Trident
Well-Known Member
If I completely encase stainless in epoxy below the waterline does this make it immune from corrosion as no oxygen can reach the metal (provided that the epoxy does not break down )
Stainless corrosion below the waterline
- Thread starter Trident
- Start date
rogerthebodger
Well-Known Member
Not in my view as the epoxy will crack and any epoxy needs a proper grit blasted surface for any epoxy to adhere.
Most stainless is polished and epoxy will not stick for long to polished stainless.
Ensure you have the correct grade of stainless below the waterline all my skin fittings are 316 stainless with no issues as long as the threads are fully sealed with sealent
This is what I did to my steel boat and below the waterline is fine after 15 years in the water
Most stainless is polished and epoxy will not stick for long to polished stainless.
Ensure you have the correct grade of stainless below the waterline all my skin fittings are 316 stainless with no issues as long as the threads are fully sealed with sealent
This is what I did to my steel boat and below the waterline is fine after 15 years in the water
Trident
Well-Known Member
Yes I know it needs roughing up and the correct epoxy product (oil rig paint companies recommend epoxy primer, epoxy paint and then a high build epoxy and finally a PU coating) but if that is done is there any way it can corrode ?
geem
Well-Known Member
I have just drilled an exploratory hole on my rudder to check the tangs for corrosion. Although there is moisture in the rudder and omotic blisters, the 44 year old 316 stainless steel is in perfect condition. No corrosion at all
rogerthebodger
Well-Known Member
Did you check where the tangs are welded to the rudder shaft as that was where the corrosion took place on my ferro boat GRP rudder due to chromium migration at the weld site
KompetentKrew
Well-Known Member
What is the application exactly? Will they be completely buried?
From experience, water will get into in chainplates.
From experience, water will get into in chainplates.
Trident
Well-Known Member
Well I said below the waterline so if chain plates are down there corrosion is probably less of an issue than drowning at that point!
vyv_cox
Well-Known Member
There are millions of uncorroded stainless steel prop shafts. No need to coat if the material is correct.
geem
Well-Known Member
Trident
Well-Known Member
But does anyone know the answer to my question?
geem
Well-Known Member
I can't give you an academic response, I can only tell, you that my rudder shaft has been encased in epoxy and is corrosion free after 44 years. I do however, believe that the stainless steel is a far higher grade than that which is now sold as 316 grade. My experience with new 316 has not been good
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Keith 66
Well-Known Member
316 is more expensive than ordinary commercial grade 304, Unfortunately some companies use the cheap stuff & pass it off as 316. Colour code on bar end is red for 304, light turquoise for 316, often plate or flat bar will also be stencilled with the number
I found this some years back when stanchions kept going rusty. I also needed some plate & bought some offcuts from a stainless fabricator not far from here, everything in their scrap bin was 304
I use Speed alloys if i ever need any and you will get what you ask for!
Im not sure about other Marcon boats but too many Sabre 27's had stainless rudder shafts with mild steel tangs welded on. When i split my rudder i found just a 4mm length of weld left on the bottom one & none at all on the top two which were totally rusted away. I can say that rudder failure was iminent!
I found this some years back when stanchions kept going rusty. I also needed some plate & bought some offcuts from a stainless fabricator not far from here, everything in their scrap bin was 304
I use Speed alloys if i ever need any and you will get what you ask for!
Im not sure about other Marcon boats but too many Sabre 27's had stainless rudder shafts with mild steel tangs welded on. When i split my rudder i found just a 4mm length of weld left on the bottom one & none at all on the top two which were totally rusted away. I can say that rudder failure was iminent!
Vilandre Ick
New Member
Yes, if stainless steel is completely encased in epoxy and no water or oxygen can reach the metal, it should be effectively protected from corrosion. Stainless steel resists corrosion mainly due to a thin oxide layer that forms when it reacts with oxygen. Below the waterline, where oxygen levels are low, stainless can be more prone to corrosion if exposed to water directly.
Snowgoose-1
Well-Known Member
I did suffer crevice corrosion one time.
Two stainless studs underwater holding a bronze spigot to the GRP deadwood for the prop shaft.
They were tapped directly into fibreglass and were about 50% gone when I withdrew them.
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Dogone
Well-Known Member
Epoxy is not impervious to O2 or water penetration. It is used to prevent osmosis as a barrier coat, but it still lets stuff through. The thicker and the more coats the better, but you won’t get hardly anything to last forever. Just look at the Roman Colosseum or the Pyramids. They are ruins and few structures have lasted longer.
vyv_cox
Well-Known Member
There is plenty of dissolved oxygen in seawater. Marine life exists even at tremendous depths. Corrosion occurs where differential oxygenation occurs, such as crevices and pits.
rogerthebodger
Well-Known Member
I looked into various epoxies with regard to how hydroscopic the epoxy was (water Absorbtion)
I found epoxy tar was the least hydroscopic but still allowed water to penetrate the epoxy.
My main interest was to protect my mild steel boat from any water penetration to prevent rusting and to ensure paint adhesion to the steel hull
I found epoxy tar was the least hydroscopic but still allowed water to penetrate the epoxy.
My main interest was to protect my mild steel boat from any water penetration to prevent rusting and to ensure paint adhesion to the steel hull
pcatterall
Well-Known Member
I thought that the stainless actually needed oxygen to remain stainless!
Halo
Well-Known Member
I think the answer is that if you encase stainless in epoxy then some water may get through and it likely won’t have enough oxygen to prevent corrosion. Probably not a good idea as without oxygen stainless will corrode
Trident
Well-Known Member
This is exactly why I am trying to get a definitive answer
In well oxygenated water 316L stainless will last decades
But for very expensive application they sell paint systems that etch prime, epoxy prime, high built prime and top coat to protect stainless . If that will genuinely keep out all water and air its great but if not it messes with the chromium oxide formation.
So hopefully someone her will know if it really works
In well oxygenated water 316L stainless will last decades
But for very expensive application they sell paint systems that etch prime, epoxy prime, high built prime and top coat to protect stainless . If that will genuinely keep out all water and air its great but if not it messes with the chromium oxide formation.
So hopefully someone her will know if it really works