The chemistry of the “no-dump” caustic etch bath is complex. There are several chemical reactions that take place at different levels of constituent concentrations and temperatures.
Larry ChesterfieldThe balance of sodium hydroxide, sodium aluminate, and chelating additives is important in preventing the galvanized or spangled appearance from occurring. If the chemistry is out of balance, it can result in the precipitation of rock-hard aluminum hydroxide in the bottom of the tank.
Galvanizing can also be caused by etching high zinc alloys (7000 Series). This can occur at a dissolved zinc concentration as low as 5 ppm (5 mg/l). To keep production going when this happens, add about 1 g/l Na2S (sodium sulfide) to the bath. This is a temporary fix until the etching bath can be dumped or 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 a preferential etch pattern on the metal.
Sodium Aluminate Can Hydrolyze
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 can “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 rapid when an unbalance of chelating additives, 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 may even shut down the process.
To help prevent this, it is necessary to maintain a bath with a sufficient concentration of chelating additives to hold the dissolved aluminum in suspension, preventing it from precipitating. Essentially, chelators are chemicals that have an affinity for metallic ions, tying them up chemically and removing them from being chemically active in the bath. There are commercial proprietary chelating additives for caustic soda etching baths that are commonly used in the anodizing industry. The proprietary additives may be added separately to the bath, or they may sometimes be purchased as part of the caustic soda itself. These baths can hold as much as 150 g/l of dissolved aluminum, more or less. The amount of aluminum that can be held in suspension depends on the concentration of caustic soda and additives 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.
Preventing Excessive Dragout
As the dissolved aluminum builds up, the bath becomes more viscous. This causes an increase in the dragout, which helps lower the amount of dissolved aluminum to within 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 solutions than others. To prevent excessive dragout, a surfactant is included in the package of additives. Surfactants help to “release” more of the solution as loads are removed from the etch bath so that excessive amounts of solution are not dragged out. The surfactants also help with rinsing. 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 (even when the production line is not operating), and always having agitation are all part of proper bath maintenance. A well-maintained bath will have little if any, sludge on the bottom of the tank.
As the amount of dissolved aluminum in the bath increases, the bath becomes less aggressive, and the appearance of the etch becomes more “satin.” 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 (≈12 to 14 oz/gal). This also helps the bath retain more aluminum.
Making Additions to the Bath
The sequence of making the additions to the bath is particularly important in bath maintenance. The proper sequence is:
- Add the sequestering additive
- Next, add caustic soda
- Then add water.
If water were to be added first, it is quite possible that the dissolved aluminum could start to fall out because the water addition can cause the concentration of the additive to be too low, reducing its ability to hold dissolved aluminum. So, first, we give the bath the extra ability to hold aluminum; secondly, we reinforce that ability by making the caustic soda concentration stronger, thirdly we can add the correct amount of water, if necessary, without fear of an out-of-balance reaction taking place
Most heavily used etching baths may have to be decanted from time to time if the bath starts to show signs of spangling when the concentration of dissolved aluminum and sodium hydroxide is high. This will depend on the type of parts and volume of parts being etched. Periodic decanting helps keep the NaOH and dissolved aluminum concentrations within the intended range and helps to remove other alloying elements and contaminants.
When the caustic etch bath is decanted, it is usually pumped into a “spent caustic holding tank.” It is then bled very slowly into the wastewater treatment stream at the first pH adjustment point in the treatment process. It is important to store the decanted caustic etch solution in the same way as if it were still in the etch tank itself. Storage conditions should be a temperature of greater than 90°F, keeping the solution agitated, and do not dilute the solution with water while it is in the holding/storage tank.
Larry Chesterfield is President and CEO of Anodizing Technologies, Inc. He has experience in a job shop performing anodizing, painting, and fabrication and now offers equipment systems design and build, anodizing technical expertise, and operations/process troubleshooting. Visit anodizingtechnologies.com or email him at larry@anodizingtechnologies.com
