How to Carefully Maintain Your Alkaline Etching Bath

Q. What should be the maximum aluminum content in an alkaline etching bath to avoid galvanized appearance (spangling) when preparing 6063 T6 extrusions for Type II anodizing? Does the chemically complex, dissolved aluminium in the bath help attain micro grain matte finishes at the higher aluminum content?

A. This is a very complex bath—chemically speaking—and there are several chemical reactions that take place at different levels of constituent concentrations and temperatures. When the bath is even momentarily out of balance, there is a danger of the galvanized appearance occurring. It is also possible that the balance of sodium hydroxide and sodium aluminate and chelating agents goes awry, resulting in the precipitation of rock-hard aluminum hydroxide on the bottom of the tank. 

Galvanizing, or spangling, is usually caused by etching high zinc alloys (7000 series). Some sources state that the appearance occurs when the dissolved zinc concentration reaches about 4 or 5 g/l. From experience, I know it can occur at much lower zinc concentrations. The quick fix is to add Na₂S (sodium sulfide) to the bath. This is a temporary fix until the etching bath can be dumped, or at least decanted, to lower the zinc concentration. Sodium sulfide combines with the zinc to form a precipitate of zinc sulfide, essentially removing the zinc from the solution. Higher levels of non-chelated dissolved aluminum in the bath have also been known to cause galvanizing, or at least a preferential etch pattern on the metal.

There is some interesting chemistry taking place in the “long-life” or “no-dump” alkaline etching baths. 
Sodium hydroxide (caustic soda) plus water and aluminum react to produce sodium aluminate and hydrogen. In this high-pH bath the sodium aluminate will “hydrolyze” or combine with water in the bath to form free sodium hydroxide and aluminum hydroxide. The aluminum hydroxide is insoluble and will “drop out” of solution. This reaction is rapid when an unbalance of caustic soda, dissolved aluminum and bath agitation and temperature occurs. Aluminum hydroxide forms a very hard scale in the bottom of the tank, which is not easily removed and will eventually shut the process down. 

To help prevent this, maintain a bath with a sufficient concentration of chelating additives to hold the dissolved aluminum in suspension, preventing it from precipitating. 

Essentially, chelators have an affinity for metallic ions, tying them up chemically and removing them from being chemically active in the bath. There are commonly used commercial chelating additives for caustic soda etching baths. These proprietary additives may be added separately to the bath, or they may be purchased as part of the caustic soda itself. These baths hold as much as 150 g/l of dissolved aluminum, depending on the caustic soda and additive concentration and bath temperature. When not in use, the bath must be kept near operating temperature and have fairly vigorous agitation to maintain the chemical balance. Without the proper additives, the caustic soda etching bath only holds about 20 to 25 g/l of dissolved aluminum.

As the dissolved aluminum builds up, the bath becomes quite viscous, causing an increase in the dragout, which helps keep the amount of dissolved aluminum in a range of equilibrium. This range can be as high as 120 g/l to 150 g/l, depending on the size and shape of the parts being etched. Some shapes drag out more solution than others.

To prevent excessive dragout, a surfactant is included in the package of additives. Surfactants help to “release” an adequate amount of solution as loads are removed from the etch bath so that excessive amounts of 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. Proper concentration of caustic soda and additives, holding the correct temperature range (even when the production line is not operating) and always having agitation are all part of proper bath maintenance.  

The more “complex” aluminum there is in the bath, the less aggressive the etch is and the more “satin” the etch becomes. As the dissolved aluminum increases, it is recommended that the concentration of NaOH (sodium hydroxide) be incrementally raised. The common initial NaOH concentration of a new bath might be about 50 g/l (7 oz/gal). As the dissolved aluminum concentration increases, the sodium hydroxide concentration should be increased and may go as high as 85 to 100 g/l (approx. 12 oz/gal). This also helps the bath retain more aluminum. 

Heavily used etching baths should be decanted two to four times per year, depending on part throughput. This keeps the NaOH and dissolved aluminum concentrations within reason and helps remove other alloying elements and contaminants.

The sequence of making the additions to the bath is particularly important in bath maintenance. The proper sequence is:

1. Add the sequestering additive.
2. Next, add caustic soda.
3. Then, add water.  

 

If water were added first, it is possible that the dissolved aluminum could start to fall out because the water addition can cause the concentration of additive to be too low, degrading its ability to hold dissolved aluminum. First, we give the bath the extra ability to hold aluminum, secondly, we reinforce that by strengthening the caustic soda concentration, and lastly, we can add the correct amount of water without fear of an out-of-balance reaction taking place.

In the past several years, the use of acid etching has been the standard in so-called “architectural” anodizing lines. This process is based on ammonium bifluoride with additives. Acid etching is followed by a brief caustic soda etch to remove the “sparkle” imparted by the acid etching. This means that the caustic etch bath use is not as heavy as it once was, making it a little easier to maintain.

 

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Originally published in the April 2017 issue.