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

 

[SJones]

https://www.eng-tips.com/search.cfm?q=galvanic+corrosion&action=search

There is a lot of garbage to sort through, but if the link in this particular thread works it will be helpful:

https://www.eng-tips.com/viewthread.cfm?qid=362955

since it saved me risking the copyright police



Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/in/drstevejones

All answers are personal opinions only and are in no way connected with any employer.

 

[EdStainless]

So this oil system is cold, correct?
I ask because external CSCC on SS is not unheard of.
Many coastal plants require the painting of the exterior of SS piping.
How exposed is this piping?
Any insulation or covers?


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P.E. Metallurgy, Plymouth Tube

 

[SJones]

And, painting the carbon steel is not a good idea either. But, it's lube oil, and unless any loss of containment becomes an ignition source, an environmental no-no, or leads to a shut down of a high revenue stream, it really shouldn't be up there in the scheme of things for a refinery where there is a lot more serious stuff going on.

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/in/drstevejones

All answers are personal opinions only and are in no way connected with any employer.

 

[Gcooper79]

Hi steve,

Thanks for the link, I’ll have a look through it when I’m back in the office on Monday.

Hi edstainless,

The oil temp will be 100deg C upstream of the cooler and 50deg C downstream. The upstream pipework will have perforated plate as personnel protection.

The min ambient temp is 17deg C with a max of 32deg C

Graeme

 

[SJones]

Your client's concern should be more centred on the bare stainless steel.

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/in/drstevejones

All answers are personal opinions only and are in no way connected with any employer.

 

[Gcooper79]

Hi Steve,

The stainless steel will be pickled and passivated. As I understand it, stainless steel in the passive state will have minimum corrosion. Is this correct?

Graeme

 

[EdStainless]

If you are near the coast then the surfaces will see salt fog regularly.
The accumulation of salt on the surfaces (and in crevices) will accelerate the corrosion of both materials.
316 will stand up to this if it is frequently washed with fresh water, otherwise it won't.

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P.E. Metallurgy, Plymouth Tube

 

[Gcooper79]

Ok,

I’m not sure how they plan to maintain the system at site but they have specified unpainted stainless steel so may be the are planning to wash it down regularly.

With regards to galvanic corrosion, the oil is not an electrolyte but would the sea fog be considered one?
If the contact between SS and CS is only at the gasket surface, would the mass of the CS valve be sufficient to minimise corrosion in respect to the gasket mass or would the SS pipe mass also count?

If there is corrosion, is the only option to use isolation kits (other than use SS valves)? I’ve seen people suggest coating the SS but what would that entail?

Sorry if these are basic questions but this is new to me and I am trying to learn.

Thanks for all the responses so far.

Graeme

 

[Compositepro]

I believe that painting stainless steel is rarely a good idea. Stainless works by developing its own protective oxide layer on its surface. Paint is likely to eventually trap salts in cracks and blisters and interfere with regeneration of the oxide layer.

 

[EdStainless]

CP, There are refineries and chemical plants along the US Gulf coast with hundreds of miles of painted 304 and 316 pipe.
In a system like this there is the likelihood of crevice corrosion in all of the flanged connections.
How are you planning on protecting them?

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P.E. Metallurgy, Plymouth Tube

 

[SJones]

If there are flanges with fasteners, then contact will not just be at the gasket surface. Marine atmospheres will deposit salts that will become concentrated over time serving both as an electrolyte and as a means of breaking down the film on the stainless steel. Some operators adopt a philosophy of painting the stainless in the belief that it reduces the likelihood of failure (possibly on the tenuous basis of simply reducing exposed surface area). Other operators would advocate thermal spray aluminium as the only coating with any reasonable chance of providing long term, reliable protection.

One issue in the scenario under discussion, is that of the stainless (cathode) being unpainted whilst the carbon steel (anode) is painted which could focus any bimetallic effects to very small pinholes in the coating. That is why it will be noted that some recommend the stainless be painted and the carbon steel left bare, or both be painted. Isolation will be prone to ineffective operation either by poor installation, or by instrumentation and earthing connections.

Again, bimetallic corrosion is a lesser issue when considering other corrosion mechanisms that could impact upon the piping system as a whole given its location.

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/in/drstevejones

All answers are personal opinions only and are in no way connected with any employer.

 

[Gcooper79]

Hi edstainless,

The flanges upstream would be covered with insulating jackets, the flanges downstream would be unprotected.

Hi Steve,

The fasteners would be coated in xylan 1070 so I assumed this would be enough of a barrier against the corrosion.

With the valves, I should have mentioned that the bodies would be painted but the raised face would be machined.

Graeme

 

[SJones]

The bolt installation process is tough on coatings and can induce damage, but that's not the main issue. You need to find out why your client is more concerned about galvanic corrosion of carbon steel valves than stress corrosion cracking of stainless steel. Try digging out a materials selection report to establish why your design office colleagues feel that this situation is acceptable.

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/in/drstevejones

All answers are personal opinions only and are in no way connected with any employer.

 

[Gcooper79]

Hi Steve,

I think the main reason the client is concerned is that they specified SS valves in the spec but our sales team submitted a deviation for CS valves which was accepted by the client. If I have to guess, I'd say the end user is pushing our client to use SS valves but as we have a contractual agreement to use CS, they don't want to pay the additional cost to upgrade. Therefore, they are looking for a valid reason why CS and SS will be ok with respect to galvanic corrosion.

Graeme

 

[SJones]

There isn't a more classic arrangement to cause this type of discussion. Since the principal contractor accepted the deviation, that acceptance should have been based on the corrosion risk associated with the change being acceptable to both themselves and the end user. If their client doesn't like it, it's not your problem. Sit back, relax, and enjoy the wailing and gnashing of teeth.

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/in/drstevejones

All answers are personal opinions only and are in no way connected with any employer.

 

[Gcooper79]

Yeah I agree, it's not really our issue but I am trying to help my client out, suppose it just my good nature!

The oil system is designed to API 614 5th edition, so I had another look through that. The specification doesn't really state the material of the valve bodies, only that the trim should be nominal 13Cr stainless steel (5.1.23) - this may be why our sales team deviated to the CS valves.

para. 5.1.24 states

Instrument valves for oil and gas service located in sensing lines downstream of a primary service block
valve may be bar-stock instrument valves, provided the instrument valves are protected against accidental
disassembly. Valves shall be stainless steel or carbon steel with corrosion-resistant plating and stainless steel
stems

But I would read that as instrument valves rather than inline valves.

Thanks for all your help.

Graeme

 

[SJones]

You are just too kind. The end user, and the main contractor (your client), should really be working with a piping service index and associated piping classes, e.g., PIP PNEM0001 Index and PNXSA0B01 (X = 1, 3, 6 for 150, 300, 600 pressure rating) for lube oil. Some piping classes would allow a carbon steel valve with stainless trim in a stainless steel piping arrangement: the PIP piping class is a case in point. However, that doesn't make bare stainless steel the right selection in the first place - see the attachment.

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/in/drstevejones

All answers are personal opinions only and are in no way connected with any employer.

 

[TugboatEng]

I'm not qualified to answer these questions but feel I have some experience. When your spec calls out a "stainless steel stem" for saltwater exposure that doesn't mean much. Our fleet of vessels (harbor tugboats) use 3" ductile iron valves with stainless trim for fuel transfer. The 400 series stainless stems and packing followers are always the failure points of these valves. Crevice corrosion destroys the stems in the packing gland and in the nut. The followers just waste away due to exposure. All bronze valves work really nicely as we use them when the budget allows.

Is there a reason copper nickel pipe is off the table? Pressure or temperature haven't been mentioned but your class 200 or sch40 pipe seems like it would cover any typical lube oil pressures. Lube oil flows tend to be laminar to eliminate erosion issues.

 

[Gcooper79]

Hi Steve,
I agree, normally we work to piping classes supplied by the client and in this case, they did specify SS valves. The problem has arisen due to our sales team deviating from this and the client accepting (tbh, I think the client was in a rush to place the order and agreed to everything without checking with their engineers!). API 614 states 'Austenitic stainless steels are typically not painted.' This may be why they have decided against painting without actually considering the environment the package will be located in.

Hi TugboatEng,

Thanks for your response. The lube system has to be designed in accordance with API 614 which states that the piping material should be stainless steel.

Graeme