Installing an Anodizing Line that Fits Your Needs

Q. We are considering adding a hardcoat anodizing system to our existing job shop anodizing operations. We understand that there are many ways to do hardcoat anodizing. Can you suggest the best process or processes that will give us the capabilities to meet as many customer requirements as possible within practical limits of space and capital?

A. There are scores of hardcoat anodizing processes, and many are designed to satisfy very specific product performance requirements. The most widely used processes are those that can provide a wide range of performance characteristics for many diverse products without compromising quality. These processes might be favored by job shop anodizers who must contend with many different part configurations and alloys from a variety of product manufacturers. Following are some basic hardcoat processes that have been around for a long time; these, and numerous variations, form the basis of processes that are still in use today:

1The Martin Hardcoat (MHC) process is one of the earliest hardcoat anodizing processes. It works well on a wide range of alloys, including 3000, 5000, 6000 and 7000 series. Anodizing of 2000 Series alloys is generally limited to those with less than 3 percent copper and less than 7.5 percent silicon. Processing conditions are:

• 15 percent (165 g/l) sulfuric acid at 45-50°F (8-10°C)
• 24 to 30 amps per square foot (ASF), 2.0 to 2.5 amps per square decimeter (ASD)

At these conditions, the anodic coating buildup is approximately 1.0 mil (25 microns) in 25 to 30 minutes. This results in a coating of good hardness. For alloys with higher copper and/or silicon, slightly lower current densities can be used, and slightly thinner and less hard coatings can be expected.

Alcoa acquired the MHC process around 1950 and made it the basis of what it called Alumilite 225 (1.0 mil) and Alumilite 226 (2.0 mil) hard anodic coating. This process uses a “mixed acid electrolyte,” and the general processing conditions are:

• 12 percent (132 g/l) sulfuric acid, plus 1 percent (40-45 g/l) oxalic acid (H2C2O4 • 2H2O)
• Bath temperature 48-52°F (9-11°C)
• 36 ASF (3.0 ASD)

Under these conditions, the coating buildup is approximately 1.0 mil per 20 minutes of anodizing time, including a two- to three-minute ramp. Again, care must be taken with higher copper and silicon alloys. This lower-sulfuric-mixed-acid electrolyte will generally produce a harder coating than the coatings produced using the MHC processing conditions. Lower bath temperature produces an even harder coating.

Easy-to-use variations of these two basic processes can also produce high-quality hard anodic coatings with excellent hardness characteristics. A few of these variations are:

• 15-20 percent (165-220 g/l) sulfuric acid at 45°F + 2°F using 24-36 ASF is a simple variation of the MHC process and allows more flexibility when anodizing some alloys that are higher in copper and/or silicon.
• The above bath containing a combination of glycerin and glycolic acid at around 3 percent (volume) of the total bath will produce very hard coatings, even at 45°F.

These two processes may be run at lower bath temperature, if desired. Some increase in coating hardness may be expected.

Hard anodic coatings produced using any of the above processes will have excellent wear characteristics. The exact combination of processing conditions and alloy will determine how hard they are and how well-suited they are for the application. There are a few important things to remember when producing hard anodized coatings:

• Recognize the conditions that affect the conductivity of the electrolyte. In most cases, higher bath conductivity results from higher acid concentration, higher anodizing temperature and lower dissolved aluminum.
• Anodize by current density (CD), not by voltage. Choose a CD that will give optimum coating characteristics for the bath conditions and alloy.
• Use the Rule of 720 or Rule of 312 (metric) to calculate the anodizing time required to achieve the desired coating thickness. Hard anodic coatings are called “precision coatings” for good reason.
• Use a rectifier that has a direct current (DC) voltage output high enough to handle the operating conditions. Higher voltages are the result of high CD, heavier coatings, colder bath, lower acid concentration and higher aluminum concentration. Although the aluminum concentration within a range of 2 to 20 g/l doesn’t affect the voltage very much, it still pays to maximize the bath conditions by using lower dissolved aluminum. The bath operates best with some dissolved aluminum. Usually, 2 g/l is a reasonable lower limit.
• Whenever possible, use aluminum racks instead of titanium ones. It is well recognized that the conductivity of titanium is much lower than that of aluminum.
• It is important to have a cooling and temperature-control system for the anodize bath that will maintain the bath’s set temperature range even at the full kilowatt output of the rectifier and at the highest ambient temperature potential of the anodizing plant. A temperature range of +3°F (+2°C) is important to maintain.
• Larger, heavier parts should use bolted connections to help prevent burning and to achieve the desired coating thickness in a reasonable amount of time.

Each of these points is deserving of a full-scale discussion in its own right.