Acid Etch, the hot topic

At the 18th annual International Anodizing conference & Exposition held by Aluminum Anodizers Council in Fort Worth last week, the hot topic was definately Acid Etch.

There were several presentations and during the Q&A session the topic came up one more time. So what is Acid Ecth and what makes it different from caustic (alkaline) etch, what is its benefits and what do we need to keep in mind when acid etching our aluminum parts.

First some comments on why we want to etch the aluminum surface before the anodizing step. After removing grease, oil, dirt and other contaminants on the surface, we are now having a clean surface to use for the formation of oxide in the anodizing tank.

Yet most of the times we need to do more about this surface to get the wanted visual appearance on the surface after anodizing.

So during the etching step the surface imperfections such as extrusion lines and scratches are smoothed out, producing a more uniform appearance changing from the brigth aluminum surface to a more matt, satin finish depending on the etching time and a couple of other parameters.

Caustic soda which is what the alkaline etch mainly consists of is very aggresive to the aluminum surface. The chemical reaction has to be inhibited to work properly but there are still a high dissolution of aluminum during the process. The amount of aluminum dissolved for a conventional, well-operated caustic etch is around 10g/ft2.

Due to the high viscosity of the caustic etch the following rinse step needs plenty of water and min. of two rinse tanks.

Introducing an acid etch step into the anodizing process will give some immediately benefits.

Some of these are less sludge and easier rinsing operation due to the fact that the acidic solution, almost neutral in some formulations, doesn´t dissolve as much aluminum as the caustic. There are an energy saving because of a lower process temperature and less fume from the process tank (10% of the fume from alkaline etching).

The reason for the significant waste treatment reduction is the fact that the acid etch results in less than 1g/ft2 dissolved aluminum to get a matte finish. The fluoride ions react and attack everything in the microstructure leading to an attractive matte finish, which differs from the one found with alkaline etching.

So the most significant difference from the caustic etch is the difference in look, because the acid etch reduces more of the extrusions effect.

One thing is important to keep in mind and that is the fact that fluoride attack the titanium racks.

The two presentations about the subject from anodizers with experience in using acid etch both mentioned that you will still need both types of etching solution in your production line. The main reasons for this are:
  • Gives a different matte finish than caustic etch
  • Needs to have the caustic etch for stripping aluminum racks
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