Dissimilar metal reactions ..

[Hood]

When i touch the bench it was leaning on and my i-pod player at the same time i get a bit of an electrical tingly sort of thing..... could it be dodgy electrics that caused it?

That can also be a major factor in corrosion on boats, often the boats electrics themselves will have leakage due to damp salty surfaces and some Harbours themselves are really bad for electrical current leaking from street lighting amongst other things.

 

[TechnicAl]

Possibly, could also maybe be that the oxide doesn't form a solid barrier but maybe porous to some extent and the reaction carries on
Another factor on fishing boats can be movement. I often see aluminium corroding when it has a plastic wear strip bolted to it. I think what happens is the water gets in between the plastic and the Alu and the movement of the plastic on the aluminium is constantly breaking down the oxide layer and not allowing it to form properly and thus the Aluminium keeps corroding.

yes maybe....there will be a reason

 

[slim_boy_fat]

It is certainly the way I have always seen it on the boats. Steel is never touched but the Alu corrodes away as per the above picture.

The ali plates they replace are called 'sacrificial anodes', so that would make sense.

 

[Hood]

An interesting, at least to me, thing that happened on a boat I fitted out was the reaction between two types of stainless steel.
It was part of a skeg and the material was sourced from the scrap yard so was of unknown grade but most likely 304 whilst the keel strip/skeg itself was made from new material and known to be 316. The boat had been laid up in the Harbour for about a year and the anodes had almost completely been eaten away, we didn't think much harm would come to the boat as nearly everything metal on it was stainless but the section I have arrowed in the picture below was eaten away almost completely.

 

[Hood]

The ali plates they replace are called 'sacrificial anodes', so that would make sense.

Anodes, at least in salt water, are usually zinc.

 

[slim_boy_fat]

Anodes, at least in salt water, are usually zinc.

 

[Windy Miller]

yes.........eventually

Ok well reading between the lines I guess it will last a while then. Hopeyby the time it falls apart the children will have grown out of it already!

 

[Hood]

In fresh water I think Aluminium anodes are used so dinnae whack yer heid too muckle.

 

[Hood]

Ok well reading between the lines I guess it will last a while then. Hopeyby the time it falls apart the children will have grown out of it already!

I would advise you to use a stainless rod for joining steel to stainless. Ideally 309 but 316 would likely do. I have seen too many fractures at welds between stainless and steel when steel rods have been used.

 

[Yamhon]

An interesting, at least to me, thing that happened on a boat I fitted out was the reaction between two types of stainless steel.
It was part of a skeg and the material was sourced from the scrap yard so was of unknown grade but most likely 304 whilst the keel strip/skeg itself was made from new material and known to be 316. The boat had been laid up in the Harbour for about a year and the anodes had almost completely been eaten away, we didn't think much harm would come to the boat as nearly everything metal on it was stainless but the section I have arrowed in the picture below was eaten away almost completely.

View attachment 188090

That might be because the large difference in size of 314 and 316 parts. I've not done much reading yet on this, but apparently the size of each electrically connected component can influence which component corrodes and the speed of which it corrodes.

 

[Hood]

That might be because the large difference in size of 314 and 316 parts. I've not done much reading yet on this, but apparently the size of each electrically connected component can influence which component corrodes and the speed of which it corrodes.

Could be, I think the combination of anodes being almost eaten away, stray electrical currents in the water/mud, boat lying next to steel pilings and finally the different grades of stainless all played a part.

 

[TechnicAl]

An interesting, at least to me, thing that happened on a boat I fitted out was the reaction between two types of stainless steel.
It was part of a skeg and the material was sourced from the scrap yard so was of unknown grade but most likely 304 whilst the keel strip/skeg itself was made from new material and known to be 316. The boat had been laid up in the Harbour for about a year and the anodes had almost completely been eaten away, we didn't think much harm would come to the boat as nearly everything metal on it was stainless but the section I have arrowed in the picture below was eaten away almost completely.

View attachment 188090

Chlorides attack stainless....316L is sometimes called Marine grade but it will still be attacked by Chlorides (saltwater).......there are grades that are saltwater resistant...Duplex is probably the most common

 

[Hood]

.......Duplex is probably the most common

Yes, also quite a bit tougher as well. Often use F51 for prop shafts in boats with high engine HP and it allows a smaller diameter shaft to be used.

 

[ronan]

We used to build sewerage treatment plant and i can remember there being issues with some stainless steel we bought corroding. I don't know what they mean by active and passive. Activated and passivated ?

We also used galvanised steel alongside the stainless steel, and they had to be protected from each other, by paint and plastic washers on fasteners etc.

 

[Windy Miller]

I would advise you to use a stainless rod for joining steel to stainless. Ideally 309 but 316 would likely do. I have seen too many fractures at welds between stainless and steel when steel rods have been used.

Hmm well I was going to mig it with ordinary ms wire. I think it’s probably too thin for a stick weld, and I’ve never done stainless with the tig. Not that I have any stainless rods anyhow!

I think I have a small reel of stainless wire somewhere but I have no idea of it’s pedigree. It’s just a tiny reel that I think I picked up in a motor factors years ago. Any ideas what type it’s likely to be?

Is Argoshield light ok for this application?

 

[TechnicAl]

We used to build sewerage treatment plant and i can remember there being issues with some stainless steel we bought corroding. I don't know what they mean by active and passive. Activated and passivated ?

We also used galvanised steel alongside the stainless steel, and they had to be protected from each other, by paint and plastic washers on fasteners etc.

I assume by Active they mean unpassivated......Stainless relies on Chromium forming Oxides on the surface....they take time to form......Passivating is artificially producing the Oxide layer by dipping in acid.......they would form anyway, eventually.

 

[TechnicAl]

Hmm well I was going to mig it with ordinary ms wire. I think it’s probably too thin for a stick weld, and I’ve never done stainless with the tig. Not that I have any stainless rods anyhow!

I think I have a small reel of stainless wire somewhere but I have no idea of it’s pedigree. It’s just a tiny reel that I think I picked up in a motor factors years ago. Any ideas what type it’s likely to be?

Is Argoshield light ok for this application?

If its Stainless it will be better than MS whatever grade it is.......probably 308L or 316L or maybe 309L if its from a Motor Factor.....yes Argoshoeld light will do, maybe a bit discoloured but will brush off

 

[Hood]

Hmm well I was going to mig it with ordinary ms wire. I think it’s probably too thin for a stick weld, and I’ve never done stainless with the tig. Not that I have any stainless rods anyhow!

I think I have a small reel of stainless wire somewhere but I have no idea of it’s pedigree. It’s just a tiny reel that I think I picked up in a motor factors years ago. Any ideas what type it’s likely to be?

Is Argoshield light ok for this application?

It will be better than using ordinary steel no matter the grade, likely it will be either 308 or 316 though.


Edit, too slow

 

[Windy Miller]

If its Stainless it will be better than MS whatever grade it is.......probably 308L or 316L or maybe 309L if its from a Motor Factor.....yes Argoshoeld light will do, maybe a bit discoloured but will brush off

It will be better than using ordinary steel no matter the grade, likely it will be either 308 or 316 though.


Edit, too slow

Thanks very much for your advice Gents, and appologies for taking this thread off course!

 

[500e]

Helpful info
https://www.bssa.org.uk/topics.php?article=89
Always wondered how the French seem to make alloy boats last so well
When we were building & maintaining boats it was a ever present thought as to stopping galvanic action.
Seen some scary SS keel bolts pulled 2/3 eaten away in 18 months also seen a 34ft sailboat total loss due to SS morning shackle coupled to a trot less than 12 months