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316 Stainless Steel is pitting and rusting in major aquarium

June 8, 2009

We've suffered a great deal here at the Shedd Aquarium as to why 316 SS is degrading submerged in artificial ocean water and above/near the ocean environment.

I wouldn't have believed it if I hadn't seen it with my own eyes. Non-contaminated, out-of-box welding materials, certified steel, and commonly purchased 316 SS accessories are pitted and rusting.

The Shedd Aquarium ozonates their water to reduce bacteria. Could this be the source of their problem?

Matthew McClay
- Chicago, IL

We've recently upgraded the Shedd Aquarium in Chicago and are battling corrosion of 316 Stainless steel. The steel has been exposed for a few months now, first to 75 degree salt water for a week (to help dissolve the "Instant Ocean" in the newly rennovated tanks) and then the temperature was dropped to 54 degrees for the animals.

I have read the posting 7117 on the corrosion of 316SS in the RO plant in Saudi Arabia, and can empathize with the failing properties of this highly regarded metal.

1. First the welds began to corrode under water and we had a diver/welder inspect and repair any pinholes that he found. He used the proper 316 SS welding electrode and is very experienced with working with on Navy vessels successfully. Those welds were then cleaned mechanically with un-contaminated wire brushes.

2. A "scum-line" began to develop at all SS tube rails where they meet the water line. The aquarium was able to lower the water a foot and we had finishing company employed to clean and passivate the ares that were affected.

3. Finally and most recently, many more areas are subject to pitting and corrosion near the water and the welds underwater are staining again. I choose to use the word staining ILO "rusting" because I feel it is too generic for our problem. The pin holes that are excreting the brown staining can be effectively cleaned with a Scotch Brite pad and rinsed clean.

Given the nature of this problem and the fact that my company does not wish to maintain this forever, can someone accurately identify the cause of and answer our problem? Did we use the correct material of SS 316?

Matt M.
Contractor - Chicago, IL

This might sound to simple but why not try some form of cathodic protection. Install some sacrificial anodes and introduce a slight current into the suffering parts. They use this form of protection on boats, piers, oil pipelines and a host of other things. The current is minimal and your fish probably won't even notice it or care. The down side is that the anodes have to be replaced every few years but I bet they would be a lot cheaper than what you are experiencing now. Talk to some folks in the marine industry, they have been doing that since the 1800s. As said, cathodic protection has been used in the oil industry for years as well and there are probably thousands of consultants and contractors there that could take one look and fix you right up. ;o)

Rod Henrickson     gunsmith Edmonton, Alberta, Canada

Matt,

Your entire post already answered the question that you put right at the end. You have clearly found that you didn't use the right material.

316 is pretty ordinary material in a chloride-bearing environment, and the need for resistance to salt water was one of many driving forces behind the development of the family of duplex stainlesses (starting with 2205 but now with a wide range of variations to suit a wide range of environments) and the family of fully ferritic nickel-free stainlesses (such as 430 and 434, the latter being notably good in chloride environments).

OK, someone is likely to point out that you should have used 316L to prevent weld decay, but that would have had no effect on the rest of the corrosion that you're seeing. It's a matter of using the right material for the application in the first place.

Bill Reynolds    consultant metallurgist Ballarat, Victoria, Australia

If the problem areas are at or near welds, then the cause us likely carbide precipitation which resulted from the high temperatures of welding. This phenomenon will definitely reduce corrosion resistance.

To avoid this problem, type 316L (L=low carbon) must be used. There is no treatment which will resolve the problem, since a metallurgical change has taken place, although passivation in situ may help.

All over passivation before the SS was put in service would have been better of course.

Jeffrey Holmes, CEF - Spartanburg, SC, USA

Dear Matt,

Yes, unfortunately, you have a problem forever with this SS . And sorry, cathodic protection wont help you in this application.
From your description,I understand, pitting corrosion has already started, and their is very little know that can stop started pitting.
It is normal that SS 316 is rusting in your application - no rust would have been abnormal. The only way to prevent/avoid corrosion in "seawater" , is by electropolishing and/or using a better alloy.

SS + marine application = passivating by electropolishing if you want to avoid corrosion problems.

Kind regards,
Marc.

Marc Quaghebeur
- Diksmuide, Belgium, Europe

There are some new nanotechnologies that could help with your problems of corrosion
You can try to paint with hydrophobic nanoparticles
look at the lotus effect

Ricardo Burstein Interplate Ltd. Bnei Berak, Israel

I agree with Matt,
I have electropolished a lot of 316 s/s products for the marine industry ranging from ocean research, scuba, salt water pool heating hardware, and fasteners. Some are still in use for many many years with no signs of corroding or failing. While I cannot attest to every combination of situations that exist. Electropolishing 316 in marine applications DOES work.

Cliff Kusch
- North Vancouver, BC, Canada