removing aluminum anodizing
In talking to a high end professional metal polisher, those aluminum automotive trim parts are technically not the same "anodize" as let say a blue anodized Aeroquip fitting.
The automotive trim part finish/process is actually called Bright Dip.
Look at a blue anodized Aeroquip fitting. Then look at a aluminum headlight bezel. You'll see the bezel has a thicker coating and it has a gloss to it. You can sand away the blue anodizing much easier than the bright dipped headlight bezel too.
You'll find many anodizing businesses that do not do Bright Dip. Also many Bright Dip places do not have vats large enough for long 3 feet plus pieces.
You are mostly correct.
Bright dip is an chemical polishing process that uses phosphoric and nitric acid heated to between 180 and 220 degrees F. It does not make the coating thicker, but it may make it more durable since the coating is now flatter and more uniform.
Bright Dip solution is so corrosive that it will etch 316 stainless steel over time. Maintaining the right balance of phosphoric and nitric acids as well as maintaining a consistent temperature is critical to a good finish. In addition with the acid at such a high temperature companies that do bright dip need to install fume scrubbers and a variety of exhaust systems to keep the workplace safe. This is why bright dip tanks are so small, it is easier to properly ventilate a small tank than a large one. Finally the bright dip process is a bit finicky and this means that few shop have the expertise to do bright dip in a production environment.
Back in the 60s a lot of automotive trim was bright dipped, it gave the appearance of stainless trim at a much lower price. At that time companies didn't have the environmental concerns either. When the bright dip solution needed to be changed you simply dumped it down the drain. Today, used bright dip solution is going to contain heavy metals like chrome and copper, inaddition it is also going to a super strong acid. So the heavy metals must be removed and the acid neutralized. Expensive.
The key benefit of bright dip trim was that not only was it cheap to produce but it was also very durable. If well maintained anodized aluminium can and does last for decades. The key exception is colored anodized parts. Since most dyes are organic in nature they can and will fade over time.
If you were to take a microscopic picture of the surface of even polished aluminum you'd see that there is a series of peaks and valleys. Chemical polishing like bright dip etches away the peaks creating a flatter surface. The flatter the surface the more reflective, thus the term bright dip.
The anodizing process is broken down into the following steps, RINSING takes place between each step.
1. Cleaning in a soap tank heated to 140 degrees F. Cleans any oils or dirt off the part. Parts should be relatively clean, heavily oiled parts may need a dedicated degreasing, or other mechanical cleaning prior to this first step.
2. Etch Tank- Sodium Hydroxide is used to etch the part to give it a uniform appearance. etching can last from 30 seconds to several minutes depending upon the finish desired. A long etch time will create a "flat" finish. Shiny = Shorter.
3. Deoxidizing- The etching process will leave trace elements on the surface including copper found in various. The deoxidizer is an acid solution at room temp.
4. Bright Dip - many shops don't bright dip. If they wish to do a shiny finish they can get a relatively bright finish by first polishing the part to a mirror finish. Then degreasing the part and then etching it for only a few seconds. It won't be as bright as bright dip though.
5. Anodize. For clear anodize the part is anodized for 10-12 minutes for colored anodize the part may be anodized for up to an hour.
6. Color- A variety of colors can be added. Longer they sit in the tank the darker the color. Hardest color? Black, a good solid black is actually pretty challenging. If the pH is off on the tank the part can come out purple or brown. Just like in third grade colors can be mixed and matched, want purple? Blue tank for a while than red tank for a while. Green? Gold(yellow) and Blue. For production runs of thousands of pieces shops will dedicate a tank for a particular color. Most job shops will have red, blue, black, and gold. You can even get a camo appearance by streaking the part with acid between colors. The acid will immediately remove the dye from the pores since the pores have not been closed in the seal tank.
7. Seal tank. Tank with a very weak nickel solution heated to 180-200 Degrees F. parts are sealed for 10-20 minutes depending upon color.
Regards,
Joe Dokes