Galvanization in the Caustic Etch Bath
Q. What is the maximum zinc level allowed in a caustic etch bath? From time to time we experience spangling (galvanization) and pitting on the surfaces of our extrusions. It seems to occur more on the big hollow sections than on the lighter, smaller surfaces. Please advise. –R.K.
A. I'm going to quote from a reference we use from time to time, The Technology of Anodizing Aluminum, Third Edition, by Arthur Brace, with whom you are probably familiar. Brace defines "spangle" as "a pronounced form of preferential etching on [an] extrusion. …It is usually due to zinc in the metal or the etch bath. As little as 5 ppm zinc in the etch can produce spangle. In the extrusion itself the level of zinc which tends to produce spangle is more difficult to fix precisely, but it tends to occur from 0.05% Zn upwards. The addition of a small amount of sodium sulfide to precipitate the zinc has been used with some success, although it has been used mainly in caustic-gluconate etches, rather than 'long-life' etchants" (p. 67).
The important thing to remember is that galvanization occurs when the bath has built up too much zinc. Either add sodium sulfide, as Brace mentions above, or dump at least some of the etch as soon as possible. This procedure even works with the so-called "no-dump" etch baths. Either procedure lowers the amount of dissolved zinc in solution. One would not normally test for zinc in the caustic etch bath. Regardless, I don't believe there is an absolute level of zinc contamination that would result in galvanization. Other factors can contribute to this phenomenon, such as the concentration of etchant (sodium hydroxide). No matter what the level of zinc in the bath, you will know it is too much when you can see the parts galvanizing.
Be aware that hydrogen sulfide gas (produced when adding sodium sulfide to the sodium hydroxide etch bath) is poisonous when concentrated. Mixed with fresh air above the tank, the gas is not concentrated enough to be hazardous. Close to the surface or inside the tank after dumping, gas concentrations can be dangerous. After dumping the bath, wash the inside and ventilate it before anyone goes into the tank. Usually, the addition of sodium sulfide should be an emergency measure during production to get through until such a time that the bath can be decanted or dumped and made up fresh without any sodium sulfide.
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