Galvanic corrosion or is it?

Sans Bateau

Sans Bateau

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Purely by chance I have found what appears to be very bad galvanic corrosion on the ends of the arms of my Walder boom brake. Picture ->

walder_zpsfe257182.jpg


Walder have been kind enough to provide me with two new arms with the swivels fitted FOC under warranty, so full marks to them. However I am reluctant to refit the Walder until I can find the source of the stray current.

The boat has a Galvanic isolator and is on shore power. For reference, the boom brake is attached halfway along the boom and the braid on braid rope part of the brake is attached to the toe rail each side of the boat, this will get wet with sea spray under certain conditions. There is no trace of any other similar corrosion in the Walder. Any ideas?
 
If its galvanic corrosion the source of the "stray" current, as you call, it will be dissimilar metals.

What are the materials of the arms and the rope guides? Stainless and aluminium?
If so you expect a little corrosion of the aluminium but not that much unless immersed in sea water.

is this one an older or a newer type or what?

boom-brake-walder-203-c.jpg
 
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Vic, no that is the larger model. The one I have does have dissimilar metals, stainless screw into alli, but there are other parts of the Walder just the same. the bracket on the boom is stainless on an aluminium boom, I dont understand.
 
I did bit more digging and found that it is how the largest size is done and also an alternative for the 403 and the 203 sizes. looks a better idea to me

I wonder why they did not use a hard plastic for the guides. There's no load on them is there?
 
Its hard to tell from photo but are these items cast? It looks a bit like the casting has been sheared by shock force and some corrosion has set in on the untreated metal exposed by the break?
 
cryan said:
Its hard to tell from photo but are these items cast? It looks a bit like the casting has been sheared by shock force and some corrosion has set in on the untreated metal exposed by the break?
Click to expand...

You may have a point ... but how you whack it hard enough to do that is difficult to imagine

If you look at the fullsize image it looks more like plastic which has be fractured/torn apart than metal !!

Certainly not corrosion.

Click this link then the image
http://i248.photobucket.com/albums/gg199/galadriel35i/walder_zpsfe257182.jpg~original

.
 
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I was looking at the photo earlier on my phone, now I am finally home and on a laptop it looks like there is some aluminium oxide around the threaded hole. It really does look like the casting has been sheared off. It would not take very much force to do this, especially if it has been weakened around the threaded hole. Its more a case of where it was hit as opposed to how hard it was hit.
I am not really familiar with the items job in life having only recently entered the world of wind powered boat ownership (and that having a dipping lug and being several hundred man hours from sailing condition). But having looked at the Walder website and watched them in action on you tube I going to guess that the item received a mechanical shock to it's bottom end which caused it to fail around the connecting bolt in the threaded hole. Could a shackle have hit it as the boom swung over?
 
cryan said:
Could a shackle have hit it as the boom swung over?
Click to expand...

It should be possible to identify it if thats the cause . The implication in the OP is that both are similarly affected so not due to some fluke occurence
 
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I have to say that it doesn't have the typical appearance of galvanic corrosion - the damage is too smooth and evenly distributed. I could believe it was crevice corrosion if it is likely that the area remains wet for significant periods. Some examples down the page at http://coxengineering.sharepoint.com/Pages/Crevice.aspx look somewhat similar. Aluminium will suffer crevice corrosion as it relies upon a passive oxide film for its resistance to corrosion.
 
This is definitely not damage as a result of any kind of collision with anything, it would be impossible. Walder have not admitted to it being a problem others have experienced, however I did notice at SIBS that the rope guides on current boom brakes are a different colour, I will ask why after SIBS. Vyv, we will be coming out of the water soon so unlikely to need the boom brake until next season, would you like me to send you the old and new parts for you to look at more closely?
 
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