Alkaline Etching

Q. What should be the maximum aluminum content in alkaline etching to avoid galvanized appearance (also known as spangling) in anodized (type II, natural, 10u) 6063 T6 extrusions? I have found conflicting advice on this. Some sources hold that aluminum conc should not be allowed to grow beyond 20-25 g/L, while others say it can be kept as high as 120 g/L without detrimental effects. Do you think complexed aluminum helps to attain micrograin matte finishes? This is what I have understood upon reading various etching additive product literature. All in all, it seems that while free aluminum may promote spangling, chelated aluminum does the opposite (enhances matt finish). Is this right? S.R.

A. I’m not sure if you are asking specifically about galvanizing (spangling) in the etching bath or if you are concerned about having to frequently decant the etching bath to keep the dissolved aluminum below the fallout concentration, so I will address both conditions.

Galvanizing, or spangling, is usually caused by zinc alloys in the metal or dissolved zinc in the etching bath. The most likely cause of high levels of zinc (> 4–5 g/L) in the bath comes from etching 7000 Series alloys, which have high levels of zinc. The zinc can be precipitated from the bath by adding sodium sulfide (Na2S). This is normally considered a temporary fix until the etching bath can be dumped or at least decanted to lower the zinc concentration. Higher levels of non-chelated dissolved aluminum also have been known to cause galvanizing or at least a preferential etch pattern on the metal.

The chemistry of the sodium hydroxide-based etching process for aluminum is complex, and I do not purport to be an expert on the various chemical reactions that take place during etching. I can, however, give you an overview that may point you in the right direction.

The basic reactions are well known in that sodium hydroxide (caustic soda) plus water and aluminum react to produce sodium aluminate and hydrogen. In this very high pH bath, the sodium aluminate will “hydrolyze,” that is combine with water in the bath, to form free sodium hydroxide and aluminum hydroxide. The aluminum hydroxide is insoluble and will “drop out” of the solution.

This reaction is very rapid once the bath achieves a certain balance of caustic soda, dissolved aluminum and bath temperature, and it forms a very hard scale in the bottom of the tank which is not easily removed and will shut the process down.

The trick in preventing this is to maintain a bath with a sufficient concentration of chelating additive to hold the dissolved aluminum in suspension, preventing it from falling out. Essentially, chelators are chemicals that have an affinity for metallic ions, tying them up chemically and, thus, removing them from being chemically active in the bath. There are commercial, proprietary, chelating additives commonly used for caustic soda etching baths. The proprietary additives may be added separately to the bath or they may be purchased as part of the caustic soda itself. These baths will hold as much as 150 g/L of dissolved aluminum, depending on the concentration of caustic soda and additive, as well as the temperature of the bath. When not in use, the bath must be kept near operating temperature and must have fairly vigorous agitation to help maintain the chemical balance. Without the proper additives, the caustic soda etching bath will only hold about 20–25 g/L of dissolved aluminum.

As the dissolved aluminum builds up in the bath, the bath becomes quite viscous. This aids in having a certain amount of solution dragout which, in turn, helps keep the amount of dissolved aluminum in a range of equilibrium, usually between 120 g/L and 150 g/L, depending on the shape of the parts being etched. Some shapes drag out more solution than others. The surfactant is also chosen for its ability to “release” an adequate amount of solution upon removing the load from the etch bath so that excessive amounts of bath solution are not dragged out. There are additional chemicals in the additive to help prevent preferential etching.

If properly maintained, etching baths such as this can last for years without ever having to be dumped completely and made up new. Maintaining proper concentration of caustic soda and additives, holding the correct temperature range and always having agitation are all part of proper bath maintenance.

To answer your last question, yes, essentially, the more complexed aluminum there is in the bath, the less aggressive the etch is, and the more “satin” the etch becomes. As the dissolved aluminum increases in the etch bath it is recommended that the concentration of NaOH be incrementally raised from about 50 g/L (7 oz/gal) as a starting point for a new bath to about 75-85 g/L (10-12 oz/gal) to maintain the degree of etch. This also helps the bath retain more aluminum. Most heavily used etching baths should be decanted two to four times per year just to keep the NaOH and dissolved aluminum concentrations within reason and help remove other alloying elements and contaminants.

The sequence of making additions to the bath is important: First, add the sequestering additive, then caustic soda and finally water. If water were to be added first, the dissolved aluminum could start to fall out because the water can cause the additive concentration to be too low, thus degrading its ability to hold dissolved aluminum. So, first we give the bath extra ability to hold aluminum, second we reinforce that ability by making the caustic soda concentration stronger, third we can add water without fear of an out-of-balance reaction taking place. I hope you are able to find the proper proprietary additives to be able to turn your etching bath into a “no-dump” bath rather than struggle with unmanageable precipitates.