Engine Mount Corrosion

[Ian_Rob]

The starboard engine mount on my Volvo D1-30B is showing signs of corrosion. The port side mount is more or less as new [see photos attached].

The boat was on a pontoon hooked up to shorepower for two years but was on a swinging mooring all last year and all this.

What is causing the corrosion?

Ian

 

[VicMallows]

My guess is that the STB has been sitting in sea water in the engine bilge due to a slight list.....but the port has not?

 

[VicS]

Salt water leaking onto it or simply splashed on to it at some time.

 

[Ian_Rob]

Well it may have been splashed at some time during a service but has never been immersed and so far as I am aware has never been significantly wet.

When I first noticed some furring on the mount I lightly wire brushed it - which in retrospect perhaps has made it more prone to corrosion by damaging the surface?

I was worried that there might be some electrical cause?

 

[VicS]

Part of any electrical connection/circuit ?

 

[Ian_Rob]

Part of any electrical connection/circuit ?

Well it shouldn't be. Nothing has been altered electrically from new. Perhaps it is just atmospheric corrosion made worse by the wire brushing??

 

[BabaYaga]

I had one engine mount corrode badly from leaking battery acid. Not just the base plate but also the dome and threaded pin. Acid can climb, obviously.
Worth a check, maybe.

 

[vyv_cox]

In general an engine mount is only going to be corroded by atmospheric corrosion, maybe exacerbated by some seawater leakeage in the engine bilge, drips from seawater pump, etc. My bilge is mostly kept dry, although inevitably there is a small amount of water in there on occasions. My engine mounts are beginning to show signs of rusting after ten years. I have now sprayed them with Waxoyl, which i have found previously to be very effective in halting corrosion.

 

[Ian_Rob]

The mounts are away from any battery acid and I have rigourously kept the engine and bilge free of water and oil.The boat is only four years old so I am a surpised by the corrosion - the more so as it first appeared the season before last.

Am I right in thinking that if it was an electrolytic issue then both mounts would be affected?

I have noticed some pink tinges on the brass/bronze[?] impeller cover plate but only very slight.

I am not sure what the finish is - it looks almost to be anodised [?] but I will wire brush it and try Waxoyl.

 

[maxi]

Electrolytic activity is probably not the cause, as the cover plate of the (Vetus?) flexible mount is well isolated by it's large block of rubber being vulcanised to the cover plate.

Poor plating of the cover plate may have been exacerbated by exposure to salt water, even a very small spray from a water pump or manifold leak.

 

[vyv_cox]

It cannot possibly be galvanic or electrolytic, which requires two dissimilar metals to be connected together and immersed in a liquid (an electrolyte!)

They are usually zinc plated. Anodising is a finish applied to aluminium, which clearly is not what your mounts are made of. The plating will be a few microns thick, not sufficient for prolonged exposure to a marine atmosphere.

Pump covers suffer from dezincification on the water side but it would be unusual on the air side.

 

[Ian_Rob]

Thanks for all the replies - helpful and reassuring.

 

[VicMallows]

Or perhaps wire brushing it ?

Edit: I of course mean that that might me a cause of the problem.......not that you SHOULD wire brush it. <grin>

 

[Chrissie]

I have seen similar before, just one mount affected, it eventually turned out to be a pin hole size leak from a faulty weld on the water cooling side of the exhaust. It was just enough to gradually cause corrosion to the whole mount.
Closely look at all adjacent hoses, for little signs of whiteish deposit, and run your engine up to temp and sit and watch the water jacket with a torch, looking for tiny bubbles or spray.