How large does an anode need to be?

[salar]

I need to protect a pair of new stainless trim tabs with zinc anodes. The tabs are 12" x 18". I was planning to use the disc type available for rudders but I can't find what size to use (salt water). Any ideas?
Thanks!

 

[Ardenfour]

Why if they are stainless?

 

[VicS]

I need to protect a pair of new stainless trim tabs with zinc anodes. The tabs are 12" x 18". I was planning to use the disc type available for rudders but I can't find what size to use (salt water). Any ideas?
Thanks!

If all parts, including any fasteners, are the appropriate grade ( 316) and precautions are taken to prevent crevice corrosion between components and under fasteners I am also wondering why anodes are necessary.

 

[VicS]

I see I have posted on this subject before ... at least regarding the need or otherwise to fit anodes on stainless steel trim tabs

Ok, a bit sacriligeous but can anyone explain why I shouldn't miss them off this year? (keeping the ones on the drive leg obviously)

My Trim Tabs aren't electrically connected to anything else - there's just a wire going into the electric actuator which is connected to the transom and tab by plastic fittings. I've been fitting a great big 100mm diameter anode to each tab every year and they do wear by a small amount.

Is there any point fitting an anode to this isolated bit of stainless steel, I can't see the electrical justification for it?

Am not an expert - best ask VicS - but if anodes are seen to wear down steadily then they are doing their job.
But if they don't wear at all. it's NOT good news - you've got problems; similarly if they erode overnight!
I need to replace my trimtab anodes, and the others, every couple of years.

Oh dear.

The direct answer to the original question is that if the trim tabs suffer from any corrosion anodes may well help to protect them. Hugo Andreae's and Nonito's posts support that.

I didn't fit them for first two years and now have small holes in my stainless steel trim tabs. Fitted anodes last year and they fizzed away to almost nothing but trim tab corrosion arrested.

My stainless trim tabs pitted in two years so I fitted anodes and this stopped any more pitting.

Definitely a good idea (for all the anodes cost anyway).

Tom

If the anodes are are only wasting away slowly then only replace them when necessary.
It does not automatically follow that because the anodes are wasting away that they are doing anything useful. A chunk of zinc bolted to stainless steel or any other metal with a potential significantly higher (see the Galvanic series ) and immersed in seawater will be eaten away. That's just the way the electrochemistry works. Likewise with anodes associated with bronze, brass or stainless props. I agree though that if they dont waste away at all they aren't doing anything useful and also if they disappear alarmingly quickly some serious problem probably exits.

I am wondering what effect high water flow velocities will have. High velocities in turbulent flows can remove the protective oxide films and result in what is sometimes called "corrosion erosion" I have seen (large ) copper water pipe work eaten into holes as a result of high turbulent flow rates down stream from bends. Whether anything similar will happen to stainless steel I dont know but maybe that is why stainless steel trim tabs corrode or erode

There are some comments to be made about things that have been said in this thread.

Anodes mounted on electrically isolated stainless trim tabs won't to be doing anything to protect the dive leg etc. They must either be mounted on the leg or nearby on the hull with a good electrical connection to the leg to do that. An electrical connection between a stainless steel trim tab and an aluminium alloy drive leg is likely to result in severe corrosion of the drive leg.

The wording of Michael_J's post troubles me. (I think Michael_ J is now A_8)

All metal under water on your boat is electrically connected through the water, saltwater even more so. The closer the metals are the more "connected" they are. This is why you need a anode on all your under water metal parts.

Anodes are also called sacrificial anodes ie they are the least "pure" and will dissolve first. There is a ranking in how pure a metal is and that decides in what order they are sacrified in this context.
Stainless steel have different qualities and can be better or worse in terms of being 100% stainless steel, if your screws holding the tabs are less quality stainless then your tabs they might be attacked more and weakened. If your tabs have small portions of regular steel in them then these portions will get eaten away.
You may have noticed getting small rust stains on your so called stainless steel stuff, that's the same inpurity.


from wikipedia:
Galvanic corrosion is a process that degrades metals electrochemically. This corrosion occurs when two dissimilar metals are placed in contact with each other in the presence of an electrolyte, such as salt water, forming a galvanic cell. A cell can also be formed if the same metal is exposed to two different concentrations of electrolyte. The resulting electrochemical potential then develops an electric current that electrolytically dissolves the less noble material.

Cheers/

For corrosion to occur between two dissimilar metals a they must be immersed, or at least in contact with, the same body of electrolyte, that being seawater as far as we are concerned, and there must be an electrical connection between them. That could be due to them being in good contact with each other or by a wire or other solid electrical conductor. Half of the electrical circuit that causes the corrosion is via the electrolyte, the other half is via the contact or the bonding between them.
True though the shorter the path through the electrolyte the greater will be the corrosion rate.

Different grades of stainless steel in contact with each other can lead to one being corroded. Fixing screws should therefore be the of the same alloy as the trim tabs.

A fairly good account of galvanic corrosion and, as you would expect, of cathodic protection by sacrificial anodes on MG Duff's website.

Some more technical reading on the National Physical Laboratory website. although the discussions on cathodic protection are mainly about "impressed current" systems rather than by sacrificial anodes

Finally don't forget the need to have a galvanic isolator fitted if your shorepower is earthed to the boats earthing or anodes and you leave it connected, in use or not.

As far as size is concerned .... Perhaps as large as reasonably practical simply because the larger they are the longer they will last

 

[AllanG]

I use the MGDuff ZD56 1kg anodes on my Bennett trim tabs, and they seem to work fine.

 

[Daydream believer]

Whatever size you decide on the design of the anode is important
As an example , my prop anodes are on the trailing ends pf the prop and are fixed with 2 ss socket screws
I have seen the anodes in various stages of wear and it seems to me that as there is not a lot of metal around the bolts the anodes drop off prior to the rest of the anode wearing away fully. The trick is to change them just prior to loss.
Some anodes are round (like balls ) so have relatively less surface are to weight whilst pthers are flatter and expose more surface to weight
I am not sure of the implications of this but it seems to me the greater surface available to loss is better than less.
Others may comment on this

 

[salar]

Why if they are stainless?

Because Bennetts Trim Tab installation instruction tell me to. However they don't say how big and I don't know if there is a formula for area to protect compared to size of anode and length of time in the water.

 

[VicS]

Because Bennetts Trim Tab installation instruction tell me to. However they don't say how big and I don't know if there is a formula for area to protect compared to size of anode and length of time in the water.

Ah Bennett trim tabs. You did not say!

In that case they are 3¹³/₁₆" ( or 3³/ ₄) x 1¹/₈" x ¹/₂" The holes are 1¹/₂" centre to centre

Or 95mm x 29mm x 13mm ( 0.2kg mass). Holes 9.5mm on 38mm centres

The bolts are ¹/₄" , 20 tpi x 1¹/₄"

They are available from Anodes direct http://anodesdirect.co.uk/joomla/in...tegory_id=40&option=com_virtuemart&Itemid=137

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[Tranona]

Because Bennetts Trim Tab installation instruction tell me to. However they don't say how big and I don't know if there is a formula for area to protect compared to size of anode and length of time in the water.

Vic has given you the answer - but there isn't a formula for working out size as anode erosion depends on so many variables that sizes are determined emprically - that is fit one and see how long it lasts - for each boat. Guess Bennett use that size because there is mixed metal in the construction of the whole assembly as the stainless steel on its own would not need them, and they would give reasonable life. Just inspect them frequently until you get an idea of the erosion rate.

 

[salar]

Thanks for your advice, everything ordered now ready for installation next weekend, weather permitting