Two problems. First one deals with the fact that we cut copper blocks that measure 3x3x3. Our customer wants us to clean the soluble oil and possibly drawing fluids. We are a machine shop with no water treatment equipment. The parts must be clean enough to braze. Second problem: Is the citric acid passivate more safe than the nitric acid? I would like to do small parts in our shop. Can you tell me what the procedure would be? W.M.
Answer: Problem 1.
There are many different types of braze processes. Some are as simple as a torch with flux while others are done under vacuum in a large furnace. The end process will affect how well you need to clean the blocks.
First lets talk about chemistry. Many aqueous cleaners would be successful at removing both the soluble oil and the drawing fluids. Depending on how much water is in the soluble oil in its as-used condition, a solvent cleaner could also be effective. There are several options here, but there are some hydrocarbon-based solvents that could be capable of cleaning your parts.
Next to consider would be equipment. The aqueous process would usually require heating, rinsing and drying. This would generally translate into a higher initial capital cost and floorspace requirements, although some cabinet-style washers can do all those steps in one system. It is possible that the solvent process could be done in a fairly simple wash system. A small parts washer is often sold to machine and automotive shops that allow the person to place a few parts at a time in a solvent tank. Options to that tank may include a pump and brush for improvement in cleaning efficiency.
Part of the equipment consideration would also be throughput and size. When you say 3×3×3, I assume you mean inches (if it were feet, it may be a little more difficult). If you only have a few parts to clean, the small solvent tank may be enough. If you have higher throughput though, you may want to consider an automated cabinet-style or conveyorized belt washer.
Finally, since you have no existing waste treatment system, it will be important to consider what you will do with your fluids once they are used. The cleaner and rinse from the aqueous system could be concentrated using an evaporator and the final concentrate shipped offsite for disposal. For the solvent system, it is generally not practical to concentrate it by the same means, so the full volume of solvent would have to be removed from your facility. However, it may be possible to find an outlet that will recycle your used solvent for a minimal cost.
To start your project, you can visit www.pfonline.com/suppliers.html to find both aqueous and solvent suppliers, as well as manufacturers of parts washing equipment.
Without knowing any more about your process, the short answer is yes, the citric acid passivation process is both safer for employees and generates a significantly less hazardous waste stream.
A couple of things to consider though are the base material and the customer specifications or expectations. The citric acid has been shown to be a generally acceptable substitute for nitric acid when passivating austenitic (300-series) stainless steel. However, I am not aware if it is as acceptable for ferritic (400-series stainless). Also, (as we have discussed in some past columns), the citric acid method has made it into some standards, but not others. For instance, ASTM indicates it is acceptable to use citric acid for passivation, while some government standards still do not call this out.