One of the most important industry discoveries has been that aluminum can be used as a cathodic material, and very efficiently. How this knowledge has been kept from so many anodizers for so many years is truly puzzling. The electrical conductivity of aluminum is over three times that of lead, and it weighs only a fraction. It is an easy metal to handle, weld, drill or bend.

Lead, the Old Reliable?

Lead has been the cathode material of choice for anodizing because it is practically impervious to sulfuric acid. However, it is a very heavy, awkward metal to handle; electrically, it is only moderately efficient. It will carry only about 180 A/in. in air without heating up. There is no need to discuss the difficulties anodizers have encountered with installation and repair of lead linings and plates. Over an extended period, some lead will show up in the sulfuric acid. The EPA, of course, takes a dim view of lead in effluents. Your waste treatment system must prevent this.

Will Aluminum Last?

People not familiar with aluminum cathodes ask, "Wont the aluminum dissolve?" Yes, it will if it is not cathodically protected. When cathodic, the rate of dissolution is very slow, so the more operating hours the better. There are two ways of compensating for inactive periods: remove the cathodes from the tank, or use a small battery charger and a relay that energizes the charger when the rectifier is off. A titanium strip at each end of the tank, connected to the anode terminal, with the cathodes connected to the negative terminal, will keep the plates cathodically protected at all times.

Which Alloy?

Alloys 1100 and 1350 are the best choices for minimum contamination of the bath because they are 99 percent aluminum. Alloys 6063 and 6101 have only slightly more alloying material, but are available in T6 condition. T6 refers to the thermo-tempered condition, which means that it is a harder and longer-lasting material. Either 6063 or 6061 can be used for the bus bar if aluminum is the metal of choice, though I recommend T6 sheet for longest life in the anodizing bath.

Anode-Cathode Ratio

A specialist with Sandoz Colors and Chemicals, in a fairly recent article, recommended a 3:1 anode/cathode ratio. It works. I know an anodizer, however, who has used aluminum cathodes for nearly 30 years in various facilities and he contends that a 1:1 ratio is best because it affords the most smooth and even oxide coating. No ratio will be exact because of varying load sizes; consequently, each anodizer must experiment to decide which ratio works best for him.

Cathode Area and Distance

Again there are choices: one school of thought is that cathodes must stay within the vertical and horizontal dimensions of the aluminum being anodized (the "work envelope"), or heavier oxide coatings will develop on areas at the envelope perimeter. Another school of thought is that aluminum should cover the walls parallel to the anode bar(s). Beautiful colorwork is being done with cathodes well beyond, above and below, the anode envelope.

The required distance, work-to-cathode, is also a subject for debate. Grubbsstatedthata10-inchminimum should be maintained because of the greater electrical conductivity of aluminum cathodes. The anodizer who has been using aluminum cathodes for 30 years says he has had work much closer than ten inches and that one can offset the increased efficiency of the aluminum cathodes by reducing the sulfuric acid content, making the bath less electrolytic. Another possibility is ramping up more slowly and using less operating voltage.

Each shop, depending on tank size and rectifier capacity, will have to do some experimenting to obtain the best results. Certainly most anodizers know exactly how much coating any rectifier setting and time will produce. These numbers will have to be revised, since use of aluminum cathodes means faster times and less power. Faster times, in turn, allow increased production, while less power means lower operating cost.

Unfortunately, there are no universal or "magic numbers," just as there are none with lead cathodes, but the good news is that power settings and times will all be less than with lead.

Installation

Sheets up to l/4-inch thick make good cathode material. Weld them to a 6061 or 6303 bar, or bolt them to a copper bar. Make sure that each bar will carry over 50 percent of the rectifier capacity. Aluminum will carry 600 A/ft; copper 1000 A/ft- when clean. Welding gives the best electrical continuity, but when the sheet wears thin, you may also have to replace the bar. If welding, use 1100 rod and weld 100 percent, so that no acid or fumes can get trapped between bar and sheet.

If bolting, bend the sheet 90 and bolt between two thinner bars. Use electrical gel, stainless bolts, nuts and heavy spring washers. Make the connections to the rectifier beyond the fume area and again use electrical gel.

If metal strips are used, there will be solution flow around the cathodes. If sheets are used, punch or drill holes to get solution flow through the cathodes. One-inch holes, with about six per square foot will suffice. The cathodes must be protected from arcing with polypropylene between the cathodes and the work. If strips are used, a perforated polypropylene sheet must be used (see Fig.1). If metal sheets are used, perforated polypropylene can be employed, or strips can be screwed to the sheet, as in Fig. 2. Location of the strips vertically and the number of strips, as well as their width, depends on tank width and depth, the type of work, racks, etc.

Background

My introduction to aluminum cathodes came from an anodizer in Anaheim, CA, who runs a Type II tank with aluminum strips superimposed on lead wallsóand it works. Many believe that this practice leads to uneven pore structure. Another anodizing shop in Burbank, CA, has its cathodes bent and sandwiched between l/4 x 4" copper bars in a plastic-lined tank (1500-amp rectifier). The owner of an anodizing shop in Santa Fe Springs, CA, informed me that he has manufactured numerous lines with aluminum cathodes.

Maintenance

The cathodes, if left in the tank when not in use, should be removed on a regular basis for inspection of welds and bolted connections to ensure good electrical continuity. If unalloyed metals have plated onto the cathodes, they can be brushed or hosed off. (Be careful that they dont go directly to sewer.) This provides an opportunity to put a caliper or micrometer on the cathodes and determine how much corrosion is actually taking place.

To Switch or Not to Switch?

This column has not recommended a specific method for changing to aluminum cathodes. Rather, it has attempted to make anodizers aware of the opportunity to save money and increase profits. By using aluminum cathodes, you should be able to produce more work with less rectifier output, which also means less refrigeration load and even more money in your pocket.

Reference

1. Charles Grubbs, Plat. & Surf. Fin., 68, 32 (Nov.1981).