S/steel rope cutter corrosion

[BlueSkyNick]

I stripped the rope cutter and thoroughly cleaned it - brick cleaner is wonderful stuff (barnacles hate it!).

I was rather surprised by the extent of corrosion inside the rope cutter corresponding to corrosion on the prop shaft. (see pic below)

Both shaft and cutter are 316 stainless steel. so why the corrosion?

 

[sailorman]

I stripped the rope cutter and thoroughly cleaned it - brick cleaner is wonderful stuff (barnacles hate it!).

I was rather surprised by the extent of corrosion inside the rope cutter corresponding to corrosion on the prop shaft. (see pic below)

Both shaft and cutter are 316 stainless steel. so why the corrosion?

crevice corrosion ???
did you use a barrier paste

 

[colhel]

Doesn't 316 need oxygen to to be non-corrosive? Just wondering if you had a good seal around the shaft you may have starved it.
I' probably talking carp cos I don't really know

 

[BlueSkyNick]

I only just bought the boat, remember!

I was contemplating some duralac when I put it back together, depending on replies here.

Mr Ambassador says it wont make any difference.

 

[BlueSkyNick]

I' probably talking carp cos I don't really know

That's why you fit in so well on this forum - one of the gang !

 

[Billjratt]

Why is the corrosion area so well defined?
It looks like the (comparitively) unclamped bit is corroding while the tight bit has survived, so we probably don't want any ingress between the surfaces.
I'd pile in the yellow stuff if it were mine. (Duralac!)

 

[john_morris_uk]

As has already been suggested, it looks like classic crevice corrosion. Put it back together with Durulac. There's plenty of meat left in the metal work by the look of things.

 

[vyv_cox]

It's a particularly nice example of crevice corrosion, which I would like to add to my website with your permission. You can see explanation and examples on the site here. You only need to exclude the water to prevent it, so bedding it on a sealant will do the job. Don't use silicone as the acetic acid cure won't help. Polysulphide or polyurethane would be best.

 

[Skysail]

I am curious as to why we don't see this more often on a cutter? There are thousands fitted and its not been noted here before.

 

[vyv_cox]

I am curious as to why we don't see this more often on a cutter? There are thousands fitted and its not been noted here before.

My guess is that this example doesn't fit the shaft perfectly, whereas mostly they do. If it has been strained, not correctly specified, etc a crevice would be created that will cause the problem. The same corrosion problem can be seen around the two central bolt holes, suggesting that these have been tightly bolted, whereas the other two have no damage. The fitting has therefore not been perfectly aligned.

 

[PCUK]

Slight deviation here - What's Duralac please?

 

[sailorman]

Slight deviation here - What's Duralac please?

calcium plumbate paste

http://www.llewellyn-ryland.co.uk/duralac.html

 

[BlueSkyNick]

I am curious as to why we don't see this more often on a cutter? There are thousands fitted and its not been noted here before.

Keith you are right there are thousands fitted, but I wouldnt mind betting there is a large proportion of them which havent been serviced or removed for quite a while !!

 

[BlueSkyNick]

It's a particularly nice example of crevice corrosion, which I would like to add to my website with your permission. You can see explanation and examples on the site here. You only need to exclude the water to prevent it, so bedding it on a sealant will do the job. Don't use silicone as the acetic acid cure won't help. Polysulphide or polyurethane would be best.

Vyv, of course you can use the picture - PM me if you would like the original high res version.

thanks for the advice, too.

Love the pic of the keel bolt, glad its not one of mine (he says, touching wood rapidly!)

 

[Neil_Y]

Classic oxygen deprived corrosion, this diagram shows it and the website explains quite well.

You have to remember that 316 is at the very bottom of the corrosion resistant steels.
Another factor to be aware of is if steel is stressed it is more likely to corrode, so over tightening can accelerate corrosion issues. Try the bent nail in salt water that we once did in metalurgy.