We are considering installing a citric acid-based passivation line. I have heard many stories about using citric acid in the passivation process—some say it is better than nitric acid and some say it's not. Can you give me some information on the process? P. R.
The discussion regarding the pros and cons of nitric and citric acid passivation has been going on for years. The nitric acid or nitric acid/chromic acid passivation process is the classic process and in fact is called out in the now canceled QQ-P-35C specification. This citric acid passivation process is a newer process. Both processes do the same thing: they remove iron from the surface of the stainless steel. From an environmental viewpoint the citric acid process is easier to work with and easier on the environment.
However, the real question is whether these two processes give comparable results.
If we look at the end results from the classic nitric acid process and the newer citric acid process, we see that results are comparable and, in many cases the citric acid process gives you better results when measured by corrosion tests of various types.
Probably one of the main reasons why the nitric acid passivation process is still used can best be described as part of “we have always done it this way” syndrome. This is not unusual thinking in the metal finishing industry and for that matter in any older, established manufacturing process.
There are still situations where the nitric acid process must be used. Certain grades of stainless steel passivated better with the nitric acid process and others passivate best with the citric acid process. Also while the QQ-P-35 specification is canceled, the replacement, AMS QQ-P-35, which in turn has been replaced by AMS 2700 is still “called out” by customers. What all this means is if your customer requires you to passivate their parts using any one of these specifications, you are stuck—you must use them. A typical formulation/process for citric acid passivation of 316 SS is as follows: Immerse the part in a 10 weight percent solution of citric acid for 30–45 minutes at 150°F. Suitable cleaning and rinsing is assumed.
There are a number of sites that you can access on the Internet that will give you additional information on the citric acid process. Go to www.pfonline.com and do a search under “citric acid passivation.” A similar search can also be performed in Google and you will have many hits that you can browse through. I am aware of at least one vendor that specializes in citric acid passivation chemistry.