preventing more corrosion on an aluminium anchor

[sarabande]

Having just bought a secondhand aluminium anchor, I want to prevent more corrosion taking place.

The anchor was taken off a sea-based yacht, with a galvanised chain and octoplait warp still attached and sea-wet. It was then put into a (not entirely dry ) garage for three years before I bought it. It is not the anchor's fault it was stored in this way

As can be seen, there is evidence of corrosion/electrolysis in various parts of the anchor. I know about hot water and gentle working to remove the by-products of corrosion, but what is the best treatment for the parts where the anodising has worn through, or where some stainless/aluminium corrosion has taken place, please ?

Water resistant grease, Duralac, or ???? Is there an anodising paint, perhaps ?

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TIA

 

[sailorman]

Duralac will be a good cheap option.
Calcium plumbate paint will work but will obviously wear away with use

 

[noelex]

With those stainless steel bolts in direct contact with aluminium a compound like Duralac or Tef-gel should be used to prevent the influence of dissimilar metal corrosion. Unfortunately it needs to be used when the anchor is first assembled it will not fix an existing problem (although it will probably prevent it getting worse). Strangely, and wrongly in my view, this does not seem to be mentioned in the assembly instructions of Fortress/Guardian anchors.

I assume the anchor is a Fortress Fx-16 or Guardian G-16. If it is the former the parts with corrosion can be replaced at minimal cost under warranty. If it is the Guardian model there is no warranty to cover this problem.

 

[Fortress Anchors]

Hi Sarabande,

Galvanic corrosion is extremely rare with our anchors, due in part because the parts are anodized (Fortress models), but more so because for this corrosion to occur, the anchor has to be in contact with a dissimilar metal in a saltwater environment for an extended period of time, which is not typical during normal use.

In the few cases I have seen during the past 19 years, galvanic corrosion occurred when the anchor was used as a permanent mooring or it was stored below deck in a leaky bilge area.

My assistant Karen received the images of your FX-16 today and we will be shipping you new parts under our Lifetime Parts Replacement Warranty.

There is a product called Flitz that is a cleaner / polisher that works with various metals, including anodized aluminum. If it is available in the UK, then you might consider giving it a try to clean and shine up your anchor:

http://www.flitz.com/flitz-polish-paste/

Safe anchoring,
Brian

 

[thinwater]

I think it is worth noting that none of the corrosion is in structural or heavily loaded areas. It probably wouldn't matter, untreated, for another 20 years. The load is all carried in one-metal areas by interlocking sections. No critical fasteners.

And no, I don't work for Fortress.

 

[NormanS]

Hi Sarabande,

Galvanic corrosion is extremely rare with our anchors, due in part because the parts are anodized (Fortress models), but more so because for this corrosion to occur, the anchor has to be in contact with a dissimilar metal in a saltwater environment for an extended period of time, which is not typical during normal use.

In the few cases I have seen during the past 19 years, galvanic corrosion occurred when the anchor was used as a permanent mooring or it was stored below deck in a leaky bilge area.

My assistant Karen received the images of your FX-16 today and we will be shipping you new parts under our Lifetime Parts Replacement Warranty.

There is a product called Flitz that is a cleaner / polisher that works with various metals, including anodized aluminum. If it is available in the UK, then you might consider giving it a try to clean and shine up your anchor:

http://www.flitz.com/flitz-polish-paste/

Safe anchoring,
Brian

Wow! That's impressive. Well done.

 

[Bajansailor]

A pal here had a Fortress anchor which was used (in association with a mooring) to hold his boat in Tropical Storm Tomas which visited us 6 years ago. The Fortress broke where the shank is attached to the anchor - he was left with literally just the shank.
We told Fortress about this, and received a similar response from Brian, who then proceeded to send us the bits needed to re-build the anchor again, free of charge.
And I seem to remember it was a Guardian type, not the 'proper' Fortress.
So another commendation for Brian and his most excellent anchors.

 

[macd]

Wow! That's impressive. Well done.

Impressive, but not altogether surprising. A few months ago Brian offered free mud-palms to a poster who'd bought a used Fortress lacking them -- and made the same offer to anyone else.

 

[Topcat47]

I'd never considered an aluminium anchor, but reading the response from Fortress, I'm definitely looking into it. That sort of company commitment shows complete confidence in the product. Well done!

 

[eddystone]

Hi Sarabande,

Galvanic corrosion is extremely rare with our anchors, due in part because the parts are anodized (Fortress models), but more so because for this corrosion to occur, the anchor has to be in contact with a dissimilar metal in a saltwater environment for an extended period of time, which is not typical during normal use.

In the few cases I have seen during the past 19 years, galvanic corrosion occurred when the anchor was used as a permanent mooring or it was stored below deck in a leaky bilge area.

My assistant Karen received the images of your FX-16 today and we will be shipping you new parts under our Lifetime Parts Replacement Warranty.

There is a product called Flitz that is a cleaner / polisher that works with various metals, including anodized aluminum. If it is available in the UK, then you might consider giving it a try to clean and shine up your anchor:

http://www.flitz.com/flitz-polish-paste/

Safe anchoring,
Brian

I wish I had known that! Having assembled my FX16 and left it unused in the locker for a year or so I decided that aluminium and s/s bolts don't mix so took the bolts off and re-assembled using Duralac paste. Unfortunately one bolt sheared in undoing and I had to get a replacement from Fortress (can't get non-metric replacement in UK) - first class customer service - wish all companies like that.
After all that faff completely forgot about the galvanised shackle but don't think this is as much an issue in contact with aluminium as stainless?

 

[macd]

There is a product called Flitz that is a cleaner / polisher that works with various metals, including anodized aluminum. If it is available in the UK, then you might consider giving it a try to clean and shine up your anchor:

http://www.flitz.com/flitz-polish-paste/

That stuff is available on Amazon.co.uk, but with a hefty mark-up for delivery of a tiny tube from the USA. Cheapest I could find was £14.99 for a large (150g) tube from a classic vehicle restoration specialist in Rochdale:
http://www.frost.co.uk/flitz-polish-paste-polishing-150g.html

 

[Fortress Anchors]

After all that faff completely forgot about the galvanised shackle but don't think this is as much an issue in contact with aluminium as stainless?

Thanks everyone for the kind words, they are greatly appreciated! :encouragement:

As I recall, an aluminum alloy is less susceptible to galvanic corrosion when it is in contact with stainless steel, however, the more important issue then becomes the structural strength of a stainless steel shackle vs. a typical galvanised steel shackle.

 

[noelex]

As I recall, an aluminum alloy is less susceptible to galvanic corrosion when it is in contact with stainless steel, however, the more important issue then becomes the structural strength of a stainless steel shackle vs. a typical galvanised steel shackle.

It is a complicated subject, but overall an aluminium anchor will be less susceptible to galvanic corrosion if a galvanised steel shackle is used and more susceptible to corrosion if a stainless steel shackle is used.

Ideally stainless steel needs to be electrically isolated from aluminium in a marine environment. If this is not possible, products like Duralac and Tef-Gel are the next best solution.

 

[thinwater]

If Tefgel is recommended for electrical contacts (it is--check the web site), then it does not isolate.

 

[noelex]

If Tefgel is recommended for electrical contacts (it is--check the web site), then it does not isolate.

Yes, that is correct. Tef-Gel does not function by electrically isolating the two metals.

Electrical isolation, if it can be achieved, is a better mechanism. It will totally prevent the sort of corrosion shown in the photo. However, electrical isolation is often not practical. Hence the popularity of products like Tef-Gel and Duralac. These don't produce electrical isolation, but work in other ways to prevent stainless steel from corroding the aluminium. Tef-Gel functions by preventing electrolytes from entering the interface of the metallic surfaces. No electrolyte = no galvanic corrosion in the same way no electrical contact = no galvanic corrosion.

There is some debate about whether Tef-Gel or Duralac is the better at preventing corrosion, but they are both effective and safer than simply connecting aluminium and stainless together. This is especially true in the the marine environment. As well as reducing the sort of corrosion shown on the Fortress anchor it will make the parts easier to disasmble in the future as the corrosive products tends to lock the stainless bolts in place sometimes making removal difficult.

 

[thinwater]

Yes, that is correct. Tef-Gel does not function by electrically isolating the two metals.

Electrical isolation, if it can be achieved, is a better mechanism. It will totally prevent the sort of corrosion shown in the photo. However, electrical isolation is often not practical. Hence the popularity of products like Tef-Gel and Duralac. These don't produce electrical isolation, but work in other ways to prevent stainless steel from corroding the aluminium. Tef-Gel functions by preventing electrolytes from entering the interface of the metallic surfaces. No electrolyte = no galvanic corrosion in the same way no electrical contact = no galvanic corrosion.

There is some debate about whether Tef-Gel or Duralac is the better at preventing corrosion, but they are both effective and safer than simply connecting aluminium and stainless together. This is especially true in the the marine environment. As well as reducing the sort of corrosion shown on the Fortress anchor it will make the parts easier to disasmble in the future as the corrosive products tends to lock the stainless bolts in place sometimes making removal difficult.

Big words for saying it is a waterproof grease, like Lanicote or Green Grease, with anti-corrosion properties. I had to point that out, because the words "isolate" are often used by others, as though these products are magical in this regard.

I would even argue, based on personal lab wash-off testing and corrosion testing, Tefgel might not be the best anti-corrosion product for this sort of use. I did not test Duralac, since the testing was focused on grease. But the point is probably mute; they are all good. And not all waterproof grease is equal, not by a long shot.

I have a set of anti-seize trials starting. That will take time.

 

[noelex]

Big words for saying it is a waterproof grease, like Lanicote or Green Grease, with anti-corrosion properties. I had to point that out, because the words "isolate" are often used by others, as though these products are magical in this regard.

Yes this is the biggest criticism of Tef-Gel. There are endless debates concerning which is best, Tef-Gel or Duralac, especially by the owners of aluminium boats.

The proponents of Tef-Gel point out it is widely used in the aviation industry on aluminium aircraft. Personally I have always used Duralac. It certainly makes a big difference, but I have no way of knowing if it is better or worse than Tef-Gel. The two products work quite differently to try and achieve the same aim.

My message would be to use something. Even waterproof grease is a help. A tube of Duralac seems to last forever so it is not expensive to use the correct product. Although yacht masts are always above the waterline (you hope ) Duralac is also the ideal material to use when attaching fittings to an aluminium mast, so the tube will not just be used on the anchor.

I have a set of anti-seize trials starting. That will take time.

More objective testing would be great to see. The anti-seizing results are important as I mentioned in my previous post, but the prevention of corrosion is more critical. The two properties are related, but not directly, so some assessment of both parameters would be helpful.

My prediction would be that waterproof grease will be as good or possibly slightly better than Duralac as an anti-seize product for a stainless bolt screwed into an aluminium part, but the corrosion protection of Duralac will be significantly superior.

 

[vyv_cox]

It is a complicated subject, but overall an aluminium anchor will be less susceptible to galvanic corrosion if a galvanised steel shackle is used and more susceptible to corrosion if a stainless steel shackle is used.

Debatable I think, and I don't know the answer. While the zinc remains on the galvanised shackle it will protect the aluminium. Once the zinc has gone the steel will be protected by the aluminium, which will corrode. Fairly straightforward.

Where the debate begins is over the state of the stainless steel. In the passive state there is a voltage of about 0.6 between it and aluminium. However, it could be argued that in a moving contact between the shackle and the anchor shank the protective oxide film could be removed, making the stainless steel state active. In this case its voltage and that of alloy steel is not very much different. Aluminium probably has an active and passive state also, since it relies on an oxide film for corrosion protection, but I have no information on the relative voltages.

I only know that my Fortress is connected to its chain and warp using a Wichard 17/4PH shackle that is stronger than many steel shackles. I have been using this combination for 16 years without visible corrosion on either part.

 

[noelex]

Debatable I think, and I don't know the answer. While the zinc remains on the galvanised shackle it will protect the aluminium. Once the zinc has gone the steel will be protected by the aluminium, which will corrode. Fairly straightforward.

Yes if using a galvanised shackle with an aluminium anchor I would be careful to replace the shackle before the zinc is worn off.

The position of the zinc coating, the underlying steel (if the galvanising wears away) and stainless steel (both active and passive) on the galvanic series is shown below.

The detailed picture is complex with factors such as rust reducing the electrical contact. A stainless shackle will not necessarily cause problems as you note, but a galvanised shackle is safer. Replace it before the zinc is worn away. The zinc will corrode before the aluminium, acting as an anode and therefore protecting the anchor. The opposite is true of stainless steel.

 

[sarabande]

really good and useful knowledge re preventing corrosion coming out for general use.

Thanks - off to order some Duralac.