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Old 18 August 2012, 05:03   #1
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Stainless steel nuts

Can anyone tell me what the diference is between A2, A4 and BZP stainless steel nuts is/are?
I want some to connect my battery terminals down. The old ones are getting a bit worse for wear.
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Old 18 August 2012, 05:14   #2
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Hi Brian. BZP is Bright Zinc plated and will rust fairly quickly on a boat. A2 is "normal" stainless, A4 is "marine grade". A quick google found this which is interesting, the useful bit is at the end:

Linky

I think that a man of your standing should have shiny nuts, so if you pick A4, your nuts will be A1
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Old 18 August 2012, 05:21   #3
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Can anyone tell me what the diference is between A2, A4 and BZP stainless steel nuts is/are?
I want some to connect my battery terminals down. The old ones are getting a bit worse for wear.
For battery terminals - I don't think you have much to worry about if you use any of the materials suggested. A2 (standard grade 304) and A4 (marine grade 316) stainless steel have the same strength, the difference is to do with the higher Molybden content which gives it more corrosion resistance. A4 stuff is used in the chemical and food processing industry, pipework. BZP is mild steel, zinc plated.

More details here: Difference between 304 and 316 Marine Stainless Steel
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Old 18 August 2012, 05:48   #4
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Brilliant, thanks Willk and thanks Spartacus.
I always promised before buying anything marine I would always consult "the Oracles" on here. This is after a disaster with Marine Megastore a bit back.
Well, while I am at it......how about someone looking at my posting in the Electronics section under Energy Squadron SFL 120 battery?
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Old 18 August 2012, 07:35   #5
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a2 is fine for most things, a4 is for things that are likely to be submersed in sea water. a4 costs more so is worth thinking about he difference.
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Old 18 August 2012, 07:55   #6
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I have ordered sizes M4,5,6,8, and 10 so if anyone wants some??
All in A4.
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Old 18 August 2012, 07:59   #7
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Hi Brian

Very important question - a lot of misconception about the use of S/S in marine applications.

All the above info

Here is my tuppence worth - A4 (316) has a far greater resistance to salt water corrosion. The rough rule of thumb within the marine industry is A2 (304) above the waterline, A4 below. In general however, A4 is the best grade in salt water applications.

Now the science bit (not again) - the corrosion resistance S/S depends on the addition of chromium to form chromium oxide on the surface to keep corrosion at bay. However chromium is a carbide former and it wants to produce chromium carbide which means less chromium oxide.
A4 is a "proofed" S/S which reduces the formation of the bad chromium carbide and potentially more good chromium oxide than A2.
However the downside of this is a reduction in mechanical properties ie a A4 (316) bolt has approx 1/3 the strength of an equivalent high tensile steel bolt and is a lot more brittle.
Finally be very wary of using any S/S below the waterline as there is also very little oxygen to maintain the vital chromium oxide film.

All done
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Old 18 August 2012, 08:18   #8
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Bad Chromium!

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bad chromium carbide and potentially more good chromium...
Down Boy!
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Old 18 August 2012, 08:23   #9
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Down Boy!

I know - I forgot to take my pills this morning - must go and marshal the troops
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Old 18 August 2012, 08:45   #10
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must go and marshal the troops
Have a Field Day!
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