Purified (< 0,01 microsiemens/cm) water corrosive?
Purified (< 0,01 microsiemens/cm) water corrosive?
(OP)
Hello,
I have seen some corrosion in silicon carbide seals that are used in process water pumps in medicine fabrication. Some people think that the corrosion is electrical and that could very well be so - I have measured high frequency currents (from frequency inverters) in the 100 mA order of magnitude.
But some guys tell me that this kind of water is very aggressive and corrodes stainless steel, and probably SiC seals as well. I know nothing about these chemical things and ask you if that is true or not. Any examples? Remedies?
I have seen some corrosion in silicon carbide seals that are used in process water pumps in medicine fabrication. Some people think that the corrosion is electrical and that could very well be so - I have measured high frequency currents (from frequency inverters) in the 100 mA order of magnitude.
But some guys tell me that this kind of water is very aggressive and corrodes stainless steel, and probably SiC seals as well. I know nothing about these chemical things and ask you if that is true or not. Any examples? Remedies?





RE: Purified (< 0,01 microsiemens/cm) water corrosive?
Oxygen is also a big factor, and doesn't affect the cond. In some cases the water becomes less corrosive if some metal ions are present-so called water hungriness. SS needs small amounts of O to remain in the passive state, which can greatly lower corr. rates. But more than that amount can/will increase them.
What temps. are involved? How are you measuring the current? How close is the corroding part located to something that could be cathodic to it? If very close (~1" or less) there may be a galvanic effect, even tho the cond. is low.
I'm not familiar with the corr. if SiC, but someone here probably is.
RE: Purified (< 0,01 microsiemens/cm) water corrosive?
Ultrapure water is also, to my knowledge, NOT corrosive toward stainless steels. There may be issues with minor quantities of metal ions released from stainless steels which may result in contamination of the ultrapure water, particularly soon after the material is brought into service, but that's not the same as saying that ultrapure water will aggressively corrode the stainless steel. It's this common misconception that your colleagues are probably repeating. Hot, oxygenated pure water IS aggressive toward copper and brasses (particularly in areas of high local velocity), but in closed deoxygenated systems both of these materials also stand up quite well- regardless of how pure the "pure" water is.
Are you certain that the damage you're seeing is corrosion? Could it not be disintegration caused by poor-quality silicon carbide? Or perhaps it's just wear? Water is very non-lubricious.
RE: Purified (< 0,01 microsiemens/cm) water corrosive?
Thanks. I do not know about the O2 contents. Will ask the specialists. The temperatures are same as environment - i.e. room temperatures. I measured the current with a rogowski coil around the shaft going from motor to impeller, the seal is between this shaft and house.
The SiC is inserted into the SS. It is a ring, so it is close to the SS. Do not know if SS is cathodic w/ respect to SiC. Do you?
Molten
Your statements support my thinking. Thanks. Interesting part that the water gets contaminated in the beginning and that this can be mistaken for corrosion. I think that it can be the explanation I need, but not quite sure yet.
We had a specialised institute (Norsk institutt for energiforsking) investigate the damages and they think that it is corrosion.
Thanks guys. A small step for mankind. A quantum leap for me
RE: Purified (< 0,01 microsiemens/cm) water corrosive?
There was a famous incident about 30 yrs ago where a university lab announced the discovery of "polymerized water," which had properties unlike any other form of water. After about six months or so, they, much to their chagrin, announced that it was simply plain water and that the beaker glass had dissolved into the water and thus had contaminated their tests.
However, the concentrations were extremely low, so it's unlikely that you would actually see physical corrosion effects. Since you are measuring currents, then it's certainly more likely that there is some sort of electrolytic reaction going on.
TTFN
RE: Purified (< 0,01 microsiemens/cm) water corrosive?
RE: Purified (< 0,01 microsiemens/cm) water corrosive?
Extremely Pure water has been known to attack silicon carbide. I'm trying to find the reference at the present, as I think there were other parameters involved.
We tried SiC/SiC seals in pure water total disaster. We went to C/Si seals evidently they are performing well.
moltenmetal,
Sodium silicate is an excellant solvent for glass, especially fiberglass.
RE: Purified (< 0,01 microsiemens/cm) water corrosive?
That could easily be the problem we are having. I have done some searching myself but not found anything substantial. I would apreciate a reference or two.
RE: Purified (< 0,01 microsiemens/cm) water corrosive?
http://www.corrosion-doctors.org/Household/Glass.htm
http://www.osti.gov/bridge/product.biblio.jsp?osti_id=787189
http://www.isis.rl.ac.uk/isis2000/highlights/GlassCorrosionH10.htm
not quite on subject:
http://www.corrosionlab.com/Failure-Analysis-Studies/28059.corrosion.zinc-housing.htm
http://corrosion.363.net/glass/accessory/resistance.htm
http://www-atalante2004.cea.fr/home/liblocal/docs/atalante2000/O3-04.pdf
http://nvl.nist.gov/pub/nistpubs/sp958-lide/181-183.pdf
TTFN
RE: Purified (< 0,01 microsiemens/cm) water corrosive?
RE: Purified (< 0,01 microsiemens/cm) water corrosive?
I can certainly understand how sodium silicate would definitely be nasty toward glass. That doesn't surprise me in the slightest.
I've personally seen SiC used successfully in pump seals and valve trims in all sorts of water services and even some highly acidic water/steam services. If you have any references on SiC corrosion in ultrapure water I'd be very interested to see them!
Thanks again, guys, this is extremely helpful and useful stuff!
RE: Purified (< 0,01 microsiemens/cm) water corrosive?
Comment: Me too!
RE: Purified (< 0,01 microsiemens/cm) water corrosive?
RE: Purified (< 0,01 microsiemens/cm) water corrosive?
I did find a reference to SiC vs SiC running dry, not very good.
Also I made a note that one of the silicon carbides absorbs water (2%-5%) but made no notes of which one sintered or reaction bonded.
I am still looking, as I am looking also for some old steel spec's.