Keeping a “No Dump” Etch in Balance

Q. We recently have had a few racks of metal that had a galvanized, over-etched appearance. The problem was coming from the etch tank and the racks were etched the normal amount of time. I had never seen that before. We added some caustic and etch additive, even though the tank chemistry was in normal operating range and the problem went away. I think we lucked out. Do you have any idea what could cause that appearance? Thanks in advance. P.P.

A. This problem is usually related to the ratio of dissolved aluminum, the concentration of NaOH and the amount of additive, or “extender” in the bath. The additive is a sequestering agent and can hold a certain amount of dissolved aluminum in solution to keep it from “falling out” and depositing a very hard (like concrete) precipitate in the etch tank. If the NaOH concentration is near the lower limit, the additive concentration on the low side and the aluminum concentration slightly higher than normal, the bright, or rough etch can occur. Sometimes it’s called “galvanizing”.

I would think since your product is mostly stock length extrusions, the dragout of caustic would be fairly consistent. However, sometimes certain shapes drag-out a lot less than others, or operator “styles” can change from time to time. If less dragout occurs, the aluminum content will go up to levels usually not seen. If this happens when the concentrations of the other chemicals in the bath happen to be at, or near, the low limit of the operating range, then the condition you describe can occur.
Remember, that as the dissolved aluminum content rises, you need to add more caustic and “extender” to maintain a consistent etch even if this results in the NaOH concentration sometimes going above the normal control limits. Apparently you experienced first hand that the addition of caustic soda and “extender” solved the problem. I don’t know what concentration of NaOH you are running. Normally, for a so-called “architectural/commercial etch” a fairly matte finish is desired. This would dictate a new bath make-up something like this:

NaOH 7–8 oz/gal (x 7.5 for g/l
Al, 10–20 g/l minimum
“Extender”, 2–3% by volume (depends on the brand. Use manufacturer’s recommendations)

A temperature of 140°F would be fairly normal, or perhaps a little higher, with maybe a 10 minute etch. (Most large etch tanks for architectural shapes also require a cooling coil to help maintain the correct temperature). As the dissolved aluminum gets to the area of 140 g/liter, or higher, the NaOH concentration should also be increased. I’ve seen as high as 12 oz/gal (90 g/liter), or more, to maintain the desired appearance of the etching. Of course, as the NaOH rises so should the “extender” concentration.

I hope this helps. Maybe you already know all this and are following these general guidelines. My response to that would be “keep going up” on the concentrations of NaOH and “extender” until you see the desired results. It never hurts to decant the etch tank if the aluminum wants to stay above 140–150 g/liter, or higher. This is not a bad idea to do once or twice a year anyway. Or you can simply try to drag-out more solution during the transfer of loads to the rinse tank. I would say, generally speaking, that if the aluminum concentration is above 160 g/liter, you are in the danger zone.

Please let me know what you do and how you make out.