Insulation kit or Isolation spool (Galvanic Corrosion)

[Jela123]

Hi guys. Can someone help with this please?

Is there a rule or standard that differentiates when to use insulation kit as compared to isolation spool or vice versa?

How do you determine which one to go for during design of piping material to avoid galvanic corrosion of dissimilar materials if we assume cost is not an impediment?
Thanks

 

[engg1405]

Hi Jela123,
By my experience, generally insulation kit will be used in piping system inside the plant which is being recommended between flanges made of different materials having moderate or high galvanic potential difference, where as the Isolation spool will be used at pipeline air to soil interface & other dissimilar dissimilar material flange joint where CP not required.

Jeyaganthan
Corrosion, Materials & Integrity

 

[Jela123]

Thanks @engg1405.

I concur with what you said regarding the isolation kit but the isolation spool explanation doesn't seem to be true because you will usually have a pipeline polyurethane wrap for the soil to air interface of pipeline to avoid soil corrosion. Have not seen where a spool within soil to air interface of a pipeline.

The issue is I have client requesting isolation spools on almost every dissimilar material connections including titanium spools.

 

[Chumpes]

Hello
we were using insulation spools between 2 pipings in order to limit CP current leakage from a CP protected piping system to the other that does not have to be CP protected. This CP current leakage can establish between the unprotected piping and the protected piping thanks to conductive fluid flowing into the pipes (water), and the result is acceleration of internal corrosion of the unprotected pipe, close to the connexion between the 2 piping systems, when an insulation kit was installed. The installation of an insulating spool will increase the distance between the protected and unprotected pipe systems, and thus the resistance of the conductive fluid between the 2 systems will increase too, which will reduce the leakage current and reduce the potential accelerated internal corrosion of the unprotected piping system.

 

[GarryB1957]

Hi Jela123,

You don't mention what the pipes are carrying but I will try to help.

Firstly Insulated Flanges and Isolation joint have many applications, primarily to seperate pipework with active CP system and pipework without CP system. However, they can be used purly to prevent or reduce galvanic corrosion between pipes of dissimilar maters.

Insulated flanges are gernerally used above ground for many reasons (to facilitate future break point etc). Where as Isolation joint (spool) is generally used beolow ground. The thing to be aware of is what is being carried inside the pipe. If the substance inside the pipe is an electrolyte (electrically conductive) and Cathodic Protection is applied to 1 section of pipe, the CP current can be drained back to source through upstream pipe, then the isolation joint is the least recommmended solution. The reason for this is at pipe joints with an electrolyte flowing through them, the pipe internal surface should be coated for a minimum lenght 2.5 time the diameter either side of the joint, to reduce the ionic electricity flow at this point. This is easily carried out to a flanged pipe after the flange has been welded on. This is not possible with and isolation joint as the final weld will remove any internal coating at that location, thus negating the coating efficiency.

Any pipework that carries an electrolyte should not been isolated underground to reduce the amount of cathodic interference with the above ground pipework.

Just as a foonote, as a Cathodic Protection Engineer I have travelled the world for work extensively. Nearly every country I have been to I have seen insulated flanges without an external debris guard. When using insusulated flanges it is vital to use a debris guard as any conductive foreign mater that gets trapped between the flanges can cause them to fail. This would require stripping the flange down, removing all foreign matter and reassembling the joint.

 

[cap1a79]

Chumpes / GarryB
CP current cannot penetrate thru the pipewall. As per basic corrosion model, in order to occur corrosion, same fluid shall be contacted with cathode and anode.
External CP current can only flow along with the external surface horizontally in the figure by the driving force between + and -. For vertical direction (thru the wall), there is no driving force nor supported corrosion model.
Generally, one of the common cases that isolation spool required is to prevent drain CP current. For example, in case below ground pipeline connected with above ground pipeline that has numerous supports and structural steels electrically contacted and exposed to soil, then CP current which is supposed to protect only underground one could protect unwanted supports & structural steels unnecessarily.

Please note that the above is my own personnel technical opinion and your input is always appreciated. Thank you for sharing the figure.

Jela,
Regarding galvanic corrosion, the below can be referred while refer to the above for isolation spool. Note that as long as the insulation kit blocks physical contact between 2 different metals, it is sufficient to prevent galvanic corrosion.

Quoted from API 571 4.3.1.3
a) For galvanic corrosion, three conditions must be met:
1) Presence of an electrolyte, a fluid that can conduct a current. Moisture or a separate water phase is usually required for the solution to have enough conductivity.
2) Two different materials or alloys known as the anode and the cathode, in contact with an electrolyte.
3) An electrical connection must exist between the anode and the cathode.


Lee SiHyoung,

 

[GarryB1957]

cap1a79,

I totally agree external CP systems do not effect the internal; structure of a pipeline as it is a seperate electrolyte and the normal requirments for a corrosion cell are not met. However an electrlitic substance within the pipe does have an effect on the amount of CP current required to protect the external pipe.

I commisioned a CP system in Algeria, some 30 gas wellheads and over 100km of variuos flow and trunk line. At commisioning of the CP system the wellheads had not been commisioned, so the pipes were internally dry and as it was new pipe with a very good coating the current required from a single deepwell groundbed was circa 3 amps. After the wellheads were commisioned with raw untreated gas, this introduce not only the gas but an electrolyte into the pipe system and the pipe potential was raised to an unacceptable level and the protection criteria was not met. A further 4 amps of CP current were required to re-establish the specified protection criterion. This is a common occurance, with ionic current flow passing through the internal electrolyte to earth and back to the anodes. This can be observed by monitoring earth rod potentials as the electrolyte is introduced into the pipe, as these earth rods will become more negative.

API571's definintion of galvanic corrosion is at best sketchy and is certainly not full. For galvanic corrosion to occur there are more than 3 criterion that need to be met, even if you class the anode site and the cathode site as a single criterion, other criterion would also be a mechanical bond between anode and cathode, an electrlyte in contact with both anode and cathode, and the avaialability of oxygen. There is no need for 2 different materails as single piece of steel will corrode, this is a galvanic corrosion caused by induvidual grains of steel having different potentials, the grains of steel with the most negative potentials will be anodic whilst the grains of steel with the most positive potentials will be cathodic.

 

[Jela123]

Thank you for the responses Chumpes,GarryB1957 and cap1a79.

My question is mainly regarding connection of dissimilar materials from galvanic corrosion point of view. Not CP related.

For example, if I have a conductive fluid passing through a titanium line outlet connected to a carbon steel line, how do I determine whether it is okay to use just isolation kits rather than using 10D internally coated isolation spool of titanium?

The 10D isolation spool of the more noble material will be much more expensive than the isolation kits so how does one justify the use of one over the other without going into using a 'hammer to kill a fly' situation?

 

[cap1a79]

GarryB1957,
Thank you very much for your input.
Would you please share any link or name of any case report you have mentioned for the effect of the electrical substance? I would research it a little bit more academically.

Meanwhile, note that the input with API 571 was the response from the query about galvanic corrosion with two dissimilar metals, NOT CP.



Lee SiHyoung,

 

[GarryB1957]

Jela 123,

Both Insulated flanges and monoblock Isolation Joint, achieve exactly the same results in electrically isolating the sections of pipe. Without other influencing factor as in my previous post I can see no technical justification for the use of the much more expensive monblock system.

 

[GarryB1957]

cap1a79,

I don't have any standards or case studies to hand on internal current drain through electrolyte past isolation. This is an experience thing with me as I only undertake field work on Cathodic Protection. However, I will ask a few of my contacts that are senior technologists with Nace and ICORR for any technical documents or case studies.

 

[Chumpes]

Hello Cap1a79

CP current cannot penetrate thru the pipewall. As per basic corrosion model, in order to occur corrosion, same fluid shall be contacted with cathode and anode.
External CP current can only flow along with the external surface horizontally in the figure by the driving force between + and -. For vertical direction (thru the wall), there is no driving force nor supported corrosion model.
Generally, one of the common cases that isolation spool required is to prevent drain CP current. For example, in case below ground pipeline connected with above ground pipeline that has numerous supports and structural steels electrically contacted and exposed to soil, then CP current which is supposed to protect only underground one could protect unwanted supports & structural steels unnecessarily.

I do not agree with you. We studied this effect and it exists, and insulation sleeves are used also against it, as well illustrated by GaryB1957 : If you were right, how do you explain external CP demand is increased by the presence of an electrolyte INSIDE the pipe while they are insulated from one another by an insulation kit ?

If the water into the pipes is very conductive, then the electrical resistance from the unprotected pipe to the protected pipe can be lower inside the electrolyte that compared to the soil, and an electric current will establish into the water inside the pipe the way it is represented on the picture. I don't talk about galvanic coupling of different metal or electrochemical cells etc. it is just simple resistance / intensity / voltage considerations...

A lower resistance for an electric current path does not mean all the current will go through this path of lower resistance, but just a larger amount.

 

[cap1a79]

Chumpes,

Still I am not convinced with that external CP current can penetrate thru pipewall and reach at the internal fluid as long as internal and external side do not contact with same fluid.

Meanwhile, have you considered temperature correction factor? Some temperature correction shall be made for the current density due to operating temperature that can affect outside by temperature gradient. Generally, around 25 percent per 10c rise in temperature is considered above 30c. My doubt is during the operation, the operating temperature affected the outer surface by temperature gradient requiring high demand of CP current. Then, it is not an internal corrosion issue at all!!

Lee SiHyoung,

 

[Chumpes]

[Still I am not convinced with that external CP current can penetrate thru pipewall and reach at the internal fluid as long as internal and external side do not contact with same fluid.]
>> why current would not pass through a conductor ?

[have you considered temperature correction factor? ]
>> Do you mean that increased temperature induces increased corrosion and therefore more electric current is required for CP ? yes, I knew it ! =)

Modification of the electric current demand may also be partially explained by a modification of the temperature, but I confirm that with exactely the same temperature, the CP system can increase internal corrosion of the unprotected pipe as described in the drawing.

 

[cap1a79]

Chumpes,
Thanks for the reply. Let us discuss more for some finding.

1.Based on basic corrosion model, corrosion cell can be valid only if cathode and anode contact with same electrolyte ( same fluid) while the figure as above does not seem like that.

2. My original doubt was the reason why current demand had been increased "only when pipeline started operation" is due to temperature gradient from the started operating internal fluid affected outer surface so that the demand was increased.

3. Do you have any reference for the conclusion you have made for the passing thru the wall of pipe?

Lee SiHyoung,

 

[Chumpes]

hello
1.Based on basic corrosion model, corrosion cell can be valid only if cathode and anode contact with same electrolyte ( same fluid) while the figure as above does not seem like that.
>> Both the anode (internal surface of the unprotected pipe) and the cathode (internal surface of the protected pipe) are in contact with the same electrolyte (water inside the piping system).

3. Do you have any reference for the conclusion you have made for the passing thru the wall of pipe?
>> item N°32 of the following link (abstracts from 2011 Cefracor conference on CP, Aix-en-Provence, France) :
Link

regards

 

[wolfzsx]

Hello,
I've found some cases about the unprotected side corrosion of the insulator joint for CP.
I think the root cause of the failure is 1) the conductive fluid which acts as the electrolyte. 2) the anode and cathode/the electrical potential between the unprotected side and protected side inside the pipe is different. 3) the connection of the outer side of the pipe. I found the corrosion occurred especially for the pipe underground.

 

[Corroneer]

@Chumpes … would you please upload a snapshot of the reference … the link does not seem to work with me.