Corrosion of 316L

[gvot]

Slide assembly for a marine access bridge to a floating vessel constructed of the following: Upper section 3/32" teflon bonded to 1/8 316 SS plate then to 3/8 Neoprene Pad to A36 backing plate. Bottom Section 1/8" 316L plate with 2B finish seal weled to A36 backing plate.

Envirorment is a commercial facility, fresh water marine with operating steel mill 3.5-miles East and operating coal fired power plant 2.5-miles West.

316L is experiencing surface corrosion and deterioration. What would cause this corrosion?

 

[stanweld]

Looks like iron contamination, more than likely due to welding and handling of A-36. Iron contamination can be chemically removed from the 316 SS.

 

[gvot]

Would iron contamination result from A36 rust settling on the 316L plate. The underside of the bridge was not painted as specified or was full of failing paint.

What method would be required to clean the iron contamination? Thank you for your response

 

[unclesyd]

Get you proprietary H3PO4 cleaner/wash primer, etc like Weedac, or Ospho, or any high H3Po4 concentration. paint it on full strength or very slightly diluted and let stand overnight then rinse. On the worst spots you may want to slightly scrub with an acid brush. You may have to do this twice. You might want to get a Citric Acid passivating solution or just let the 316L sit for a couple of days.
The CS is a little different in that the best method id to blast the existing rust off. After you do this you can use the H3PO4 material a a wash primer on the bare clean CS before you apply the paint primer.
You could clean the SS by lightly blasting but you will still have to treat the SS with the H3PO4 material.

Please post if you can blast, if not we can go to a different approach for the CS.

 

[strider6]

i don't understand if the material is immersed in seawater or if it's an atmospheric exposure..


S

Corrosion & Rust Prevention Control

 

[gvot]

unclesyd: "lightly blast" would this be with abrasives or would water work? We are basically over water and containemnt/cleanup would be difficult.

strider6: atmosperic fresh water enviorment on the Great Lakes. No submersion and only possible chemical exposure would be to de-iceing products.

 

[drrust]

This looks like what happens to 300-series stainless when it stays wet and not clean.

It you want to see pictures of stainless in atmospheric corrosion, contact the NASA Kennedy Space Center. They run tests on this material and on more corrosion-resistant materials in atmospheric exposures. Their corrosion website URL is: corrosion.ksc.nasa.gov If you don't see what you want there, the home page on that site should have contact information for Louis MacDowell or Jerry Curran. They can probably lead you to more information.

I get the impression this is away from seawater, which is OK. The stainless industry deals with discoloration called rouging (light rust that can be powdery) in many environments. Stainless isn't stainless--it just resists corrsion better than the A36 carbon steel, etc. that it is often used with.

BTW the stainless in your picture isn't pretty,but it doesn't surprise me. I have attached your picture with two arrow and letter labels. Please confirm that A and B point to 316 stainless. I see no structural problems, just dirty metal.

If you have a galvanic corrosion problem, it's not in the pictures and probably doesn't exist as a PROBLEM. I live on the coast in Florida, and stainless fasteners are used all the time on carbon steel and aluminum. No problem if the sun comes out and things dry out. The area around the fasteners isn't pretty, but the things hang together and work. There's lots of this stuff at Port Canaveral, just 3 miles from where I live.

 

[gvot]

drrust: A points to the A36 backing plate and B points to the 316L in question. Concerns are not wirh durability but with surface distress. Under your "A" caption is a polished portion of the 316L that functions as a low friction slide with a teflon plate. The surface distress appears to be causing major issues with the coefficient of friction.