Olive Drab Color Measurement
I am a new process engineer assigned to support the chemical finishing area at a metal fabrication facility. We are currently running into color issues with our olive drab bath, aluminum green #9 used after sulfuric anodize. The parts that are processed in this bath are both castings and wrought as well as different alloys.
At times, parts will appear brown, orange, and even red and blue. I understand that the olive drab is composed of three different dyes. Under what conditions would any of the three individual dyes become depleted in the bath? Looking at the vendor literature, the dye is expected to have a short life. Yet this is contrary to the experience of our technicians. This bath has been in operation for several years and our technicans say more trouble is encountered with a newer bath.
We have begun a "design of experiment" to investigate the major contributing factors that would cause large variations in color. The current factors under investigation are anodize time (i.e. anodize thickness), dye time, dye temperature, and dye pH. Contaminants have been suggested as another item to investigate. What contaminants would be present? How would we measure and control the contamination level?
We began our DOE with a new dye bath but were only able to obtain brown/orange coupons when processing at optimum conditions. We have since taken a 2-gallon sample of the current processing bath and started to use it. These later coupons have been green. Therefore, I am forced to conclude that this bath changes as it ages. I can assure you, I am quite interested in knowing what would cause this bath to stabilize especially since it is expected to be short lived.
I also need to know what other manufacturers use olive drab. Do they measure the color produced? Are spectrophotometers used? Is it a comparative measurement (i.e. requiring a standard)? What other options are available to implement tighter control over the color achieved by the bath?
Thank you for your help.Eden L. Smith
Raytheon Metal Fabrication - Dallas, Texas
The first thing you must do is find someone (I would recommend knowing this yourself) who can make up a new bath so that it gives you the coating color for which it was sold.
This is not black magic.
Get the technical bulletin, start with standard coupons of a simple or pure alloy, and learn that first. DOE comes after that.
Falls Township, Pennsylvania
If there is a very large color change, you may want to simply increase the dump schedule on the olive drab chromate bath. These baths are relatively inexpensive and are supposed to be changed out regularly. You will also want to make sure that the plater does not change chemical vendors unless you get notification. There can be a significant color change from Brand A to Brand B.
Exact chromate color is difficult to control. Getting nitpicky about the specific shade of green can be unproductive. I would imagine that the desired controls your DOE will yield will also be costly. I would suggest that you learn to live with a reasonable range of green.
We use zinc olive drab and we allow a good latitude on acceptable color. We do not measure color beyond a simple visual inspection.
Rochester Hills, Michigan
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