CS valve and SS pipe galvanic corrosion with low conductivity (tap) water as fluid

[jupemu]

Hi.

I'm wondering what's the possibility of galvanic corrosion when welding CS (1.0345 / A285) valve to SS (1.4307 / 304L) pipe with 309L filler metal?

H2O conductivity is 30 mS/m (300 μS/cm), temperature 50° C (122 °F), pressure 3 bar (43.5 psi).

 

[EdStainless]

That is low conductivity, but it is likely that you have dissolved oxygen and the temp is fairly high.
Why do you want to do this?

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

 

[jupemu]

CS valve is cheaper. There's a vacuum degasser in the system.

 

[moltenmetal]

Unless this is giant, why on earth would you not just buy a CF8M cast valve and be done with it?

 

[blacksmith37]

A small anode ( valve) and a large cathode (pipe) is not a good plan.

 

[mk3223]

IMO, the money saved for using CS valve in the SS piping may not offset the potential system failure caused by the corrosion damage, especially the system may be designed for 20+ years of life.

 

[TugboatEng]

I feel like a stainless valve in steel piping is acceptable due to the low surface area of the valve to the piping but a steel valve in stainless piping is going to decay quickly. Stainless steel valves are really inexpensive in class 150 so I'm going to assume your system is much higher pressure. If it is higher pressure and the water is treated a carbon steel valve may work but now you have to deal with the labor of cutting it out and welding it back in if it fails. You will find the cost of replacement will far exceed the cost of purchasing the correct part.

 

[jupemu]

Water analysis report says conductivity is actually <150 μS/cm and dissolved oxygen is <20 μg/kg. There's Hydrazine (200 μg/kg) in the water. pH is 9.5.

But put Class 150 valve in.

 

[EdStainless]

Typically when someone has to do this we would suggest coating the inside of the SS pipe back about 10x the diameter in an effort to suppress the localized galvanic attack.
But this can cause problems with failures of the coating or crevice corrosion under the coating.

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

 

[Chumpes]

hi
in absence of galvanic coupling, your water will be NOT corrosive to carbon steel due to oxygen scavenger (hydrazine), very low oxygene and alkaline buffered pH at 9.5.
your water is similar to boiler water that is not corrosive to carbon steel if well treated : carbon steel is our usual material selection for such systems even at 50°C.
I don't think galvanic effects between stainless steel and carbon steel will be a problem because galvanic effects would accelerate existing corrosion of carbon steel that is already very low... cathodic reactions on the stainless steel surface will remain low due to lack of oxygen and buffered pH, thus the driving force for galvanic coupling remains low.

 

[cap1a79]

Although I am afraid that my post is too late(as I was actively engaged last few days ago), but I hope below will be helpful.

Actually, essential factors that galvanic corrosion can happen are the presence of electrolyte & "physical connection between two different metals".
As it were, metal loss due to galvanic corrosion means movement of electron from cathode to anode thru elecrolyte. Generally, one of the common zones of galvanic corrosion is at flange joint.

For this case as there is welding(309L) related between two different metals, the exact applicable mechanism would be "PWC (Preferential Weld Corrosion)". Actually, the mechanism of PWC is not simple. There are multiple variations to affect the degree of the corrosion mixing together and every each case should be reviewed and confirmed as well. (I have seen 3 days advanced training course to handle only PWC.) If you refer to below link, several cases were reported even in low conductivity environment(CO2 containing), deoxidation product,biocide, etc. Again, it is not simple mechanism.

https://www.twi-global.com/technica...he-preferential-weld-corrosion-in-c-mn-steels

I think nowadays dissimilar metals' welding is being ruled out due to the complicating varizables. (The application of DMW is applied mostly for the case where liquid water (electrolyte) is not present without high temperature, H2S and hydrogen exposure.)

For your case also, I would recommend to find solution with same metals' welding to avoid any uncertainities.


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