possible outdrive corrosion

[greycaster]

Hi, I am new to this forum and hope somebody will be able to give me some advice. My boat was out the water last week and the outdrive and hull were both antifouled by the local marineroes at the marina I use. They have put the boat back in the water but I have noticed that they have painted the hull right up to the outdrive. The hull is painted with Hempel hard racing antifoul and the drive is treated with Prop-Shield. As the Hempel is copper based will I have to remove it from around the drive, if so how long can I leave it like this as I am away from home for 2 weeks.

 

[oldsaltoz]

You only have problems if the antifoul has copper in it's mix.

From memory you need the outdrive antifoul to extend 300mm onto the hull, then you can apply copper based antifoul outside that zone.

 

[carlton]

The spec sheet for that antifoul says...

Description: HEMPEL S HARD RACING 7142H is a hard antifouling (insoluble matrix) based on cuprous oxide. It changes to final colour after approximately 1 week of exposure to water.

Recommended use: As an antifouling for boats of glass fibre, wood, plywood, steel and ferro-cement. Do not use on aluminium or other light-alloy metals. Risk of corrosion in case of direct contact.

Usual practice is to leave a 1" gap around the outdrives when antifoul is copper based.

Might be an idea to have a word with the gaffer.

 

[oldgit]

The spec sheet for that antifoul says...



Usual practice is to leave a 1" gap around the outdrives when antifoul is copper based.

Might be an idea to have a word with the gaffer.

I always left a 1" gap around outdrive,.Is the boat left in or out of the water when not in use ?

 

[greycaster]

live and learn

The boat is in the water year round, guess i will sail across to the nearest island where there is shallow water, jump in and scrape off a 1" gap all round.

 

[oldgit]

The boat is in the water year round, guess i will sail across to the nearest island where there is shallow water, jump in and scrape off a 1" gap all round.

Probably the simplest thing to do,to avoid long term problems.

 

[coreng]

The boat is in the water year round, guess i will sail across to the nearest island where there is shallow water, jump in and scrape off a 1" gap all round.

Agree. Hempel Hard Racing should not be used on aluminium. Just keep it away...

PS It is strange your professional painters don't do this...

 

[Elessar]

Hi, I am new to this forum and hope somebody will be able to give me some advice. My boat was out the water last week and the outdrive and hull were both antifouled by the local marineroes at the marina I use. They have put the boat back in the water but I have noticed that they have painted the hull right up to the outdrive. The hull is painted with Hempel hard racing antifoul and the drive is treated with Prop-Shield. As the Hempel is copper based will I have to remove it from around the drive, if so how long can I leave it like this as I am away from home for 2 weeks.

We always leave a 1" gap because customers worry if we don't. It is accepted good practice to leave the gap.
Yet to see any evidence of it causing problems if you paint right up to the drives though.
Get your painty people to put in writing that it won't cause a problem and if they won't get them to scrape it.
I certainly wouldn't lose sleep for the next fortnight.

 

[greycaster]

Thanks

Thanks for the replies, I think I will scrape it off just to make sure.

 

[coreng]

Might be interesting to understand why you should leave a gap, whatever customers may think... Natural potential difference between copper and aluminium is over 300 mV : this means copper corrodes aluminium if they are in contact and immersed (200 mV difference between 2 metals are necessary for corrosion to occur).

 

[VicS]

Might be interesting to understand why you should leave a gap, whatever customers may think... Natural potential difference between copper and aluminium is over 300 mV : this means copper corrodes aluminium if they are in contact and immersed (200 mV difference between 2 metals are necessary for corrosion to occur).

Can you explain just what causes the problem with copper based antifouling paints and aluminium

The antifouling paint does not contain metallic copper. It contains cuprous oxide, Cu2O. I would think the paint film is non conducting.

Trilux antifouling is marketed as suitable for use on aluminum out-drives etc. That contains cuprous thiocyanate!

 

[coreng]

This is correct. Though a lot of paint manufacturers don't want to see their copper based paints on aluminium. Mainly to ensure safety of scratches.

 

[VicS]

This is correct. Though a lot of paint manufacturers don't want to see their copper based paints on aluminium. Mainly to ensure safety of scratches.

But what is the mechanism by which a paint containing cuprous oxide is detrimental to aluminium ( while one containing cuprous thiocyanate is not)

 

[coreng]

Neither theory nor technology of the cathodic protection of aluminum is as well developed as it is for mild steel. Oxidation of aluminium with copper based paints depends on many parameters such as material’s and sulfides concentration, pH or water velocity. It is indeed a complicated chemist’s affair, which I am not…

For the rest, and as it seems we don’t control all parameters, we just try to keep copper well away from steel or aluminium…

 

[VicS]

Neither theory nor technology of the cathodic protection of aluminum is as well developed as it is for mild steel. Oxidation of aluminium with copper based paints depends on many parameters such as material’s and sulfides concentration, pH or water velocity. It is indeed a complicated chemist’s affair, which I am not…

For the rest, and as it seems we don’t control all parameters, we just try to keep copper well away from steel or aluminium…

I'll take that as a "dont know" then.

Pity I as hoping that an expert in the field would be able to explain it to me.

I'd be interested if you do come across any relevant information.

 

[coreng]

Some more

Because cuprous thiocyanate contains less ions than oxide it then requires less epoxy barrier. According to International, 8 mils are required with thiocyanate versus 22 mils with oxide.