Internal corrosion in SS 316L tank at liquid interface 9

[ABCcorrosion]

Guys,
There was ring of rust (corrosion) found at the internal surface of a SS 316L tank at its liquid/air interface. This tank is 3 ft in diameter and 5 ft in height. This tank is above ground standing on three legs on a third floor of a building; it is also connected to piping system, and is grounded. This tank contains a solution of ALPO4 and NaCl with a pH 5.2. I was wondering if catholic protection or sacrificial anode can be used at arrest this corrosion. Please suggest any solution.
Thanks

 

[ABCcorrosion]

Sorry I forgot to add the most important point, we do not want to contiminate the product inside of the tank.

 

[rustbuster]

If is is only occuring at the interface I would look into padding the tank with an inerts gas like N2. removeing the O2 from the head-space should solve your problem.

Cathodic protection would contaminate the product as the anodes corrode.

Good luck

 

[ABCcorrosion]

Thanks for your input.
My product has Cl- OH- H+ ions padding with N2 will still stop corrosion?.
Can we use Impressed Current Cathodic Protection to the SS vessel which can inhibit rust inside at the liquid/air interface?

 

[unclesyd]

Theoretically, impressed current will stop SCC with resultant corrosion from the Chlorides, but I've never seen it work in production equipment. As posted by rustbuster any anode with the exception of Platinum or in some cases graphite will corrode to some degree. You will always have the interface with the wet and dry zone.

I would look to replacing the tank with a material that can handle the chlorides. I would seriously look at Titanium.

 

[ABCcorrosion]

Thanks for the reply,
Q1. how about using Hastelloy alloys for pH range of 5-7 in the presence of Chloride ions
Q2. Is Titanium effective for pH of 5-7 in the presence of Chloride ions

Please advise
Thanks

 

[unclesyd]

For Hastelloy C to work the environment has to be oxidizing, period. Haynes does make other alloys like 22 that could work. Checkout the Haynes website for corrosion data and if you don't see what you need give them a call.

 

[EdStainless]

At room temp and a pH of 5 you should be fine using a 6%Mo super austenitic stainless (AL-6XN or 25-6M0 or 1925hMo).
You don't need Ti or a Ni based alloy.

Can you tell, is the corrosion on the zone just above the liquid, or just below?

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Plymouth Tube

 

[strider6]

it would be necessary to know also the operating temperature, and chlorides concentration..

S

Corrosion & Rust Prevention Control

 

[ABCcorrosion]

Hi,
The corrosion on the zone just above the liquid. The temperature is room temperature with 15% Nacl sulution.
Thanks

 

[unclesyd]

Your problem is in the splash zone which is sort of a no man's land when it some to protection. The only practical way to alleviate the majority of the problem is through a change in material as suggested above.

Th splash zone with is a problem is an area where you can have problems with many material that would other wise be acceptable in the liquid or vapor space. the reason being that all the bad actors tend to concentrate in this area, so if by some means you can keep this area flushed with the process fluid if will help with when using any material of construction.

 

[ABCcorrosion]

Thank You all for the valuable inputs.

 

[strider6]

you can also evaluate the use of a coating.. or apply an overlay in the spalsh zone..

S

Corrosion & Rust Prevention Control

 

[ABCcorrosion]

Hi,strider6 (Materials),
Good advice thanks, can you suggest what type coating should be applied and how it is done.
Thanks

 

[drrust]

The original post states that the corrosion is at the air-liquid interface. No cathodic protection system can work in the absence of an electrolyte, so it cannot protect any metal that isn't wet.

The post doesn't indicate that the liquid level varies, so splash zone (still wet from waves or tidal motion) doesn't apply.

Raul Castillo, a NACE corrosion instructor with Dow in Freeport, Texas, is a world-recognized expert on CP in process equipment. If you have any questions on water-phase corrosion, he can help. His contact information should be easy to find on the Internet.

 

[EdStainless]

There are coatings that are commonly used in water service, they have NSF approvals. But I wouldn't put them over 316. Any slight pinhole or crevice at an edge and you could get very rapid localized attack.
If the liquid level is fairly constant I would look into splicing a band of 6% Mo super austenitic SS into the tank.

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Plymouth Tube

 

[strider6]

i think that the choice of coating vs clad should take into consideration the possibility to make frequent inspection and maintenance of the coating..
if this is not possbile or you want to fit and forget.. then i suggest the metal clad..

here you can find a descritpion of coating for SS

http://www.bssa.org.uk/topics.php?article=119

S

Corrosion & Rust Prevention Control

 

[drrust]

The above advice on painting stainless steels has a link to a BSSA site that discusses painting stainless in ATMOSPHERIC EXPOSURES. The closest parallel I can personally come up with is the several major oil companies I deal with that paint their stainless steel lines underneath insulation (the concern is corrosion under insulation and condensed water with minerals leached from the insulation). Here the standard approach is to coat as they would carbon steel with a system that is rated for seawater immersion service at the temperature of interest.

I'm sorry, the data I have is proprietary, so I can't reference these internal reports/guidelines.

If it were my tank I'd use a cladding and forget it. Materials are cheaper than labor, and you don't want to spend a lot of time on inspection. Remmember, the tank is small enough to stand on legs.