Hull Blisters,Hole in prop

[GrahamHR]

they look like aly props to me but surely if you have stainless props you would not expect galvonic corrosion on them. The anodes and then the drives should go long before the stainless as the current should flow to stainless from aluminium. Destroying the more reactive metal in the proccess..

As I mentioned, there is no electrical continuity between the stainless part of the props and the rest of the drive/ anodes. So, the anodes do not protect the stainless steel. The anodes do protect the aluminium alloy hubs and the prop nuts. What I have looks like crevice corrosion to me.

Graham

 

[Renegade_Master]

Can I ask where

LJS will no doubt be back.... but I would guess Spain.

LJS Spain ?????

 

[ian38_39]

As I mentioned, there is no electrical continuity between the stainless part of the props and the rest of the drive/ anodes. So, the anodes do not protect the stainless steel. The anodes do protect the aluminium alloy hubs and the prop nuts. What I have looks like crevice corrosion to me.

Graham

But you would still need a less reactive metal than stainless for there to be any galvonic corrosion problems with the stainless props.
Stainless props do tend to eat anodes as they are pretty close to the top of the food chain but I cant think what would be causing the corrosion you have other than low grade stainless.

 

[GrahamHR]

Crevice corrosion of stainless does not need another external metal present; basically in areas where the sea water is stagnant, it becomes deficient in oxygen and the chloride ions attack the stainless. There is no oxygen to reform the protective oxide layer. On the props there are stagnant areas between the plastic sleeve and the prop and if there are any surface defects, crevice corrosion can take place. All the corrosion I have originates from that interface area. There is no corrosion whatsoever on the blades or the outside of the hubs of the props I have; only on the inside of the hubs.

Graham

 

[ian38_39]

You are obviously a lot more informed than me on corrosion but unless it is electrolysis, which it seems from what you say it is not, then how would it help if the props were bonded to the anodes?

 

[GrahamHR]

You are obviously a lot more informed than me on corrosion but unless it is electrolysis, which it seems from what you say it is not, then how would it help if the props were bonded to the anodes?

Anodes wouldn't help much at all!

The first indication I had that something was happening was when I found what looked like a tiny rust stains on the surface. I poked at them and found there was a thin skin of stainless with a pretty big cavity behind them, full of a black sludge, with what can best be described as "wormholes" downwards from those cavities into the hub, leading to where the corrosion had started I presume!

Instead of 2 dissimilar metals in contact in the same environment, it's one metal exposed to 2 different environments ( oxygen depleted and oxygen rich on the outside; it's like the stainless forms it's own corrosion cell).

Graham

 

[longjohnsilver]

Mayflower marina Plymouth.

Only a thousand miles out !

Well OG it might as well be Spain for you, don;t expect we'll see you there any time soon!