The Role of Citric Acid in Parts Cleaning

Q: Can you control Citric Acid with Conductivity?
A: Although the concentration of Citric Acid correlates nicely with conductivity, the soils that you generally clean with Citric Acid solutions interfere. When oxides are cleaned with Citric Acid solutions, they dissolve into the solution and raise the conductivity. Very soon you cannot differentiate between the contributions the cleaned oxides make toward the conductivity reading and the contribution from the amount of citric acid in the solution. This soil interference makes conductivity an inappropriate choice for measuring the concentration of Citric Acid Cleaners. Using pH is a popular alternative. However, pH does not have enough resolution to properly monitor cleaning baths. The difference in concentration between a pH of 3 and a pH of 3.5 can be very significant, sometimes a factor of 5x. Additionally, pH is a logarithmic scale, not linear. This makes it difficult to easily adjust the bath. Titration is the preferred method of monitoring citric acid solutions. Although it is a bit more tedious, it has good resolutions, appears on a linear scale, and has less soil interference than conductivity.
Q: Can Citric Acid Clean oils?
A: The short answer to this question is no. As described earlier in the presentation, Citric Acid alone is a bad degreaser of parts. However, if you work with your chemical supplier there are some citric acid formulations with added surfactantcy and other additives that help these solutions clean light oils. The downside is that the bath will load soil quickly and  you will have to change out the bath more frequently because cleaning performance will drop off quicker than if you are using an alkaline degreaser.
Q: Can Citric Acid Clean Water-Soluble Coolants?
A: Citric Acid solutions do slightly better with water-soluble coolants than oils. However, the same cleaning drop-off happens with water-soluble coolants and oils. The water soluble coolants quickly saturate the cleaning solution and prevent the citric acid from cleaning and/or brightening the parts as well or as quickly as with a fresh bath. Again, working with your chemical supplier, you can achieve a hybrid product that will clean water-soluble coolants effectively, but it will most likely be at the expense of bath life.
Q: Can I Clean and brighten my parts in one step?
A: This is the number one reason we generally get asked the first two questions. As I outlined above, when you try to do multiple cleaning processes with citric acid, the bath life degrades rapidly. Additionally, a multi-step process gives you far greater control over the process. You will be able to monitor concentrations much more effectively. This will help you minimize the amount of chemical usage. Your parts will be much more consistent from load to load and bath to bath as well. Not only will the performance of both the cleaner and the brightener last longer, but the cleaning drop-off from the start of the bath to the end of the bath will be much less. The same will be true for the brightening process. Ultimately it is a decision each plant must make for itself. However, in the long run, we find plants are much happier when they employ a multistage process. Those plants that have a single-stage process get by, but they always tend to be fighting the process and results.