A2 vs A4 stainless bolts

machone

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Renovating my old steel boat I just bought some stainless nuts and bolts for deck fittings, they cost about 20euro. Driving home I wished I'd bought a4 as these are 'better' for marine environments due to more chromium and longer lasting corrosion resistance. Thing is, will the a2 bolts give me hassle on a steel boat or should I stick with them, the cost is triple for the a4 but I'd told myself to do everything I could to not let the boat get in the state it was again?
 

Lon nan Gruagach

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Steel on steel, Whitworth thread and red lead oxide putty. I helped renovate a 150 year old water wheel and the nuts and bolts undid with spanners. What ever keeps your hull good is good for your bolts and mixing metal is always going to kill one part or the other.
 

lpdsn

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Renovating my old steel boat I just bought some stainless nuts and bolts for deck fittings, they cost about 20euro. Driving home I wished I'd bought a4 as these are 'better' for marine environments due to more chromium and longer lasting corrosion resistance. Thing is, will the a2 bolts give me hassle on a steel boat or should I stick with them, the cost is triple for the a4 but I'd told myself to do everything I could to not let the boat get in the state it was again?

Where on earth are you buying your bolts if A4 is triple the cost? I don't know a Dutch supplier, but there should be one on the web. I use www.seascrew.com in the UK and www.inox.ie in Ireland, which should give you some useful price comparison.

A4 is better for external use and you might get a bit of surface rust with A2, but shouldn't be too much. The difference between A2 & A4 is mainly, I believe, Molybdenum.
 

yachtorion

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Someone may be along to correct me in a minute but kind of similar to what Dougal said... I'd be concerned about Galvanic corrosion between the mild steel hull and stainless bolts. I'd have thought they might be prone to leaking once they've eaten a bit of the deck away.

Of course something like Duralac would probably solve that. But then so would using cheap mild steel bolts and painting with whatever you use for the hull... like Dougal mentioned.

On the chart below A2 being 304 and A4 being 316 (generally, I think). Going by what Wikipedia says about Galvanic Corrossion, "For harsh environments, such as outdoors, high humidity, and salt environments, there should be not more than 0.15 V difference in the anodic index.", stainless and mild steels are a bad combo in a marine environment (https://en.wikipedia.org/wiki/Galvanic_corrosion).

Corros1.gif
 
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PCUK

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Huge mild steel area against minuscule stainless area. The mild steel will protect the stainless but is so massive the stainless will not effect it. In effect they will cancel each other out. A2 is absolutely fine above the waterline. Even A4 rusts if never cleaned.
 

rogerthebodger

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I also have a steel boat and I fitted all underwater fittings in 316 stainless steel. As said 304 is OK above the waterline. It is also common to weld stainless steel fittings to mild steel even below the waterline providing stainless is 316 and the weld rods are the correct type for welding stainless to mild steel.

Also the galvanic corrosion of stainless also depends on if the stainless is active or passive as its galvanic voltage is different.

http://www.bssa.org.uk/topics.php?article=101

Has some interesting info on welding stainless to mild steel.
 

PCUK

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I too welded my stainless mooring bollards onto my steel decks and over the years they had to take massive strain and never showed any signs of moving (or corrosion).
 

machone

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Thanks for the replies. Good to know the stainless won't affect the hull. These nuts and bolts are above the water line - vents and port holes so I'll stick with them I think. Hopefully will be cleaned or painted soon enough.
 

Birdseye

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Renovating my old steel boat I just bought some stainless nuts and bolts for deck fittings, they cost about 20euro. Driving home I wished I'd bought a4 as these are 'better' for marine environments due to more chromium and longer lasting corrosion resistance. Thing is, will the a2 bolts give me hassle on a steel boat or should I stick with them, the cost is triple for the a4 but I'd told myself to do everything I could to not let the boat get in the state it was again?

A4 is more corrosion resistant because of the molybdenum content. As for your situation it depends on how / where you are using the bolts. If for example, you are using them on deck the only likely issue is more surface rusting / staining. Nothing like as bad as mild steel but the A2 wont stay shiny. If you are using for a structural application often wetted by sea water then I would pay for A4.. I wouldnt use either below the water line or where they would be permanently sat in a pool of stagnant water

Cost wise, the material for A4 was about 50% more than that for A2 and the fabrication cost is the same so triple is a monumental rip off
 

macd

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A4 is more corrosion resistant because of the molybdenum content. As for your situation it depends on how / where you are using the bolts. If for example, you are using them on deck the only likely issue is more surface rusting / staining. Nothing like as bad as mild steel but the A2 wont stay shiny. If you are using for a structural application often wetted by sea water then I would pay for A4.. I wouldnt use either below the water line or where they would be permanently sat in a pool of stagnant water

Cost wise, the material for A4 was about 50% more than that for A2 and the fabrication cost is the same so triple is a monumental rip off

+1 to all that.
And to confirm: chromium content of A2 and A4 is the same. It's the tiny bit of moly that makes the difference.
 

Yngmar

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After 14 years, the differences seem to be (observed on various hose clips in engine room and bilges):

A4 stainless: Shiny band and dulled/minimally corroded screw that turns when asked to.
A2 stainless: Clip gone dull grey but not corroded, screws somewhat corroded, turns after some Plusgas and force.
Mild steel: Clip rusted halfway through, screw corroded, won't turn and head breaks off when force applied. Dremel with metal cutting disc works well for cutting them off.

There's also a lot of cheap ones where the band was good stainless steel but the worm screw was some lesser alloy that corroded badly. This results in the same Dremel and replace job later on. Only one clip had rusted through and let the hose go entirely (luckily not at the bottom end).

Based on this, I'm replacing them all with A4 stainless, which inox.ie sells at what seems pretty reasonable prices.
 

vyv_cox

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Hose clips described as 'stainless steel' quite often have screws of either mild steel or a 400 series stainless. Both are strongly attracted to a magnet, which is worth taking with you to the chandlery.
 
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