Q. Our company has to make a major decision because we are going to try and replace the nitric acid (20%) bright dip line that we have used to clean brass fittings for the last 40 years. It’s basically a large in-line basket dip system and runs eight hrs/day. Can you direct us to any good reference materials? We have been pulled in all directions with citric acid or peroxide cleaning and know about the “appearance” differences, but then it comes back to speed and, of course, our chemical supplier thinks we need to stick with nitric acid. We also need to consider that we run stainless steel parts through this for passivation, and the nitric does work well in cleaning flux from brass and copper brazed assemblies. Any suggestions you have would be appreciated. R.J.
A. It sounds like you are doing a lot with this one cleaning process—possibly too much. All three processes will work for general clean-up on brass fittings. But if they are heavily oxidized from a torch brazing process, flux removal and brightening may not be possible in the citric acid tank alone, especially without longer times and elevated temperatures.
Also, I am not sure the current process is doing much good for your stainless passivation. The dissolved copper from the bright dipping may not allow the metal to build up as effective an oxide layer, or may incorporate the copper into the oxide. (I’m not sure about this one.) Additionally, you are presumably removing flux residue following the brazing operation in order to eliminate the potential corrosive effects from chloride-based fluxes which are commonly used for this type of process. As a result, you will have a tendency to build up excessive amount of chlorides (or borates if a “non-corrosive” flux) in the cleaning tank, which can also have a negative impact on the stainless substrate.
If you intend to continue with the three processes in this tank (bright dipping brass and copper, passivating stainless steel and flux removal), I suggest you eliminate the peroxide bright dip from consideration, since that will not be very effective at passivation and the peroxide could degrade more quickly.
Of the two, the citric acid can perform all three processes and is less hazardous, but would likely require elevated temperature and possibly a longer time than the nitric. At that point it becomes an economic decision to consider.
I suggest you incorporate additional measures to deal with your flux residue to eliminate much of it from going into your process tank. Two potential ideas include adding a quench station immediately following the brazing operation, or soaking the parts in hot water followed by a rinse. This would be effective at removing a significant portion of the flux residue before it makes it into your process tank, extending the life of it.