anoxic seawater corrosion of duplex SS Pump. Should I be concerned?

[KRamses]

Hi guys,

I have a pump with all wet end components made from CD4MCU duplex stainless steel (UNS number J93370). I would like to use these pumps (because I have them already) to pump brackish water with following conditions: Temp: 11 C, pH = 7.1, Chloride = 11000 mg/L, sulfide = 2.2 mg/L. I am thinking that sulfide is low enough that this should be ok and as far as I can tell... it is ok according to ISO 15156 but am still looking for second thoughts on this. Thanks,

 

[EdStainless]

Continuous duty or intermittent? Continuous is much better.
The low temp and neutral pH work in your favor, the low sulfide actually hurts as it helps buffer some of the effects of Cl.
Have the castings been annealed? Were they bought with any requirements on the corrosion resistance?
You will get pitting attack, this is beyond what this alloy will handle long term, but they will work for a while.
What about other wetted parts? are they at least as corrosion resistant?

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[KRamses]

Thanks Ed,

It will be continuous duty (run for seven months then shut down for the rest of the year). All of the wet components (impeller, bowl, etc.) are CD4MCU but not annealed. Ideally this pump would last for 5-10 years in operation... any bets on reaching that life?

Also, curious about low sulfide being a negative with the chloride. I assumed that the only good amount of sulfide was 0

 

[cap1a79]

Generally, the most common option for sea / brackish water is SDSS and my experience as well. Referring to API 938-C, it says that SDSS is most common option and has "excellent" resistance to general and localized corrosion in polluted and unpolluted sea water while DSS is difficult to predict as sea water is a highly complex situation due to aeration, biofouling control. I hope the below reference is helpful for you.

Meanwhile, note that the application of ISO-15156 is only for the environment where oxygen is not present.(Refer to Note 4 of ISO-15156-3,A.1.3.) Sometime, it becomes main reason why NACE MR 0175 / ISO-15156 cannot be used when selecting materials for utility water services which are having oxygen.


(Quoted from API 938-C, clause 5.2)

Seawater corrosion is a highly complex situation where aeration, filtration, biofouling and biofouling control, and salinity lead to corrosion behavior that is not necessarily directly proportional to temperature. This makes the acceptability of 22 % Cr DSSs difficult to predict, hence, 25 % Cr Super DSSs that meet the minimum PREN of 40 are more commonly used in seawater-cooled heat exchangers.
(Omitted)

The super and hyper duplex SS grades (PREN ≥40) have shown excellent resistance to general and localized corrosion in polluted and unpolluted seawater. They can also be considered to have good resistance to stress corrosion cracking in seawater service (see Section 5.3).

(Unquoted)

Reference
1. API 938-C, Use of Duplex Stainless Steels in the Oil Refining Industry, API TECHNICAL REPORT 938-C THIRD EDITION, FEBRUARY 2015, American Petroleum Institute


Lee SiHyoung,
Engineer - Material & Corrosion Specialist
WorleyParsons Oman Engineering,
cap1a79@gmail.com