316 vs. 17-4 Corrosion 4

[Bicky22]

Hi,

I have two parts, one made from 316 SS and one made from 17-4 SS. They were both exposed to the same substance, I don't know what this substance was but it corroded the 17-4 until it fell apart and left the 316 SS in mint condition. Does anyone have any thoughts on what kind of of substance or chemical could cause a complete break down of 17-4 and leave 316 unaffected?

Thanks all.

 

[EdStainless]

Anything slightly acidic with some chlorides in it.
At room temp 50ppm Cl and pH=6 will not hurt 316 if it is clean.
17-4PH will be seriously attacked in this environment.
Though there are probably about a hundred more that will do the same.


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Plymouth Tube

 

[Bicky22]

Thank you for the help, I do appreciate it.

Can I ask what about the 17-4 would leave it so susceptible to this corrosion compared to 316? Would it have to do with the copper in 17-4? or the Molybdenum in 316? I can actually see the copper color in the 17-4 where it has corroded.

 

[EdStainless]

What was the 17-4 in contact with?
No, the Cu is fully in solution, you can't separate it.
SS gets corrosion resistance from Cr and Mo.
17-4 has no Mo, low Cr, and the precipitates that form when you age it have even lower corrosion resistance.
316 actually does not have that good of corrosion resistance, you have to be very careful.

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Plymouth Tube

 

[Bicky22]

I have no idea what it actually was in contact with, I am trying to get answers to that but so far no luck. Just like the substance is unknown, I am unsure how often it was in contact. It could have been submerged for 3 full years or just been in contact for a few minutes.

I do know This corrosion happened over 3 years time. It was roughly 1/8" thick of 17-4 corroded to nothing.

This was a broad question in that regard, I don't deal with metallurgy on a day to day basis so I was unsure if this was a common issue between the two types.

 

[Gregorydas]

I think Hydrochloric acid can cause the damage of 17-4.. I am not sure about it

 

[Guest102023]

The copper colour you see is copper ions (generated by corrosion) plating on to the metal surface. I've seen it happen in admiralty brass corrosion failures, especially when cleaning material in fairly strong acid.

If you can salvage an intact piece of metal, a laboratory might be able to differentiate between possible corrosion mechanisms, which include general (uniform) corrosion, pitting, or intergranular attack. If the two parts were in contact then galvanic effects come into play.

The temper of the 17/4 may also be a relevant factor, although by the sounds of it this alloy would have failed in any temper.

"If you don't have time to do the job right the first time, when are you going to find time to repair it?"

 

[MagBen]

Almost at any condition, the corrosion resistance for 316 is better than 17-4. 17-4, comparable to 302/304, is desgined mainly for high strength with some corrosion resist

 

[tbuelna]

More details about the application where these parts were used over the 3 year period would be helpful, especially some photographs. What was the function of each component? What other materials/metals were these components in direct contact with? Your post mentioned the parts were "submerged", so is it safe to assume the environment they were exposed to was some type of liquid? Since you know what material the components were manufactured from, do you have engineering documentation for them you can share with us?

 

[Bicky22]

Thank you for all your input I do appreciate it.

To answer brimstoner: The 17-4 and 316 were in contact. I will look into having a lab analyzing the type of corrosion, however as of now, this whole issue isn’t of the utmost priority so it might not get that far. The 17-4 was tempered to H1025. The 316 was untampered.

tbuelna: 100% agree with you.. More details would help, however I don’t have any details about the field use. The end user has been hush hush about details.
If I get more details I can share I will. The 316 was a small shaft that rotated the 17-4 bi-directionally. No Idea how often it was actually rotated (low torque) . Other materials I know it encounter are Brass and Viton. I believe it was some sort of liquid, but like I said the end user hasn’t shared any details, so I don’t know for sure or how often it was exposed.

What would you specifically be looking for on the eng documentation? I may be able to.

The lack of details was one of the reasons I went to the forum with this question. I was curious to see if anyone has encountered this same issue before with these two materials.

Thank you all again for the help

 

[tbuelna]

The 316 shaft and 17-4 bushing are not a bad galvanic compatibility match on their own. But what material were the 17-4 bushings installed into?

The engineering documentation I would be looking for would be detail drawings for the individual parts, assembly drawings, and design requirements.