Queries on Galvanic Corrosion

Because your client is not very knowledgeable?

Does the insulation gasket you have also include insulating washers and bolt covers to provide electric isolation? Perhaps if you can show the diagarm below (or google cathodic protection isolation flange) the client will trust you. Otherwise, as long as the insulating spool piece does not reduce the integrity of the system go for it...the customer isn't always right but a happy customer tells their freinds (I would just make sure to explain as well as possible and make sure the client knows you are doing it because they insist...that way when their freinds tell them they are stupid the friends will know you at least tried).

Drew K

"I am trying to figure out why he doesnt want me to use insulation gaskets."

Because he's the customer, and he's paying for it. He's also paying you to produce equipment to his specification, not your guess about what he wants.

Tell him it will cost more his way...see what he says.

What is your aversion to the spool?

They generally provide a better isolation than a simple flange gasket, and are much easier to test for such continued isolation.

If it being specified, then I would expect that the extra cost would also be covered.

Depending on the fluid being transported and the velocity of same, crevice corrosion of the SS is possibly a bigger concern.

I would go with #4 and #5.

This is a fair request by your client, I do not know what type of flanges you are using, plus what is the substance flowing through the pipes. I am sorry to say that one of the suggestions is to isolate the bolts is also incorrect, in order to get total isolation from each alloy you would need to insert a insulation spool which is designed to take the same pressures. Galvanic corrosion is a huge study alone which corrosion engineers having to be up to date with as designs change yearly. Both Robert Baboian and Steven L. Pohlman have explained this in great detail in the ASM HandbookVolume 13 Page 83, they have covered the following: Galvanic series, Polarisation, Area distance and geometric effects, Modes of attack, Predicted Galvanic Corrosion and methods of control. I do not know if you are an engineer if so you research from the above will give you your answer, however if your are not a Mechanical or corrosion engineer you might need to seek their advice on why I am recommending that a isolating spool is the solution I do however think it is a very good question to bring up and I am sure lot of people will learn from this to prevent possible disaster or fatal injury if this is not addressed properly in the Oil and Gas industry. Both Crevice and pitting corrosion if unchecked is dangerous.

So you are saying all the isolation flanges I have encountered on fuel and water systems are incorrect? I have seen isolation flanges with nonconductive gaskets, insulating washers and bolt sleeves used much more than insulating spools (can they be used with low flashpoint fuels?).

Drew K

oh , c'mon , give a first time poster a break (meaning, go easy on 'im)...

... or we might never see what he's capable of on a Good Day.

Feb 22, 2013 ^^^

No! I really am curious...I am still new at the cathodic protection game and didn't have anybody to ask the question of (that knew that a cathode was) back when I saw those particular isolaiton flanges.

Drew K

The more distance you put between the two dissimilar alloys the better as far as galvanic isolation is concerned. Suffice it to say that in this present case, I think the answer where they use an isolation gasket, with bolt covers and isolation washers will do the trick. If the alloys were further apart in the galvanic series, I would think that an isolation spool would be in order.

B e t t e r ... "tab-in" here. 8 responses, and nobody yet has asked:

Is the pipeline above ground, below, submerged...?

What is the pipe carrying...? What SIZE is it...?

There ARE valid reasons for NOT using a simple Isolation_Flange ... but, as in MANY instances, "Circumstances Prevail".

((( Know what they ARE before assuming / answering...! )))

{ Nobody mentioned that Standards DICTATE : "No isolation flanges" in an area that might have or develop an explosive atmosphere."...}

Come back, vanuta ...

What's so special about a spool? It entails two sets of glanges, gaskets, and bolts. One way to look at that is twice as many failure points.

. . . well , I WHOLEHEARTEDLY agree with you on that statement ("for what it is worth" ... "standing-alone") ... but/yet , since I never mentioned "...a spool piece", I am assuming that you intended to elicit a response from somebody else...?

............

Can you please further explain "{ Nobody mentioned that Standards DICTATE : "No isolation flanges" in an area that might have or develop an explosive atmosphere."...}

Before I got started with cathodic protection I have seen them used in jet fuel pumphouses where an underground cathodicaly protected pipe enters the pumphouse through a concrete wall. The pumphouse was an area that could develop an explosive atmosphere. They had installed some sort of bonding strap over the isolation flange, exept the bonding strap had some sort of fuseable insulator incorporated in it.

Drew K

...... "shame-on-me"... tho' I try to be as complete as I can with such responses, I did err by not completing the statement:

Isolation flanges should not be used where an explosive atmosphere might exist, *without* the addition of a bonding strap (to preclude any possibility of a static charge on one side discharging across to the other, thereby providing an ignition source to the potentially explosive atmosphere).

I "tabbed-into" the discussion due to seeing so many responses that were (as is all-too-often the case) skipping-over the need for more input! (G-I-G-O)

((Of course , once the metallic pathway is re-provided (by a bonding strap), the corrosion issue "returns" ... again, dependent upon so many other factors that may-or-may-not exist , at this point it is still simply one , Big , "maybe"...))

And, if the owner/contractor has a "Money-is-no-object" attitude, there are ways to "spend-around" (i.e., work-around) the bonding-issue.......

Y e t ... we still have not heard back from the OP for ANY further input ... and so, it is "put-to-rest" for now....

I wish I had a picture of the bonding straps they used...it confused me greatly at the time. I had asked all the questions I knew to ask and got the best answers available at the time.

I know much more now and and even more confused by by what I remember (could be because of faulty memory). I do recall asking why the new bonding straps were different (all the old flanges had a copper strip bond). The new ones seemed to have a clear plastic cover over some sort of fuse-able link. The had installed the new one with the isolation flanges, I don't know if we had cathodic protection before I saw the flanges but they pointed the rectifier out to me when I asked.

Any idea what type of bonding strap I was looking at?

Drew K

re: "...what type of bonding strap..."

Since we don't see said items in the underwater realm, chances are that I've never seen what you are referring-to ('cept mebbe "in-passing" at one of the Corrosion Expos).

However, I might hazard to guess that such 'fusible links' might be used to preclude any heavy-duty stray currents from "above-ground" making their way below the surface. We all know what happens if such a charge decides to LEAVE a pipeline through a single , small holiday in the coating...

[This is exactly the right sort of question to direct towards the Corrosion Engineer for the project...(!)]

I wonder how the guys who pointed-out the rectifier would've answered if you'd asked them : "Can the cathodic current pass-through the strap and protect everything both below and above ground...?"

(!) Many Most people are blissfully ignorant when it comes to such basics!

YOUR CLIENTS ARE RIGHT!!

-This insulation spool piece ,is a good solution against corrosion!!

Any way the proposed sketch with insulating parts -is the second best (from experience....) As was allready stated -corrosion also depends on fluid quality flowing in pipes.