We are manufacturing SS components which we are pickling in HF/HNO3 soln for approx three mins. After this we are cleaning the components in caustic soda solution (pH 9.4) after which we are passivating in citrisurf at 80°C for 30 mins. However we find that our components are failing in 5% NaCl test at 35°C. On observing at 40x, we find small microscopic pits which are the points of corrosion. We assume that these pits are a result of pickling. Before pickling, we are using ALOX in certain processes done on the components. J.P.
The above question was taken from the online Products Finishing forum. To join the discussion, go to the Cleaning & Pretreatment Forum. The first question I would have is: why are you performing the pickling operation? Is there heat tint from welding or some other oxide on the surface that needs to be removed? If not, I would eliminate that step.
If you need to stick with the pickling, the HF/HNO3 is a good choice, but I would minimize the concentration as much as possible. I would then follow that with a very thorough rinsing, but would avoid the use of the caustic solution. If you are doing a good job rinsing, the caustic will not be necessary to remove residual acid from the parts and is a wasted step.
Another variable to check would be the concentration of the Citrisurf. It could be that you simply need a higher concentration of it in the passivation tank. Also, another advantage of removing the caustic soda process step is to eliminate that as a source of trouble for your passivation tank. If you were to drag some of the caustic solution into the passivation tank, it would partially or completely neutralize the citric acid solution rendering it partially or completely useless.
If these steps do not work, you will have to spend additional time studying the process. Some things to try would be:
- Use a nitric acid passivation to see if you get improved results. If not, you know that it is not the passivation step, but must be another part of the process.
- Use alternate methods to check the degree of passivation. ASTM A 967 lists a variety of tests that can be used to deter- mine if a stainless steel surface is ade- quately passivated or if there is residual iron left on the surface from welding or another fabricating operation.