Stainless Steel / Carbon Steel Galvanic Corrosion

[Gcooper79]

Hi I posted this earlier in the wrong forum,

I am designing a lube oil pipe system to be installed in a refinery close to the coast. The oil system is fabricated from unpainted 316/316L stainless steel pipework and painted ASTM A105N carbon steel valves with stainless steel trim (Company standard) - the flanges and valves will be connected with SW gaskets comprising of stainless steel inner and outer rings / 316 windings with graphite filler. Bolting will be ASTM A193 Gr B7/A194 Gr 2H, Xylan 1070 Coated.

My client is questioning how we protect against galvanic corrosion.

From my understanding the factors which affect galvanic corrosion are electrical potential of the materials, mass of the materials, contact surface area of the materials, process medium, and the local environment of the pipe system.

My questions is, that as the contact between the materials is relatively small (windings surface area) would there be any significant corrosion or would I need to provide isolating kits?

Thanks in advance for any advice provided.

Graeme

 

[TugboatEng]

The key word is "should". There are many reasons not to run stainless steel but so far none to not run 90/10 cuni. I can understand that some petroleum products are not compatible with copper alloys. Some examples would be crude oil due to sulfides or odorized LP gas. Lube oil should be very inert in copper. If you're painting in broad strokes, yes stainless covers all cases but there are many cases where other materials will work, possibly better.

Speaking of painting, my understanding is that it's generally not wise to paint 300 series stainless steel as coatings will have defects and these defects can become sights of crevice corrosion or SCC.

Copper alloys and SS couple well in most applications so I think you would still be able to use any off the shelf SS sensors and valves and not have to source brass or bronze components.

 

[GabrieleB]

It is not a good solution to connect AISI 316 pipe with Carbon Steel body valves. For sure the surface and the possibility of humudity can increase or not the problem. Remember that galvanic corrosion can occur only in presence of water beetweend them. If you don't have the conductive solution (water) you will not have corrosion. Therefore if the environment is usually "dry" you will not experience big galvanic corrosion phenomena. In any case the potential risk is present and the use of AISI 316 valves body with AISI 316 PIPE would be the commonly accepted solution to avoid the risk.

 

[Gazzaj2101]

The simple answer is yes fit an isolating flange kit on the flange connections. If you achieve electrical isolation between the carbon steel valve and the stailess steel pipework, then the issue of galvanic corrosion occuring between the anode (carbon Steel valve) and the cathode (ss316 pipe) in largely mitigated.

It should be noted that an isolating kit for this type of connection comes in 4 parts, the gasket needs to be electrically isolating, there needs to be isolating sleves for the bolts, isolating washers for the bolt head and nut and finally a protection seal to seal the joint of the flanges to prevent foreign matter and water ingress.

The size of the mechanical connection between the carbon steel valve and stainless steel pipework would not mitigate galvanic corrosion occuring as the current flow in this type of corrosion cell is extremely small, so any direct mechanical connection between the 2 types of material will allow galvanic corrosion to occur. The level and rate of galvnic corrosion will be dependant on the amount of electrlyte and the resistivity of the electrolyte. It should be noted that even in deserts with high temperature and low humidity, corrosion occurs, just because we cannot see of feel the presence of an electrolyte, does not meen it is not present.

As pointed out by many others there are many other corrosion mechanisms to be considered on this type of installation, but electrical isolation of the 2 different materials crosses off the galvanic interaction issue. I hope this is of some help.