A PROCEDURE FOR ELECTROCHEMICAL POLISHING OF AN ALUMINIUM SUBSTRATE TO OBTAIN A SPECULAR SURFACE THEREOF

Abstract: The present invention concerns the removal/reduction of the directionality or anisotropy of a surface and making the surface specular on a substrate material consisting of aluminium or an aluminium alloy. The central process stages are: chemical polishing, chemical etching, preferably using phosphoric acid, and electrochemical polishing, preferably using phosphoric acid. Alternatively, the chemical polishing could be replaced by another chemical etching and electrochemical polishing. The present invention has made it possible to produce bright surfaces with good optical qualities on substrate materials of inexpensive Al alloys which may contain recycled aluminium. With the improved process, it is possible to achieve a considerable increase in brightness both relatively (from the starting material to the finished material) and absolutely.

Description: A PROCEDURE FOR ELECTROCHEMICAL POLISHING OF AN ALUMINIUM SUBSTRATE TO OBTAIN A SPECULAR SURFACE THEREOF The present invention concerns a procedure for removing/reducing the directionality or anisotropy of a surface and making the surface specular on a substrate material consisting of aluminium or an aluminium alloy. The substrate material can be produced by casting, extrusion, rolling or other shaping techniques and the surface properties of the substrate material may be of greater or lesser significance. For example, stringent requirements have to be made for the surface properties of a substrate material which is to be used for reflector purposes. Materials which are to reflect visible light are particularly surface-critical and the description which follows will concern such materials in particular. In order for a material to be suitable as a reflector material, such as for lighting purposes, it must meet a number of requirements. The material must have high reflectivity for light and it should preferably have certain light dispersion properties, depending on the application. Moreover, the material must have sufficient scratch resistance to allow cleaning. Any layers on the surface of the material must adhere well to the substrate. The material should also be as light as possible and may typically be based on rolled aluminium. Already as early as in the thirties, it was realised that aluminium meets many of the above requirements. The reflectivity of this material is exceeded only by silver. The shaping properties of aluminium are good and its weight is low. For functional and aesthetic reasons, it used to be desirable that lighting materials had as bright a surface as possible. Chemical and electrochemical polishing processes were developed which made it possible to transform the relatively rough rolled surface of a highly pure material into a bright surface. In addition, these processes contributed to increasing the reflectivity of the rolled substrate. Aluminium is a soft material and when it comes into physical contact with other materials, damage occurs, for example in the form of scratches. In addition, a bare aluminium surface is exposed to corrosion. For most applications of aluminium, including for lighting purposes, it is, therefore, necessary to protect the surface. The most common methods of protection are anodisation or lacquering. After 1945, aluminium started to be used on a large scale for lighting purposes. The production of reflectors in aluminium was a typical one-off process. First the reflector parts were shaped to form the desired geometry and then they were assembled to form a complete reflector or screen. The screens were then polished chemically or electrochemically and subsequently protected by anodisation. Lacquering was also used as a treatment after the electrochemical process. A common feature of the one-off or batch processes was the long treatment time required to polish the relatively rough starting material. With the chemical processes then in use, the quality of the starting material was decisive if the treatment time was to be reduced. Gradually, rolling mills improved their processes and became able to supply materials with considerably greater brightness. Thus it became possible to reduce the polishing times, which gradually allowed for the introduction of continuous brightening processes for aluminium strips. Continuous plants for chemical or electrochemical polishing and anodisation were taken into use in the mid-fifties. Continuous surface treatment was considerably more cost-effective than the traditional one-off processes. Rolling mills, particularly in Europe, continued to improve their processes and the need for polishing became less and less. Today, the brightness of the material is mainly created in the rolling mill. Lines for electrochemical polishing and anodisation have become less important as a consequence. They are mainly used to remove a surface layer in order to increase the reflectivity of the material. The brightness is usually not altered significantly as a consequence of this. Perhaps the most important task of the brightening line today is thus to form protection for the aluminium by means of the anodisation which normally follows. Specular materials are produced today by cold-rolling pure aluminium alloys (typically 99.8% Al or purer, for example AA 1080) in a rolling mill. The material's reflectivity is subsequently increased by means of chemical or electrochemical treatment, which involves removing a thin layer of the material's surface without altering the brightness significantly. The material is then protected by anodisation or lacquering. The requirements for the rolling quality and alloy quality (among other things, its purity) are usually regarded as being fundamental for a satisfactory result. The surface is polished either chemically or electrochemically. The electrochemical polishing process is generally preferred when polishing a bright starting material and has the potential to produce a more specular surface than chemical polishing. Chemical polishing, in turn, can be used to advantage to polish raw materials with a somewhat rougher rolling quality. Chemical polishing is today regarded as the best process for giving relatively rough surfaces a considerable increase in brightness. However, as rolling mills now usually supply very bright material, chemical polishing is gradually becoming a less relevant process. If the starting material is already bright, the electrochemical process will produce a brighter end product than the chemical process. Both chemical and electrochemical polishing may result in topographical disturbances or so-called gassing defects. In today's production processes, it is desirable to avoid these defects. This is done by using a starting material of a sufficient quality and by correcting the process conditions in the process bath. The problems associated with gassing defects are discussed and described in the following article: "The incidence and avoidance of gassing defects in chemical and electropolishing of aluminium, A.W. Clifford and D.J. Arrowsmith, University of Aston in Birmingham, Dept. of Metallurgy and Materials, 23 February 1978". The tests described in this article were carried out in acid mixtures which are difficult to regenerate. Particularly in processes which involve major removal of substrate material, it will be of great significance that the chemicals can be regenerated expediently. In order to obtain a satisfactory result with the processes described in the article, the treatment time will easily be as much as 10 minutes. With treatment times of this order, these processes will not be feasible in connection with continuous treatment of substrate materials (strip process). Furthermore, aluminium materials with a low content of alloy elements are preferably used as, according to the article, the alloy elements (in particular the size of the intermetallic particles) are assumed to be of major significance in relation to the formation of defects on the surface of the substrate. The article supports the general view that a sufficient purity of the aluminium material is necessary in order to be able to control and prevent the development of defects so as to achieve a satisfactory end product. The present invention concerns a procedure which makes it possible to produce a material with qualitatively good optical properties based on aluminium alloys of semi-bright rolling quality, which is a considerably rougher quality than that which is currently used for the production of specular lighting materials. Moreover, in accordance with the present invention, it is possible to use a less pure, less expensive starting material than previously, for example qualities such as AA 1200 (Al < 99.2%) or AA 1050 (Al < 99.5%) for the production of specular material. Moreover, it is possible to use recycled Al metal, for example AA 1200 recycled, which results in environmental gains. Moreover, the process uses chemicals which can be regenerated, which results in both environmental and financial gains. In accordance with the present invention, this has been made possible by means of a new, improved chemical/electrochemical polishing process. In accordance with the present invention, the brightness may be increased more than by means of processes of the prior art on the basis of the specified raw materials and available treatment times. This applies both to the relative increase in brightness between the starting material and the finished material and to the absolute brightness in the finished product. With the improved process in accordance with the present invention, the treatment time can also be reduced in relation to processes of the prior art, which makes it possible for the process to be used in a plant for the continuous treatment of a substrate material. The present procedure involves treatment of the material in several successive stages. The main stages of the process involve chemical polishing and chemical etching, immediately followed by electrochemical polishing, preferably without the material being exposed to air. Alternatively, a process can be used involving chemical etching, electrochemical polishing and another chemical etching and electrochemical polishing. The starting material of the process is characterised by two properties which distinguish it from the norm in today's production of lighting materials: - The rolling quality is considerably rougher and - The alloy is less pure than normal. While lighting products are produced today in rolling mills without the subsequent chemical or electrochemical process altering the starting material's surface topography significantly, an essential feature of the present invention is that it allows the use of a less expensive raw material by improving the chemical or electrochemical process. As mentioned earlier, such processes are associated with the emergence of gas bubbles, which can produce gassing defects in the finished product. However, the process proposed allows better control of the emergence of gas bubbles on the surface of the material so that it is possible to produce a specular product without having to make stringent requirements for the quality of the starting material. The present invention will be described in further detail in the following. The alloy which is used may preferably be AA 1200 recycled. However, AA 1200 based on a primary metal or AA 1050 can also be used. It is also possible to use a recycled alloy of type AA 3105. The rolling quality must be relatively good for the end result to be satisfactory. The material used in the example consists of a semi-bright-rolled quality. This quality lies between mill finish (MF) and bright-rolled quality. The optical properties of the raw material may be as follows: Total reflection 11 ': Approximately 80% Brightness at 20° [21 : In rolling direction (RD) : 1050- 1200 GU Transverse to rolling direction (TD): 1000-1150 GU Roughness: Ra [3] : 0.60-0.12 micrometres [1]: measured in accordance with DIN 5036 part 3 [2]: measured in accordance with DIN 67530 [3]: measured by using a drag pin Before the actual polishing process begins, the material is degreased. This is done mainly in order to avoid rolling oil and dirt entering and contaminating the process bath. Degreasing may be done by using organic solvents, hot water (possibly with tenside), an acid or a basic solution. After degreasing, the material is subjected to a process stage which involves chemical polishing. The material is brought into contact with a bath which may, for example, consist of the following: Concentrated phosphoric acid, H 3 PO , (85% weight) 3.5% weight concentrated nitrous acid, HN0 3 , (65%) 0.5 g/1 Cu (added as CuSO 4 -5H 2 O) 8 g/l Al. A number of chemical polishing methods, based on commercially available solutions, may probably be used instead of the above method. The important element is that the chemical polishing stage is combined with the subsequent processes so that the end product is better than is possible with the prior art techniques. Pretreatment with chemical polishing is necessary to achieve a specular product from the given starting point. The polishing time and the temperature in the bath may vary somewhat; typical values for the production of specular products are 60 seconds at 80°C. Agitation in the chemical polishing bath will be necessary. The chemical polishing process may be stopped by rinsing, for example by bringing the material into contact with distilled water. It is important to stop the polishing fast so that transfer etching does not ruin the surface. Transfer etching can be a problem when the temperature and acid concentration are as high as in chemical polishing. It is then possible to desmut the material using nitric acid. This may be necessary, among other things, to remove copper from the surface. Desmutting improves the material's optical properties after chemical polishing and cleans the surface before the further treatment. After this treatment stage, the material is rinsed and dried. Before the electrochemical polishing stage, brief chemical etching is carried out. The etching takes place preferably in the same bath as that in which the electrochemical process is to take place. An important feature of the present invention is that the material is not removed from the bath between etching and electropolishing. If the material is exposed to the air before the electrochemical polishing, the quality of the end product will be altered, usually for the worse. The bath for the chemical etching may consist of phosphoric acid, water and a certain quantity of dissolved aluminium. The acid bath may have different concentrations and temperatures and the dwell times may also be varied. Variation of the parameters provides the opportunity to avoid gas-generated elevations on the surface of the material. It is assumed that the generation of gas bubbles is associated with defects and inhomogeneities in the surface itself. Chemical etching before electropolishing will condition the surface in such a way that optimal avoidance of gassing defects can be achieved. If the material is exposed to air so that a natural oxide layer is formed, this conditioning may be ruined. Control of the emergence of gas bubbles can be achieved in particular by means of the use of prior etching. It is important, in connection with the production of the specular quality, that the dwell time in the acid etch is not so long that coarse etching topography is produced. If too coarse an etching topography is formed, this cannot be removed even by thorough electropolishing. The electropolishing is done in the same bath as the chemical etching. In practice, this will be done by switching on the power for the electropolishing after a certain time in a one-off process. For a continuous strip process, this must be done differently. For example, the strip can pass through different zones in the same vessel so that it is not exposed to a significant electric field before the etching process has been carried out. Another solution is for the strip to pass from one etching vessel to another while it is kept moist and is not exposed to the air. A third solution, perhaps the easiest solution in technical terms, is to carry out the prior chemical etching by spraying the strip with phosphoric acid just before it goes down into the electropolishing bath. The electropolishing itself can be carried out both in constant current and in constant voltage mode. It can also be carried out in different acid concentrations and with different additives in the bath. Moreover, the voltage and/or current can be varied to achieve different surface qualities. For a strip process, it is important for the process time to be kept as short as possible so that the production rate results in an economical process. Process times of approximately one to two minutes are usually required but there is potential for improvement of the products if longer process times are allowed. This applies to both the chemical polishing and electropolishing. Longer times can be used for a one-off process. An alternative way is to carry out a double electropolishing, each time with a preceding chemical etching. Also according to this alternative method the material is first degreased. A quick chemical etching is carried out, preferably in the same bath as for the following electrochemical process. As earlier mentioned, it is important that the material is not exposed to air between chemical etching and electropolishing. After the first electropolishing the process is repeated by a new chemical etching followed by a new electropolishing. In practice, this is carried out in a process line consisting of two identical steps. Alternatively, the material could be wound up after the first electropolishing for thereafter being processed in the same line another time. The bath composition for such a double chemical etching and electropolishing is the same as ealier described. The total process time is also of the same order as for the earlier described process. It should be understood that, in connection with the process described above, it is possible to use accessory agents, for example to adapt the viscosity in the bath to a desired value. Such agents will represent the prior art for experts and will not, therefore, be described in further detail here. Moreover, the present invention is not restricted to processing strips or producing lighting products. Thus the present invention may be used in one-off manufacture of various products. These may be extruded products for various structures such as buildings or mechanical structures. For example, the present invention may expediently be used in connection with the surface treatment of profiles, for example of alloy type AA 6060. Furthermore, the present invention may be used for cast parts which can be used in mechanical or building structures. Products produced in accordance with the present invention may preferably be finished by lacquering. However, anodisation may also be used. The choice of type of finishing will depend on the quality of the raw material, such as its purity. The present invention shall be described in the following using two examples.. The examples concern the production of a specular material involving restrictions to the exposure time (continuous strip process). The objective of the process is to increase the brightness as much as possible within the available process time, to avoid the processed material becoming too directional (either by the rolling scores still being visible or flow marks being produced in the process) and to bring the total reflection up to an acceptable level for the lighting product. Example 1 Process conditions Starting material: AA 1200 semi-bright-rolled, optical specifications below Chemical polishing: Concentrated phosphoric acid, H 3 PO , (85%) weight) 3.5%) weight concentrated nitrous acid, HNO 3 , (65%>) 0.5 g/1 Cu (added as CuSO 4 5H 2 O) 8 g/l Al Temperature: 80°C Exposure time: 60-120 seconds Chemical etching: Bath: concentrated phosphoric acid (85% weight) Low Al concentration: 0-8 g/1 Al Temperature: 55°C Exposure time: 10 seconds Electrochemical polishing: Same bath as for chemical etching Exposure to air to be avoided Exposure time 60-120 seconds Voltage: 18 V Finished product A product is produced with an isotropically bright appearance. The final brightness value is approximately 1500 GU (R20). The total reflection in the finished product is affected by the oxide layer generated on the surface during the electrochemical treatment. The quality of this finished product is improved by extending the process times. However, a longer process time is not desirable in this case. The results are shown in Table 1 : Table 1. Example 2 Process conditions Starting material: AA 1200 semi-bright-rolled, optical specifications below Chemical etching: Bath: concentrated phosphoric acid (85%> weight) Low Al concentration: 0-8 g/1 Al Temperature: 55°C Exposure time: 10 seconds Electrochemical polishing: Same bath as for chemical etching Exposure to air to be avoided Exposure time 60-120 seconds Voltage: 18 V Chemical etching: As above Electrochemical polishing: As above Finished product A product is produced with an isotropically bright appearance. The final brightness value is approximately 1500 GU (R20). The total reflection in the finished product is affected by the oxide layer generated on the surface during the electrochemical treatment. The quality of this finished product is improved by extending the process times. However, a longer process time is not desirable in this case. The results are shown in Table 2: Table 2.

Claims: Claims 1. A procedure for removing/reducing the directionality or anisotropy of a surface and for making the surface specular on a substrate material consisting of aluminium or an aluminium alloy, characterised in that the substrate material is etched, preferably by using a solution comprising phosphoric acid, after which the substrate material is subjected to electrochemical polishing by using a solution which may preferably consist of phosphoric acid. 2. A procedure in accordance with claim 1, char ac t e r i ze d i n t h at the substrate material is subjected to a chemical polishing before the etching process 3. A procedure in accordance with claim 1, characterized in that the substrate material is subjected to a new etching and electrochemical polishing directly after the first treatment. 4. A procedure in accordance with claims 1-3, characterised in that the surface of the substrate material is not exposed to air between the etching and the electrochemical polishing. 5. A procedure in accordance with claim 1, characterised in that the substrate material may consist of an aluminium alloy which has been recycled, for example AA3105. 6. A procedure in accordance with claim 1, characterised in that the substrate material may consist of a profile alloy such as AA 6060. 7. A procedure in accordance with claim 1, characterised in that the substrate material to be treated has a brightness which is lower than 1200 GU. 8. A procedure in accordance with claim 1, characterised in that the substrate material consists of a rolled aluminium strip and the procedure is carried out as a continuous strip process. 9. A procedure in accordance with claim 1, characterised in that the chemicals from the individual process stages are regenerated and reused.

Inventors: HALLENSTVET MERETE, LEIN JOHN ERIK, MAARDALEN JOSTEIN, SOERAKER JAN MORTEN, SPOOREN CORNELIS, HALLENSTVET, MERETE, LEIN, JOHN, ERIK, MAARDALEN, JOSTEIN, SOERAKER, JAN, MORTEN, SPOOREN, CORNELIS
Applicants: GLAMOX FABRIKKER AS, HALLENSTVET MERETE, LEIN JOHN ERIK, MAARDALEN JOSTEIN, NORSK HYDRO AS, SOERAKER JAN MORTEN, SPOOREN CORNELIS, HALLENSTVET, MERETE, LEIN, JOHN, ERIK, MAARDALEN, JOSTEIN, NORSK HYDRO ASA, SOERAKER, JAN, MORTEN, SPOOREN, CORNELIS
Application year: 1998
Main classification: C25F3
IPC codes: C25F3