Galvanic corrosion: Cooper and Mild Steel

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PhillM

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I have removed my forward chain plates and am adding an extra plate to help spread the load over multiple

I have had it all shot blasted to remove old powder coating and paint and its being welded up this week. Once done I plan to have them powder coated and bed them in with 3M 5200 standard.

After blasting, the chain plates have come up grey but the cover plates (that fit around the chain plates and cover the hole through the deck) have come up salmon pink. I suspect that they are copper.

Will I have a problem with the mix of metals?
 
It's not an ideal combination but there is only 0.2 volts between the two, so could be worse. The powder coating will provide a sound insulating layer between the two, which will reduce any galvanic effect considerably. Careful application of the sealant should prevent most water ingress, a further benefit.

Presumably this combination has been in place for some years? How has it performed previously?
 
vyv_cox said:
It's not an ideal combination but there is only 0.2 volts between the two, so could be worse. The powder coating will provide a sound insulating layer between the two, which will reduce any galvanic effect considerably. Careful application of the sealant should prevent most water ingress, a further benefit.

Presumably this combination has been in place for some years? How has it performed previously?
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Could be 0.3 to 0.4 volts depending on the table you consult.

The difference between cooper and mild steel is usually considered sufficiently large to pose a corrosion problem surely ?


016_galvanic_series.gif
 
I consulted the galvanic series in seawater chart. Copper is shown as -0.32 to -0.4, mild steel -0.58 to -0.72. So yes, in the worst case it could be up to 0.4 difference. I was trying to make the point that if there had been no significant corrosion up to now I saw no reason why it would suddenly start after the refurbishment programme.

It was common practice on tankers for brass nuts to be used on steel bolts. They didn't last for ever but were far better than steel on steel, which corroded together in a short time.
 
There is a very good compound to use between the two, me and friends call it 'the mustard' it is a similar colour and consistency, sorry I cannot recall its proper name, but gooling should find it.
 
vyv_cox said:
Presumably this combination has been in place for some years? How has it performed previously?
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I think that's the key question. Looking at the original spec these lower stays were not present, so I presume a previous owner fitted them. The owner prior to me was a blacksmith and looking at the job itself it does look more "home made" than I would have expected. So my guess is that they have been in place for around 10 - 12 years.

There was quite a lot of corrosion but it looked to me as it it started at the top where the stainless fitting had worked through the powder and then water had got into cracks and started to run down. I didn't see much around the base itself, but I didn't look close enough to be certain the that damage wasn't actually from the base and had worked its way up.

Perhaps I should ask the question another way: Given that the chain plate is mild steel and is powder coated, WHICH metal should the surface plate be made of?

As I have time on my side, should I consider getting a couple of new cover plates cut out, using the old ones as templates and use them instead?
 
In complete ignorance - is it normal to have mild steel chain plates? Or is there not something 'better'. Though as they have lasted 10-12 years then it must be adequate and maybe I have answered my own question.

Jonathan
 
PhillM said:
Perhaps I should ask the question another way: Given that the chain plate is mild steel and is powder coated, WHICH metal should the surface plate be made of?

As I have time on my side, should I consider getting a couple of new cover plates cut out, using the old ones as templates and use them instead?
Click to expand...

J Neeves said:
In complete ignorance - is it normal to have mild steel chain plates? Or is there not something 'better'. Though as they have lasted 10-12 years then it must be adequate and maybe I have answered my own question.

Jonathan
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I think I would consider galvanizing for both the chain plates and steel cover plates. Otherwise, if there is any reason why powder coating is preferred, steel cover plates powder coated although if the cover plates can be fitted so that there is no metal to metal contact with the chain plates then the original copper cover plates should cause no problems.

It seems odd to mix steel and copper if there is no good reason to do. If they are both used powder coating, while it remains in good and undamaged condition, and avoidance of direct contact should prevent any corrosion problems.
 
Salmon pink sounds more like de-zinced brass. Try tapping one with a small hammer,copper will be very malleable compared to brass.
 
J Neeves said:
I had wondered why not galvanised high tensile steel plates that are then power coated. I had also wondered what purpose the copper cover plates served.

Jonathan
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The cover plates got over the chain plate and offer protection to the sealant / bedding compound that makes the hole in the deck where they poke through water tight.

Thanks all for your advice. I believe that copper was used because it's more malleable and do can be bent to fit over the sealant.
 
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