Chloride corrosion pitting on SS316 6

[Rachbay]

Hi,
I am working on a project, the situation is: water coming from a wastewater treatment plant with chloride levels between 200 and 500ppm for most of the year, but for 3 weeks, the chloride level reacheas a maximum of 3000 ppm because of de-icing salt melting (according the informations that I received). For the 200 to 500 ppm chloride levels, the 316 can handle that with some precautions such as a good flow and non stagnant water. But for the 3 weeks where the chloride can reach a maximum of 3000ppm do you think that it can be acceptable?

 

[EdStainless]

Not in 316, and especially not if there are any welds anywhere.
Look at using 2205 and remember to redesign the wall thickness, you can use a lot less metal.
Long term 500ppm is pushing it for 316 unless this is always cold and above pH=7.

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[mk3223]

But for the 3 weeks where the chloride can reach a maximum of 3000ppm do you think that it can be acceptable?

It sounds that you had a doubt about it, and you have answered the question.

 

[MJCronin]

IMHO, lined carbon steel pipe would be a good choice here ... Temperatures must be modest

What does everyone else think ???

MJCronin
Sr. Process Engineer

 

[moltenmetal]

The trouble with pitting is that once initiated, it is autocatalytic.

Of course the pH, flowrate, presence of other oxidants, and most importantly the temperature also greatly matter.

If de-icing salt is present, it's likely also at a very low temperature. I don't think you have to worry about chloride induced pitting for short durations at 3000 ppm while also at low temperature, assuming conditions are not stagnant and there's no risk of under-deposit or microbial initiators. That assumes you do a good job of internal purging and then passivation after welding though- EdStainless's caution is valid.

2205 would be swatting a fly with a sledgehammer in this case.

 

[EdStainless]

A non-metallic would be a prudent solution to this.
I have seen this happen with 316 condenser tube in power plants. In Jan or Feb you have a melt off, the Cl would spike over 2000ppm and the max outlet water temp would only be about 65F. And condensers were destroyed.
Overkill? well this is likely a 40+ year service life so sch10 2205 may be a very reasonable option.
Any system using coating will require ongoing inspection and repair.

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[oal223]

3000 ppm is too high for 316, as per ASM, the maximum 316 can withstand is 1000 ppm for pitting corrosion at ambient temp, this limit decreases as temperature increase.

Also, for the presence of welding and to avoid Cl-SCC, API 581 shift the cracking susceptibility from medium to high as chloride exceeds 1000 ppm. (Thus assuming the waste water pH is lower than 10) and temp is lower than 66 C.

2205 OR 904L can serve the purpose.

 

[Rachbay]

thank you Guys for your answers, you are confirming my point of view. But during my discussion with my customer I explained that 3000 ppm of chloride is high for SS316 but according to his information, the waste water plant from where this liquid is coming are using SS316 without issues. This can be explained with the low temperature.
now this sames water is passing through a plate heat exchanger (gas-liquid) and the outlet same water will have a max temperature of 90°C. I recommanded for that the duplex 2205. Do you agree that it could handle these temperature?

 

[oal223]

as per below chart (reference ASM 13), 2205 will works fine.

 

[EdStainless]

The crevice lines (dotted) are very close to reality. You have to use these because as soon as there are any deposits or fouling you move into this corrosion mode.
Now for a war story
In the 1970's (early days of SS condenser tubing) a power plant was built using 316 condenser tubing and seawater for cooling. They has a system for keeping the ID of the tubes clean by recirculating sponge balls and a fresh water flush for shutdowns. The plant worked fine for 5 or 6 years, then the ball clean system failed. They shut down for a few hours to fix it without using fresh water flush. They started back up and within a week they had thousands of leaks.

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[weldstan]

316 SS has been misapplied for decades and still is today. So many engineers are still deluded that it is the best (cheapest) material in chloride environments. With Duplex SS has been in use for 40+ years, shouldn't it be time that 316 would not so often misapplied?

 

[TugboatEng]

I don't think 2205 is a standard material for plate and frame heat exchangers. 254 and Ti are the chloride resistant alloys available from a leading manufacturer. Titanium is used exclusively for the marine industry.

 

[Rachbay]

What is the difference in price between the 2205 and the 254?

@ weldstan: definitely I agree with you
@ TugboatEng: I think "97522 CORROSION FAILURES IN PLATE HEAT EXCHANGERS (NACE paper)" is interesting we can found some case histories about plate heat exchanger

 

[EdStainless]

About 3x in price.
2205 is a duplex, 22%Cr, 5%Ni, 3%Mo
254Smo is a poor mans 6%Mo alloy, a better one is 25-6Mo or AL-6XN.
But either way these alloys are roughly 20-25%Cr, 20-25%Ni, 6-7%Mo.
So they do have significantly better pitting resistance than 2205, they are usually referred to as superaustenitic alloys and with the expectation of 254 are very well suited applications even more aggressive than seawater.
With Cr costing $0.75/lb, Ni $6/lb, and Mo $12/lb you get the idea.

The duplex SS are the most efficient use of alloy additions for corrosion resistance.

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The duplex SS are also the most difficult to weld.
Material costs in welding are rarely the dominant cost factor.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[EdStainless]

I believe that the high alloy SS grades regardless of variety (superaustenitic, superferritic, and superduplex) are the hard ones to weld. They all have issues with segregation and secondary phases.
The lean duplex and 2205 are no harder than any other SS grade. Keep things clean, use good gas, and watch heat input. The same rules apply. The only difference is that you use some nitrogen welding duplex, though it doesn't hurt the austenitics to use it there also.

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When you have stringent gas purity requirements, fussy heat input controls and corrosion testing for not just the PPQ but for individual welders, it can get hairy. Of course the responsible WE could choose to hold his nose, close his eyes, and not visit the shop

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[moltenmetal]

2205 is a very efficient use of the expensive alloying constituents- a triumph of metallurgical soup-making. It is, however, not the easiest thing to weld leaving both weld and HAZ of adequate corrosion resistance. ironic metallurgist has this one right in my opinion.