Stainless question from a non-materials engineer

[cr1973]

Any thoughts as to corrosion resistance of 17-4 stainless versus 304 SS? This would be for a hydrant rod/stem. The rod might sit in stagnant municipal water at times, and fluctuate between a wet and dry conditions, typically dry though. Temps would be cool, from about freezing to maybe 60 or 70 degrees F.

 

[EdStainless]

It depends somewhat on the HT condition of the 17-4PH, but in general it is very similar in corrosion resistant to 304.
But that is assuming that both were properly heat treated and cleaned (especially free from any heat tint).
That said 17-4PH even in a lower strength condition (H1150) has a min yield strength of 115ksi min, and to near that with 304 would take some heavily cold worked rod.
I would rather use 17-4PH in the H1150 condition than cold worked 304.
That said there are other options, perhaps a duplex stainless (LDX2101). Even annealed these alloys will have a yield strength of 75ksi which may be enough, and they have better corrosion resistance, and don't require HT or cold work.

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P.E. Metallurgy, Plymouth Tube

 

[racookpe1978]

So, based on that, you (Ed Stainless) would NOT go with a galvanized carbon steel rod?

 

[metengr]

Good point,racookpe1978. I can tell you this, the hydrant in front of my house that contains municipal water does not contain an austentic stainless steel stem.

 

[EdStainless]

Most of them are actually plain steel, no galv. And they are about 4 times the size that they need to be to allow for corrosion.
That said I have seen stainless trim ones used in places with really bad water.
I am pretty sure that those had NiAl bronze valves in them, not common brass.

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P.E. Metallurgy, Plymouth Tube

 

[cr1973]

Thanks for the replies guys. We have had several stems rotted away to the point where it looked like a beaver went at, chewed up like an apple core, near the bottom of the stem which I assume was the water line of any stagnant water sitting in the bottom. These stems were made of plain steel. We have changed our spec to require stainless 304 or 316 as a result, and a supplier is now asking if 17-4 would work in lieu of 304 or 316.

Edstainless, your post was 100% over my head. So generally speaking would 17-4 be an appropriate substitute for 304 in terms of corrosion resistance?

 

[EdStainless]

If you use 17-4PH make sure that it is in the H1150 condition.

One thing to think about is what do these connect to? If the bottom of the rod is frequently wet, and it connects to a steel part, then the stainless will accelerate corrosion of that steel part.

How long do you want these to last? How quickly did the last ones fail? 2 months, 2 years, 20 years?

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P.E. Metallurgy, Plymouth Tube

 

[cr1973]

These rods are typically tied to a bronze valve plate via a stainless steel pin, and I'll try to find out if their 17-4 allow is in the h1150 condition. We've seen these things fail as early as 12 years, usually in the 15 or 20 year range though. Expected life should be about 80+ years.

 

[EdStainless]

Some waters are going to be much more aggressive so I could see this happening.

On option would be to go with steel and double galvanize. But in the high corrosion areas the Zn (galv) will be lost and when it is gone you will just have bare steel again.
The hesitation about SS is that any corrosion will be localized pitting. So 99% of the surface can be perfect and at one place you would have a large pit. In this application that isn't the end of the world so I am not too concerned.

If the existing 17-4PH is on some other heat treat condition it can just be re-aged at 1150F to get into that condition. This material has more toughness and is very resistant to cracking.

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P.E. Metallurgy, Plymouth Tube

 

[cr1973]

Thanks again

 

[MagBen]

I see no problem to replace 304 with 17-4, both of which have a similar general corrosion resistance. In term of strengths, 17-4 maybe a bit overkilled. H1150 will give you the best corrosion, but least strengths, which is what you want.

 

[mrfailure]

From what you described (i.e. wet/dry areas, stagnant water, and the fact it looks like a beaver chewed on it), it is possible you have microbiologically-induced corrosion, in which case the choice of material (17-4, 394, stainless) will not make a difference. MIC can be tested nondestructively using a sample sent to a MIC lab.