Oxalic Acid Anodizing—the Basics
What are the general processing conditions for oxalic acid anodizing? Are they essentially the same as for sulfuric acid? Can titanium racks be used successfully in this process? T.C.
Oxalic acid can be used as an anodizing electrolyte. Historically it has been used as a single electrolyte in concentrations of from 3% up to about 10% by weight. Also used as an additive (1–4%) to sulfuric acid (12–15% by weight) to produce hard coatings at 36 ASF current density. In either case (straight oxalic, or as an additive to sulfuric acid), the voltage is much higher than in just sulfuric acid alone.
In dilute oxalic acid, voltages can go as high as 100–200 depending on the bath concentration and temperature. This process is used (had limited usage) to produce gold to bronze, and even black, architectural finishes in the 1950s and 1960s. The mixed oxalic/sulfuric electrolyte was developed and commercialized as a Type III (hardcoat) finish by Alcoa in the 1940s and 1950s. The use of oxalic acid by itself, or as an additive, is mostly outdated. Titanium can be used for fixturing.
Another fact about anodic coatings produced in oxalic acid is that the coating is not as transparent as coatings produced in sulfuric acid. Oxalic acid anodic coatings have good durability as exterior architectural finishes. The pore size, barrier layer thickness and cell size are all larger than with sulfuric acid coatings. Dissolved aluminum in the bath lowers bath conductivity, as with sulfuric acid, but it does not appreciably affect corrosion resistance. The bath is sensitive to chloride contamination just as sulfuric acid electrolytes are.
Electropolishing can be a pretreatment for anodizing or a substitute for bright dipping. Either way, it improves the surface of the aluminum...
The year 2020 will be here before you know it, signaling the beginning of a new decade and bringing changes to the world as we know it.
The cornerstone of quality and productivity for any finishing operation, process control is a plater’s key to success. To find out how far techniques have come, where they’re headed in the future, and how platers can raise the bar, Products Finishing convened a panel of experts for a roundtable discussion on the topic. With well over 100 years of combined plating experience, experts Greg Arneson, Art Kushner, Peter Gallerani and Joelie Zak share their thoughts.