Stainless steel 304 "Stainless" properties

[GN1]

Hi,

my first post. I hope some one can help.

I work for a manufacturer and we make a particular item from 304 stainless. We have two parts we put together and then stamp (rivet) to hold in place.

When we perform a 48 hour salt spray we occasionally find the there are small 'rust or brown' stain marks around this stamped area.

If we salt spray the virgin parts without stamping we never see this issue.

Does anyone know why this might occur? I'm assuming its down to a structural change in the material but any ideas or insight could be valuable.

We also manufacture the same part in 316 and never see the issue with riveted parts after salt spray.

Thanks.

 

[gleem]

It is my understanding that stainless steel is corrosion resistant because of a chromium oxide layer on the surface. This layer can be breached by scratching (or other mechanical stresses like stamping?) This breaching exposes iron atoms to the atmosphere resulting in oxidation and thus rusting. This can be corrected by a process called passivation which reestablishes the protective layer. This done by treating the surface with citric or nitric acids.

316 SS does contain more nickel and molybdenum which might contribute to it being more impervious to corrosion than 304 SS.

 

[anorlunda]

Metallurgy is not my field. But I have seen similar corrosion in 316 as around stress points like holes. That leads to stress corrosion cracking and that is said to begin with putting. Therefore, see

https://en.m.wikipedia.org/wiki/Pitting_corrosion#Mechanism

 

[ChemAir]

'rust or brown' stain marks around this stamped area.

In addition to the above, if there are dark parts where a Steel tool strikes the stainless fairly firmly, it can be from very small particles of steel residue left where the riveted area is struck. These can leave rust stains if the residual steel is not removed. Spraying carbon steel tool marks on a piece of stainless steel with salt solution would seem to be a good way to get them to show up quickly.

If this is the problem, I'd suggest cleaning the areas that turn brown with a stainless steel wire brush before washing, chemically treat the struck area with something that will dissolve/remove the loose (carbon) steel, or determine if there is alternate tooling/barriers that will not cross-contaminate the surface.

 

[gmax137]

Many years ago I was guiding a federal inspector on a tour at a plant in Florida. a lot of the equipment at this plant is outdoors. He stopped me at one point, he was looking at small stainless steel lines with Swagelok fittings. "Why are these rusty?" he asked, pointing to some rust in the crevice where the tubing entered the fitting. I explained: we are outdoors, in south Florida, it is wet and salty here. "Well," he says, "these should be done in stainless steel, shouldn't they?" I told him they are stainless, 316. His walkdown report said "system engineer is ignorant."

Thanks for reviving that memory

 

[Asymptotic]

What is the rivet material? Could galvanic corrosion between dissimilar metals also be in play?

This cite may also be of interest.

Influence of the cold working induced martensite on the electrochemical behavior of AISI 304 stainless steel surfaces
https://www.sciencedirect.com/science/article/pii/S2238785418303958

 

[GN1]

Thank you for all of the replies. I'm taking this all on-board.

This is in the UK and both parts are 304 SS. One part is slotted and the other has legs. The leg goes through the slot and is rivets, pushed down onto the part with a rotational action until it spreads out and cannot be removed.

 

[russ_watters]

It is my understanding that stainless steel is corrosion resistant because of a chromium oxide layer on the surface. This layer can be breached by scratching (or other mechanical stresses like stamping?) This breaching exposes iron atoms to the atmosphere resulting in oxidation and thus rusting. This can be corrected by a process called passivation which reestablishes the protective layer. This done by treating the surface with citric or nitric acids.

AKA rouging:
https://www.bssa.org.uk/topics.php?article=105

 

[Astronuc]

"Well," he says, "these should be done in stainless steel, shouldn't they?" I told him they are stainless, 316. His walkdown report said "system engineer is ignorant."

One would expect 316 to eventually corrode if in service at elevated temperatures near the ocean. It's corrosion resistance is much better than 304, but there are better alloys, derivatives of 316, or high nitrogen stainless steels like AL-6XN (6 Mo, developed for naval landing gear), or SMO-254/-654 (7 Mo), and some high-Cr duplex stainless steels, that even greater corrosion resistance.
https://www.imoa.info/molybdenum-uses/molybdenum-grade-stainless-steels/steel-grades.php
https://www.notzgroup.com/media/wysiwyg/PDF/NME/15_Outokumpu_Ultra_range_Datasheet_May_2015.pdf

There's a reason that marine structural alloys are tested at Kure Beach (marine environment) in North Carolina!
http://corrosionjournal.org/doi/abs/10.5006/0010-9312-1.4.178
https://www.starnewsonline.com/news/20040220/laque-center-plans-for-future-move-amid-coastal-change

It could be a martensite transformation, but it could be sensitized material as well. 304/316 have C up to 0.08 wt%, which can render them more susceptible to pitting corrosion if sensitized (carbide precipitation on grain boundaries associated with Cr depletion in the matrix near the carbides), or damage to the surface, or the Cr level is too low, or some combination thereof. Galvanic corrosion could be a factor if the rivet material is substantially different. Based on the stamping operation, one might consider if the surface has become active as opposed to passive.

 

[Tom.G]

Not my specialty so please take this as a wild guess.
If the cause is damage to the surface passivation layer, perhaps a delay before salt spray would allow the passivation to reform.