Microsiemens and Fluid Conductivity.

Conductance as a Water Quality Measurement

Thank you, Yeah I did run across that site in my research. That graph appears to be the voltage output of their conductivity meter for a given microsiemens value.

I think the most usefull information at this point might actually be a list of materials with their microsiemens values.

I may just be shooting in the dark... but when all else fails I typically turn to CR4 for guidance, and more often than not all you great individuals show me the way!

Thanks again!

DEIONIZED WATER

Re: The problem I have now in convincing the "powers that be" that the fluid is in fact above a "critical" level of conductance, which has not been established. Does anyone know where information can be found which might have some data concerning the acceptable levels of conductance that a liquid can have before the potential for accelerated corrosion may occur?

I think you're going to have to define criteria for "accelerated corrosion level" in your process. Any conductance will result in some flow of electrons (in the presence of electric potential). More conductance will result in a greater flow of electrons, proportional to the conductance (in accordance with ohms law, except that you want to use units of conductance (mhos) instead of ohms, and "convert" ohms law for conductance instead of resistance. I can't remember the symbol for conductance, but let's assume it is C, then instead of E=IR (and I =E/R) you want to consider that I=EC.

And, rate of corrosion is directly proportional to the flow of electrons.

So, now you have to look at how much corrosion your process can tolerate, and over how much time--how often do you want to repair / replace vessels, piping, etc. Or how much corrosion allowance are you willing to add to the thickness of vessels and piping (and pump and valve bodies, etc.).

Or, how much can you reduce the conductance by some means of treatment or similar?

I was going to rate my comment OT, but then I realized it would count as 5 OT votes, and it really isn't OT, nor is it wrong, but it is certainly incomplete (as others have noted with their comments).

I was considering only corrosion by electrolysis, and there are other means of corrosion, like--I'm not sure of the right word, but corrosion simply due to acid / base reactions.

See comments #6 and #7 to this thread, for example.

Critical level is entirely depending on your application.

I had optimised and EDM machine and among many steps necessary was lowering the conductivity of the water from 15 to 10µS.

But this is still a lot of conductivity if "no" corrosion is allowed.

No corrosion and some conductivity is possible only if using higher frequency currents together with graphite electrodes. This is used in gyrocompasses where the current to operate the gyros is transmitted through a liquid (for flotation of the sensitive capsule).

There have been severe problems with tiny currents corroding important metallic structures also in "inert" fluids!

Most of the gyros that died during Hubble Space Telescopes mission had fouled electrical wiring where the wires are fed through an inert fluid!

RHABE

Perhaps I am reading what you wrote incorrectly: I would have said that a fluid with a lower conductivity would be more corrosive (we say that these are "ion hungry"), take for example Deionized water or pure steam - both of these have very low conductivities and are extremely corrosive. Typically in power plants the boiler feedwater is quite low in conductivity and NH3 is added which brings up the pH level and also brings up the conductivity. It is the pH level that they are worried about in order to prevent corrosion to the steam distribution system - they are not worried about the conductivity. I would suggest that you perhaps should be looking at the pH instead of the conductivity as any disolved ion will lead to an increase in conductivity and without knowing the source of the ion then it would be almost impossible to determine if the solution was more corrosive or not.

I assume that you are not concerned with microbiological induced corrosion.

However like I said, I may have misread your post and if so please accept my apology.

This doesn't make sense, which is probably why the internet trawl has yielded so little.

Materials are selected for their purpose on the basis of their compatibility with the process fluid, as corrosion will tend to denature both the solid and the fluid. If there is a risk of corrosion, change either the one, or the other, or perhaps both, so as to maintain the process. Conductivity is one thing. pH and temperature are others, and the forum is given nothing to go on with these two measurements. As an illustration:

• 2000μS/cm at pH=7.0 and 15degC is brackish water, which enhances the corrosion of mild steels and has no effect on 316L and similar. Now the farther the pH is from 7.0, the higher the conductivity as there are more ion-forming species present in the solution. With low pH and high conductivity, say goodbye to both sets of steels. With high pH and high conductivity, both sets of steels will survive.

PWSlack, Do you have any references? The brackish water comment is exactly what I am looking for, you hit it on the money, now I just need some verifiable references to fall back on.