EP3597795A1 - Anodization method for corrosion protection of aluminium or aluminium alloy elements used in an aircraft structure - Google Patents
Anodization method for corrosion protection of aluminium or aluminium alloy elements used in an aircraft structure Download PDFInfo
- Publication number
- EP3597795A1 EP3597795A1 EP19187128.4A EP19187128A EP3597795A1 EP 3597795 A1 EP3597795 A1 EP 3597795A1 EP 19187128 A EP19187128 A EP 19187128A EP 3597795 A1 EP3597795 A1 EP 3597795A1
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- EP
- European Patent Office
- Prior art keywords
- subjecting
- dipping
- subsequent
- anodization
- water
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- 238000000034 method Methods 0.000 title claims abstract description 31
- 238000002048 anodisation reaction Methods 0.000 title claims abstract description 28
- 229910052782 aluminium Inorganic materials 0.000 title claims abstract description 16
- XAGFODPZIPBFFR-UHFFFAOYSA-N aluminium Chemical compound [Al] XAGFODPZIPBFFR-UHFFFAOYSA-N 0.000 title claims abstract description 16
- 230000007797 corrosion Effects 0.000 title claims abstract description 16
- 238000005260 corrosion Methods 0.000 title claims abstract description 16
- 239000004411 aluminium Substances 0.000 title claims abstract description 14
- 229910000838 Al alloy Inorganic materials 0.000 title claims abstract description 13
- 238000007598 dipping method Methods 0.000 claims abstract description 30
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 claims abstract description 30
- 238000005406 washing Methods 0.000 claims abstract description 20
- 239000002253 acid Substances 0.000 claims abstract description 16
- 239000011651 chromium Substances 0.000 claims abstract description 12
- VYZAMTAEIAYCRO-UHFFFAOYSA-N Chromium Chemical compound [Cr] VYZAMTAEIAYCRO-UHFFFAOYSA-N 0.000 claims abstract description 10
- QAOWNCQODCNURD-UHFFFAOYSA-N Sulfuric acid Chemical compound OS(O)(=O)=O QAOWNCQODCNURD-UHFFFAOYSA-N 0.000 claims abstract description 10
- 229910052804 chromium Inorganic materials 0.000 claims abstract description 10
- 230000003647 oxidation Effects 0.000 claims abstract description 10
- 238000007254 oxidation reaction Methods 0.000 claims abstract description 10
- 238000007789 sealing Methods 0.000 claims abstract description 10
- FGUUSXIOTUKUDN-IBGZPJMESA-N C1(=CC=CC=C1)N1C2=C(NC([C@H](C1)NC=1OC(=NN=1)C1=CC=CC=C1)=O)C=CC=C2 Chemical compound C1(=CC=CC=C1)N1C2=C(NC([C@H](C1)NC=1OC(=NN=1)C1=CC=CC=C1)=O)C=CC=C2 FGUUSXIOTUKUDN-IBGZPJMESA-N 0.000 claims abstract description 7
- FEWJPZIEWOKRBE-UHFFFAOYSA-N Tartaric acid Natural products [H+].[H+].[O-]C(=O)C(O)C(O)C([O-])=O FEWJPZIEWOKRBE-UHFFFAOYSA-N 0.000 claims abstract description 6
- 239000001117 sulphuric acid Substances 0.000 claims abstract description 6
- 235000011149 sulphuric acid Nutrition 0.000 claims abstract description 6
- 235000002906 tartaric acid Nutrition 0.000 claims abstract description 6
- 239000011975 tartaric acid Substances 0.000 claims abstract description 6
- -1 zirconium ions Chemical class 0.000 claims abstract description 6
- 238000009835 boiling Methods 0.000 claims abstract description 5
- 238000005238 degreasing Methods 0.000 claims abstract description 5
- 238000001035 drying Methods 0.000 claims abstract description 5
- 238000005554 pickling Methods 0.000 claims abstract description 5
- 229910052726 zirconium Inorganic materials 0.000 claims abstract description 5
- PNEYBMLMFCGWSK-UHFFFAOYSA-N aluminium oxide Inorganic materials [O-2].[O-2].[O-2].[Al+3].[Al+3] PNEYBMLMFCGWSK-UHFFFAOYSA-N 0.000 claims description 11
- 238000006243 chemical reaction Methods 0.000 claims description 11
- 229910052593 corundum Inorganic materials 0.000 claims description 7
- 229910001845 yogo sapphire Inorganic materials 0.000 claims description 7
- 238000003487 electrochemical reaction Methods 0.000 claims description 6
- 150000003839 salts Chemical class 0.000 claims description 6
- 150000004673 fluoride salts Chemical class 0.000 claims description 5
- 238000003486 chemical etching Methods 0.000 claims description 2
- 150000004760 silicates Chemical class 0.000 claims description 2
- GNFTZDOKVXKIBK-UHFFFAOYSA-N 3-(2-methoxyethoxy)benzohydrazide Chemical compound COCCOC1=CC=CC(C(=O)NN)=C1 GNFTZDOKVXKIBK-UHFFFAOYSA-N 0.000 claims 1
- 150000002222 fluorine compounds Chemical class 0.000 abstract 1
- 239000000243 solution Substances 0.000 description 13
- 239000010410 layer Substances 0.000 description 7
- KRVSOGSZCMJSLX-UHFFFAOYSA-L chromic acid Substances O[Cr](O)(=O)=O KRVSOGSZCMJSLX-UHFFFAOYSA-L 0.000 description 5
- WGLPBDUCMAPZCE-UHFFFAOYSA-N Trioxochromium Chemical compound O=[Cr](=O)=O WGLPBDUCMAPZCE-UHFFFAOYSA-N 0.000 description 4
- 230000000711 cancerogenic effect Effects 0.000 description 4
- 150000001875 compounds Chemical class 0.000 description 4
- 239000003973 paint Substances 0.000 description 4
- HEMHJVSKTPXQMS-UHFFFAOYSA-M Sodium hydroxide Chemical compound [OH-].[Na+] HEMHJVSKTPXQMS-UHFFFAOYSA-M 0.000 description 3
- 229910045601 alloy Inorganic materials 0.000 description 3
- 239000000956 alloy Substances 0.000 description 3
- 231100000315 carcinogenic Toxicity 0.000 description 3
- JOPOVCBBYLSVDA-UHFFFAOYSA-N chromium(6+) Chemical compound [Cr+6] JOPOVCBBYLSVDA-UHFFFAOYSA-N 0.000 description 3
- 239000000463 material Substances 0.000 description 3
- 230000001590 oxidative effect Effects 0.000 description 3
- 239000011241 protective layer Substances 0.000 description 3
- 239000007921 spray Substances 0.000 description 3
- 231100000331 toxic Toxicity 0.000 description 3
- 230000002588 toxic effect Effects 0.000 description 3
- MHAJPDPJQMAIIY-UHFFFAOYSA-N Hydrogen peroxide Chemical compound OO MHAJPDPJQMAIIY-UHFFFAOYSA-N 0.000 description 2
- FYYHWMGAXLPEAU-UHFFFAOYSA-N Magnesium Chemical compound [Mg] FYYHWMGAXLPEAU-UHFFFAOYSA-N 0.000 description 2
- FAPWRFPIFSIZLT-UHFFFAOYSA-M Sodium chloride Chemical compound [Na+].[Cl-] FAPWRFPIFSIZLT-UHFFFAOYSA-M 0.000 description 2
- QAOWNCQODCNURD-UHFFFAOYSA-L Sulfate Chemical compound [O-]S([O-])(=O)=O QAOWNCQODCNURD-UHFFFAOYSA-L 0.000 description 2
- 239000003929 acidic solution Substances 0.000 description 2
- 150000007513 acids Chemical class 0.000 description 2
- 230000015572 biosynthetic process Effects 0.000 description 2
- NINOVVRCHXVOKB-UHFFFAOYSA-N dialuminum;dioxido(dioxo)chromium Chemical compound [Al+3].[Al+3].[O-][Cr]([O-])(=O)=O.[O-][Cr]([O-])(=O)=O.[O-][Cr]([O-])(=O)=O NINOVVRCHXVOKB-UHFFFAOYSA-N 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
- 230000036571 hydration Effects 0.000 description 2
- 238000006703 hydration reaction Methods 0.000 description 2
- 239000000314 lubricant Substances 0.000 description 2
- 229910052749 magnesium Inorganic materials 0.000 description 2
- 239000011777 magnesium Substances 0.000 description 2
- 230000007246 mechanism Effects 0.000 description 2
- 229910052751 metal Inorganic materials 0.000 description 2
- 239000002184 metal Substances 0.000 description 2
- 229910044991 metal oxide Inorganic materials 0.000 description 2
- 150000004706 metal oxides Chemical class 0.000 description 2
- 239000000203 mixture Substances 0.000 description 2
- 239000003921 oil Substances 0.000 description 2
- 229910021653 sulphate ion Inorganic materials 0.000 description 2
- 206010058467 Lung neoplasm malignant Diseases 0.000 description 1
- MXRIRQGCELJRSN-UHFFFAOYSA-N O.O.O.[Al] Chemical compound O.O.O.[Al] MXRIRQGCELJRSN-UHFFFAOYSA-N 0.000 description 1
- 206010073310 Occupational exposures Diseases 0.000 description 1
- QCWXUUIWCKQGHC-UHFFFAOYSA-N Zirconium Chemical compound [Zr] QCWXUUIWCKQGHC-UHFFFAOYSA-N 0.000 description 1
- 238000010521 absorption reaction Methods 0.000 description 1
- 230000001154 acute effect Effects 0.000 description 1
- 239000012670 alkaline solution Substances 0.000 description 1
- 230000004075 alteration Effects 0.000 description 1
- 239000001164 aluminium sulphate Substances 0.000 description 1
- 235000011128 aluminium sulphate Nutrition 0.000 description 1
- VXAUWWUXCIMFIM-UHFFFAOYSA-M aluminum;oxygen(2-);hydroxide Chemical compound [OH-].[O-2].[Al+3] VXAUWWUXCIMFIM-UHFFFAOYSA-M 0.000 description 1
- 239000010407 anodic oxide Substances 0.000 description 1
- 238000007743 anodising Methods 0.000 description 1
- 229910001593 boehmite Inorganic materials 0.000 description 1
- ZCDOYSPFYFSLEW-UHFFFAOYSA-N chromate(2-) Chemical class [O-][Cr]([O-])(=O)=O ZCDOYSPFYFSLEW-UHFFFAOYSA-N 0.000 description 1
- 150000001845 chromium compounds Chemical class 0.000 description 1
- BFGKITSFLPAWGI-UHFFFAOYSA-N chromium(3+) Chemical group [Cr+3] BFGKITSFLPAWGI-UHFFFAOYSA-N 0.000 description 1
- 230000001684 chronic effect Effects 0.000 description 1
- 239000011248 coating agent Substances 0.000 description 1
- 238000000576 coating method Methods 0.000 description 1
- 238000007739 conversion coating Methods 0.000 description 1
- BUACSMWVFUNQET-UHFFFAOYSA-H dialuminum;trisulfate;hydrate Chemical compound O.[Al+3].[Al+3].[O-]S([O-])(=O)=O.[O-]S([O-])(=O)=O.[O-]S([O-])(=O)=O BUACSMWVFUNQET-UHFFFAOYSA-H 0.000 description 1
- 230000000694 effects Effects 0.000 description 1
- 238000009661 fatigue test Methods 0.000 description 1
- 239000004519 grease Substances 0.000 description 1
- 230000036541 health Effects 0.000 description 1
- 231100000003 human carcinogen Toxicity 0.000 description 1
- 150000004679 hydroxides Chemical class 0.000 description 1
- 239000008235 industrial water Substances 0.000 description 1
- 239000003112 inhibitor Substances 0.000 description 1
- 230000002401 inhibitory effect Effects 0.000 description 1
- 230000037041 intracellular level Effects 0.000 description 1
- 201000005202 lung cancer Diseases 0.000 description 1
- 208000020816 lung neoplasm Diseases 0.000 description 1
- 229910001092 metal group alloy Inorganic materials 0.000 description 1
- 231100000219 mutagenic Toxicity 0.000 description 1
- 230000003505 mutagenic effect Effects 0.000 description 1
- 231100000675 occupational exposure Toxicity 0.000 description 1
- 238000010422 painting Methods 0.000 description 1
- 239000011148 porous material Substances 0.000 description 1
- 238000003825 pressing Methods 0.000 description 1
- 230000008569 process Effects 0.000 description 1
- 239000008213 purified water Substances 0.000 description 1
- 229910052761 rare earth metal Inorganic materials 0.000 description 1
- 230000009467 reduction Effects 0.000 description 1
- 238000011946 reduction process Methods 0.000 description 1
- 210000002345 respiratory system Anatomy 0.000 description 1
- 239000012266 salt solution Substances 0.000 description 1
- 239000011780 sodium chloride Substances 0.000 description 1
- 235000011121 sodium hydroxide Nutrition 0.000 description 1
- 238000001179 sorption measurement Methods 0.000 description 1
- 239000000126 substance Substances 0.000 description 1
- 238000004381 surface treatment Methods 0.000 description 1
- 231100000419 toxicity Toxicity 0.000 description 1
- 230000001988 toxicity Effects 0.000 description 1
- 238000011179 visual inspection Methods 0.000 description 1
Images
Classifications
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- 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
- C25D11/04—Anodisation of aluminium or alloys based thereon
- C25D11/18—After-treatment, e.g. pore-sealing
- C25D11/24—Chemical after-treatment
-
- 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
- C25D11/04—Anodisation of aluminium or alloys based thereon
- C25D11/06—Anodisation of aluminium or alloys based thereon characterised by the electrolytes used
- C25D11/08—Anodisation of aluminium or alloys based thereon characterised by the electrolytes used containing inorganic acids
-
- 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
- C23G1/125—Light metals aluminium
-
- 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
-
- 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
- C25D11/024—Anodisation under pulsed or modulated current or potential
-
- 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
- C25D11/04—Anodisation of aluminium or alloys based thereon
- C25D11/06—Anodisation of aluminium or alloys based thereon characterised by the electrolytes used
- C25D11/10—Anodisation of aluminium or alloys based thereon characterised by the electrolytes used containing organic acids
-
- 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
- C25D11/04—Anodisation of aluminium or alloys based thereon
- C25D11/16—Pretreatment, e.g. desmutting
-
- 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
- C25D11/04—Anodisation of aluminium or alloys based thereon
- C25D11/18—After-treatment, e.g. pore-sealing
- C25D11/24—Chemical after-treatment
- C25D11/246—Chemical after-treatment for sealing layers
-
- C—CHEMISTRY; METALLURGY
- C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
- C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
- C25D5/00—Electroplating characterised by the process; Pretreatment or after-treatment of workpieces
- C25D5/10—Electroplating with more than one layer of the same or of different metals
- C25D5/12—Electroplating with more than one layer of the same or of different metals at least one layer being of nickel or chromium
-
- C—CHEMISTRY; METALLURGY
- C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
- C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
- C25D5/00—Electroplating characterised by the process; Pretreatment or after-treatment of workpieces
- C25D5/34—Pretreatment of metallic surfaces to be electroplated
- C25D5/42—Pretreatment of metallic surfaces to be electroplated of light metals
- C25D5/44—Aluminium
-
- C—CHEMISTRY; METALLURGY
- C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
- C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
- C25D5/00—Electroplating characterised by the process; Pretreatment or after-treatment of workpieces
- C25D5/48—After-treatment of electroplated surfaces
- C25D5/50—After-treatment of electroplated surfaces by heat-treatment
Definitions
- the present invention relates to an anodization method for corrosion protection of aluminium or aluminium alloy elements used in an aircraft structure.
- anodization methods have been developed to provide a thin (a few microns) protective layer of metal oxide, which protects the underlying aluminium/aluminium alloy from corrosion.
- This layer of metal oxide also facilitates the subsequent painting of the aircraft structures and also increases the surface electrical resistance of the aircraft structure.
- the known anodization processes comprise a plurality of steps, including:
- This method uses very dangerous compounds, such as H 2 CrO 4 , commonly called chromic acid where the chromium has an oxidation number of +6; it is a highly oxidizing species.
- Aluminium chromate will also be formed according to the following mechanism: chromic anhydride -> chromic acid -> aluminium chromate CrO 3 -> H 2 CrO 4 -> Al 2 (CrO 4 ) 3
- chromium with an oxidation number of +6 has been classified by IARC as a human carcinogen (Class I).
- hexavalent chromium is one of the possible causes of lung cancer, as it is mutagenic and carcinogenic.
- the respiratory system is the main target of the toxic and carcinogenic action, and acute and chronic occupational exposure occurs above all by absorption through inhalation.
- the toxicity of the hexavalent form at the intracellular level appears above all with the numerous molecular and structural alterations caused by the unstable [Cr(V) and Cr(IV)] and stable [Cr(III)] forms resulting from the reduction process.
- US2015020925 describes a method for the surface treatment of an aluminum, magnesium or one of its alloys, to protect the part from corrosion.
- the method consists in consecutively immersing the part in the following two baths:
- the method can be performed for the chemical conversion of aluminum or its alloys and magnesium or its alloys, on parts that have not been previously treated or after anodizing the piece to seal the anodic layer.
- the anodization method for corrosion protection of an aluminium or aluminium alloy element used in an aircraft structure comprises the following steps:
- Drying the element (block 180).
- the drying can be carried out in a dust-free environment, for example an oven that reaches a temperature between 45 and 65 degrees Celsius for at least 20 minutes.
- step b) and step d) the following further steps may also be carried out, optional to step c), which is carried out anyway: rapid alkaline chemical etching (block 200) with a caustic soda based solution in order to prepare/activate the aluminium/aluminium alloy surfaces for the electrochemical treatment in step d).
- step c rapid alkaline chemical etching
- this step is carried out by dipping the element in an alkaline solution having a temperature of approximately 60 degrees for 30-60 seconds;
- Aluminium desmutting and rinsing (block 210).
- this step of clearing the blackening due to the preceding alkaline bath is performed by dipping the element in a clearing solution (based on ferric sulphate and a mixture of acids) at room temperature (approximately 25 degrees) for 5-10 minutes.
- a clearing solution based on ferric sulphate and a mixture of acids
- the method of the present invention in particular the sealing (step f), does not use highly toxic, and in particular carcinogenic compounds, such as chromium with an oxidation number of +6.
- the sealed oxide layer has a sensibly constant thickness of a few microns (typically from 2 to 7 microns) and good adhesion characteristics. The elements are therefore effectively protected from corrosion.
- the Applicant performed a series of mechanical fatigue tests on samples subjected to the method of the present invention.
- the tests were carried out on cylindrical specimens treated according to the method of the present invention in accordance with standard EN6072 provided in the aeronautical field. Results were plotted in a Wohler curve at various load levels.
- Tests for adhesion of the paint to the elements treated according to the method of the present invention were also carried out. These tests were carried out in accordance with ISO 2409 standard after dipping in demineralized water at 23 degrees Celsius for 14 days. The test consists in squaring off, after the dipping in water, the paint layer with a six-blade cutter, arranging and pressing a layer of tape against the squared paint and then tearing off the tape quickly. The tests gave a positive result, with a detachment of the coating paint of less than 5%.
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- Chemical & Material Sciences (AREA)
- Engineering & Computer Science (AREA)
- Chemical Kinetics & Catalysis (AREA)
- Materials Engineering (AREA)
- Metallurgy (AREA)
- Organic Chemistry (AREA)
- Electrochemistry (AREA)
- General Chemical & Material Sciences (AREA)
- Mechanical Engineering (AREA)
- Inorganic Chemistry (AREA)
- Chemical Treatment Of Metals (AREA)
- Cleaning And De-Greasing Of Metallic Materials By Chemical Methods (AREA)
Abstract
Description
- This patent application claims priority from Italian patent application no.
102018000007314 filed on July 18, 2018 - The present invention relates to an anodization method for corrosion protection of aluminium or aluminium alloy elements used in an aircraft structure.
- As is known, in order to protect aluminium or aluminium alloy elements used in an aircraft structure, anodization methods have been developed to provide a thin (a few microns) protective layer of metal oxide, which protects the underlying aluminium/aluminium alloy from corrosion. This layer of metal oxide also facilitates the subsequent painting of the aircraft structures and also increases the surface electrical resistance of the aircraft structure.
- Typically, the known anodization processes comprise a plurality of steps, including:
- a) subjecting the element to a degreasing step by means of an alkaline bath for removing contaminating elements, such as for example oils, fats, lubricants, protective layers, dusts and residues in general - then subjecting it to a first washing in water;
- b) subjecting the element to an acid pickling step. Then extracting the element from the acid solution and subjecting the element to a subsequent washing in water. This step contributes to the removal of natural oxide, thermal oxides, traces of materials deposited as a result of mechanical processing, scratches, discolourations, mild corrosion;
- c) subjecting the washed element to a subsequent electrochemical treatment step by dipping the element in a chromic acid solution (using chromium with an oxidation number of +6 - hexavalent chromium) and applying an electric potential to this element;
- d) subjecting the element to a subsequent second washing in water;
- e) dipping the element in a bath in which a solution of a chromium compound with an oxidation number of +6 (hexavalent chromium) is present, in order to carry out a post-anodization sealing step;
- f) extracting the element from the bath of step e) and subjecting it to a third final washing.
- This method uses very dangerous compounds, such as H2CrO4, commonly called chromic acid where the chromium has an oxidation number of +6; it is a highly oxidizing species.
- The chemical reaction that occurs is the following: Electrochemical reaction at the anode:
2Al + 3H2O = Al2O3 + 6H+ + 6e-
- Electrochemical reaction at the cathode:
6H+ + 6e- = 3H2
- Resulting anodization reaction:
2Al + 3H2O => Al2O3 + 3H2
- Aluminium chromate will also be formed according to the following mechanism:
chromic anhydride -> chromic acid -> aluminium chromate CrO3 -> H2CrO4 -> Al2(CrO4)3
- Furthermore, on the basis of experimental and epidemiological evidence, chromium with an oxidation number of +6 (hexavalent chromium) has been classified by IARC as a human carcinogen (Class I).
- With regard to the effects on health, several studies have demonstrated that exposure to hexavalent chromium is one of the possible causes of lung cancer, as it is mutagenic and carcinogenic. In fact, the respiratory system is the main target of the toxic and carcinogenic action, and acute and chronic occupational exposure occurs above all by absorption through inhalation. The toxicity of the hexavalent form at the intracellular level appears above all with the numerous molecular and structural alterations caused by the unstable [Cr(V) and Cr(IV)] and stable [Cr(III)] forms resulting from the reduction process.
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US2015020925 describes a method for the surface treatment of an aluminum, magnesium or one of its alloys, to protect the part from corrosion. The method consists in consecutively immersing the part in the following two baths: - first aqueous bath at low temperatures containing a corrosion inhibiting metal salt and an oxidizing compound to form a conversion coating on the surface of the part containing oxides, hydroxides and fluorides based on Zirconium / Chromium;
- a second aqueous bath, kept at a temperature below 80 ° C and containing an oxidizing compound and a rare earth salt corrosion inhibitor. Through the presence of hydrogen peroxide, the oxidation of chromium is obtained from the trivalent to the hexavalent form and the formation of chromates.
- The method can be performed for the chemical conversion of aluminum or its alloys and magnesium or its alloys, on parts that have not been previously treated or after anodizing the piece to seal the anodic layer.
- Therefore, there is a need to develop a method which does not use toxic/carcinogenic materials and allows the formation of an oxide layer that provides good protection to the underlying aluminium/aluminium alloy.
- The above object is achieved by the present invention in so far as it relates to an anodization method for corrosion protection of an aluminium or aluminium alloy element used in an aircraft structure, comprising the following steps:
- a) subjecting the element to a degreasing step by means of an alkaline bath (block 100) for removing contaminating elements;
- b) subjecting the element to a subsequent first washing in water (block 110);
- c) subjecting the element to an acid pickling step (120) by dipping the element in an acid solution and then extracting the element from the acid solution and subjecting the element to a subsequent washing in water;
- d) subjecting the washed element to a subsequent electrochemical treatment step in a tank (140) by dipping the element in a solution of tartaric acid (C4H6O6) and sulphuric acid (H2SO4) and applying an electric potential to said element;
- e) subjecting the element to a subsequent second washing in water (150);
- f) dipping (block 170) the element in a bath in which a solution of chromium, with an oxidation number of +3, and zirconium ions and fluorides is present, in order to carry out a post-anodization sealing step;
- g) extracting the element from the bath of step f) and subjecting it to a third final washing in water and a subsequent dipping in a tank of boiling water, which provides a second sealing step, and then drying the element (block 180) .
- The invention will now be illustrated with reference to the accompanying figures wherein:
-
Figure 1 represents a non-limiting embodiment showing the main steps of the method according to the present invention; and -
Figure 2 specifies one step of the method of the present invention. - With reference to
Figure 1 , the anodization method for corrosion protection of an aluminium or aluminium alloy element used in an aircraft structure comprises the following steps: - a) subjecting the element to a degreasing step by means of an alkaline bath (block 100) for removing contaminating elements, such as for example oils, fats, lubricants, protective layers, dusts and residues in general. Typically, step a) is carried out by dipping the element in the alkaline bath for a time interval of 10 - 20 minutes. Typically, the alkaline bath has a temperature of approximately 55 ± 5 degrees Celsius.
- b) subjecting the element to a subsequent first washing in water (block 110). Typically, step b) is carried out with water for a time ranging from 2 to 5 minutes at a temperature below 35 degrees Celsius.
- c) subjecting the element to an acid pickling step (block 120) by dipping the element for approximately 5-10 minutes in an acidic solution based on ferric sulphate and a mixture of acids maintained at a temperature comprised between 20°C and 40°C and then extracting the element from the acidic solution and subjecting the element to a subsequent washing in water (
block 130 following block 120) for 4 - 10 minutes at room temperature and assessing the film of water. Checking by assessing the film of water on the surface of the part during the rinsing ensures the effectiveness of the pretreatment. Step c) contributes to the removal of natural oxide, thermal oxides, traces of materials deposited as a result of mechanical processing, scratches, discolourations, mild corrosion; - d) subjecting the washed element to a subsequent electrochemical treatment step in a tank (block 140) by dipping the element in a solution of tartaric acid (C4H6O6) and sulphuric acid (H2SO4) and applying an electric potential, wherein the aluminium/aluminium alloy element behaves as the anode (positive pole) whereas the negative electrode (or cathode) is represented by the tank. Step d) is carried out with a solution having a temperature comprised between 36 and 39 degrees Celsius. The typical concentration of tartaric acid is 72-88 g/l and that of sulphuric acid is 36-44 g/l.
The chemical reaction occurring in step d) is the following:- Electrochemical reaction at the anode:
2Al + 3H2O = Al2O3 + 6H+ + 6e-
- Electrochemical reaction at the cathode:
6H+ + 6e- = 3H2
- Resulting anodization reaction:
2Al + 3H2O => Al2O3 + 3H2
- Aluminium sulphate, which contributes to the protection of the underlying metal/metal alloy, will also be formed according to the following mechanism:
3H2SO4 + 2Al ←→ Al2 (SO4)3 + 3H2
Figure 2 ):- applying on the element the voltage within one minute from the dipping of the element in the solution;
- subsequently and without interruption applying on the element an increasing voltage with a ramp not exceeding 3 volts per minute;
- subsequently and without interruption applying on the element a constant voltage (approximately 14 Volts) for approximately 20 minutes, and thereafter;
- gradually reducing the applied voltage to a null value in approximately one minute;
- removing the element from the solution within 3 minutes from the switching off of the voltage
- Electrochemical reaction at the anode:
- e) subjecting the element to a subsequent second washing in water (block 150). Typically, step e) comprises an optional rinsing step by dipping in industrial water (
block 150a) at room temperature and a subsequent rinsing step by dipping in purified water at room temperature (block 150b).
Subsequently, the washed element is subjected to visual inspection by an operator (block 160) and, if this step is fulfilled, the following step is carried out: - f) dipping (block 170) the element in a bath in which chromium, with an oxidation number of +3, zirconium ions and fluorides (resulting from salts and fluorozirconates/silicates) are present, in order to carry out a first post-anodization sealing step;
The chemical reaction that occurs is the following:
4Al2O3 + 24F- + 3Zr+4 + 4Cr+3 → 8AlF3 + 3ZrO2 + 2Cr2O3
Typically, the dipping time in step f) is comprised between 2 and 20 minutes. It is suggested a time comprised between 2 and 3 minutes. - g) extracting the element from the bath of step f) and subjecting it to a third final washing and a subsequent dipping in a tank of boiling water (temperature comprised between 95 and 100 degrees Celsius, pH ranging from 4.5 to 7, for approximately 30 minutes), which provides a second sealing step, according to the following reaction:
Al2O3 + H2O -> 2 AlO (OH) (an aluminium oxide hydroxide-böhmite) - Drying the element (block 180). Typically, the drying can be carried out in a dust-free environment, for example an oven that reaches a temperature between 45 and 65 degrees Celsius for at least 20 minutes.
- Between step b) and step d) the following further steps may also be carried out, optional to step c), which is carried out anyway:
rapid alkaline chemical etching (block 200) with a caustic soda based solution in order to prepare/activate the aluminium/aluminium alloy surfaces for the electrochemical treatment in step d). Typically, this step is carried out by dipping the element in an alkaline solution having a temperature of approximately 60 degrees for 30-60 seconds; and - Aluminium desmutting and rinsing (block 210). Typically, this step of clearing the blackening due to the preceding alkaline bath is performed by dipping the element in a clearing solution (based on ferric sulphate and a mixture of acids) at room temperature (approximately 25 degrees) for 5-10 minutes.
- From the above description it appears that the method of the present invention, in particular the sealing (step f), does not use highly toxic, and in particular carcinogenic compounds, such as chromium with an oxidation number of +6. The sealed oxide layer has a sensibly constant thickness of a few microns (typically from 2 to 7 microns) and good adhesion characteristics. The elements are therefore effectively protected from corrosion.
- The Applicant performed a series of mechanical fatigue tests on samples subjected to the method of the present invention. The tests were carried out on cylindrical specimens treated according to the method of the present invention in accordance with standard EN6072 provided in the aeronautical field. Results were plotted in a Wohler curve at various load levels.
- By comparing the data, the treatment according to the present method was thus shown not to alter the fatigue resistance of the sample, which complies with the CS 25.571 aeronautical directives. Corrosion tests, which were carried out in a salt spray chamber for ASTM B 117, also performed well. In this case, a salt spray chamber was used, which was capable of providing a controlled saline environment fed by a 5% NaCl salt solution. Tests for permanence in the salt spray for 336 hours were brilliantly passed.
- Tests for adhesion of the paint to the elements treated according to the method of the present invention were also carried out. These tests were carried out in accordance with ISO 2409 standard after dipping in demineralized water at 23 degrees Celsius for 14 days. The test consists in squaring off, after the dipping in water, the paint layer with a six-blade cutter, arranging and pressing a layer of tape against the squared paint and then tearing off the tape quickly. The tests gave a positive result, with a detachment of the coating paint of less than 5%.
Claims (12)
- An anodization method for corrosion protection of an aluminium or aluminium alloy element used in an aircraft structure,
comprising the following steps:a) subjecting the element to a degreasing step by means of an alkaline bath (block 100) for removing contaminating elements;b) subjecting the element to a subsequent first washing in water (block 110);c) subjecting the element to an acid pickling step (120) by dipping the element in an acid solution and then extracting the element from the acid solution and subjecting the element to a subsequent washing in water;d) subjecting the washed element to a subsequent anodization step in a tank (140) by dipping the element in a solution of tartaric acid (C4H6O6) and sulphuric acid (H2SO4) and applying an electric potential to said element that acts as anode;e) subjecting the element to a subsequent second washing in water (150);f) dipping (block 170) the element in a bath in which a solution of chromium, with an oxidation number of +3, and zirconium ions and fluorides is present, in order to carry out a post-anodization sealing step;g) extracting the element from the bath of step f) and subjecting it to a third final washing in water and a subsequent dipping in a tank of water having a temperature comprised between 95° and 100°, which provides a second sealing step, and then drying the element (block 180). - The anodization method as defined in claim 1, wherein step c) is carried out by dipping the element in an acid bath for a time interval of 5 to 10 minutes.
- The anodization method as defined in claim 1 or 2, wherein step c) is carried out by dipping the element in an acid bath having a temperature of 20°C to 40°C.
- The anodization method as defined in any one of the preceding claims, wherein step d) is configured to perform the following chemical reactions:Electrochemical reaction at the anode:
2Al + 3H2O = Al2O3 + 6H+ + 6e-
Electrochemical reaction at the cathode:
6H+ + 6e- = 3H2
Resulting anodization reaction:
2Al + 3H2O => Al2O3 + 3H2
- The anodization method as defined in any one of the preceding claims, wherein step d) is carried out using the following parameters:applying to the element the voltage within one minute from the dipping of the element in the solution;applying to the element an increasing voltage with a ramp not exceeding 3 volts per minute;subsequently and without interruption applying to the element a constant voltage for approximately 20 minutes, and thereafter;gradually reducing the applied voltage to a null value;removing the element from the solution within 3 minutes from the switching off of the voltage.
- The anodization method as defined in any one of the preceding claims, wherein step d) is carried out using a solution having a temperature ranging between 36°C and 39°C.
- The anodization method as defined in any one of the preceding claims, wherein step g) is carried out in a tank of boiling water with a temperature higher than 95°C and a pH ranging between 4.5 and 7 for approximately 30 minutes.
- The anodization method as defined in any one of the preceding claims, wherein in step d) the concentration of tartaric acid is 72-88 g/l and the concentration of sulphuric acid is 36-44 g/l.
- The anodization method as defined in any one of the preceding claims, wherein step f) performs the following chemical reactions:
4Al2O3 + 24F- + 3Zr+4 + 4Cr+3 → 8AlF3 + 3ZrO2 + 2Cr2O3
- The anodization method as defined in any one of the preceding claims, wherein between step b) and step d) the following further steps are also carried out:alkaline chemical etching (200) in order to prepare/activate the aluminium/aluminium alloy surfaces for the electrochemical treatment in step d); andaluminium/aluminium alloy desmutting and rinsing (210).
- The anodization method as defined in any one of the preceding claims, wherein said boiling water has a pH ranging between 4.5 and 7.
- The anodization method as defined in any one of the preceding claims, wherein said fluorides result from salts and fluorozirconates/silicates.
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JP2022083947A (en) * | 2020-11-25 | 2022-06-06 | ミクロエース株式会社 | Anodization treatment method of aluminum or aluminum alloy, and hole sealing treatment of anodic oxide film |
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WO2003040437A1 (en) * | 2001-11-06 | 2003-05-15 | The United States Of America As Represented By The Secretary Of The Navy | Post-treatment for anodized aluminum |
US20060226020A1 (en) * | 2005-04-08 | 2006-10-12 | Kioritz Corporation | Plating method for aluminum alloy material |
US20150020925A1 (en) | 2012-02-10 | 2015-01-22 | Mecaprotec Industries | Method for the surface treatment of parts made of an aluminum or magnesium alloy |
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US20160017510A1 (en) * | 2014-07-21 | 2016-01-21 | United Technologies Corporation | Multifunctional anodized layer |
AU2016210539B2 (en) * | 2015-01-19 | 2020-10-08 | Council Of Scientific And Industrial Research | A process for the preparation of corrosion resistant sealed anodized coatings on aluminum alloy |
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WO2003040437A1 (en) * | 2001-11-06 | 2003-05-15 | The United States Of America As Represented By The Secretary Of The Navy | Post-treatment for anodized aluminum |
US20060226020A1 (en) * | 2005-04-08 | 2006-10-12 | Kioritz Corporation | Plating method for aluminum alloy material |
US20150020925A1 (en) | 2012-02-10 | 2015-01-22 | Mecaprotec Industries | Method for the surface treatment of parts made of an aluminum or magnesium alloy |
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WO2024003504A1 (en) * | 2022-06-30 | 2024-01-04 | Safran Landing Systems | Aluminum alloy part and associated manufacturing process |
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KR20200010107A (en) | 2020-01-30 |
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