Self passivation of Stainless Steel 316L - questions 3

[TomaszKruk]

hello,

kinda newbish questions, but the specific answers are hard to come by from what I could google. Hope someone will take time to answer them.

1) I'm currently on a job, when we are to modify an existing demi water installation. Having removed some pipes I noticed signs of oxidization on welds (from residual oxygen, nothing horrible - light yellow color). The installation was supposedly passivated during comission (way, way back). I was wondering - from my knowledge proper passivation includes washing the pipe with acid, and would mean that the discoloration of welds would be removed in the process. Could it have been that the pipes were not passivated at all? And still did not corrode badly? We plan to passivate the thing and our sub-contractor's brochures state that passivation should include cleaning the pipe with acid, and would lead to clean, "natural" stainless steel surface even on welds (it wouldn't polish the welds obviously - just remove any discoloration).

2) I read conflicting info on self passivation of stainless steel - would stainsless steel self passivate on weld surface? Found info both ways - that it would, or that the chromium layer would not rebuild on its own on the weld surface.

3) Could stainless steel self passivate in your typical demi water circuit? I don't know the exact specs of the installation, so it's hard to determine if the water contains sufficient amounts of oxygen.

4) From what I've managed to gather by taking a sneak peak at their storage room they use stuff containing blach (chlorium) to desinfect the pipes. Shouldn't that mess with unpassivated welds? Would the self passivation on welds be sufficient to protect the pipe from periodic contact with solutions of chlorium?

5) I reat somewhere that corrosion products of stainless steel would not readily contaminate the water unless the corrosion happened due to contact with carbon steel. Is this right?

I'm new to this whole corrosion thing and find it fascinating so far. Never thought there was so much to it till I got to see it for myself.

All in all - I'm wondering if we're looking at installation done right, or that corrosion in this case would be so slow that it doesn't show yet. I was led to believe that passivation was super crucial, and should be done immidately after welding. Now I'm wondering.

Cheers and thanks for any info,

 

[EdStainless]

One issue is that we use the word passiveation two ways with SS.
When the system was built it was passivated wither with Nitric acid or a Citric acid mixture. This is a cleaning process to remove free iron from the surface. It will not remove oxides from welding (that requires pickling, usually nitric+HF is used as a paste). Once the surface if free from iron a nice uniform protective layer of Cr oxide can form.
The second use for passive is for SS to develop a natural protective oxide film that will allow it to be chemically passive (not actively corroding) in the process environment.
Clean SS will naturally become passive in air, but for prolonged use in oxygen free or chemically reducing environments the oxide layer can weaken and breakdown. Heated high purity water contains very little free oxygen and is a very good solvent and over time may break down the passive film.
If they sanitize with some sort of hypochloride solution they will be fine as long as the concentrations are appropriate. If they get too high they will begin seeing attack of the heat tinted welds (darkest ones first) as well as some crevice corrosion issues.
If the system has been in service some time there is no way to tell if the original passivation was done correctly, operations related changes will have changed conditions enough.
Feel free to reach out with more questions, this is and entire field of work.

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P.E. Metallurgy

 

[TomaszKruk]

Thank you for our answer.

How does weld surface behave, though? Does it self passivate? If I understand your post correctly - it does self passivate, but the weld surface remains somewhat vulnerable to corrosion, if right (or wrong), particular conditions happen.

I think they chill their water in order to slow the biological processes in it. That would mean it's less agressive right?

What I don't know is the oxygen content. And what it would do to the pipes. I've read in one place, that water lacking oxygen becomes corrosive (normal water, stagnant in a pipe). Somewhere else I've read that DI water IS more agressive when it has oxygen in it. Kinda confusing. Is it one of those "neither too much or too little is ok" situations?

Regards,
Thomas

 

[EdStainless]

The issues with welds is two fold. First there is the oxide formation. This oxide is mostly Cr oxide. When it is a light heat tint it may range from very pale yellow or gold to a blue or purple color. This layer is clear and the color is related to the thickness. When it gets thicker than this it is usually brown to black and it is not transparent.
Either way the Cr comes from the metal underneath it. The result is that you have a thin layer of metal that will be 2-5% Cr lower than the bulk alloy so it will have lower corrosion resistance. Then the heat tint is a light yellow (or straw) the layers are so thin that the impact is very minor.
Some people try to grind off heat tint. The problem is that the oxide and Cr depleted material just get re-embedded in the surface. To remove this tint requires pickling (usually Nitric + HF), some times to minimize pickling they either blast or grind first. When doing blasting or grinding it is critical that the media is either new or has only been used on SS.
The biggest issue with the surface forming a natural passive oxide layer (a few atoms thick) is defects in the surface. The most serious are embedded iron in the surface. This become a break in the passive film, and it easily corrodes. In some cases the corrosion products from this just result in rust spots, in other cases it leads to localized pitting. How does iron get there? Every time that the SS is touched with a steel tool (clamp, wrench, lifting chain, support, and so on) microscopic amounts of iron are transferred.

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P.E. Metallurgy

 

[EdStainless]

When you talk about corrosivity of water it becomes complicated quickly. As people are talking about different types of corrosion and over different time frames.
As we look at water conditions there are a zillion things to consider.
On the worse end, chlorides will tend to de-passivate the surface, and any fouling (scale or biological) will create crevice conditions (low Ox, low pH, concentrated impurities) and more rapid corrosion.
In 'clean' systems there are variations in TDS (is it just good soft water with what trace minerals, high purity, or DI), pH, and oxygen are the biggest ones.
Yes, very high purity water (WFI and purer) is common in pharmaceutical, semi-conductor, and optics applications. This water will have a resistivity anywhere from 1Mohn/cm2 to 18Mohm/cm2. When you are at the high end of this nearly everything will dissolve in the water and keeping ti clean is a real science.
Remember that all corrosion of SS in water is pitting, localized breakdown of the passive film and then the difference in galvanic potential between those (now anodic) spots and the rest of the surface will drive pit formation. And once pits form their growth is auto-catalytic (the oxygen is low preventing re-passivation, metal ions and solution impurities concentrate, pH goes down) so even if you 'fix' the macro environment the pits will continue to corrode and grow.


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P.E. Metallurgy

 

[TomaszKruk]

Both posts are great. Thanks for explaining this stuff.

 

[TomaszKruk]

It's a bit dumb to ask the questions, since there's no conclusions to be drawn, but maybe you guys can look at this weld and tell me what it looks like to you. The pipe was installed on what is supposedly very high quality water (everything is vague, sadly). We're required to provide much higher quality, which I have nothing against since we're being paid for that, but I want to know for myself - is this weld good enough for pharmaceutical standard? I look at this and see good source of contamination, but I honestly lack real life experience enough to tell. My assumption would be that the Client overestimates his requirements.
Either I get my expectations biased based on books, standards and promotional material or the client doesn't have water as pure as they state.

Also - is that orange spot on the weld a sign of pitting corrosion?

 

[EdStainless]

I am not impressed with the weld. It was not automatic and multiple stops and starts tend to leave higher risk for issues.
the orange could be a corrosion pit, or it could just be some surface material corroding.
At he ends of welds it is easy to get either small voids or slag pits, these are often the weak spots.
The weld color doesn't look too bad, the texture is not good. The 'fogging' next to the welds make me suspicious of how clean the tubing was prior to welding (may be the mills fault, may be the welder).

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P.E. Metallurgy

 

[Compositepro]

The orange spot appears to be slight rusting at a crevice caused by a lack of full weld penetration at that spot.

 

If the picture shows a root pass made with GTAW from the outside then it is WAY too big. And it is ugly.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[TomaszKruk]

Thanks for all the answers. Maybe I will be able to post something done by our guys soonish.