Oxalic Acid Anodizing—the Basics

Question:

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? 

Answer:

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 The use of oxalic acid by itself, or as an additive, is mostly outdated.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.