I discovered Mr. Hass's work in 1986 while searching library Journals for information on aluminization of mirrors. I was investigating ways to improve the optics for the Fly's Eye ultra high energy cosmic ray observatory at the University of Utah. For the record, Georg (sic) Hass also originated the use of SiO and MgF protective coatings widely used for aluminized mirrors. I actually spoke with Mr. Hass at the time (he was in his eighties). As far as I know the tenchnique was dead until I revived it for the Fly's Eye. Using his original recipe I anodized the 63" diameter mirrors used for the Fly's Eye in 1987. Prior to that time the mirrors had been coated with bare aluminum only. After a few months in the field they would deteriorate significantly. The bare aluminum mirrors were stripped (using an NaOH solution) and re-aluminized every year. Within a couple of years after anodizing the mirrors we had quadrupled the amount of data taken in the previous history of the Fly's Eye. All of the 113 mirrors which I anodized in 1987 using Georg Hass's recipe are still in the field (although only 37 are currently in use) and they are still in good condition. Over the years I have distributed the recipe to other research groups including Teresa's group at Whipple.
The basic process used for anodizing mirrors is called "Barrier Layer Anodization" it is different from the anodization of aluminum typically used in industry. In standard anodization of aluminum the electrolye used is a strong acid, Sulphuric Acid. The oxide is somewhat soluble in this electrolyte. As long as a voltage is applied the oxide will continue to grow. As it grows it forms two distinct regions. Near the aluminum is a hard amorphous layer of oxide (the barrier layer) and above this is a highly porous layer. An anodized aluminum part is commonly immersed in a dye to color the part and then dipped in boiling water to seal the dye into the porous oxide. Barrier layer anodization on the other hand uses a weak acid such as Boric Acid or Tartaric Acid in which the oxide is nearly insoluble. In this type of electrolyte the barrier layer oxide will grow rapidly to a specific thickness determined by the applied voltage and then almost completely stop.
Georg Hass's original recipe called for using a 0.2 Molar solution of Tartaric Acid titrated to a slightly acidic pH of 5.5 - 6.5 using Ammonium Hydroxide. The bare aluminum surface of the mirror is submerged in this Ammonium Tartrate electrolyte. The mirror is made the anode (+) and a piece of fairly pure aluminum wire is used for the cathode (-). I used a current limited power supply to set a constant oxide growth rate (constant current) of from 250 A per minute to 1000 A per minute until the desired voltage was reached. The thickness of the oxide formed is approximately 14 A per volt.
I learned through further investigation into barrier layer anodization that electrolytes such as the one used by Georg Hass suffer from several problems. For one, it will incorporate small amounts Tartaric acid into the oxide. This shows up in an entertaining way. If you use his original recipe to anodize aluminum in a darkened room the oxide will produce an eerie blue fluorescence as it grows. Use a piece of aluminum foil and really kick up the current. This is because the Tartaric acid ions stuck in the oxide layer are being excited by collisions with electrons. The oxide layer formed is also not as solid, amorphous, and resistant to corrosion as one would like.
My primary desire for my mirrors was to get the best resistance to corrosion and the highest reflectivity at 337.1 nm. I have investigated a number of different elctrolytes for anodization. I used a novel approach to measure the resistance to corrosion of my anodized mirrors. Although anodized mirrors are much more resistant to corrosion than bare aluminum mirrors, they will still succumb to a strong solution of sodium hydroxide after some time. What I did was to coat hundreds of microscope slides with aluminum. I anodized the slides using various electrolytes at the same voltage. I then measured the time required to dissolve the anodized aluminum coating using a fixed concentration of NaOH solution. Eventually, I settled on the following electrolyte which is similar to Georg Hass's original recipe. It produces an oxide coating without included acid ions (no fluorescence) and which resists attack by the NaOH solution for 3 to 4 times the length of time as those produced using Georg Hass's original electrolyte.
My current electrolyte used for anodizing aluminum mirrors
1. Prepare a 0.2 molar Tartaric acid in dH2O solution.
2. Titrate it to a pH 5.5 - 6.9 using Ammonium Hydroxide.
3. Mix this Ammonium Tatrate solution 1:1 by volume with Propylene Glycol.
This electrolyte will also produce oxide coatings at about 14 A (1.4 nm) per volt.
Teresa expressed concern about loss of reflectivity across the visible spectrum. I do not think this is a bigger problem with the oxide coatings than with other protective coatings. I do not have a set up to measure the reflectivity over the visible wavelengths. I suspect that by using significantly thinner oxide layer than the one I use (1000 A) you should be able to achieve good reflectivity across the visible spectrum. What thickness is typically used for SiO and MgF coatings?
-- Stan Thomas <thomas@cosmic.physics.utah.edu>