2205 SS in a High Chloride, Low pH Environment

[jbl123]

I am installing an FRP mixing system in a manganese particulate removal wastewater treatment tank for a power plant. The wastewater will have a chloride content up to 3500 mg/l and a pH range of 2.0 -7.1. The liquid temperature in the tank ranges from 20-25ºC. There is scant information on the low pH outfall water from the power plant but I am assume that the low pH is due to hydrogen sulfide carried over from the cooling process.

I am planning on using 2205 stainless steel for the material of support brackets for the FRP mixing system. Is this material suitable for the ranges of pH and chlorides? Are there better alternatives?

Additionally, I was going to use 2205 stainless hardware for the supports, but I have been pricing the hardware and have been getting prices that are 20-25 times that of type 304 stainless hardware. Is there a suitable alternative material that may be more price competitive? Would Hastelloy C276 be a suitable alternative for the hardware?

 

[moltenmetal]

2205 should be a good fit at first glance, but your experience matches mine. The price of the material in its finished product tends to be greatly higher than would be assumed from its alloy contents, probably because it is not (yet) widely used enough.

Titanium hardware would be an option, and I gather that since it is used in the aerospace industry it is more common and not as expensive as some of the alternatives such as C276.

Depending on the design loads, there is also FRP hardware available.

 

[EdStainless]

What fasteners do you need? aren't you welding most of the supports?
Go ahead and use any Ni-Cr-Mo alloy for hardware. Ti would work also.
It is a crime, 2205 hardware should be ~33% higher than 316.

At the low temp this should work. The Cl level is reasonable and I hope that your pH is not usually 2.

My concern would be places where the 2205 is only wet some of the time. When these areas dry off you will get concentration of impurities. Rather soon you may have 30% salt at low pH and start to get some pitting.
If there are places like this you need to think about giving them a regular wash down with clean water.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[TVP]

I can only echo what Ed already mentioned: 20-25x price is a crime, and pitting may be a concern with drying/concentration. Otherwise, 2205 duplex should be ok at 25 C.

 

[jbl123]

Thanks for all the input.

The hardware is for the clamps that hold the pipe saddles and for the support bases for the mixing system.

This system operates continuously and should be submerged almost 100% of the time. I will include a note in the O&M manual that requires a washdown of the system whenever the tank is drained.

I am going to look at titanium and hastelloy as alternates for the hardware material. Hopefully that will bring the price down from outrageous to merely unreasonable.

 

[CoryPad]

When you say hardware, I assume you mean threaded fasteners. If so, contact Bufab for stainless fasteners. You should be able to get suitable parts for less money than titanium or C-276.

 

[Metaljon]

2205 would be marginal for this application for corrosion pitting. I would use SAF 2507.

 

[MJCronin]

I agree with Metaljon ....this material is not a good choice for this combination of pH and chlorides.

Based on the recent experience of Power Plant WFGD Scrubbers constructed from 2205, long term guarantees are only valid with a pH above 5.0 and chlorides less than 15,000 ppm ( temperatures are less than ~130F.

http://goliath.ecnext.com/coms2/gi_0199-14853134/CORROSION-CONTROL-COATING-PROTECTS-POWER.html

EPRI is investigating the causes of these failures.....see page 10 of this document:

http://mydocs.epri.com/docs/Portfolio/PDF/2012_P087.pdf