Corrosion pieces

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Big Pete
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Corrosion pieces

Post by Big Pete »

Can anyone help me?
According to the drawings of the sea water system on my ship, Corrossion pieces were fitted in the pipe systems. Most of the system is made of 90% Copper, 10% Nickel Alloy, with chemical dosing pipes in stainless steel.
There are some pipes in Galvanised steel, which I suspect must be replacements for the original corrossion pieces.
In a Galvanised steel system I would expection corrossion pieces to be made of soft or wrought iron.
Has anyone any knowledge of the type of material that would be used for corrosion pieces, to act as sacrificial anodes, in a Cu/Ni alloy system?
Is there any standard interval for renewing these corrosion pieces/pipes?
Should each pipe be electrically bonded to the adjacent pipes , to ensure protection against Galvanic action?
Thanks for any sugestions. :?
It is always better to ask a stupid question than to do a stupid thing.
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The Dieselduck
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Re: Corrosion pieces

Post by The Dieselduck »

I wasn't sure what exactly you mean in your question. But I have used iron bars as sacrificial anodes in a Cupronickel piping system. At least that's what I was told to do, and I did not see any subsequent problems.

I was looking online for the Noble Chart, I believe it is call, which list in order of their galvanic action, the various metals commonly used in our trade. I couldn't find the simple one I was thinking of, but you can check out this one on Wikipedia (http://en.wikipedia.org/wiki/Galvanic_series), which list iron as being less noble than Cu/Ni and therefore a suitable sacrificial anode.
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JK
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Re: Corrosion pieces

Post by JK »

I am not sure either what you are asking, which is more my reading comprehension then your engineering, I assure you!
Personally I suspect the galvanized pipe is replacement for the original piping due to the ordering lead time and cost of the CuNi piping-but could be completely wrong.
The CuNi SW piping on our vessels has lasted upwards of 20 years. There are jumpers that go across the flanges that otherwise would be isolated by the jointing.
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Re: Corrosion pieces

Post by Big Pete »

Hi Guys,

Thanks for your replies, they confirmed the way I was thinking.
In the drawings the corrossion pieces are just short lengths of pipe, in the middle of the pipework between the sea inlet and the cooler and between the cooler and the overboard discharge. On the drawings we have a total of 6 plate coolers fitted with these corrossion pieces.
These are the ones that are now Galvanised Steel. (and corroding through!)
The ship was only built in 2000 so I do not think the CuNi pipes have worn out been replaced with steel and that these have then rusted through.
Most Plate Heat exchangers in sea water systems are fitted with a bobbin piece/spool pieces/(short connecting pipes with a flange at each end) cast in soft iron that connects the end cover of the heat exchanger to the sea water pipes and acts as a sacrificial anode. These are small and easily accessible for changing and provide protection against Galvanic corrossion to the rest of the pipework, which as we all know is often in awkward places.
I have often seen these replaced with Galvanised steel on grounds of cost and faster availability, and I assume the same has happenned here. ( Also because people do not recognise what they are, or understand their importance)
However the vessel I am serving on is a DP2 offshore construction vessel and we do not want to lose cooling pipes, and consequently 50% of the machinery, when we are working.
I couldn't see how effective protection against Galvanic action could be obtained unless there was continuous electrical connection from sea inlet to overboard discharge.
I guess I will have to try and contact the shipyard that built the ship to get a definitive answer. :)
Thanks again for the help. It's a great website. :D
It is always better to ask a stupid question than to do a stupid thing.
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JK
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Re: Corrosion pieces

Post by JK »

Actually when I read you original post, I thought what dumbass would design a seawater system meant to have piping fail :roll: 'cause we all know it FAILS at the WORST possible time. Then I thought it was such a dumbass idea that I had to have misunderstood your post.It is even worse to hear that some dumbass designer actually thought is was a good idea. :lol:
I would put CuNi pipes in there.
As for the iron piece to the cooler we tend to coat the inside of them with a apexior, so they don't rot out. With titanium plates in the coolers, we have not had problems. Of course we have central cooling on all of our ships so the SW piping is right there in front of our faces and visible.

I still can't believe someone would put a sacrificial pipe in a sea water system.....what if your valves didn't shut tight? We all know what happens after 3-4 years of in the water service.
Then again I'm only a marine engineer...in my organization they seem to think the guys with the iron rings are the end all, be all. (Or is that be all, end all :oops: )
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Re: Corrosion pieces

Post by Pacific Barbarian »

I currently sail on AHST vessels. Corrosion pieces are the nnorm in our sea water systems. Planned maintenance schedule to inspect and replace at regular intervals, not sure how often. Spares are kept on board, normally 2 in use, 2 spare.

I read in above posts that some people actually galvanize these pipes, not knowing what they are, thinking it will last longer. WRONG :lol:

Look at this chart, what does it tell you?

Noble (Cathode) :mrgreen:

Graphite

Platinum

Ni-Cr-Mo alloy

Titanium

Ni-Cr-Mo-Cu-Si alloy

Ni-Fe-Cr alloy (type alloy 825)

Stainless Steel Alloy"20"

Stainless Steel type 316, 317 in passive state

Nickel-Copper Alloy, type Monel

Stainless Steel, type 302, 304,321, 347 in passive state

Silver

Nickel 200

Ni-Cr alloy, type Alloy 600 in passive state

Ni-Al bronze

Copper-Nickel 70-30

Stainless Stell, type 430

Copper-Nickel 90-10

Stainless Steel, type 410, 416

Bronze

Aluminium Brass

Copper

Tin

Naval Brass, Red Brass

Stainless Steel type 316, 317 in active state

Al-Bronze

Stainless Steel, type 302, 304,321, 347 in active state

Low Alloy Steel

Mild Steel, Cast Iron

Al-Alloys

Zinc

Magnesium

Active (Anode) :oops:

To galvanize the pipes you are creating a serious sacrificial anode, as the rate of corrosion will actually be increased! The zinc coating just speeds up the whole process. Better of sticking to original mild steel pipes so it can do what it was designed to do! Zinc is lower on the chart than mild steel.

Have you guys ever seen "engineers" :shock: put galvanized pipe fitting in sea water systems, eg galvanized plugs on sea water pumps? Well, not for very long anyways! Mild steel will last longer. It seriously winds me up, for they go, as you know, not during convenient times. Stick with brass plugs, not eveen stainless, for stainless also corrodes!!!

Brush up on the materials guys!
Big Pete
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Re: Corrosion pieces

Post by Big Pete »

Galvanized piping works very well so long as the entire pipe is galvanised, and therefore at an equal electrical potential. Once there is any local damage to the galvanising it creates a corrossion cell, and you have a problem.
Personally I prefer to replace corroded steel pipe with thick wall pipe that is not galvanised.

Stainless steel is only stainless in an oxygen rich atmosphere, a protective oxide layer forms which protects the metal from further corrosion. In an oxygen depleted location any damage to the oxide layer will not repair itself and the steel will rot out.

The best example I have seen was on a ship that had stainless filters in the main sea intakes. The bottom part of the filter baskets were below the water flow in stagnant water and rotted out. The upper part was in the water flow and was as good as new.

I have been on a couple of ships that had plastic sea strainers, which surprised me at first, but they worked well, and obviously no corrossion problems.

Thanks for your comments.
BP
It is always better to ask a stupid question than to do a stupid thing.
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