316 Stainless Not Conducting in Water 2

[seurban]

I have an odd issue that I hope someone here can shed some light on. I have 1/16" OD 316 stainless steel rods that I'm using as probes to test whether a device is submerged in water or not. There is a positive and negative spaced a bit apart and I'm checking for conductivity/resistivity. I've made a few prototypes and the issue I'm having is that on 2 of the 3 protos everything starts out fine, but then the resistance starts to rise while in the water until it no longer conducts at all. If I scrape the probes a bit they conduct again, but the resistance climbs again. It takes a minute or two to be fully non-conductive. My only theory so far is that there is an insulating oxide forming on the surface, but I did not expect that at all, especially since there are TDS meters that use stainless steel probes. And they are used to measure conductivity/resistivity, so they can't be having this issue.

They one prototype that isn't having this issue has two positive probes (connected directly on the inside) and two negative probes (also connected). This was done for redundancy and increased surface area, but maybe it also is preventing this resistance increasing issue? Again, it seems unlikely since TDS meters have only one of each.

I did have little bit of difficulty soldering to the rods. I think the soldering iron I was using was a little weak and combined with the large cross section of the rods it was hard to get the rods hot enough to melt solder. Consequently, they spent a relatively long time heated almost to the point of melting solder (I'll have to check which solder I was using, but that's probably about 180°C, 356°F). Maybe this has something to do with it? Some I did better than others, maybe that's why some work and others don't?

Also, after the probes have stopped conducting in water, if I connect the probes with a metal tool they conduct immediately with very low resistivity, so the rest of the circuit is fine and if there is a film/layer it can't be all that thick since I don't have to press very hard to get it to conduct.

I am going between salt and fresh water, but I did rinse the saltwater off and it does this in the fresh water too. And what the heck, 316 SS? I thought you were supposed to be pretty good in saltwater! The tests have just been done in small glasses so far (can't imagine that has anything to do with it, but I'm stumped).

I am just using a multimeter for now, which I assume uses a very low voltage and current (I’ll check tomorrow what it is). Hopefully this isn’t the issue, since I also want to keep them as low as possible to conserve battery.

Any thoughts, explanations, magic solutions? Hopefully this is the right section to ask this in. If it’s not corrosion perhaps I’ll try the electrical section. Thanks.

 

[EdStainless]

What potential do you have on the rods?
What did you use as flux for the soldering? Hint it must be a very strong acid flux.
The natural film that forms on SS is a semiconductor.
But with 316 in salt water the corrosion currents may be higher than your system is trying to measure.
316 will not stand up in salt water.
Many of these sensors either use the 4 probe arrangement or use an AC signal.
Have you thought of using CuNi? Or NAB? or some other Cu based alloy that is seawater resistant.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[seurban]

Thanks for your thoughts, Ed.

The rods have about 400 mV going across them from the multimeter I'm using (haven't put together the full circuit yet). Coincidentally that matches the voltage of the conductivity probes on a water meter we have. The current is 0.13 mA.

I do need to use a better solder, thanks for the hint. I had been just using hardware store grade Rubyfluid paste flux, but just order some stainless steel flux.

It's interesting that you say 316 will not stand up in salt water. From what I've seen it is frequently used in that environment, but perhaps not with electricity? It is also frequently used to measure conductivity, but I guess not in salt water.

Good thought on using an AC signal, though it will complicate the electronics. The 4 probe arrangement could be interesting (I assume you mean this:

https://en.wikipedia.org/wiki/Four-terminal_sensing

, not the 4 probe setup I had where they were just redundant). We don't need to be very precise with our measurements, since the difference between submerged and not should be big, and all we need is a Yes or No, but if that solves the issue I'll go for it.

We have started looking into other materials, including CuNi. Haven't looked at NAB yet. It seems to me that my issue may be that many oxide films are non or semi-conductive, even if they offer good protection from a structural standpoint. So if these or other materials can continue to conduct after oxidation that would be ideal. And the material doesn't need to be all that strong, since I should be able to protect the probes. If I stuck with stainless steel, do you think passivation would help? Or is the oxide film that would create just bad news all around?

 

[moltenmetal]

Steady polarized DC is the wrong way to measure conductivity in water for sure. You'll end up with the problems Ed mentioned. AC conductivity is generally used.

 

[EdStainless]

With AC as you switch polarity you are overcoming the polarization of the surface film.
The oxide film on SS is much less conductive than the hydroxide films on Cu alloys.
316 will survive in cold seawater if there is very little oxygen (deeply submerged) or if it gets washed off frequently (boat fittings).

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[Gazzaj2101]

From a Cathodic Protection point of view 316SS may not be suitable for this application due to when the 316 is immersed in a water based electrolyte there is an initial corrosion product formed on the 316 surface. This product is an oxide layer that is highly resistant to the passing of an electrical current, which in turn inhibits the corrosion process.

Additionally the measuring equipment is supplying a 400mV DC electrical potential. therefore the 316 probe attached to the positive side of this equipment effectively becomes an anode, thus accelerating the production of the oxide layer, whilst the probe connected to the negative (nuetral) side of the test equipment becomes a cathode and the oxide layer is much reduced or even eliminated, corrosion being eliminated cathodically.

The oxide layer produced on stainless steel is very thin in the micron range and has poor strength and adhesion, it can virtually be removed by rubbing a finger on the surface of the material.

AC current may be a way forward, however I would percieve this to be a more expensive peice of equipment, which if you are looking for a commercial application may be detrimental.

I am not a metalurgist, however I would suggest a suitable probe for this type of application could well be a titanium probe, coated with mixed metal oxide (MMO). This type of material is widely used in the manufacture of Cathodic Protection Anodes, used on both land based and marine applications. I am not sure of the effect of salt water on titanium in an anodic application, however I do know marine MMO anodes have some of the titanium exposed to the electrolyte and they consistently last for 20 years at high current outputs. Also the MMO does not suffer the normal oxidisation issues associated with many other anodic materials, additionally anodic depletion is very low.

I hope this is of some value and answers more questions than raises new issues.