The anodizing process has many variables which must work together to produce quality coatings. Pretreatment, anodizing, post treatmentóall must be accurately controlled.

The anodizing tank itself has the largest number of variables, therefore it is a good starting point for identifying and eliminating problems. Each significant variable will be discussed, concluding with optimum running procedures recommended by industry leaders. By concentrating on these variables and understanding their effects, the quality of anodic coatings produced can be substantially improved. Major variables in the anodizing tank are:

1. Current density

2. Cathode to anode ratio

3. Electrolyte temperature

4. Electrolyte concentration

5. Aluminum content of electrolyte

6. Cathode-to-anode relationship

7. Agitation

Current Density

Amps per square foot is one of the simplest variables to control, but it requires calculation from load to load. These calculations can become extremely time consuming, especially to the job shop anodizer who might run numerous different parts in the same load. As a result, many jobs are run at constant voltage instead of constant current. This produces adequate but not superior quality. The most commonly used voltage is 15, and running at constant voltage will produce anodic coatings that vary from load to load. Although finishes from two different loads may appear identical, quality may vary substantially. This includes variation in durability, corrosion and abrasion resistance, and pore structure, which has a direct relationship to dye absorption characteristics and performance.

Keeping in mind that quality is the ultimate goal, try running by current density. Pick any load at random and determine current density while running. Simply add up the surface area of the parts, plus that of the racks, if they are aluminum. (Remember an advantage of titanium racks: their surface area is not included in this calculation.) Take the running amperage at 15 volts and divide by the calculated surface area of the load. The result is the operating current density. Compare your result with 12 to 15 A/ft2, the recommended current density for sulfuric acid anodizing.

If you have been running loads by voltage and your result falls within the above range, luck is on your side.

Should your result fall outside the recommended range, take it as a hint that the quality of the work you are doing can definitely be improved. You will see more uniformity of finish and get more consistent anodic coatings from load to load when running at constant amperage.

Anode-to-Cathode Ratio

The effective cathode area is directly related to the construction of your anodizing tank. As general rule, each load should have three times as much anode as cathode, or an optimum anode-to-cathode ratio of 3:1. You are not, of course, going to reconstruct your anodizing tank for each load just to satisfy this ratio, but there are ways of staying close to the recommended value. Always try to run loads of similar surface area; this will keep the anode as constant as possible. The benefit of calculating proper surface area is twofold. First, it ensures that the load is running at optimum current density. Second, the calculated surface area can be compared immediately with that of the cathode, which is known. Should the cathode area need to be adjusted, plastic shields can be inserted in the ends and bottom of the anodizing tank. Cathode area should be charted with different combinations of shielding for quick determination of the effective cathode area required for each load.

Electrolyte Temperature

As with other variables in the anodizing tank, temperature must be controlled to ensure consistent quality. The heat generated during the anodizing process must be removed to maintain the temperature in the recommended range. The amount of cooling required is directly related to the size (in watts) of the load. The optimum temperature range is 68 to 72 F. This is by far the easiest of the variables to measure, but not always the easiest to maintain. Of course, anodizing times can be adjusted to compensate for temperature variations, but the cooling system should be designed to make this unnecessary.

Electrolyte Concentration

Sulfuric acid concentration has a direct relationship to the conductivity of the electrolyte, the current density, and the diameter of the anode pore. These parameters increase or decrease as the concentration increases or decreases. The optimum range is 15 to 17 percent sulfuric. Good procedure calls for frequent monitoring and recording of the acid concentration. This will allow an approximate addition schedule to be developed that can even be automated.

Al Content of Electrolyte

The aluminum content of the sulfuric acid electrolyte should be maintained between 2 and 12 g/L. A low aluminum content helps to minimize burning, while the initial voltage is being increased. More than 12 g/L may cause dulling of bright or critical work. The aluminum content should be monitored as carefully as the acid concentration. Expert opinion is that 7 to 9 g/L is the optimum range.

Cathode-to-Anode Relationship

Normally, cathodes should be placed on the two larger sides of a tank. If, however, the entire anodizing tank is being used as the cathode, shields may be inserted in the ends or bottom of the tank, as mentioned earlier. The goal of cathode positioning is equal distances from the anode and as unobstructed as possible.

Agitation

The amount of tank agitation employed should be as much as required to keep tank temperature uniform, but not so much as to knock parts off racks. Agitation also moves spent solution from recesses and heated electrolytes from work surfaces.- An adequate supply of clean, oil-free, low-pressure air is necessary. The most desirable air agitation is by small-bubble effervescence throughout the solution. If necessary, the air should be filtered prior to use.

References

1. "Aluminum Finishes Process Manual," Reynolds Metals Company, (1973).

2. S. Wernick and R. Pinner, The Surface Treatment and Finishing of Aluminum and its Alloys, Robert Draper Ltd., Teddington, England, 1974; pp. 263, 267.