"Bleach Test" Feedback
I was surprised at the interest in the so-called "Bleach Test" that I wrote about in the February issue. I'm glad that my comments and answers to your questions generate some conversation and controversy from time to time. I encourage more of your comments. One reader in particular hit on what may be a satisfactory answer to the question of what is the bleach test. He stated that the bleach test is ASTM B136-84. At first I did not agree, but after considered thought I decided that he had a good point. The title of this ASTM standard is Standard Method for Measurement of Stain Resistance of Anodic Coatings on Aluminum. This standard is widely known to anodizers as the Modified Stain Test. It is a fairly quick "production line" method used to determine if the anodized parts are properly sealed. I suppose one could call it the bleach test, but most anodizers would not recognize this standard by that name.
There are caveats in the standard about the types of anodized finishes not to test using this method, which are too numerous to list here. Basically, the test method involves putting a drop or two of 40 ñ5 mass percentage nitric acid (HNO3) on the part to be tested. (Note: This would be a 50/50 volume solution of nominal 70% nitric acid and deionized water.) Make sure the part being tested is cooled to room temperature. After 2 min thoroughly remove the nitric acid with clean water and put one drop of a solution of blue 2LW dye on the same area where the acid resided. (See the ASTM standard for proper preparation of this dye solution.) Leave the dye spot on the part for 5 min, and then clean the dye off with water. Dry the part and visually check for any remaining dye stain. If there is a visible stain, the area may be cleaned with powdered pumice and water to achieve more thorough stain removal. The bottom line is that if you can see any remnant of the stain, it is an indication that the part is not properly sealed. On dark-colored coatings an indication of a good seal is no discoloration of the finish. Please do not use this shortened description of the procedure. Get a copy of the ASTM B136-84 Standard.
This important first step can help prepare the metal for subsequent surface finishing.
Electropolishing can be a pretreatment for anodizing or a substitute for bright dipping. Either way, it improves the surface of the aluminum...
Question: What is the best way to strip an anodize coating?