[sweweld]

Recently a job came in, it was a rudder bearing for a 47-foot sailing yacht.
It was seriously damabed by corrosion. Previously the owner had a problem with galvanic corrosion.
The galvanic problem was solved and the rudder bearing was repaired/welded by some other firm, which I do not know of.
This was 2-3 years ago.

I was involved, because the owner had observed some brown iron oxide creeping out on the bolts holding it in place.
The bolts and nuts were new, and of 316L grade.
The bearing is mainly an axial bearing keeping the rudder from falling out.
I disassembled the two halves, and observed some serious surface corrosion, apart from the previous crevices from the galvanic corrosion.

Bearing halves

rudderbearing.jpg (47.58 KiB) Viewed 867 times

This bearing has been in service since 1989, and it has mainly been in the area around Florida US. The current owner bought the boat in Florida and sailed it across the Atlantic to Sweden and the Baltic sea five years ago. Everything was fine until the galvanic problem a few years ago. The salinity around Florida and the Atlantic is many times higher than in the Baltic sea. And the bearing was ok until the galvanic problem.

The first welding shop that made repairs on it made a really bad job.

Poor job

poor_repairjob.jpg (57.91 KiB) Viewed 867 times

You can clearly see the band of heat tint on the surface

Heat tint

heat_tint.jpg (59.49 KiB) Viewed 867 times

Here is the craters from the previous galvanic corrosion problem

It was obvious that they did not use pickling or some other form of passivation.
Before I even started to work on it I used pickling paste on it twice. Most of the iron oxide dissapeared and it started to look like stainless steel again.
But there still were some deep black pores in the surface. I had to use a carbide burr to dig them all out. I planed the surfaces with a big belt sander, and filled the craters with 316LSi wire and TIG.I drilled out some of the holes that were corroded inside, and inserted bushings of 316L in some and welded.
There were some serious caves in the holes that were threaded, I just inserted/welded the bushings and used nuts instead of threading.

I think the previous shop skipped the passivation, since there was serious surface corrosion around the welded places. And of course welded to hot for too long. Almost all welds were on edges and corners where you have to be careful with heat input.
And possibly worse they welded with the wrong filler, 304 instead of 316L.

I have made every thing possible to salvage the part, since it will be expensive and hard to make a replacement that fits nicely.

I have some concerns though, and thought I would post some questions here.

Is it possible that the previous galvanic corrosion has degraded the corrosion resistance of the steel so much that it is not usable any more?
Controlled galvanic corrosion is used for passivation, "electro polishing". Is this uncontrolled galvanic corrosion eating much deeper into the steel, or is it just taking layer by layer of the surface?
Is my attempt at passivation with the pickling paste enough?

If they welded with 304, do I have any hope to restoring the corrosion resistance. Do I have to clad all affected areas with new 316L filler?

Just to be on the safe side, and also to confirm that there is no more galvanic action going on, I fitted small Zinc anodes on each halve.
That way we can asses the situation the next time the yacht goes out of the water.

[Otto Nobedder]

I read this yesterday, and decided to give it some serious thought before replying.

You repeatedly refer to 304 as a filler; I would expect 308 was used, as this is the most common filler rod for stainless, particularly for 304 base metal. The iron oxide is highly suspect; The filler rod may well have been a cheap Chinese grade, whose import does NOT require meeting ASTM standards regarding the metallurgy. I would go over all the filler metal with a magnet. The welding process should make the metal SLIGHTLY magnetic. I have encountered counterfeit stainless that is as strongly magnetic as mild steel. If you encounter this, you'll never achieve the corrosion resistance you need without a complete resurfacing with 316.

I dislike few things as much as trying to fix someone else's half-ass work.

Good luck,

Steve

[sweweld]

I have not been in the welding business for that many years, and have only used 316L filler since 99% of my work is on marine parts. That's why I referred to 304, as a lesser corrosion resistant filler...

Never crossed my mind, but I will definently try the magnet trick, thanks!
Luckily the previosly welded areas are few and quite small, so maybe it's not so time comsuming to just clad them with 316L filler.
But as you say, it is never fun to fix someone elses half-ass job!

Tried to attach more than 3 images yesterday, but failed somehow. Here is the crater image...

Galvanic craters

galvanic_craters.jpg (54.08 KiB) Viewed 841 times