Posted 12/02/2014:

Possible cause electrolysis.

Lars – Yes, my answer mentions electrolysis as a possible reason, although I  referred to this as “some galvanically very active substances”.

Electrolysis is a rather broad term, and can be divided into electrolytic and galvanic corrosion. 

The electrolytic variant is caused by a leak in the electrical on board system, appears unlikely here because it would require cabling in the area, and occur on the wet side inside the tube. 

Galvanic corrosion is caused when a galvanically nobler material is protected by a less noble sacrificial one, for example the propeller shaft zinc is a sacrificial material. On order to cause corrosion here on the stainless a nobler material would be required on its surface in moist conditions, and one that comes to mind is graphite, which is used in some greases. 

But why would someone put graphite grease on the outside of this tube? On the other hand the corrosion on the outside of the larger Y-tubes suggests that graphite may have been used as sealant in the joint between hose and tube, but not on the smaller tubes, which do not show corrosion. 

From past experience it is not clear to everybody that substances containing graphite must be avoided in these applications, and it is possible that somebody was not aware of this.

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IMPORTANT FEEDBACK FROM LARS:

It is assumed you have all checked your Y-drains after being made aware of this case of serious corrosion. Probably nothing found as nobody reported back – very good if this is the case, and would suggest that there are some peculiar reasons for these problems on Savannah Girl.

Weld decay, scientific name carbide precipitation, is a known problem if the welding procedures are improperly done on stainless, but this is supposed to affect the welds very locally, and here pitting is visible on the outside far away from the welds.

Also, the upper ends of the bigger Y-tubes are not welded, but still show loss of material specifically under the hoses, while the smaller Y-tube ends do not show any corrosion.

This would suggest that either some strong chemicals, or some galvanically very active substances have been in local contact with the affected areas.

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Posted 29/01/2014:

Dear members of the Swan 46 organization: 

I would like to share a problem that I had with the custom stainless steel pipe that is under the steering quadrant in a custom “Y” shape that collect the water from the cockpit and the steering well.

The pipe as you can see in the pictures developed very serious electrolysis and to replace it the boat has to be slipped.

I encourage all owners to take a look under the pipe, because mine looked perfectly ok from the top but on closer examination under it, only achievable with a mirror, it looked very bad as you can appreciate in the picture attached.

The solution was rather simple, but very hard to access the spot.

The old pipe was cut were the material was still in good shape, an extra heavy duty reinforced rubber pipe (exhaust type) 3 inches in diam. internal was used to join the existing pipe with the new custom made fitting in stainless steel 316 of course.

Four clamps, 2 for each end were used plus silicon to be extra safe.

To do this job, in my case the autopilot ram had to be disconnected, maybe in some other boats it may not be necessary.

I decided I did not want to touch the pipe that was in good condition once cut, because it was fiberglassed at the factory in a very uncomfortable place.

Should you feel like removing the entire pipe and re glass it, the whole rudder will have to come down.

The approach taken is very similar to joining other pipes in the boat, like in the engine and when shaft leaves the hull.

I hope this short message helps anyone with a similar problem, I do not mind at all get into deeper detail should you need to replace this pipe.

Kind regards,  

Gustavo Sjöberg / Savannah Girl