cleaning or pickling metallic material with solutions or molten salts with alkaline solutions
- C23G1/22—Light metals
Definitions
- the present invention relates to a method for forming a conversion coating on the outer surface of magnesium or a magnesium alloy. More specifically, the present invention relates to a method for forming a conversion coating on the outer surface of magnesium or a magnesium alloy in an environmentally friendly manner to improve the corrosion resistance of the magnesium or magnesium alloy.
- Magnesium is one of the most abundant elements found on the earth. Magnesium is the most lightweight structural metal material having a density of about 1.74 g/cm 3 . Based on these advantages, magnesium is currently used in a variety of applications, including aircraft, automobiles and portable device, and its usages and application are increasingly extended.
- Magnesium alloys however, have the drawback of poor corrosion resistance on account of their material properties. For better corrosion resistance, magnesium alloys should undergo chemical surface treatment. A series of conversion coating and finish coating steps is one of the most popular chemical treatment processes.
- Conversion coating is a metal surface treatment process in which a coating is formed to a thickness of hundreds of nm on the surface of magnesium by various chemical reactions.
- the kind of the coating varies depending on a conversion coating solution used.
- conversion coating is characterized in that a coating is formed on the surface of a metal by chemical reactions without the need to apply external electricity.
- Such conversion coating processes of magnesium alloys are typically divided into chromate conversion coating and non-chromate conversion coating processes.
- the first chromate conversion coating process termed Dow1 or Dow21, were developed by Dow Chemical in the 1970s. According to the representative Dow's process, the surface of a metal is treated in an acidic solution containing sodium bichromate as a major component. The acidic solution is easy to handle and manage and the surface-treated metal has the advantages of good corrosion resistance and coating adhesion. However, the presence of hexavalent chromium, which was found to be toxic, in the acidic solution limits the use of the Dow processes.
- Non-chromate conversion coating processes are known to use various metal salts, such as manganese and zirconium salts, rare earth salts and organic salts.
- coating solutions used in the non-chromate conversion coating processes are highly priced and are difficult to handle.
- Chromate conversion coatings are green or reddish grey and non-chromate conversion coatings are grey or ivory in color. That is, chromate conversion coatings do not sufficiently express the inherent color of magnesium, impairing the attractive appearance of magnesium alloys.
- An object of the present invention is to provide a method for forming a conversion coating on the outer surface of magnesium or a magnesium alloy to improve the corrosion resistance of the magnesium or magnesium alloy.
- Another object of the present invention is to provide a method for forming a conversion coating on the outer surface of magnesium or a magnesium alloy in an environmentally friendly manner.
- Another object of the present invention is to provide a method for forming a conversion coating on the outer surface of magnesium or a magnesium alloy to minimize the damage to the appearance of the magnesium or magnesium even after surface treatment.
- Still another object of the present invention is to provide a method for forming a conversion coating on the outer surface of magnesium or a magnesium alloy to maximize the attractive appearance of the magnesium or magnesium alloy.
- a method for forming a conversion coating on the outer surface of magnesium or a magnesium alloy comprising: degreasing the outer surface of magnesium or a magnesium alloy, followed by water washing; etching the degreased magnesium or magnesium alloy to remove an oxide coating present on the outer surface thereof, followed by water washing; removing smut present on the outer surface of the magnesium or magnesium alloy (desmutting), followed by water washing; forming a conversion coating on the desmutted outer surface of the magnesium or magnesium alloy, followed by water washing; and forming a coating on the conversion coating.
- the corrosion resistance of magnesium or a magnesium alloy is improved, the damage to the appearance of magnesium or a magnesium alloy is minimized even after surface treatment, and the attractive appearance of magnesium or a magnesium alloy can be maximized.
- the degreasing may be performed using a first aqueous solution containing 30-60 g/L of potassium hydroxide (KOH), 150-200 g/L of sodium hydroxide (NaOH), 20-30 g/L of an amine oxide, 5-10 g/L of sodium gluconate (C 6 H 11 NaO 7 ) and a wetting agent.
- KOH potassium hydroxide
- NaOH sodium hydroxide
- the first aqueous solution can facilitate the removal of oily components from the surface of the magnesium or magnesium alloy and can minimize the discoloration of the metal material.
- the etching may be performed using a second aqueous solution containing 600-800 g/L of nitric acid (HNO 3 ), 200-400 g/L of phosphoric acid (H 3 PO 4 ) and a wetting agent.
- the second aqueous solution may be diluted with distilled water to a pH of 1.0-2.0.
- the second aqueous solution may be in the temperature range of about 35 to about 50° C.
- the second aqueous solution can facilitate the removal of an oxide coating present on the surface of the magnesium or magnesium alloy and can minimize the discoloration of the metal.
- the desmutting may be performed by immersing the etched metal in a third aqueous solution containing 50-200 g/L of potassium hydroxide (KOH) and 5-10 g/L of a wetting agent.
- KOH potassium hydroxide
- the third aqueous solution can prevent the formation of a hydroxide coating on the surface of the metal while preventing the metal from being desmutted.
- the conversion coating may be formed using a fourth aqueous solution containing 600-800 g/L of nitric acid (HNO 3 ), 200-400 g/L of phosphoric acid (H 3 PO 4 ) and 0.5-10 g/L of hydrofluoric acid (HF).
- the fourth aqueous solution may be diluted with distilled water to a pH of 2.0-4.0.
- the conversion coating may be formed in the fourth aqueous solution at 20-60° C. Within the limited temperature range of the fourth aqueous solution, the conversion coating can be formed to a constant thickness at a controlled rate while minimizing the damage to the appearance of the metal.
- FIG. 1 is a magnification image showing the surface texture of a magnesium alloy plate whose surface is treated by a method for forming a conversion coating on the outer surface of magnesium or a magnesium alloy according to an embodiment of the present invention
- FIG. 2 is a flow chart illustrating a method for forming a conversion coating on the outer surface of magnesium or a magnesium alloy according to an embodiment of the present invention.
- FIG. 1 is a magnification image showing a surface texture of a magnesium alloy plate whose surface is treated by a method for forming a conversion coating on the outer surface of magnesium or a magnesium alloy according to an embodiment of the present invention
- FIG. 2 is a flow chart illustrating a method for forming a conversion coating on the outer surface of magnesium or a magnesium alloy according to an embodiment of the present invention.
- a first aqueous solution capable of degreasing the surface of a molded magnesium alloy is prepared to remove oils, fats and impurities attached to the molded metal surface and to prevent the discoloration of the metal.
- the additive contained in the first aqueous solution is a wetting agent.
- the wetting agent may be a surfactant for allowing the first aqueous solution to permeate the surface of the metal.
- Various wetting agents may be used depending on conditions required in the invention.
- the removal of oily components present on the surface of the magnesium alloy may be retarded or the metal material may be discolored. Therefore, it would be desirable that the first aqueous solution fall within the composition indicated in Table 1.
- water washing can be preformed (S 210 ). The water washing enables complete removal of the first aqueous solution remaining unremoved on the surface of the magnesium alloy.
- a portion of the strongly alkaline first aqueous solution remaining unremoved on the surface of the magnesium alloy can react with a strongly acidic solution to be used in the subsequent step to impede chemical reactions on the surface of the metal and to make the metal surface uneven. Therefore, the water washing can contribute to an improvement in the efficiency of the subsequent step.
- a second aqueous solution having the composition indicated in Table 2 is prepared to remove the magnesium oxide coating formed on the metal surface (S 220 ).
- the additive contained in the second aqueous solution is a wetting agent.
- the wetting agent may be a surfactant for allowing the second aqueous solution to permeate the metal.
- Etching of the surface of the magnesium alloy with the second aqueous solution enables removal of the oxide coating formed on the metal surface at an appropriate rate while preventing the discoloration of the metal.
- water washing can be performed (S 220 ). As explained previously, the water washing can prevent a portion of the second aqueous solution remaining unremoved on the metal surface after removal of the oxide coating from causing damage to the appearance of the metal.
- a third aqueous solution having the composition indicated in Table 3 is prepared to remove smut present on the surface of the magnesium alloy (S 230 ).
- the third aqueous solution is used to remove the smut (desmutting) from the metal surface.
- the additive contained in the third aqueous solution may be a leveling agent that makes the metal surface flat.
- the composition of the third aqueous solution as indicated in Table 3. If the third aqueous solution contains KOH in an amount smaller than the lower limit, only a portion of the metal surface may be desmutted. Meanwhile, if the third aqueous solution contains KOH in an amount larger than the upper limit, a hydroxide coating may be formed on the metal surface. Therefore, it would be desirable that the third aqueous solution falls within the composition indicated in Table 3.
- a coating can be formed on the conversion coating (S 250 ).
- R.N. is a rating from 10 to 0 which represents a degree of corrosion in terms of a ratio between the effective area and the corroded area. The greater the R.N. value, the smaller the corroded area (i.e., less corrosion).
- the inherent color of the surface of the magnesium alloy can be maintained unchanged and the damage to the metal surface can be minimized.
- the magnesium alloy can be used as a material for a variety of devices due to its improved corrosion resistance.
- the conversion coating can be formed in an environmentally friendly manner because a highly toxic conversion coating agent, such as chromate or chromic acid, is not used.
- a conversion coating formed by the method according to the embodiment of the present invention can improve the corrosion resistance of the magnesium or magnesium alloy because of it is highly reactive with an acid and an alkali and shows strong activity.
- the method enables the formation of a conversion coating on the metal surface in an environmentally friendly manner without the need to use of an environmentally harmful substance, such as chromic acid.
Abstract
Description
- 1. Technical Field
- The present invention relates to a method for forming a conversion coating on the outer surface of magnesium or a magnesium alloy. More specifically, the present invention relates to a method for forming a conversion coating on the outer surface of magnesium or a magnesium alloy in an environmentally friendly manner to improve the corrosion resistance of the magnesium or magnesium alloy.
- 2. Description of the Related Art
- Magnesium is one of the most abundant elements found on the earth. Magnesium is the most lightweight structural metal material having a density of about 1.74 g/cm3. Based on these advantages, magnesium is currently used in a variety of applications, including aircraft, automobiles and portable device, and its usages and application are increasingly extended.
- Magnesium alloys, however, have the drawback of poor corrosion resistance on account of their material properties. For better corrosion resistance, magnesium alloys should undergo chemical surface treatment. A series of conversion coating and finish coating steps is one of the most popular chemical treatment processes.
- Conversion coating is a metal surface treatment process in which a coating is formed to a thickness of hundreds of nm on the surface of magnesium by various chemical reactions. The kind of the coating varies depending on a conversion coating solution used. Unlike electroplating wherein external electricity is applied, conversion coating is characterized in that a coating is formed on the surface of a metal by chemical reactions without the need to apply external electricity.
- Such conversion coating processes of magnesium alloys are typically divided into chromate conversion coating and non-chromate conversion coating processes.
- The first chromate conversion coating process, termed Dow1 or Dow21, were developed by Dow Chemical in the 1970s. According to the representative Dow's process, the surface of a metal is treated in an acidic solution containing sodium bichromate as a major component. The acidic solution is easy to handle and manage and the surface-treated metal has the advantages of good corrosion resistance and coating adhesion. However, the presence of hexavalent chromium, which was found to be toxic, in the acidic solution limits the use of the Dow processes.
- Under these circumstances, low-chromate and non-chromate conversion coating processes have been developed. Non-chromate conversion coating processes are known to use various metal salts, such as manganese and zirconium salts, rare earth salts and organic salts. However, coating solutions used in the non-chromate conversion coating processes are highly priced and are difficult to handle.
- Chromate conversion coatings are green or reddish grey and non-chromate conversion coatings are grey or ivory in color. That is, chromate conversion coatings do not sufficiently express the inherent color of magnesium, impairing the attractive appearance of magnesium alloys.
- An object of the present invention is to provide a method for forming a conversion coating on the outer surface of magnesium or a magnesium alloy to improve the corrosion resistance of the magnesium or magnesium alloy.
- Another object of the present invention is to provide a method for forming a conversion coating on the outer surface of magnesium or a magnesium alloy in an environmentally friendly manner.
- Another object of the present invention is to provide a method for forming a conversion coating on the outer surface of magnesium or a magnesium alloy to minimize the damage to the appearance of the magnesium or magnesium even after surface treatment.
- Still another object of the present invention is to provide a method for forming a conversion coating on the outer surface of magnesium or a magnesium alloy to maximize the attractive appearance of the magnesium or magnesium alloy.
- According to an embodiment of the present invention, there is provided a method for forming a conversion coating on the outer surface of magnesium or a magnesium alloy, the method comprising: degreasing the outer surface of magnesium or a magnesium alloy, followed by water washing; etching the degreased magnesium or magnesium alloy to remove an oxide coating present on the outer surface thereof, followed by water washing; removing smut present on the outer surface of the magnesium or magnesium alloy (desmutting), followed by water washing; forming a conversion coating on the desmutted outer surface of the magnesium or magnesium alloy, followed by water washing; and forming a coating on the conversion coating.
- According to the method of the present invention, the corrosion resistance of magnesium or a magnesium alloy is improved, the damage to the appearance of magnesium or a magnesium alloy is minimized even after surface treatment, and the attractive appearance of magnesium or a magnesium alloy can be maximized.
- In the step of degreasing and water washing, the degreasing may be performed using a first aqueous solution containing 30-60 g/L of potassium hydroxide (KOH), 150-200 g/L of sodium hydroxide (NaOH), 20-30 g/L of an amine oxide, 5-10 g/L of sodium gluconate (C6H11NaO7) and a wetting agent. The first aqueous solution can facilitate the removal of oily components from the surface of the magnesium or magnesium alloy and can minimize the discoloration of the metal material.
- In the step of etching and water washing, the etching may be performed using a second aqueous solution containing 600-800 g/L of nitric acid (HNO3), 200-400 g/L of phosphoric acid (H3PO4) and a wetting agent. The second aqueous solution may be diluted with distilled water to a pH of 1.0-2.0. The second aqueous solution may be in the temperature range of about 35 to about 50° C. The second aqueous solution can facilitate the removal of an oxide coating present on the surface of the magnesium or magnesium alloy and can minimize the discoloration of the metal.
- In the step of desmutting and water washing, the desmutting may be performed by immersing the etched metal in a third aqueous solution containing 50-200 g/L of potassium hydroxide (KOH) and 5-10 g/L of a wetting agent. Within this composition, the third aqueous solution can prevent the formation of a hydroxide coating on the surface of the metal while preventing the metal from being desmutted.
- In the step of forming a conversion coating and water washing, the conversion coating may be formed using a fourth aqueous solution containing 600-800 g/L of nitric acid (HNO3), 200-400 g/L of phosphoric acid (H3PO4) and 0.5-10 g/L of hydrofluoric acid (HF). The fourth aqueous solution may be diluted with distilled water to a pH of 2.0-4.0. The conversion coating may be formed in the fourth aqueous solution at 20-60° C. Within the limited temperature range of the fourth aqueous solution, the conversion coating can be formed to a constant thickness at a controlled rate while minimizing the damage to the appearance of the metal.
- These and/or other aspects and advantages of the invention will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which:
-
FIG. 1 is a magnification image showing the surface texture of a magnesium alloy plate whose surface is treated by a method for forming a conversion coating on the outer surface of magnesium or a magnesium alloy according to an embodiment of the present invention; and -
FIG. 2 is a flow chart illustrating a method for forming a conversion coating on the outer surface of magnesium or a magnesium alloy according to an embodiment of the present invention. - Exemplary embodiments of the present invention will now be described in detail with reference to the accompanying drawings. However, these embodiments are not intended to limit the scope of the invention. The same reference numerals denote the same parts throughout the drawings.
-
FIG. 1 is a magnification image showing a surface texture of a magnesium alloy plate whose surface is treated by a method for forming a conversion coating on the outer surface of magnesium or a magnesium alloy according to an embodiment of the present invention, andFIG. 2 is a flow chart illustrating a method for forming a conversion coating on the outer surface of magnesium or a magnesium alloy according to an embodiment of the present invention. - It is to be understood that although an explanation is given of a method for the formation of a conversion coating on the surface of a magnesium alloy in order to explain the drawings, a conversion coating can be formed on the surface of a magnesium plate according to conditions required in the invention.
- Referring to
FIGS. 1 and 2 and Table 1, a first aqueous solution capable of degreasing the surface of a molded magnesium alloy is prepared to remove oils, fats and impurities attached to the molded metal surface and to prevent the discoloration of the metal. -
TABLE 1 Composition Amine Sodium Treatment Treatment KOH NaOH oxide gluconate Additive Temp. time 30-60 g/L 150-200 g/L 20-30 g/L 5-10 g/L 3-5 g/L 35-50° C. 0.5-2 min - The additive contained in the first aqueous solution is a wetting agent. The wetting agent may be a surfactant for allowing the first aqueous solution to permeate the surface of the metal. Various wetting agents may be used depending on conditions required in the invention.
- If the first aqueous solution is out of the composition indicated in Table 1, the removal of oily components present on the surface of the magnesium alloy may be retarded or the metal material may be discolored. Therefore, it would be desirable that the first aqueous solution fall within the composition indicated in Table 1.
- After the removal of oils, fats and impurities from the surface of the magnesium alloy using the first aqueous solution, water washing can be preformed (S210). The water washing enables complete removal of the first aqueous solution remaining unremoved on the surface of the magnesium alloy.
- A portion of the strongly alkaline first aqueous solution remaining unremoved on the surface of the magnesium alloy can react with a strongly acidic solution to be used in the subsequent step to impede chemical reactions on the surface of the metal and to make the metal surface uneven. Therefore, the water washing can contribute to an improvement in the efficiency of the subsequent step.
- After completion of the degreasing, a second aqueous solution having the composition indicated in Table 2 is prepared to remove the magnesium oxide coating formed on the metal surface (S220).
-
TABLE 2 pH range adjusted by Treat- Treat- Composition water ment ment HNO3 H3PO4 Additive dilution temp. time 600-800 g/L 200-400 g/L 5-10 g/L 1.0-2.0 35-50° C. 0.5-2 min - The additive contained in the second aqueous solution is a wetting agent. As explained in the first aqueous solution, the wetting agent may be a surfactant for allowing the second aqueous solution to permeate the metal.
- Etching of the surface of the magnesium alloy with the second aqueous solution enables removal of the oxide coating formed on the metal surface at an appropriate rate while preventing the discoloration of the metal.
- After completion of the etching, water washing can be performed (S220). As explained previously, the water washing can prevent a portion of the second aqueous solution remaining unremoved on the metal surface after removal of the oxide coating from causing damage to the appearance of the metal.
- After completion of the etching and the water washing, a third aqueous solution having the composition indicated in Table 3 is prepared to remove smut present on the surface of the magnesium alloy (S230).
-
TABLE 3 Composition Treatment Treatment KOH Additive Temp. Time 50-200 g/L 5-10 g/L 35-50° C. 0.5-2 min - The third aqueous solution is used to remove the smut (desmutting) from the metal surface. The additive contained in the third aqueous solution may be a leveling agent that makes the metal surface flat.
- It would be desirable to adjust the composition of the third aqueous solution as indicated in Table 3. If the third aqueous solution contains KOH in an amount smaller than the lower limit, only a portion of the metal surface may be desmutted. Meanwhile, if the third aqueous solution contains KOH in an amount larger than the upper limit, a hydroxide coating may be formed on the metal surface. Therefore, it would be desirable that the third aqueous solution falls within the composition indicated in Table 3.
- After removal of the smut from the surface of the magnesium alloy, water washing is performed on the magnesium alloy (S230). Thereafter, a fourth aqueous solution having the composition indicated in Table 3 is prepared to form a conversion coating on the outer surface of the magnesium alloy (S240).
-
TABLE 4 pH range adjusted by Treat- Composition water ment Treatment HNO3 H3PO4 HF dilution Temp. Time 600-800 g/L 200-400 g/L 0.5-10 g/L 2.0-4.0 20-60° C. 10 sec-4 min - By the use of the fourth aqueous solution, a coating can be formed on the conversion coating (S250).
- Thereafter, a salt spray test is conducted to evaluate the corrosion resistance of the metal and a test is conducted to evaluate the adhesion between the coating and the metal. The results are shown in Table 5. In Table 5, R.N. is a rating from 10 to 0 which represents a degree of corrosion in terms of a ratio between the effective area and the corroded area. The greater the R.N. value, the smaller the corroded area (i.e., less corrosion).
-
TABLE 5 Step Cross Cut Salt spray time R.N. 1 Coating after formation 100/100 24 hr 10.0 of conversion coating 36 hr 9.4 - The inherent color of the surface of the magnesium alloy can be maintained unchanged and the damage to the metal surface can be minimized. The magnesium alloy can be used as a material for a variety of devices due to its improved corrosion resistance. The conversion coating can be formed in an environmentally friendly manner because a highly toxic conversion coating agent, such as chromate or chromic acid, is not used.
- As is apparent from the foregoing, a conversion coating formed by the method according to the embodiment of the present invention can improve the corrosion resistance of the magnesium or magnesium alloy because of it is highly reactive with an acid and an alkali and shows strong activity.
- In addition, the method enables the formation of a conversion coating on the metal surface in an environmentally friendly manner without the need to use of an environmentally harmful substance, such as chromic acid.
- Furthermore, even after the treatment of the outer surface of magnesium or a magnesium alloy, the inherent color of the magnesium can be maintained unchanged.
- Although the present invention has been described herein with reference to its preferred embodiments, these embodiments do not serve to limit the invention. Accordingly, those skilled in the art will appreciate that various modifications are possible without departing from the spirit of the present invention as disclosed in the accompanying claims.
- The various embodiments described above can be combined to provide further embodiments. All of the U.S. patents, U.S. patent application publications, U.S. patent application, foreign patents, foreign patent application and non-patent publications referred to in this specification and/or listed in the Application Data Sheet are incorporated herein by reference, in their entirety. Aspects of the embodiments can be modified, if necessary to employ concepts of the various patents, application and publications to provide yet further embodiments.
- These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.
Claims (7)
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KR1020090113144A KR101067744B1 (en) | 2009-11-23 | 2009-11-23 | Method for conversion coating of surface to magnesium or magnesium alloy |
KR10-2009-0113144 | 2009-11-23 |
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US20110120595A1 true US20110120595A1 (en) | 2011-05-26 |
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US12/785,235 Abandoned US20110120595A1 (en) | 2009-11-23 | 2010-05-21 | Method for forming conversion coating on outer surface of magnesium or magnesium alloy |
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US20040244875A1 (en) * | 2003-06-09 | 2004-12-09 | Mitsuhiro Yasuda | Method of surface treating metal and metal surface treated thereby |
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JPH06220663A (en) * | 1993-01-13 | 1994-08-09 | Nippon Light Metal Co Ltd | Smut removal from mg alloy surface |
JPH06330341A (en) * | 1993-05-25 | 1994-11-29 | Nippon Paint Co Ltd | Pretreatment of magnesium or magnesium alloy material before coating |
JP2001288580A (en) * | 2000-03-31 | 2001-10-19 | Nippon Parkerizing Co Ltd | Surface treating method for magnesium alloy and magnesium alloy member |
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