Expert: Angel Evangelista - 11/7/2008

Question
QUESTION: i'm an electronics student.

i was building a water lvl controller(based on water conduction). i'm sending an AC signal to the Stainless steel probes to prevent corrosion. but however, probes are still electrolysing. i even used a capacitor to block any DC in the AC signal, but still there is some electrolysis. what can i do to prevent it completely?

i also tried increasing the frequency of the ac signal (square wave of 1250Hz, 10Vp-p) i was using, but still corrosion exists.

also, when corroding, i find air bubbles on one of the probes very much more compared to the other. does it mean there is still some dc in the ac i'm sending to the probes?

pls help!! many many thanks in advance..


ANSWER: Regardless of the probe material, you will experience ELECTROLYSIS OF WATER with either AC or DC current.  If you are using household tap water, municipal water supplies, bottled water supplies, or a natural water source, your electrolysis will occur more readily.  The bubbles you are seeing, particularly on one probe are Hydrogen AND Oxygen molecules.  Since its AC, the "hot" anode, the one actually supplying the AC current is inducing the electrolytic reaction of the water, causing its separation into hydrogen and oxygen molecules.  The oxygen coupled with the presence of an electric potential is oxidizing/corroding your probe, regardless of what metallic probe material you use.

To thwart electrolysis, consider using ultra-purified water in an ultra-clean container with ultra-clean probes.  Anything that the probes, container or wires were in contact with or washed with will contaminate the ultra-pure water.  To get ultra-pure water, ask your chemistry lab if they can provide a gallon of it in a clean container (that you might have to supply).  The point of using ultra-pure water is that it will have less than 1PPM, maybe even a few PPB of dissolved solids – hardly any ions will be present in the water.  Without the ions, ultra-pure water will not conduct electricity as readily, and electrolysis will occur at a far lower rate.  You may have to tune your meter sensitivity however since much less current will also conduct in ultra-pure water.

Next, consider a non-metallic semi-conducting material for your probes to prevent oxidation corrosion.  A semi-conductive material however will require higher voltage, but that could be compensated by using less current.

But in the first place, why are you considering electric probes for determining water level?  Or are you trying to measure any liquid level, not just water?  Industry level meters utilize nuclear gauges, electromagnetic, and densitometers to detect liquid levels.  Densitometers and nuclear level gauges have been quite successful in recent years.  Densitometers utilize the fact that gases (like air) and liquids have distinct density breakpoints inside a vessel, and even successfully distinguish an aerosol or foam from the true liquid level.  Prior level float technologies sometimes succumbed to aerosol and foam problems as well as dirty liquid service issues.  Nuclear gauges are prominent where the liquid and/or gas regimes are very dirty or in production equipment that was impossible or difficult to service or shutdown.

Anyway, hope this helps!

---------- FOLLOW-UP ----------

QUESTION: hello,

i have reduced corrosion of probes by reducing the current passing thru the probes. but NOW, corrosion is visible - only at the water-air interface of the probes. and its kind of eating away the probe, slowly though. bubbles are not visible anywhere else on the probe(tested for 2 days).

my question is - why corrosion only at the interface?

Answer
Q: I have reduced corrosion of probes by reducing the current passing thru the probes. but NOW, corrosion is visible - only at the water-air interface of the probes. and its kind of eating away the probe, slowly though. bubbles are not visible anywhere else on the probe(tested for 2 days).

my question is - why corrosion only at the interface?

ANS: Hmmm, that is interesting, but logical.  Closer observations may require a microscope, but I'll bet there are micro/tiny bubbles also being formed below the air-water interface.  As the tiny bubbles merge and grow, they exert a higher bouyancy and migrate up towards the water's surface.  But this "micro-bubble phenomena" doesn't explain what you're seeing in this case.  

Let's examine the path electrons take inside your probe - a conductor, and from one conductor to the next.  

Your probe does not conduct electrons into the air because air has extremely high resistance, so electricity therefore continues to flow down the probe...until...it meets the water's surface.  Water, being naturally more conductive than air (and salt water being much more conductive than ultra-pure water) now provides the next path of least resistance for the flow of electrons.  If you are seeing bubbles only at the air-water interface where the probe breaks the water's surface, then the water is more conductive than the remainder of probe below the water's surface.  The vast majority of the electrons are flowing straight through the water where the probe first contacts the water because that is the path of least resistance.  The restof the submerged probe and the deeper water all possess a higher resistance path.

You can test this by finding that ultra-clean container of ultra-pure water and immersing the probes in it.  You should see bubbles forming along the entire length of the submerged probe, because ultra-pure water will have much higher resistance than the probe metal.  Conversely, you can also try turning the water into a concentrated brine solution, by dissolving a lot of salt into the water.  This has the effect of making ALL of the water highly conductive and allowing most of the submerged metal to provide a path of least resistance along its entire submerged length.  But, a brine solution will also more quickly oxidize and corrde the entire probe - the salt contributes to this even activity more, both electrolytically and chemically.  Chemically, it forms a non-conductive salt of the probe's metal that deposits itself back onto the probe, (this depends on the metal; iron and copper do this readily).

Hope it helps!