Getting to the facts about surface roughness and passivation for 316(L)
Getting to the facts about surface roughness and passivation for 316(L)
(OP)
I work for a pharma company and I quickly became involved in discussions about what the L in 316L meant, Ra values, electropolihing, and passivation for the first time. The basic driver for surface roughness is cleanability and the corrosion resistance treatments are to ensure the purified water/water for injection/purified steam do corrode the metal due to their low ion content (i.e. hungry water syndrome).
I still have some lingering questions:
1) Is the only advantage of chemical passivation vs regualr air exposure the decreased time to form the oxide layer?
2) When trying to achieve a certain Ra value, is the mechanical polishing doing the lion's share of polishing while the electropolishing might get you the last 1 micron?
3) If I electropolish, why am I passivating again? This confuses me because electropolishing should passivate at the same time or am I off base?
I believe the pharma industry has accepted certain practices (like what is described in q3) as some form of GxP, but I need to know the facts, Jack!
Thanks
I still have some lingering questions:
1) Is the only advantage of chemical passivation vs regualr air exposure the decreased time to form the oxide layer?
2) When trying to achieve a certain Ra value, is the mechanical polishing doing the lion's share of polishing while the electropolishing might get you the last 1 micron?
3) If I electropolish, why am I passivating again? This confuses me because electropolishing should passivate at the same time or am I off base?
I believe the pharma industry has accepted certain practices (like what is described in q3) as some form of GxP, but I need to know the facts, Jack!
Thanks





RE: Getting to the facts about surface roughness and passivation for 316(L)
No.
Passivation is not about forming the oxide barrier- the stainless steel alloy does that on its own. Passivation is about removing surface contaminants, so that the oxide barrier completely coats the entirety of every surface of the part.
This is correct. Electropolishing removes a very very small amount of material- if you electropolish a part that is heavily burnished or scratched, you will have a very shiny, heavily burnished or scratched part.
The electropolishing process is the approximate equivalent of the buffing stage of polishing a car- all of the previous polishing steps need to be correctly executed. Any scratches visible before electropolish will still be visible after.
If you electropolish, you do not need to passivate afterward. Electropolishing provides the same cleaning as the passivation process does.
RE: Getting to the facts about surface roughness and passivation for 316(L)
Also, is rouge considered surface contamination since passivation processes can remove it? (I have to search E-T about rouge separately, because that is another area that seems unclear for no good reason in pharma).
RE: Getting to the facts about surface roughness and passivation for 316(L)
I mean all of the above. Correctly passivated parts should be free of scale, loose contaminants, and embedded debris from machine tools or other processes.
Whether or not you consider rouging to be a problem depends on the specifications of what you're designing- but electropolishing does reduce the rouging tendency in stainless steels, and you've helped yourself out by selecting 316, as it will be less susceptible to rouging than 304.
RE: Getting to the facts about surface roughness and passivation for 316(L)
My company is a large pharma company, so our design spec guidelines are usually good(those docs say 316L is mandatory for all product contact surfaces that are part of welded assemblies, 304L if not welded), but at my local site there isn't much true understanding about these issues. Unfortunately, my site is the only one in North America so my international colleagues sometimes screen my emails haha!
I also want to clarify one point. My second quote says passivation process removes rouging and you mentioned electropolishing, so am I wrong about passivation not removing rouge?
RE: Getting to the facts about surface roughness and passivation for 316(L)
RE: Getting to the facts about surface roughness and passivation for 316(L)
If the Fe is coming from actively corroding stainless steel then you have another problem.
I know, everyone uses Ra for surface measurements. All that it measures is flatness, not texture. We had a customer who would buy pickle finished 316L tube and then EP it. It would still have an Ra of >70 microinches but when you looked at samples under an SEM at 100-500x you could not see any surface features. The micro crevices are what you are trying to eliminate.
The selective dissolution of Fe from the surface of SS during passivation is minor. A passivation is cleaning, and it should never etch the surface. If there is attack then you are doing things wrong. After all if it does not come off of the surface in nitric acid will it come off in your process?
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P.E. Metallurgy, Plymouth Tube
RE: Getting to the facts about surface roughness and passivation for 316(L)
Concerning surface roughness, I used the below source to understand what the profilometer measurements mean. Assuming the measurements taken are the arithmetical mean, I would think that stark surface features of the metal is directly impacting the flatness.
https://my.misumi-ec.com/pdf/tech/press/pr1167_116...
Thanks for the engagement everyone!
RE: Getting to the facts about surface roughness and passivation for 316(L)
If you read the standard you will see that below 25uin (0.6um) the values are largely just noise.
I have customers that insist that 10uin is 'smoother' than 15uin, but when you examine and measure them optically they may be identical or they may be completely different, but you can't tell from the profilometer values.
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P.E. Metallurgy, Plymouth Tube
RE: Getting to the facts about surface roughness and passivation for 316(L)
RE: Getting to the facts about surface roughness and passivation for 316(L)
We had many customers that required us to mechanically polish tubing, even when the as-manufactured surface met the Ra. And while the mechanical polish lowered the apparent Ra, it created millions of micro crevices that did not help.
Small peaks and tears are to be expected in any mechanically polished surface. If there are laps or smears then the process is very poor should not be used. This is why buffing a surface to make it 'smooth and shiny' is so bad. You smear a lot of surface metal when buffing and the crevices formed are deep and tight making them impossible to clean and a great place for corrosion to initiate.
The ability of EP to actually improve a surface depends on the process parameters and controls (V, A, flow, temp, ...). Unless someone had a documented process, with good controls, and the before and after SEM pictures to back it up then they may be doing nothing but making it shiny, and not really removing the microscopic surface texture at all.
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P.E. Metallurgy, Plymouth Tube
RE: Getting to the facts about surface roughness and passivation for 316(L)
Are those features (individual peaks and valleys) well below the micron scale? Usually 0.22 micron filters are used for sterile filtration, so if my surface can't harbor particles and bugs in the tenths of microns, then the wave and lay measurements should be enough to gauge cleanability of the surface. Otherwise, how am I getting away with NOT getting SEM images
http://www.nist.gov/calibrations/upload/89-4088.pd...
RE: Getting to the facts about surface roughness and passivation for 316(L)
RE: Getting to the facts about surface roughness and passivation for 316(L)
You have to separate systems that all EPed, and those that are just mech pol. In EP systems the cleanability will be great weather the surface Ra is 0.2u or 1.2u. There won't be any crevices tight enough to trap contaminants and prevent cleaning. Smooth open features that are 100 or 1,000 times the size of contaminants are not a problem, even if your profilometer tells you that it is 'rough'.
In a mech pol system it is a bit different. Once contaminated these can be a real problem to clean.
But in either case if the system is kept clean then it will be fine, and most plants are actually very good at this.
How do you quantify cleanability? Ever done tracer tests on your systems? I have seen a few done, with varying results.
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P.E. Metallurgy, Plymouth Tube
RE: Getting to the facts about surface roughness and passivation for 316(L)
We electropolish all metal surfaces and equipment since 99% of it is 316L SS and we can do so. However, I don't understand why EPed surfaces retain good cleanability. I imagine the spiky areas have a higher current flux and thus more energy goes to these areas and facilitates the mass transport limited dissolution. However, the preferential leveling of these areas doesn't creat wider creavices from existing ones, it just brings down the roughness by reducing the positive displacement from the level. Am I missing something?
As I understand it, my company's stance on clean validation is to perform cleaning performance qualification with swab tests and other tests if needed and then validate that cleaning method if all results pass.
RE: Getting to the facts about surface roughness and passivation for 316(L)
RE: Getting to the facts about surface roughness and passivation for 316(L)
It is almost never from the corrosion of that stainless surface, it is nearly always transported Fe from other places in the system. When the Fe in solution gets someplace where temperature, pressure, pH, velocity, gas content, or some other characteristic changes it will form insoluble iron oxide ans settle on the surface.
When you get these ultrafine particles on a very polished surface they cannot be wiped off (sometimes you can get a little to wipe). The only way to remove them is with acid.
In most systems they only bother the Quality people and not the product.
Over 90% of the cases that I have seen the source of the metal was a pump impeller (case 316L is not good enough), with valves and spray ball being nearly all of the others.
In high purity steam systems you will get dark oxides forming. These are very stable and should never be touched.
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P.E. Metallurgy, Plymouth Tube