Question re galvanic corrosion

[citygent111]

Hi all, please could someone explain this to me?! I have a 19ft boat at Conwy Marina (so saltwater), but she is not connected to mains electricity. I fitted a new set of anodes in March. Am I likely to need new anodes when she's lifted for winter?

I've tried to get my head round the whole corrosion thing, but chemistry was never my strong point, and my last lesson was almost 40 years ago!

Thanks

 

[vyv_cox]

What are the anodes connected to?

 

[Tranona]

Depends on what they are protecting. You don't say what sort of boat it is, but an anode is normally used to protect a propeller from the shaft on a shaft drive boat or on a sterndrive motor boat the drive housing from the shaft.

If the former then usually a hull mounted anode will last more than a year, an anode on the shaft a shorter time (because it is smaller) and the latter type also shorter time. you will only know if they need replacing when you lift the boat so plenty of time to get new ones if needed before you launch.

 

[rogerthebodger]

For galvanic corrosion to take place you need 4 things.

2 dissimilar metals immersed in an electrolyte Isea water) and an electrical connection between the dissimilar metals

If any one is missing you generally not experience galvanic corrosion.

The questions by vyv_cox and Tranona are the ones you need to answer first

My boat is steel, and I looked into galvanic corrosion extensively so to protect my steel boat from galvanic corrosion

 

[wonkywinch]

Did you ever make a potato battery at school? Our boat seems to want to continue that experiment in the marina despite fitting a galvanic isolator and more recently not using shore power as a habit.

 

[citygent111]

Depends on what they are protecting. You don't say what sort of boat it is, but an anode is normally used to protect a propeller from the shaft on a shaft drive boat or on a sterndrive motor boat the drive housing from the shaft.

If the former then usually a hull mounted anode will last more than a year, an anode on the shaft a shorter time (because it is smaller) and the latter type also shorter time. you will only know if they need replacing when you lift the boat so plenty of time to get new ones if needed before you launch.

Ok a bit more info too - it's a 19ft Fletcher speedboat effectively. GRP/Firbreglass hull. Mercruiser inboard engine with Alpha 1 drive. The anodes are the standard kit supplied, so one goes behind the prop, then one near the skeg and a couple a bit higher up (been 6 months since I've seen them!)

 

[Alex_Blackwood]

Ok a bit more info too - it's a 19ft Fletcher speedboat effectively. GRP/Firbreglass hull. Mercruiser inboard engine with Alpha 1 drive. The anodes are the standard kit supplied, so one goes behind the prop, then one near the skeg and a couple a bit higher up (been 6 months since I've seen them!)

You will know when you see them again, seriously almost impossible to tell until then.

 

[trapper guy]

as i understand things in terms of chemistry, galvanic corrosion doesnt even need an electricity supply, as the positive and negative ions in the metals produce their own supply, all it needs is an electrolyte, which can be as simple as moisture in the air.
when typical air contains as much or more than 80% moisture, corrosion will occur regardless.

in terms of attachment to masts, for such things as steps, i often hear the arguments of what fittings are best, there is a simple answer to every such question and that is, the fixings should be, as far as possible, the same kind of metal. so for steps i would opt for aluminium steps and aluminium rivets, as from what can be seen on this chart, pretty much everything bar aluminium will react with aluminium.
sure, you can get zinc plated screws, but plating i only good for so long, and the action of inserting them can scratch the plating through to parent metal.

https://www.simpletwig.com/blog/wp-content/uploads/2017/05/GalvanicActionChart.png

 

[trapper guy]

Ok a bit more info too - it's a 19ft Fletcher speedboat effectively. GRP/Firbreglass hull. Mercruiser inboard engine with Alpha 1 drive. The anodes are the standard kit supplied, so one goes behind the prop, then one near the skeg and a couple a bit higher up (been 6 months since I've seen them!)

there is no anode in the engines sea water cooling supply??

 

[Tranona]

there is no anode in the engines sea water cooling supply??

If there is it is nothing to do with the anodes on the drive leg

 

[colind3782]

there is no anode in the engines sea water cooling supply??

Many engines don't have them like my Sole Mini 34

 

[vyv_cox]

as i understand things in terms of chemistry, galvanic corrosion doesnt even need an electricity supply, as the positive and negative ions in the metals produce their own supply, all it needs is an electrolyte, which can be as simple as moisture in the air.
when typical air contains as much or more than 80% moisture, corrosion will occur regardless.

Galvanic corrosion never depends upon an external electricity supply. Electrolytic corrosion occurs when there is an external supply.

 

[madabouttheboat]

If the boat is in the water for the whole summer, I would change them regardless. Lets say they are 40% worn. That might be ok for another season if nothing changes, but if they disappear the expensive drive is next in line.

 

[colind3782]

I had a propshaft anode fall off a few years ago. Now I put two on the shaft!

 

[citygent111]

Thanks all, will stick to an annual change i think!

 

[vyv_cox]

I had a propshaft anode fall off a few years ago. Now I put two on the shaft!

If you use the two hammer method your shaft anodes will never fall off.

 

[colind3782]

If you use the two hammer method your shaft anodes will never fall off.

I know that but I guess the yard doesn’t!