Dissimilar Metals Galvanic Corrosion: SS Tubing Connected to CS Pipe - Is it a concern? 10

[Instrument1352]

Hi,

In general connecting or touching dissimilar metal is not allowed due to galvanic erosion issues. What about the stainless steel instrument impulse line connected to the carbon steel process lines which is very common. I am aware that ideally an insulating mean (e.g. an insulating flange kit) can be used but that is not the common practice as it would be a very expensive practice if used all over the plant.

Another example is having carbon steel line with a stainless steel orifice plate where usually no insulating flange is used. Also looking at standard transmitters, for example in a Rosemount 3051, the aluminum case is not insulated from the SS process connection. There are numerous such examples. I have seen all these in Oil & Gas facilities for the past many years.

One can argue, the galvanic corrosion is a concern in presence of air and water but it is never guaranteed that this does not exist.

I am confused if the general practice of connecting SS tube to CS pipe is right or not. What about having SS orifice plate or SS valve in a CS line?

I appreciate if you could help me understand what the correct practice should be, keeping cost in mind.

Regards,
Instrument Guy

 

[EdStainless]

If you connect a small stainless line to a large steel pipe, and there is an electrolyte present, then the steel will suffer accelerated corrosion near the connection. However since the pipe is large and heavy this is usually of little significance.
Now if you had a full flow meter in SS and put it into a steel line that would be a very different story.
The ratio of the areas is critical. As long as the anode (more reactive) area is much larger than the cathode (more noble) area then there is usually very little risk from galvanic corrosion.

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Plymouth Tube

 

[tbuelna]

...I appreciate if you could help me understand what the correct practice should be, keeping cost in mind....

The correct practice would be to use compatible materials at the faying surfaces, or to apply adequate corrosion protection to prevent moisture intrusion between the faying surfaces. Carbon steel and 18-8 cres have a galvanic potential difference of around 0.35V, which is not too bad. As a rule of thumb, a difference of around 0.25V or less is considered compatible, as long as the local environment is not too aggressive. With a threaded connection between carbon steel and 18-8 cres, applying a sealant to the threads at installation should provide adequate protection against moisture intrusion into the faying surfaces between the two materials.

As for "keeping cost in mind", consider what a failure due to corrosion will cost your company in terms of repairs, downtime, damage to nearby equipment/facilities, credibility, etc. In these terms, carefully applying some sealant at installation of a threaded connection or spending a few extra bucks on an insulated flange kit seems like cheap insurance.

Hope that helps.
Terry

 

[Instrument1352]

Hello Terry,

Thanks for your comment.

You are right on the spot with the "Cost considerations". What was wondering me was that even among the big names such as Shell, BP, ENI, Total, Exxon Mobil (you name it!) everybody does this practice to directly connecting the SS tube to CS pipe and I am convinced with the comments by EdStainless (Plymouth Tube).

I had spoken to numerous people in the industry and almost nobody knew what is the correct reason why this is the industry practice. Many had an idea that it is ok to have small piece of SS connected to a large mass of CS but could not really back up their "belief", but just after I posted this question I studied further and came to know the small "area ratio" or "surface area ratio" is the key as addressed by EdStainless.

Cheers

 

[EdStainless]

The other consideration is the conductivity of the solution. You need an electrolyte for corrosion.
Concentrated acids are non-corrosive because they are not electrical conductors.
If you are dealing with non-conductive solutions then it isn't an issue.

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Plymouth Tube

 

[Instrument1352]

That is a valid argument Ed, however the treaded connections in the field are always exposed to ambient condition i.e. humidity and air. I am convinced with your notes on the area ratio though.

 

[moltenmetal]

Humidity isn't enough- you need liquid electrolyte bridging both components for galvanic corrosion to occur.

In conductive liquid services, or in external environments (i.e. offshore) where carbon steel is already a marginal selection due to general corrosion of the carbon steel, a dissimilar metal pairing between carbon steel and stainless steel is bad news. In gas or nonconductive liquid services or in external environments where you are not depending 100% on a coating for protection of the base carbon steel, these junctions are more a theoretical than a real concern UNLESS the anode/cathode area ratio is also unfavourable.

 

[Duwe6]

The c/s pipe to to s/s instrument tubing question has popped up a couple of times in here, Thus far, nobody has cited any failures. In my 3 decades, it has been the site standard everywhere i've been - nukes, coal-power generation, HRSG's, petro-chem. The 0.35V differevtial does 'bother' me, but experience shows that it is satisfactory.

I dunno - she just works good

 

[EdStainless]

I have seen lines opened up and there are signs of accelerated local corrosion on the ID near the SS penetration, but with heavy wall pipe it simply isn't a big issue.
I have seen SS lines attached to CS compression fittings and the fittings corroded enough to leak.
Be careful where you make the transition.

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Plymouth Tube

 

[Instrument1352]

Thanks everybody,

Here is a definitive answer to the query.

[URL unfurl="true"]http://www.npl.co.uk/upload/pdf/bimetallic_20071105114556.pdf[/url]

Special thanks to EdStainless for the clear explanations & notes.

Now I know what should I do as long as it is concerned to me.

Cheers,

 

[Instrument1352]

Hi,

I am surprised to have read this:

"Be aware that it is often a safety requirement for
all equipment to be earthed. This is common in chemical plant
and on offshore platforms and completely negates any
attempt at insulation. An example is the separation of
dissimilar metal pipes with an insulated flange coupling. Both
sides of the flange can still be connected if both are connected
to earth. The use of coated spool pieces and other methods
of separating dissimilar metal pipes are discussed in detail by
Francis (10)." Ref: [URL unfurl="true"]http://www.npl.co.uk/upload/pdf/bimetallic_20071105114556.pdf[/url]

I have not yet read the original ref by Francis but I had an application where the Utility Water was fed via 1" SS tubing and using an isolating flange connected to a galvanized steel fire water hose reel and at the end, both were connected to earth! All in vain?

This is a serious issue since many believe using an isolating flange would solve the problem of dissimilar metals and never think of the fact that often both sides are connected to earth.

Appreciate your thoughts.

 

[Instrument1352]

For clarification, I am aware that where there is Cathodic Protection, normal earthing is not the practice. My concern is where insulating joints are used but there is no cathodic protection and for safety we are obliged to ground any steel work including piping.

Cheers