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Passivating 304 Stainless?

S

steamandsteel

Aluminum
Joined
Oct 21, 2017
Location
Wichita, KS
Hello,

Looking for anyone that has had experience with passivating 304 stainless, and how necessary it is to "keep stainless stainless"
Will be in a marine environment on occasion(salt and fresh water) but most times drydocked on land.


If necessary, I'm leaning towards citric acid as it would be done at home and I'd like to keep hazmat to a minimum.
Some lengths of extruded tubing with various weldments is all I need to do
Could fit within a large pipe or long vat/tray more than likely.
 
I have only passivated SS with nitric acid so I can't speak about how effective citric acid would be. We used concentrated nitric acid followed by copious amounts of water, testing the rinse water with pH test strips to determine if all the acid was gone. I'm really not sure if all that is necessary however. SS kind of gets pasivated by itself somewhat like aluminum does. Not really passivated perhaps but it oxidizes to form a protective coating.
 
Citrisurf 77 is commonly used in industrial applications, and it looks like you can buy the stuff online. Should do exactly what you need.
 
I used to work in the pharma industry and we passivated stuff with nitric acid. Then as time went on we were forced to abandon that and move to citric acid passivation. We used CitriSurf. Personally I think it is crap and did not do a thing but it's safe and so who cares. I recently toured the shop of an anodizer that I have been sending some work to and they do passivation. I asked him what they used and he said nitric acid. I asked him about citric acid passivation and he said he tried it but didn't think think it did much so they continued with nitric acid passivation.

Also, we had issues with 304/316 stainless in an environment with chlorine. I don't know but I wonder if sea water will eat it away fast enough that the passivation layer is of no use. I don't know.
 
304 is tough- it will water spot and rust on those spots in the marine environment.
Get as high a polish as you can and keep it waxed.

316 much more forgiving- still a high buff and really watch for any scratches in an anoxic environment- crevice corrosion can run through a 1” rod and it will look like it broke like glass.

On the juice our fabricator is using I don’t know- will ask him.
316 is his material though.
 
Marine grade is 316 SS. I hear 304 is a no no for anything marine based. It may corrode no matter what you do to it.

I only have my customers to go on for this. I recommended 304 a few times cause I have a bunch on the floor. I was told they cannot use anything other than 316 and it has to be passivated.

Passivation out here I believe is $75-$150 lot charge.
 
304 and 316 all suck in a room with a barrel of chlorine with the bung off.

I did a lot of work for food facilities and we had passivated stuff rusting, some 304, some 316. We even had a solid 316 valve body heavily rusting away. The 304 and 316 propress was never effected though. The 316 pipe was rusted to hell.

Sent from my SM-G960U using Tapatalk
 
Bondo said:
304 and 316 all suck in a room with a barrel of chlorine with the bung off.

I did a lot of work for food facilities and we had passivated stuff rusting, some 304, some 316. We even had a solid 316 valve body heavily rusting away. The 304 and 316 propress was never effected though. The 316 pipe was rusted to hell.

Sent from my SM-G960U using Tapatalk
Click to expand...

How did you guys passivate the rusty stuff?
 
I use a plater that uses nitric acid bath.

The room I am referring too had glass bead shot, passivated, plain mill finish, mix of 304 and 316. It seemed as though it was completely random where the rusting happened. The 316 valve body was about 30 feet from the barrel, the overhead gantry was 75 feet away in the other direction which was rusting as bad as the valve. Some plain mill finish 30 feet farther and no rust, then mill finish 15 more feet and its rusting. Lots of spots between.

I did some passivated and some mill finish depending on customer speed request.

They use the citric stuff to clean it from time to time.

Also, everything is designed on an angle for water drainage.

Sent from my SM-G960U using Tapatalk
 
Bondo said:
The room I am referring too had glass bead shot, passivated, plain mill finish, mix of 304 and 316. It seemed as though it was completely random where the rusting happened. The 316 valve body was about 30 feet from the barrel, the overhead gantry was 75 feet away in the other direction which was rusting as bad as the valve. Some plain mill finish 30 feet farther and no rust, then mill finish 15 more feet and its rusting. Lots of spots between.
Click to expand...

Would have been interesting to take a XRF (or better, a proper lab analysis) of some samples to confirm the metallurgy. I'd suspect there'd be deficiencies of some sort for the parts that rusted the worst.
 
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