US 3372008 A
Description (OCR text may contain errors)
United States Patent F METAL REFLECTOR AND METHGD OF MANUFACTURING SUCH REFLECTORS Cornelis Korver, Eindhoven, Netherlands, assignor to North American Philips Company, Inc., New York,
N.Y., a corporation of Delaware No Drawing. Filed May 10, 1962, Ser. No. 194,217 Claims priority, application lgletherlands, May 29, 1961, 2 5, 87 3 Claims. (Cl. 29183.5)
The invention relates to metal reflectors and to a method of manufacturing such reflectors.
Metal reflectors for outdoor use and for use in damp spaces have to satisfy stringent requirements. They have to be capable of withstanding any type of climate and elevated temperatures while retaining their reflecting power. Materials which might be particularly suitable for such use are aluminium casting alloys consisting mainly of aluminum with magnesium and/or silicon as main alloying element or elements. In addition to satisfactory machinability, satisfactory corrosion resistance, which in some cases can be improved by the provision of an oxide film, for example by anodizing, and sufficient mechanical strength at the temperatures which may occur in practice, the use of an aluminium casting alloy has the advantage that the reflector, which frequently has a complicated shape, can be simply manufactured by casting in sand or a permanent mold.
However, the said alloys and many other alloys, such as brass, V A-steel, cast iron, and similar materials suffer from the disadvantage that articles manufactured from them cannot be polished so as to be highly reflecting. For example, the coefficient of reflection, expressed as a percentage, of a known aluminium alloy containing 1.0% of magnesium and 5% of silicon after polishing is at most about 60%, whereas the coeflicient of reflection of pure aluminium after polishing may be from 80 to 90%.
A further disadvantage consists in that an anodizing treatment of an aluminium alloy performed to increase the corrosion resistance reduces the coeificient of reflection by about one half, while some alloys do not form a continuous oxide film during this treatment.
It has now been found that reflectors comprising a support made of a material which itself cannot be polished so as to become highly reflecting when provided with a reflecting layer consisting of pure or substantially pure aluminium coated with a protective oxide film, are particularly suitable.
Mirrors and reflecting layers produced by deposition of pure or substantially pure aluminium from vapour are known. It is also known to anodize aluminium so as to grow an oxide film which imparts an increased corrosion resistance to the metal. However, in practice it has been found that such an oxide film having a thickness which does not reduce the reflectivity to an unserviceable value, that is to say, a thickness of the order of magnitude of from 5 to microns, is not capable of withstanding temperatures exceeding about 150 C. for a prolonged period of time. Above the said temperature cracks are produced in the oxide film so that the protective action preventing further oxidation is lost and the reflecting power of the mirror is decreased.
Surprisingly, it has now been found that reflectors in accordance with the invention can be heated to temperatures of up to about 300 C. for a prolonged period of time without the oxide film on the aluminium mirror losing its protective action against further oxidation and without the reflectivity deteriorating perceptibly.
Such a support is preferably made of an aluminium casting alloy containing magnesium and/ or silicon as main alloying element or elements. Examples of such alloys are 3,372,008 Patented Mar. 5, 1968 the aluminium alloys containing 3.0% of magnesium, less than 0.6% of manganese and less than 0.3% of silicon, or about 0.4% of managanese and from 12.0 to 13.5% of silicon, or 1.0% of magnesium, from 0.2 to 1.0% of manganese and 5% of silicon, and the like.
In order that the invention may readily be carried into effect, an embodiment thereof will now be described in detail in the following example.
A parabolic reflector was made by casting a molten alloy having the composition 1.0% of magnesium, 0.6% of manganese, 5.0% of silicon and the remainder aluminium, in a suitable mold and cooling in the mold, after which the inner side was polished. The coeflicient of reflection of the material expressed as a percentage was 57%. Part of the reflector was anodized until an oxide film having a thickness of 1 micron was obtained. The coefficient of reflection now was 25%.
Another part of the reflector was degreased with the aid of an organic solvent, for example trichloroethane, and heated to a temperature of from 250 C. to 275 C., after which this part was treated in a vacuum with the aid of a glow discharge for 10 minutes. After cooling, aluminium was deposited from vapour so as to form a coating having a thickness of about 1.5 micron. This mirror had a coeflicient of reflection of 84%. After anodizing in a solution of ammonium tartrate (pH 5.5) with a voltage of 110 volts (aluminium electrode) until an oxide film having a thickness of 1 micron was obtained, the coefficient of reflection still was In many cases an oxide layer having a thickness of 0.1 micron proves sufficient, as the following corrosion resistance tests show.
An anodized reflector surface provided with an oxide film of thickness 0.1 micron after an endurance test of 4 weeks in a climate chamber, in which the relative humidity was from to and the temperature was 42 C., did not exhibit measurable losses of light-reflecting properties. This climate chamber test had been preceeded by a temperature test at 300 C. for 2 hours and at 250 C. for 24 hours.
In case of pitting, that is to say, the occurrence of local points of attack owing to the porosity of the test casting, the rate of extension and expansion of the centers of attack was not increased. This was proved by immersing a test article in a solution containing 25 gms. of common salt, 15 mls. of glacial acetic acid, 7 mls. of 30% hydrogen peroxide and 1 gr. of crystallized copper sulphate per liter of distilled water, for 8 hours.
The deterioration of the light reflectivity was only 2% compared with the initial value.
Comparable results were obtained with reflectors the supports of which were made from other metals, for example brass.
What is claimed is:
1. A metal reflector comprising a cast reflector body of aluminum and having alloys chosen from the group consisting of an alloy having 3.0% of magnesium, less than 0.6% of manganese, less than 3.0% of silicon and the remainder aluminum; an alloy having about 0.4% of manganese, from 12% to 13.5% of silicon and the remainder aluminum; an alloy having about 1.0% of magnesium, from 0.2% to 1.0% of manganese, about 5% of silicon, and the remainder aluminum; a reflecting layer of alumi num on said cast aluminum body and an oxide layer coating on said reflecting layer of aluminum.
2. A metal reflector as claimed in claim 1 wherein the thickness of said oxide layer is approximately 0.1 micron.
3. A method of manufacturing a metal reflector comprising casting a metal reflector body from an alloy chosen from the group consisting of an alloy having 3.0% of magnesium, less than 0.6% of manganese, less than 3.0% of silicon and the remainder aluminum; an alloy having about 0.4% of manganese, from 12% to 13.5% of silicon and the remainder aluminum; an alloy having about 1.0% of magnesium, from 0.2% to 1.0% of manganese, about 5% of silicon, and the remainder aluminum, polishing the surface of said body which is desired to be reflecting, degreasing said surface, treating said surface with a glow discharge in a vacuum, vapor depositing aluminum upon said surface, and anodizing said polished aluminum reflective surface whereby said reflective surface is coated With an oxide layer.
References Cited UNITED STATES PATENTS 2,017,757 10/1935 Keller et al. 2,040,617 5/1936 Mason et al. 2,084,327 6/1937 Edwards 88-105 2 2,108,604 2/1938 Mason 88-105 2,443,196 6/1948 Raines et a1. 2,513,149 6/1950 Coutant et al 88-105 X FOREIGN PATENTS 337,558 11/1930 Great Britain. 1,03 6,081 4/1953 France.
OTHER REFERENCES Hass et al., On the Structure and Properties of Some Metal and Metal Oxide Films, article in Journal de Physique et le Radium, v01. 11, No. 7, July 1950, pp. 394402 cited.
15 HYLAND BIZOT, Primary Examiner.
DAVID L. RECK, RICHARD O. DEAN, DAVID H.