Stainless Steel and Corrosion

Can you post a picture, or describe the deposits?

Is the picture not showing up? I'm talking about the rainbowed colouration visible in the picture. Here's a link in case the picture didn't embed properly... Link

The link is blocked by my Web filter, and no, the image did not embed properly.

Image is fine must be his filters.

Darn! I curse Thee Websense! *while shaking fist violently in air*

Questions: What grade of SS is the part and is this part totally submerged? Regarding the observed stain (and discontinuity of the apparent discolorizing film), how thick is it and can it be removed by rubbing or how? What solution is in the central area of the part. I am having trouble visualizing the orientation of the part and where it is being contacted by the water or other solutions. Please advise.

Depending on the SS grade and the water chemistry this could be an oxidation product of some sort.

Hello, I greatly recommend to check the chlorides concentration in water because SS 316 is attacked by them. Please check also pH and sulphates. The combination of these two anions produces corrosion under scale and makes SS 316 brittles.

Armando Hernandez

Senior Process Engineer

I couldn't see the pic either, but you can look after galvanic corrosion also depend of the electrolyte the water has.

Any ideas on what could be causing this discolouration?

Visually it looks like "staining" rather than corrosion. Is there any noticable pitting or roughening of the surface? If the surface is still smooth when the part is cleaned then you have deterioration of something else, not corrosion of the SS.

If there is surface roughening, crevacing, or pitting then the other comments about checking the liquid chemistry would be the correct next step.

Armando, I think you are on the correct track. Anything with chlorides can cause this type of discoloration. It is possible that there is some sort of crevice that may be storing the chlorides and releasing them with temperature changes.

At first view the pattern of the discoloration suspiciously look like a weld and the HAZ of weld.

The OP may try to find the details. Even a etching may show it.

And if not properly welded the SS is prone to the corrosion on these zones (elements migration, improper chemistry, oxidations during welding due to absence of inert media...)

The other portion is the presence halide (chloride, fluorides) or strong oxidisers. In fact the common bleaching powder and the chlorination of water may be the source.

But my vote goes to welding (looking at the pattern)

Looks like interference colours of an oil or other thin film. Unlikely to be corrosion. We do see it occasionally on polished ss cookware when it comes out of the washing tunnel.

bioramani

Any thin film can give such a pattern of colors. It need not be chemical or an oil. I recall research texts on corrosion where such thin films were generated on mild steel by various heat treatments.

I still would like to know the the EDAX probe values show for the surface in the various areas.

Was this part replaced? Often? Did the last one do this?

During machining, some of the tooling may have been left embedded in the ss surface.

Maybe a cutting oil around the seal?

Good luck!

Thanks for all the suggestions...

The part is totally submerged in water and it is mounted such that the flow of the staining is downward. The center part of the part in the picture is a piece of quartz.

The stain will come off with hard scrubbing.

It is a machined part that is press fit together, so there is no HAZ from welding.

Would chlorides cause the discoloration pattern or would it usually just cause reddish-orange rust?

If it was something precipitating out of the water, wouldn't it be more consistently over the whole part?

It has happened on more than one part, but not on all parts (they are installed in different water conditions, i.e. different sites will have different water chemistry).

I am inclined to think it something non-water soluble leeching out of the teflon or viton. Why would it show up like a drip or a flow when it is constantly submerged in water?

Instinctively I don't think it looks like corrosion, but it does look like the top ID chamfer has significant pitting. I find it odd that it deposits on the quartz as well as the 316 SS.

Hopefully that clears up most of the questions raised. Any more input?

If I can't come up with a satisfactory answer, does anyone have suggestions on who I could talk to in order to get a better answer? The local university? Some independent lab that has expertise? Suggestions?

Thanks for the feedback so far!

Nothing leaches from Teflon or Viton. Were the parts thoroughly degreased before putting to use. I still hold that it is an iridescent colours of some thin film, rather like the colours of a soap bubble. Bioramani

Hi,

what is beneath the quartz-centerpart?

Either something beneath is corroding, very likely that Teflon or Viton seal is not completely tight.

Or the SS is corroded (not really much). I estimate the thickness in the range of 0.5 to 2µm.

The deposition mechanism is by oxidation so you will find it on any substrate near by.

(In my searching for beautiful minerals there is often this effect: pyrite or CuFe-sulfides are corroded by water plus oxygen, the soluble iron^II-salts precipitate as soon as there is some more oxidation ( to insoluble Fe^III within minutes to days.)

So you can be sure that this is iron-hydroxide, mostly Fe-OOH. You can test yourself: by heating the scraped off material in a flame this will be converted to magnetite Fe₃O₄ that is strongly magnetic and clearly attracted by any permanent magnet.

Another possibility is with K3Fe(CN)6 or K4Fe(CN)6 - this will yield massive blue nontransparent precipitates. As this is very sensitive, make sure you have a reference by testing a probe without the brown iridescent material. And make your unknown substance soluble by acid.

If there is any pitting the material is from the iron content of the SS-steel. The nickel and chromium will not go into solution but remain in black powdery layers.

RHABE

Did you test the water for iron-oxide or any type of salt/saline content? Stainless steel CAN corrode if the nickel content is not high enough.

Iron hydroxide dissolved in the water, especially with some H2S present, can result in similar staining.

Looks as if your gasket leaks slightly, which gives the directional pattern.

Do you have a water analysis for the situations where this staining shows?