A2 (304) vs A4 (316) stainless - ok to mix?

Lomax

Lomax

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Need to buy some stainless bolts & locking washers to replace missing screws on a marine cooker, and since I would get a 20% discount if spending £50 I figured I might as well fill in a few gaps in my assortment. But many items are only available in A2 (304) stainless, and I'm unsure how much this matters. Ideally, all the nuts, bolts and washers in my "stainless" box would be A4 (316), so I'd never have to give this a second thought. For example, what will happen if I use an A2 locking washer for an A4 bolt, in an external location? I imagine the washer would corrode slowly, eventually failing to provide the locking it's there for? And what about galvanic corrosion, will mixing A2 & A4 cause one or the other to be attacked more in the presence of water?
 
We once examined the difference between the two and found that it is possible to have good A2 which is the same as 'bad' A4, the parameters can cross. However, we think that studding in particular can be low quality. I have A2 above the water and no signs of degradation. Below WL all A4. Bought readily at FWB.
 
Thanks, good to hear! Although prices have fallen dramatically in recent years, there's still a premium to pay on A4. I'm new to owning a steel hulled boat though, and still haven't grasped the implications of mixing metals/alloys wrt galvanic effects. Is there enough of it between steel, stainless 304 and/or stainless 316 to need to take it into account? My guess is "no", but would be good to have this confirmed.
 
304 (A2) and 316 (A4) are almost identical but for the addition of a 2% or so of molybdenum to the latter. Similarly, their galvanic potential is almost the same.

Given the choice I'd always use 316 over 304 on the exterior, and go to some lengths to do so underwater, principally because of its greater resistance to crevice corrosion. Unfortunately, in much of the Med, 316 fasteners are difficult to source. (It also became clear to us that the previous owner of the boat couldn't tell a galvanised coach bolt from a copper nail.) The result is that we have plenty of deck fasteners that must be a mix, yet I've noticed no undue corrosion. 304 in the salt environment usually develops a rust-brown bloom, but rarely does it get worse than that if exposed to air. Nor can I say that 304 in contact with 316 shows markedly accelerated corrosion.

But, as said, I'd take special care with fasteners underwater, or in any hidden place that might be subject to damp.
 
Lomax said:
Need to buy some stainless bolts & locking washers to replace missing screws on a marine cooker, and since I would get a 20% discount if spending £50 I figured I might as well fill in a few gaps in my assortment. But many items are only available in A2 (304) stainless, and I'm unsure how much this matters. Ideally, all the nuts, bolts and washers in my "stainless" box would be A4 (316), so I'd never have to give this a second thought. For example, what will happen if I use an A2 locking washer for an A4 bolt, in an external location? I imagine the washer would corrode slowly, eventually failing to provide the locking it's there for? And what about galvanic corrosion, will mixing A2 & A4 cause one or the other to be attacked more in the presence of water?
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I would be surprised if your cooker was 316 - its significantly more expensive. But dont worry - use either or a mix of both inside the boat.
 
howard easton said:
I would be surprised if your cooker was 316 - its significantly more expensive. But dont worry - use either or a mix of both inside the boat.
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Hehe, well, I wasn't thinking about the cooker here, or even what grade is appropriate inside - I was mainly curious about whether A2/304 would be ok outside, and whether mixing A2 & A4 outside might accelerate corrosion. Mac pretty much answered these questions already. I think you can tell the difference between the two with a magnet - IIRC 316 should be completely non-magnetic, while 304 is very weakly magnetic - but for simplicity's sake it might be better to only have 316 fasteners on board. I'm buying an assortment of M4-M12 nyloc nuts to have to hand, and the extra cost of getting them only in 316 might outweigh the hassle of keeping track of what is what. From what I can see 316 is about 1/3 more expensive than 304. This looks like a good deal to me: http://www.ebay.co.uk/itm/191982011454
 
You cannot rely on magnetism to tell the difference. 300 series stainless steels are nominally austenitic = non magnetic but there may be some retained ferrite = magnetic. Retained ferrite is more likely in 304 than in 316. Turnings can reach temperatures above the austenitising temperature of 723C and may therefore become more ferritic. There is no galvanic potential between the two, but this is not really an issue inside where the fastenings are unlikely to be immersed in seawater(!)
 
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