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Ted Mooney, P.E. RET
Pine Beach, NJ
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for Metal Finishing since 1989
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Pickling SS 304
Q. We are pickling our SS 304L components in HF ( 3%) and HNO3 ( 10%) soln at room temp for 3 mins. After this we are rinsing the components in caustic soln ( PH 10) for approx. 2 hrs. After this we rinse the parts in plain water and passivate the parts in citric acid formulation ( which is obviously bought from a reputed manufacturer). After doing hot passivation for 30 mins we rinse again 3 times in hot water. Then we put the parts in 5 % NACL test at 35 C.out of 200 parts we have approx. 6 parts catching rust. On observing the parts at 40X we can see that at the points of corrosion we have a small pit. Generally there is only one spot on the part where the corrosion is found. We feel that these small pits are a result of pickling. Surface roughness of parts is approx. 0.2 or better RA.
Can anyone suggest why this rust is taking place. We feel that our passivation liquid is very sticky and it may be sticking in some small holes which even after 3 rinses are not getting cleaned sufficiently. As this is batch production it is not possible to rinse separately. Is there a better method of cleaning the parts after passivation? Will DI water affect passivation if we choose to rinse in DI or will it be a better option? Could the corrosion be due to some acid remains in the small holes, although we are doing caustic wash after pickling? Any help is welcome.
Alam Karchiwallamanufacturers and traders - Karachi, Pakistan
2004
A. The process you are using is very good, although I am unsure why you are pickling in HF. The problem is not because of poor rinsing, although you may want to use DI water if you want the best rinsing and spot free parts afterward. Any citric left on the parts would not cause this problem, even if you did not get it rinsed well enough. Your rinsing should always remove all of the citric product. You could try ultrasonics in the rinse if you need help there.
It sounds like the pits you are getting are probably coming from inclusions in the steel. This is not uncommon when you have the pit formation that quickly. You can test for this on untreated parts to see if you have inclusions. The only other thing I can think of is that you have something in the rinse water that is causeing the problem, but I doubt it. You have to realize, of course, that 304L stainless steel is not perfectly resistant to NaCl solution, especially if there is some air or oxygen present in the solution. It will definitely not hold up to salt spray on the ocean. You may have to use 316 to get the resistance you want.
Lee Kremer
Stellar Solutions, Inc.
McHenry, Illinois
2004
We want uniform quality from pickling
Q. Hi!
I'm a chemical engineering student and I'd like to ask if anybody can help me: why we do not get the same quality during pickling method? We'd like to pickle tiny stainless steel parts. Every time we use an ultrasonic cleaner as a part of the method and a standard HF/HNO3 solution in a dilution of 1:3 with distilled water.
I'd like to ask if anybody knows why we get spots (darker areas on the parts) after etching? We use a standard degreasing product, a standard pickling product with an ultrasonic device and mainly that's all.
Anyway, this problem occures ocasionally.
Thank you for your kindness!
Anita
- Budapest, Hungary
July 6, 2015
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