Chemical Treatment
Prior to Anodizing
Chemical treatment of the part significantly affects its matteness or brightness (specularity). Chemical treatment is done prior to anodizing and is a very important factor in the final appearance of the anodized part. The pre-anodize chemical treatment of aluminum is often called the cleaning, clean-up or pretreatment phase of the process.
Cleanliness of the part is critical to quality in anodizing. A typical chemical treatment process might be as follows:
Treatment in an inhibited acid or alkaline cleaner to remove dirt and oils
Deoxidize in strong acidic solution to remove natural oxides or heat-treat scale
Chemical etch or brightening.
Etching
Etching is frequently accomplished in a weak solution of caustic soda (sodium hydroxide). Etching removes metal uniformly and roughens the surface of the metal to give a uniform, matte finish. Acid etching also may be done. Treatment in solutions such as ammonium bifluoride or trisodium phosphate produces a white, matte (satin) finish without as much metal removal as alkaline etching. Different anodizers customize the cleaning and etching process to meet the needs of the products they process. Some examples of chemical finishes:
Fine matte etch: (AA) C21; Alcoa R2
Medium matte etch: (AA) C22; Alcoa R1
Note: MIL-A-8625 does not specify pretreatment processes by number or type designaion.
There are other etching processes that may be used; each has a designation that may be called out by using the AA or Systems originated by Alcoa.
Brightening
Brightening is the micro-leveling (micro-smoothing) of the aluminum surface by either chemical or electrochemical means. High luster is created because the process removes the micro-peaks but does not affect the micro-valleys. The surface is smoothed or leveled, which renders high luster to the surface. Both of these methods of achieving a specular finish may be specified by the AA designation C31. The Alcoa designation for this finish is R5. There are a number of other chemical bright dip solutions and electrobrightening baths, which may be specified by designation or by trade name.
Different mechanical finishes may be combined with satin or bright chemical finishes to produce a variety of appearances. For example, the AA designation M21C21 indicates a “smooth specular buffed” (M21) mechanical finish followed by a fine matte (C21) chemical finish. The specifier, working in conjunction with the finisher, may select the exact type of “fine matte chemical finish” to achieve the desired look. This is best accomplished by producing sample coupons of different versions of the finish and choosing the one that comes closest to the desired appearance. The nomenclature originated by Alcoa for the above finish would be A1R1 or A1R2, “polished and buffed with light etch” or “polished and buffed with satin etch.” See the Appendix for a detailed list of finishes, designations, and nomenclature.