Sacrificial anodes on deck?

[SVKemily]

Hi everyone,

I'm working on rust remediation above the waterline on my 31 year old, 32' steel sailboat. I have a stainless steel bowsprit which is welded to the mild steel hull, and as you might expect there's quite a bit of corrosion in the mild steel near the weld. In fact, I've chipped through the hull in a couple of spots. There are a few other places on deck where stainless is joined directly to mild steel, and although the corrosion hasn't gotten as bad in those other places, it is definitely progressing.

I'll remove loose rust, treat the rusty metal and replace the paint in all of those areas, and will replace the steel that's welded to the bowsprit, of course.

I have also been wondering about placing small sacrificial anodes on deck, near the areas where stainless meets mild steel, or alternatively using some kind of zinc-rich galvanizing primer in those areas. The nice thing about sacrificial anodes would be that I could see how much is left and could replace them without redoing all of the paint.

Does anyone have any experience with these options, or other rust countermeasures specific to areas where stainless and mild steel are touching?

Cheers,
SVKemily

[fstbttms]

Unless the anode and the part to be protected are both immersed in an electrolyte, you are wasting your time.

[SVKemily]

This is the thing I'm wondering about. Would the anode provide protection when temporarily immersed in an electrolyte, ie when the area gets wet from splashes or rain? The steel would only rust under those conditions anyway.

[nuku34]

Having owned and cruised a steel yacht for many years, I agree that anodes on deck will do nothing to inhibit rust formation. I sincerely doubt that there is a major on-going galvanic reaction between the stainless and the mild steel causing the rust near the bowsprit.Something else is going on there.
Sorry for the bad news, but the proper way to treat rust is first sandblast the area. No other prep process will completely get rid of the rust and give a "profile" in the steel to help the first layer of zinc-rich epoxy primer mechanically "key" into the steel. Wire brushing/grinding just embeds the rust into the steel and creates a smooth surface making it hard for the paint to stick. I carried a small portable sandblaster which I could hook up to a compressor when I got to a boat yard.

[RaymondR]

Make sure that when you paint it you carry the coating over onto the stainless steel.

[Fore and Aft]

SVkemily just bolt some anodes on, make sure you put a couple on the topsides as well. Sailors all like to have something new to gossip and yarn about. If you can keep a straight face you should tell everyone they work really well!
Cheers

[SVKemily]

Thanks for the replies! Nuku34, do you have a sandblaster recommendation? I bought a low-end one from Harbor Freight and have had terrible luck with it so far. Also, what primer and paints did you use?

Fore and Aft - I like the way you think!

[nuku34]

I used a cheap portable sandblaster; it looked like a paint sprayer and had a hose that you put into a bucket of sand. It sucked up the sand by siphon action. They are a bit fiddley to get working properly: good pressure and dry sand are key. They only do a small area and are no substitute for a proper commercial blaster, but for small areas and especially for inside the boat they can be useful.

I used a 2 part zinc rich epoxy primer, any of the major brands will do and use a polyurethane top coat from the same brand.

[Pauls]

Sand blasters require a little attention to detail. The sand must be dry. And you need a large enough compressor to maintain a decent air pressure while blasting, I'd say somewhere around 60 PSI is a good minimum. I've used a cheap Harbor Freight blaster for many years, it works fine. Pay attention to the details of the valve settings. You need to understand what the different valves do if you're going to operate it well.