METHOD FOR COMPRESSING A COMPONENT MADE OF ALUMINUM AND/OR AN ALUMINUM ALLOY

Abstract

The invention relates to a method for compressing a device component of aluminum and/or an aluminum alloy, comprising an oxide layer on the surface that is produced in a uniform manner by anodizing. The compression method comprises a hot water compression of the component in a bath of completely desalinated water while using positive pressure for a period of at least one minute. Due to the pressure application in a closed compression chamber, the compression process is accelerated and the capacity of the anodizing unit is increased.

Description

The invention relates to a method for sealing a device component made of aluminum and/or an aluminum alloy, in particular a decorative part or a functional part, with a very high corrosion stability.

High glazing, mat glazing or, respectively, silk glazing decorative parts, which are formed from aluminum sheet metal or aluminum profiles, are disposed in the outer and inner region of many motor vehicles. The decorative surfaces are produced by polishing and glazing anodizing treatment. These possibly also colored surfaces are optically demanding and fulfill a high quality standard. These are surfaces, which exhibit a completely uniform layer thickness and have neither waves nor edge superstructures nor edge alignments. They feel metallic and therewith with a high value, and there is talk of so-called “cool touch”. The surfaces exhibit in addition a good corrosion stability based on a combination of cold and hot sealing treatment processes.

Improvements of the sealing treatment method are in particular directed to an increase of the corrosion stability of anodized and densified device components, in particular in the alkaline region. A method is known frorm the European patent application EP 1 407 935 A1 wherein non densified glazed anodizing treatment surfaces are coated with a transparent “lacquer” (Aluceram), that is a lacquering method follows to the anodizing treatment. It is disadvantageous that the non densified or, respectively, part densified anodized goods cannot be transported as is desired and handled as desired prior to the layer application with these lacquers, since the capillary action of the open pores entails an irreversible soiling of the parts. Furthermore, only the view sides are coated for reasons of process technology and costs. An increased tendency to corrosion of the backside results therefrom, in particular in combination with other metals or working materials containing free carbon, which stand in direct contact with the non densified or partially densified backside in the presence of a conductive electrolyte, for example a salt solution. The thus coated parts corrode at view faces and at non view faces to a different degree such that an inhomogeneous overall picture can be generated.

Furthermore, there existed tests to increase the corrosion stability by additions during the cold sealing process. Corrosion stability up to pH values of 13.5 can be obtained with a combination of a cold sealing process and a hot water sealing treatment process, wherein in the cold sealing process for example nickel fluoride containing products are added and the cold water sealing treatment is performed with completely desalted water in combination with nickel acetate and possibly with a further hot water sealing treatment step. The pores of the anodizing layer are in this case closed or, respectively, covered by a covering layer, wherein the covering layer contains nickel containing compounds in addition to aluminum oxide hydrate (Böhmit). This covering layer takes care that highly alkaline solutions cannot attack. This nickel containing covering layer however is little stable, such that a small mechanical load leads to the elimination of this layer, which then destroys the increased corrosion stability and therefore is unsuitable for the application with construction parts in the motor vehicle region.

The generation of a stabilized, glass like modified oxidized layer is known from the unpublished German patent application DE 10 2007 057 777.1-45. In this method, construction parts with a very high corrosion stability relative to acid and alkaline media are obtained, and in particular an alkaline resistance at pH values of 13.5 is accomplished.

Furthermore, a vapor sealing treatment is known which can be performed at temperatures above the boiling point of the water. However, the possibility does not exist here to bring materials, for example silicates into the sealing treatment layer by way of the vapor, since these substances are not carried along in the vapor.

Usually, several sealing treatment steps are performed in all known sealing treatment processes for the treatment of device components of aluminum and/or aluminum alloys with a high sealing treatment quality and in particular a high corrosion resistance. Various flushing vessels and several sealing treatment vessels are furnished, in addition to the anodizing vessel and possibly various different vessels for pretreatment, for the treatment of device components in an anodizing treatment plant. Usually, the longest sealing treatment time is required for a hot water sealing treatment such that several hot water sealing treatment vessels are furnished in an anodizing plant for a continuous flow through of the products.

It is an object of the invention to furnish a method for sealing treatment of device components out of aluminum or an aluminum alloy with high sealing treatment quality, in particular good corrosion resistance. It is a further object to accelerate the sealing treatment process and thus to increase the plant capacity and to reduce the cost for each piece.

This object is obtained with a method with the features of claim 1. The new improved sealing treatment substitutes the usually employed hot water sealing treatment and in addition leads to a reduction of the necessary processing time.

In a first step of the invention sealing method, the porous oxide layer obtained by the anodization, wherein the oxide layer usually exhibits a layer thickness of from 2 to 30 micrometers, preferably a layer thickness of from 5 to 7 micrometers in case of naturally colored parts, and with colored parts exhibits a layer thickness of from 12 to 15 micrometers, is subjected to a known cold sealing step. Preferably, sealing products with nickel fluoride are here added.

In the following, a hot water sealing treatment is performed after multiple flushing in fully desalted water. This new hot water sealing treatment according to the present invention is performed at increased temperatures and under application of over pressure in a closed chamber. The temperatures lie in the region above 100 degrees centigrade, preferably in a region above of from 100 degrees centigrade to 140 degrees centigrade. The increased temperature serves for increasing the reaction speed. The over pressure amounts preferably to from 1 bar to 2 bar. In case of a lower over pressure than 1 bar, the shortening of the hot water sealing treatment time is not such significant that the additional plant expenditure for the pressure generation would pay off. Over pressures of more than 2 bar are associated with the disadvantage that the plant technical expenditure through the high effective forces unreasonably increases. The optimum operating pressure has to be determined depending on the employed chemicals, substrates and layer thicknesses.

The hot water sealing treatment can be performed in different pH regions. According to a preferred embodiment, the pH values are situated in the region from 6.0 to 7.0. In contrast, pH values resulted in the basic region with a silicate sealing treatment, since the dissolved silicates are basic.

The sealing treatment bath contains fully desalinated water. Known surfactants can be added. Additionally, glass like substances of one or several such alkaline silicates can be brought into the covering layer for increasing the stability in the alkaline region. The glass like substances are preferably entered as an aqueous solution in concentrations of from 5 to 20 grams per liter into the hot water sealing treatment bath. The thus densified parts show in this case no attack in a test in an acid medium with a pH value of 1.0 for ten minutes and in a following test in an alkaline medium at a pH value of 13.5 for ten minutes.

The sealing treatment time amounts to between 0.5 and 3 minutes in a hot water sealing treatment according to the invention at 1 bar over pressure and temperatures of 120 degrees centigrade per 1 micrometer layer thickness of the anodization layer. The reduction of the sealing treatment time becomes clear when comparing this result with the known hot water sealing treatment without pressure application, where the sealing treatment time per 1 micrometer layer thickness of the anodization layer amounts to between 2 and 6 minutes.

The hot water sealing treatment according to the present invention is performed in place of a known hot water sealing treatment. As already described, the flow through time of device components in an anodization treatment plant are reduced and simultaneously the quality of the sealing treatment is improved. The sealing treatment layer obtained by the invention method is without a gap and reaches up to the floor of the pores of the anodization layer. A very homogeneous sealing of the pores with aluminum oxide hydrate is accomplished with the process of the present invention. The conventional known hot water sealing treatment does not or not always achieve this goal, amongst others based on process inherent residual amounts of chemicals, for example acids from the bright bath, which can collect in the capillary floors of the pores and which are not displaced, but more likely included in the layer. According to the invention method, the sealing treatment media, for example fully desalinated water or in case of addition of alkali silicates also these silicates, are improved entered into the pores through the pressure. Residual amounts of treatment substances, deposited in the pores, which have not been removed by the various flushing processes, are displaced or assimilated. In addition the reaction speed and the reaction completeness is increased by the possible increased processing temperature above 100 degrees centigrade, which represents the usual boiling temperature of the water under normal pressure. This way the sealing treatment times of about 1 to 3 minutes per micrometer can be achieved, which corresponds to a shortening of time of up to 50 percent.

COMPARISON EXAMPLE 1a

A piece of aluminum sheet metal with the dimensions 40 by 100 by 2 mm of an Al 91.9Mg0.8 alloy is mechanically polished and chemically pretreated in a known fashion. Then an anodically generated oxide layer is generated on this piece during a direct current sulfuric acid treatment. The layer thickness is about 7 micrometers. After the flushing of device component A, the porous oxide layer is subjected to a cold sealing step.

• Temperature: 28-32 degrees centigrade
• pH value: 6.2-7.0
• sealing time: 4-8 minutes
• addition 4-8 g per liter sealing treatment agent (sealing salt with nickel fluoride) an after sealing treatment is performed through a hot water sealing treatment temperature: 95-100 degrees centigrade
• pH value: 6.25±0.2 sealing time: 21 minutes
• sealing treatment bath: fully desalinated water
• 2-3 ml/1 deposit prevention agent

COMPARISON EXAMPLE 1b

an equal piece of aluminum sheet metal is treated as in the comparison example 1a, wherein only the hot water sealing treatment is different, namely a first hot water sealing treatment under the following conditions:

• temperature: 70-80 degrees centigrade
• pH value: 5.7±0.3
• sealing time: 3 minutes
• sealing treatment bath: fully desalinated water
• 15-20 grams per liter nickel acetate

and a second hot water sealing treatment for after sealing under the following conditions:

• temperature: 95 to 100 degrees centigrade
• pH value: 6.2±0.2
• sealing time: 21 minutes
• sealing treatment bath: fully desalinated water
• 2-3 milliliter per liter of deposit prevention agent

EMBODIMENT EXAMPLE 1 ACCORDING TO THE PRESENT INVENTION

a like piece of aluminum sheet metal as present in the comparison example 1 was treated, wherein only the hot water sealing treatment is different. This is followed by the new hot water sealing treatment, which is performed under the following conditions:

• temperature: 120 degrees centigrade
• pressure: 1 bar over pressure
• pH value: 6.2±0.2
• sealing time: 14 minutes
• sealing treatment bath: fully desalinated water
• 2-3 milliliters per liter surfactant mixture (deposit prevention agent)

If one compares the hot water sealing treatment times, then the advantageous reduction of the treatment time from 21 minutes or, respectively, 24 minutes to 14 minutes becomes clear.

EMBODIMENT EXAMPLE 2

A piece of aluminum sheet metal with the dimensions 40×100×2 mm of an Al99.9Mg 0.8 alloy is mechanically polished and in a known way chemically pretreated. Then an anodically generated oxide layer is generated on this piece during a direct current-sulfuric acid treatment. The device component B is additionally led to an electrolytic and adsorptive coloring method. The layer thickness lies at 15 micrometers. After the flushing of the device component, the porous oxide layer is subjected to a cold sealing step as recited in the comparison example 1.

An after sealing treatment is performed by way of a hot water sealing treatment.

• Temperature: 95-100 degrees centigrade
• pH value: 6.2±0.2
• sealing time: 45 minutes
• sealing treatment bath: fully desalinated water
• 2-3 milliliter deposit prevention agent per liter

EMBODIMENT EXAMPLE 2 ACCORDING TO THE PRESENT INVENTION

A like piece of aluminum sheet metal is treated as in the comparison example 3, wherein only the hot water sealing treatment is different.

The new hot water sealing treatment follows, which is performed under the following conditions:

• temperature: 120 degrees centigrade
• pressure: 1 bar over pressure
• pH value 6.2±0.2
• sealing time: 60 minutes
• bath: fully desalinated water
• 8 grams per liter sodium silicate and potassium silicate mixture sealing treatment 0.2-0.3 milliliter per liter surfactant mixture

If one compares the hot water sealing treatment times, then the advantageous reduction of the treatment time from 45 minutes to 30 minutes becomes clear.

COMPARISON EXAMPLE 3

A piece of aluminum sheet metal with the dimensions 40×100×2 mm of an Al99.9Mg0.8 alloy is mechanically polished and chemically pretreated in a known fashion. Then an anodic generated oxide layer is generated on this piece during a direct current sulfuric acid treatment. The layer thickness lies at 7 micrometers. After the flushing of the device component, the porous oxide layer is subjected to a cold sealing step as performed in the comparison example 1.

An after sealing treatment is performed through a hot water sealing treatment

• temperature: 94-100 degrees centigrade
• pH value: 10.4-10.8
• sealing time: 21 minutes
• sealing treatment bath: fully desalinated water
• 8 g per liter sodium silicate
• 0.2-0.3 milliliters surfactant mixture per liter

EMBODIMENT EXAMPLE 3 ACCORDING TO THE PRESENT INVENTION

a like piece of aluminum sheet metal is treated as in the comparison example 4, wherein only the hot water sealing treatment is different.

The new hot water sealing treatment follows, which is undertaken under the following conditions:

• temperature: 120 degrees centigrade
• pressure: 1 bar over pressure
• pH value: 10.4-10.8
• sealing time: 14 minutes
• sealing treatment bath: fully desalinated water
• 8 g per liter sodium and potassium silicate mixture
• 2-3 ml per liter surfactant mixture

If the hot water sealing treatment times are compared, then the advantageous reduction in treatment time from 21 minutes to 14 minutes becomes clear.

The porous oxide layer is densified under pressure at the device component densified according to the present invention. Since water as is known has a higher boiling point under pressure as compared with under standard conditions, the sealing treatment according to the present invention can be performed at temperatures of 100 degrees centigrade or higher, whereby the reaction speed is increased, whereby the reaction, on which the sealing treatment is based, runs quicker and in addition more completely. An improved and more homogeneous sealing treatment results based on the applied pressure. The residues of the treatment media possibly remaining in the pores of the anodizing layer are better displaced and represent no local microscopic sealing treatment errors in the finished product, which act as weak positions with respect to the corrosion stability. In the example 3 according to the present invention of the silicatic sealing treatment, the glass like substance added during the hot water sealing treatment is better entered into the pores of the oxide layer and/or built on the surface layer through the pressure and the temperature.

Test on Thermal Crack Stability:

All construction components both of the comparison examples as well as also of the embodiment examples according to the present invention are stored at 100 degrees centigrade for 60 minutes. All construction parts do not show optically any heat cracks.

Test for Acid Resistance and Combined Acid-/Heat-/Alkali Loading:

All construction parts are subjected to 5 cycles of the Kesternich test according to standard DIN 50018 KFW 2.0S. Thereafter no part shows optical changes. The construction parts according to the present invention show also no changes in a test according to the standard TL 182 of the Volkswagen AG, that is a treatment over a ten minute time period in an acid medium, which exhibits a pH value of 1.0, a following heat dislocation aging and a ten minute submerging in a medium with a pH value of 13.5. Also the construction parts of the comparison examples 1b and 3 meet the test requirements, however not if previously also an abrasion test had been performed. The protective effect on the construction parts treated according to the present invention remains present in contrast thereto also where previously an abrasion test had been performed, since here the protective effect is not only associated with the surface, but is also built in the pores. The construction parts of the comparison examples 1a and 2 fail in this test completely.

Test of Resistance Versus Salt Containing Media:

All construction parts are subjected to a salt spray test according to DIN 50017 SS over 480 hours. Thereafter, no part exhibits optical changes.

Test of Resistance to Alkali:

All construction parts are stored in an alkaline test solution with a stochiometrically set pH value of 13.5 at temperatures from 18-20 degrees centigrade for ten minutes. The alkaline test solution comprises a 0.317 molar solution, wherein one liter solution contains

• 12.7 gram sodium hydroxide
• 4.64 gram sodium phosphate dodeca hydrate (corresponds to 2 gram sodium phosphate)
• 0.33 gram sodium chloride
• and the balance contains distilled water.

The construction parts according to the present invention and the construction parts from the comparison examples 1b and 3 show after 10 minutes no changes or changes removable by polishing. The anodizing layer is not damaged relative to the starting state with the layer thickness practically unchanged.

The construction part of the comparison example 1a changes after 4 minutes and the construction part of the comparison example 2 changes after 3-4 minutes. The transparent sealing treatment layer becomes cloudy, and part of the anodizing treatment layer is completely removed after the overall test duration of ten minutes.

Test of Alkali Resistance after a Preceding Mechanical Loading:

The construction parts from the comparison example 3 and of the embodiment examples 1, 2, and 3 are led through a device according to Amtec-Kistler, which represents a washing road simulation. Here, ten double strokes are exerted on the surface of each construction part. In the following, the construction parts are stored in the above described alkaline test solution with a measured pH value of 13.5 at temperatures of 18-20 degrees centigrade for ten minutes.

The construction part of the comparison example 3 and the construction parts out of the embodiment examples 1 and 3 according to the present invention show after ten minutes a slight change nearly completely reversible by polishing. The construction part of the embodiment example 2 according to the present invention meets the test at a pH value of 12.5, whereas the comparison example 2 meets the test at a pH value of 11.5.

All construction parts can be employed as decorative parts or as functional parts, since they exhibit a heat crack stable and corrosion resistant surface. The construction parts treated according to the present invention show equally good or better properties as the construction parts treated in the comparison methods. The in part better properties of the construction parts according to the present invention are obtained based on the improved sealing treatment. The construction parts treated according to the present invention are obtained in a significantly shorter process time.

The construction parts treated according to the invention method represent and make available decorative parts with a high sealing treatment quality, in particular with a high corrosion resistance and simultaneously shortened process time of the anodizing treatment process.

The invention is not limited to the process conditions described in the embodiment example. These conditions can be varied corresponding to the application purpose of the construction part. For example the layer thickness of the anodization layer of a decorative part can lie between 2 and 30 micrometers, whereby the treatment times are changed.

The capacity of an anodizing treatment plant can be increased by faster flow through times of the construction parts to be treated through the improved treatment quality at a reduced process time. The hot water sealing treatment step is usually the step of longest duration in the overall process such that several hot water sealing treatment vessels are furnished in anodizing treatment plants constructed according to the known method. A lesser number of hot water sealing treatment vessels can be furnished in an anodizing treatment plant with the faster hot water sealing treatment according to the present invention method. This is opposed by the higher plant technical expenditure for a sealing treatment under increased temperature. The energy balance is favorable with the method according to the present invention based on the reduced flow through times despite the increased entry of used energy.

Claims

1. Method for sealing a construction part out of aluminum and/or an aluminum alloy with an oxide layer uniformly generated on the surface of the construction part by anodization of the surface of the construction part with a layer thickness of 2 to 30 micrometers, comprising at least the step

hot water sealing the construction part, namely a sealing treatment in a bath of fully desalinated water, wherein an over pressure is applied and process temperatures above 100 degrees centigrade are applied for a time period of at least 1 minute.

2. Method according to claim 1 wherein additionally a cold sealing of the construction part is performed in a bath of fully desalinated water, preferably under the addition of a nickel fluoride containing sealing treatment agent, at temperatures from 25 to 35 degrees centigrade and pH values from 6 to 7 for a time period of at least 4 minutes.

3. Method according to claim 1, wherein temperatures above of 100 to 140 degrees centigrade are employed, preferably temperatures above of 100 to 120 degrees centigrade.

4. Method according to claim 3, wherein an over pressure of from 1 bar to 2 bar is applied in a closed chamber for the hot water sealing treatment.

5. Method according to claim 1, wherein the hot water sealing treatment time amounts to 0.5 to 3 minutes for each micrometer of layer thickness of the anodized oxide layer.

6. Method according to claim 1 wherein a glass like substance is added to the hot water sealing treatment bath for obtaining a particularly high resistance to alkali of the construction part.

7. Method according to claim 1, wherein a compound of one or several alkali silicates is added to the hot water sealing treatment bath, preferably sodium silicate and/or potassium silicate, particularly preferred as an aqueous solution with a concentration of 8 to 16 grams silicate per liter of fully desalinated water.

8. Method according to claim 1, wherein the hot water sealing treatment is performed at pH values of 10 to 11, preferably at pH values of 10.4 to 10.8.

9. Method according to claim 1 wherein the device component is subjected to an electrolytic and/or adsorptive coloring step after the anodizing and prior to the sealing treatment.

10. Method according to claim 1, wherein one or several surfactants are added to the hot water sealing treatment bath.