Threaded fasteners for cast iron keel?

So, I find myself in need of a bit of repair work, which would be simpler if not for the submersion element.

I need to replace the pivot point in my cast iron swing keel, it's so bad that it's about to tear out. The plan is to notch out the rusted bad metal, tap threads into the good metal, and bolt in a block of fresh steel, everything coated in POR-15.

I'm scratching my head on what best to use for the 4 bolts, due to galvanic corrosion. I don't want the threads eaten away.

Stainless and Monel seem like they may eat away the iron threads.

Titanium is on the table, but that's on a rumor that it's oxide film creates a barrier against galvanic corrosion...?

Steel holds well and should play nice with he iron, do zinc/cadmium coatings help enough? I plan to coat everything in copious amouts of POR-15.

Will adding a zinc anode nearby take the beating away from the bolts?

Thanks for your time.

 

Is he pivot point a notch? You can always replace the bad metal with welds.

 

Someone who can weld, or has access to a qualified welder, may be able to do that, yes. lol

The pivot point is a through hole, badly rusted and almost ready to break out the trailing edge of the keel.

 

It is cast iron, welding practically only possible with (nearly) pure nickel electrodes, which leave a soft repair material. An insert/bushing of harder alloy might be held in place by nickel welding though.

But these measures, involving materials with different galvanic potential, do increase the risk for further corrosion. It is a tricky choice.

 

If the boat is near Cincinnati it will be used on fresh water, so galvanic corrosion is less of an issue.

 

Lol It's on a trailer so I can take it far far from Cincinnati. A lotnof thought went behind the question posed in the first post.

 

The keel could be cast mild steel instead of iron. A spark test will let you know which is it. Keels are often cast from scrap metal, and steel is more common.

 

100% sure it is iron, I've never heard of cast steel on a small boat.
Steel is very difficult to cast, it has a higher melting point and experiences more shrinkage. I only see cast steel in more expensive applications, such as Navy parts.

 

Sure, but all it takes is a grinder to test it.

 

There's no electrolite in the threads to start galvanic corrosion. I know this because like any normal person you will use lubricant on them, either grease or loctite or wet epoxy. There are only two areas of concern, the seam between the two pieces and the bolt head. The seam can be taken care by either a shrink fit or epoxy, and the heads you countersink deep and fill the hole with epoxy, tar, cement, etc.
It doesn't really matter if the keel is cast iron or cut from a steel plate, the answer is always the same, normal steel bolts properly installed are enough. The bolts should be sized such that a single one is sufficient, then you have a safety factor of 4. The replacement plate should be of the same material as the keel. If you want to be fancy you install it as a dovetailed shrink fit (it's absolutely possible to do that at home with handtools).
What does matter is that your picture seems to show evidence of galvanic corrosion, either from stray currents or maybe from a bronze pivot pin and its stuffing boxes.

 

Excellent, I appreciate the input. This was the theory but I didn't know for sure if this was enough protection. I'm actually planning to "pot" the whole repair like an electrical component, in POR-15. Everything will be coated, however the pressure faces of the screw threads will inevitably have contact with one another.
Each bolt can handle 2x load, and I'm using 4, so that's a safety factor of 8.

The pivot pin is 316ss and did a number on that hole over the past 40 years. Part of the slotting (I believe) comes from slag inclusions in the iron casting adjacent to the hole, which offer a vector for corrosion to work its way deeper into the casting. Pressing in a bushing is definitely on the table, the trick is finding a good middle-man material.

 

If it took 40 years to corrode the keel, you shouldn't worry too much about what material the bolts are.