Corrosion of 9% Nickel Steel in Seawater used for Hydrotest 1

[stanweld]

What are the recommended corrosion preventive methods for 9% Nickel steel LNG tanks during filling for and during hydrotest with an anticipated maximum exposure time of 60 days? Is 9% Nickel Steel highly susceptible to MIC?

 

[metengr]

Stanweld;
I would be very concerned exposing the 9% Ni-bearing steel to sea water because of susceptibility to local corrosion pitting attack promoting MIC. Since moly is not an alloy addition, nickel in this case will not enhance corrosion resistance, it only serves to promote toughness.

I would not use sea water. I would make arrangements to use some other medium.

 

[JoeTank]

Sea water has been used often for hydrotest of LNG tanks, but, I am not aware of the water being left in the tank for the length of time you indicate. After emptying, the tank is flushed (sparyed down) with fresh water.

Steve Braune
Tank Industry Consultants

http://www.tankindustry.com

 

[metengr]

For the typical duration of seawater exposure during a hydrotest as mentioned above, I would not see a problem. However, 60 days exposure would allow plenty of time to incubate pitting/MIC damage.

If you absolutely have to use sea water and leave it in the LNG tank for 60 days exposure, I would investigate some type of impressed current cathodic protection system to prevent any surface pitting/MIC. This type of cathodic protection system could be easily attached to the tank and monitored for the 60 day duration.

 

[JoeTank]

metengr,
Any thoughts on the costs of a CP system to temporarily protect an big LNG tank. They are often in the 150-200 ft diameter range. Shell heights could be in the range of 120-150 feet. And the shell is typically unlined. There is often 304 internal piping too that could be inplace. Would you consider an inhibitor?

Steve Braune
Tank Industry Consultants

http://www.tankindustry.com

 

[YLWEA]

I agree with previous comments about the potential corrosion resulting from prolonged exposure to seawater. C.P. could be expensive for a vessel that size, particularly if the internals are uncoated; that's a lot of bare steel to protect. I would consider chlorination of the seawater and/or corrosion inhibitors, although you would have to check into the effect of any chemical treatment on the water disposal requirements.

 

[stanweld]

Thank you all.
We have considered an impressed current CP system, which has been used to protect LNG storage tanks during hydrotest with seawater. I am concerned with MIC, especially on the thin bottom plates, although I don't expect MIC pitting to be nearly as acute as in stainless steel. We have also considered film forming, biodegradable inhibitors. I suspect that an impressed current CP system together with a biocide would offer adequate protection. Or should we employ CP, biocide and inhibitors?

Note that nearly 38 million gallons of water are required for each test and we will have multiple tests to perform. Fresh water is unavailable in the quantity required. The 60 days maximum hold in one tank is to permit testing of subsequently erected tanks with the same treated water, which may be seawater, brackish surface water or well water, potentially high in iron and sulfates but having less than 1000 ppm Cl. Any further thoughts as to water treatment and protection using brackish surface water or non-potable well water?

 

[metengr]

Stanweld;
In talking with our Chemical Engineering Specialist, he suggested using sodium hypochlorite to sterilize any bacteria and use the impressed current CP system. Regrading disposal of the treated sea water, neutralize the hypochlorite with sodium meta-bisulfite. This should allow you to discharge the treated sea water with minimal environmental impact.

During transfer from tank to tank, periodically check the hypochlorite concentration to assure effectiveness.

 

[adil2005]

hello,

I am working for a society wich is building storage tanks (LNG), the problem we are is the same that "stanweld".
One year ago, you want to know how protect storage tanks against corrosion by sea water.
Can you help me in telling what solution did you used (inhibitor or CP system)and if the problem was solved.
Thank you all

 

[shine26]

Hello,

I need to know what the price of 9% nickel steel is. Is it traded as a commodity? Could anyone recommend a source for historic prices?

Thanks!

 

[stanweld]

Right now, the only available manufacturer appears to be Arcelor out of Belgium. The Japanese and Korean manufacturers are not even supplying for internal use. ISG, now a Dutch entity, (the old Leukens mill in Coatesville ,PA)has also demonstrated a lack of interest in manufacturing the alloy. 9% Ni steel manufacturers, other than Arcelor, appear more interested in supplying carbon and Cr-Mo plate steels and have robust bookings thereof. Price is highly volatile.