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Passivation and corrosion

 

We are manufacturing stainless steel 316L balls. We forge the balls, then pass them through a process to remove the forging ring, then anneal the balls to make them non magnetic( we use neutral atm- N2), and finally grind and lap the balls. We then test the balls in concentrated nitric acid to see if there is any colour change. If there is no colour change we assume that balls will not get corroded, also as we use neutral atmosphere , we feel that there is no need for passivation. However our customer checks the balls by putting them in salt water overnight and a few balls show corrosion. Firstly is it correct to put product in salt water, and if passivation is done will it ensure that balls will not get corroded in salt water. Also how many hrs should it withstand in salt water?

M Ganga
- Gujrat, Pakistan

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Hello Mr. Ganga,

First you ask if it is proper to put the balls in salt water and the answer is yes. It is a widely used test that is simple and effective.

Second you ask if passivation will stop the corrosion in salt water. The answer here is yes, but not for ever. The balls will eventually start to corrode.

Your last question is how long should they withstand the salt water treatment. They should be able to last for at least a few days if they are properly passivated and you are using the proper grade of stainless steel.

At this point I would like to take issue with you on your statement that because you are using a neutral atmosphere you do not need to passivate. Sorry but you are wrong. The reason for passivating is to remove any free iron from the surface. In your processing you are almost certainly introducing free iron into the surface and this must be removed or you will always fail the salt water test. You can do this by cleaning the balls very well and then soaking them in 20% nitric acid for about 45 minutes at a temperature above 72 fahrenheit. The balls should then be protected from any further iron contamination. After passivating they must not even touch anything made of iron and they must be protected from grinding dust that may contain iron or they will fail. If you treat your balls in this way you should have no more problems. Good luck.

John Holroyd
- Elkhorn, Wisconsin

Thanks for the reply , but as I have written we are already checking ( a samples from the batch ) in conc. Nitric acid and even after 20 minutes it shows no rusting or colour change. If there are iron particles etc on the surface then it should change colour in conc. nitric acid. No doubt we are checking only sample from the entire batch qty.

M Ganga
- Gujrat, Pakistan

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Dear Mr. Ganga:

John is correct in all counts. You do not, however, have to use the hazardous nitric acid to passivate the balls. Billions of balls are made and passivated with proprietary citric acid formulations that give BETTER results without the hazards. Typical passivation will be 10-15 min. in an appropriate solution at 120-160 F. Let us know if we can help.

Lee Kremer
Stellar Solutions, Inc.

McHenry, Illinois