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Stainless Steel as material of construction in Wastewater Treatment Facilities

Q. We have just installed a 3-compartment fiberglass chlorination/dechlorination building at a wastewater treatment plant. We feed chlorine gas into solution from one compartment and sulfur dioxide gas into solution from the other. The third compartment in the center of the building houses the piping, rotometers, gauges, valves, etc. The unit has been operating for three months. Most of the stainless steel installed inside the compartments have red spots on it. This red looks like "blood" when wiped off with the hand. Also, if you wipe it on your pants it is difficult to remove. Other items such as hose clamps and Unistrut supports have black "pits". Please let me know what may be the cause of this...

Also, we replaced the stainless steel about two weeks ago thinking we may have used a bad steel and and the items are corroding again.

What is causing this an how can we resolve this problem?

David Colson
PEC Engineers - Baton Rouge, Louisiana

A. I've never seen this blood colored corrosion myself, and you haven't said what grade of stainless steel you are using, David. But anything less than 316 would be questionable for chemical service. My first piece of advice would be, wherever possible, to use PVC, or vinyl ester fiberglass.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
 

A. Even 316 SS is attacked by chlorides. I also have never seen red corrosion as described, but I would assume that it's a byproduct of chloride attack.

As the previous answer stated, next time use all plastic, FRP with a 411 Derakane resin is the most suitable material.

Stephen Bell
KB Technologies, Inc. - Lowell, Massachusetts

A. David,

As those Gurus said, use plastics. PVC or a vinylester (vinylesters being better than polyesters where alkalines are concerned).

Regarding Unistrut supports, the Aickinstrut brand is a range of supports, bolts, and channels mainly in PVC and urethane. Even the pipe clamps (up to 6" dia, going by memory) are a glass filled urethane, I believe).

If you have any piping of stainless (you didn't indicate what all the s.s. components were), use PVC. Joining PVC to an FRP 'tank compartment' is not a problem to the cognoscenti but would be to an ordinary fibreglass fabricator. You'd need someone with dual laminate experience

Cheers !

P.S. 'Red' spots on stainless? Doesn't that sound like iron oxide or rust spots to you?

Freeman Newton [deceased]
(It is our sad duty to advise that Freeman passed away
April 21, 2012. R.I.P. old friend).

A. I have seen exactly what you describe as the red stain which looks like blood when you touch it with your hand. I see this in waste water treatment plants. It is an apt description, and the continuous condensing moisture in these plants, along with the acid atmosphere from the addition of sulfuric acid is tough on every structural metal.

  
Tom Pullizzi
Falls Township, Pennsylvania