Metal finishing Q&As since 1989
-----
How can I best protect stainless steel from acid fumes?
2005
I have a fume hood that is stainless steel backed. This fume hood unfortunately often contains acid fumes, from heating acid solutions
(in a chemistry lab). The fume hood stainless steel back has been corroded very badly over the years. I have removed all the rust and would like to know what is the best finishing to apply to protect the stainless steel?
Even if it is not a total fix, anything to best protect the backing would be great. At times very concentrated acids may be in use (sulfuric, nitric, hydrochloric).
Thank you.
chemistry lab - Toronto, Ontario, Canada
2005
Dear Rob !
When I read your epistle of woe, I can't help thinking that whoever bought that s.s. hood had rocks in his head.
Common assay acids are HCl, aqua regia, nitric, sulfuric, HF and, my old favorite, Perchloric. And HCl has a particular love for stainless
... and HF + nitric will gobble up titanium. And sulfuric at full concentration is boiled off at over 220 degr. C .... or was it 320
?
These will EAT up any metal, let alone lowly 3l6 s.s.
There is no easy solution. There is no paint nor cladding you can use.
The answer lies in getting a plastic (and heaven forbid using fibreglass) hood made of PVC ... and if you are using 4-1/2 kW hotplates, ah, oh dear, then the hood has to be of a dual laminate construction along with tempered glass shelving.
Back in the 70's I supplied these hoods for Laurentian university.... and in the 80's and 90's many of them
abroad, too .... it was called a PER.LAB and PLACER in Vancouver are using these for nigh on 25 years without any failure.
Freeman Newton [deceased]
(It is our sad duty to advise that Freeman passed away
April 21, 2012. R.I.P. old friend).
Rob - Sounds like the hood isn't getting quite all of the fumes if the stainless in question is on the outside. If you have gotten several years with what seems to be surface rust (you were able to clean it off), my suggestion would be to look in the phone book under industrial paint suppliers. You shouldn't have any problem finding a good epoxy that will take care of your problem. Be sure to get good instructions on surface preparation as that is the key to any successful coating.
David JayeHouston, Texas
2005
2005
Hi again Rob,
David Jaye brought up a point about the 'back' corroding away but I'd presume, hope anyway, that you meant the internal back of the hood.
Good ventilation means a minimum of 100 cfm/sq.ft of open area and more for Assay acid usage.
Re his suggestion of epoxy. Ah, yes, that would give you some protection but for how many years I don't know.
Earlier all-PVC hoods had the rear plenum made of asbestos due to the hotplate heat problem ... these had a main slot which was fibreglassed over to prevent the acids from wicking down BUT after a few years the acids had ablated away the frp. Gone. Completely. Vanished !
It doesn't matter if you use an ordinary boat isopthallic resin or a good acid polyester resin or an alkali resistant vinyl ester resin
... OR EPOXY ... those horribly strong acids will eventually attack them.
The advice of a well prepared surface makes sense. I'd use a good epoxy AND at least one matte plus perhaps a surfacing veil to build up some 'meat' and to prolong the life. At least epoxy has one big advantage over those other resins, it's bond strength to the steel is excellent.
Freeman Newton [deceased]
(It is our sad duty to advise that Freeman passed away
April 21, 2012. R.I.P. old friend).
Thank you both for your input on the subject.
I agree that using s.s. in the first place was quite an error in its own.
To clarify, yes it is the internal backing of the fume hood (and sides) that is the issue.
We do have many other fume hoods in the lab that are of plastic/fibreglass origin and are much more resistant to the fumes. However this fume hood is also used for this application. We are in the process of having it replaced so I am essentially looking for a short-term solution. This short-term solution could however be needed for a couple years!
After investigating finishes, it also crossed my mind to use a PVC covering. Epoxy also crossed my mind but I was not sure how long that would last.
I thank you both again for your input, I think I will take some of that advice and try to find an appropriate covering for the fume hood.
chemistry lab - Toronto, Ontario, Canada
2005
Rob,
I agree with the others that you need some other material, but in regards to a short term solution you can easily clean and repassivate the stainless as needed using a citric acid formulation. Let us know if we can help.
Ray Kremer
Stellar Solutions, Inc.
McHenry, Illinois
2005
Q, A, or Comment on THIS thread -or- Start a NEW Thread