Galvanic corrosion for material choice

[adrich91]

Good afternoon,

I have a couple of questions and proposals related to the phenomenon of galvanic corrosion that I would like to know your opinion.

Context: At the moment I have a cast iron pump, 3 way valves in carbon steel flanged, other 2 way valves in stainless steel, carbon steel piping and some accessories (pressure gauge, level gauge, etc.) in stainless steel. With these materials, I want to assemble a small plant for the transfer of fluids (aromatic hydrocarbons C10 more or less) that will be outside (humidity, rain...but not sea water).

I have always been taught that it is not ideal to mix materials when dealing with metals with different potential to avoid corrosion phenomena, but here I have several doubts:

1. If the connections between the valves and the piping are by flange (one flange would be carbon steel and the other stainless steel), is it enough to put a dielectric gasket to avoid galvanic corrosion? Or is it advisable to coat everything with some kind of paint?
2. If the connection between the carbon steel pipe and a stainless steel valve is welded, would it be enough to use some kind of welding material to insulate the two metals?
3. In the case of buying new valves, I understand that it would be better if they were made of carbon steel as the pipes, but if it is not possible, I understand that between cast iron and stainless steel would be better cast iron because it has a potential difference more similar to carbon steel, right? Nevertheless, In terms of INTERNAL corrosion between the material and the fluid, obviusly stainless steel is preferable to cast iron, but I don't know if it compensates due to higher galvanic corrosion between stainless steel and carbon steel

In principle these are the doubts that I would like to corroborate. I don't have much knowledge in terms of corrosion, so any opinion or comment is as always welcome.

Thank you guys

Kind regards,

 

[EdStainless]

I presume that your process side will always be dry, is this correct?
So we are talking about external corrosion.
Are all of the CS to SS joints horizontal?
Allowing things to drain (no puddling) helps a lot.
The SS is likely to cause some localized accelerated corrosion of the CS in areas directly adjacent to the joint.
But this will mostly be cosmetic, more rust staining.
There could be some attack of the bolts in flanges, how about coated bolts?
You could use isolation kits on the flanges, but they are a real pain.
Painting the SS might be a better option.
If this was internal I would suggest coating the inside of the CS piping for 10-20 diameters from the SS parts.