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Caution: Corrosion When Wet

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Beth Stackpole
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Blogger
Keeping the environment at bay
Beth Stackpole   3/9/2012 6:36:19 AM
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I'm sure engineers across disciplines and industries can appreciate your tales of water ingress protection. Lots of perspective advice, too, as to how to prevent or at least trouble shoot this issue. I'm sure water isn't the only environmental component that finds a way to wreck havoc on engineering efforts--what are some of the other major environmental influences that post common challenges for engineers?

GlennA
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Gold
Pressurized tank
GlennA   3/9/2012 10:31:22 AM
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One of the machines that I worked on had a pressurized hydraulic tank.  About 4 psi from the plant compressed air supply was supposed to keep water from getting in.  However, the design team had upgraded the reservoir to have a vented cap.  When I advised them that a vented cap would not hold pressure, they told me " you don't understand".  When I installed this version of machine I advised the customer not to try to pressurize the tank - to turn that regulator down to zero.  On one of my service calls for a similar machine, the hydraulic oil looked milky.  I found the plant had wet air.  The air drop did not have a drip leg or drain valve.  The machine's water trap was full of water and the automatic drain had been turned off, and the lubricator was also full of water, instead of oil.  And the pressurization regulator was on.  So effectively they had been adding water to the hydraulic tank through the pressurization system that was supposed to keep the water out.

JimT@Future-Product-Innovations
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Blogger
Gasket material: TPE vs. LSR
JimT@Future-Product-Innovations   3/9/2012 11:34:43 AM
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During a recent contract, I was hired to design an enclosure intended to pass IP64 and temperature cycling extremes. Since I had experience designing both water-resistant, and water-proof (ambiguous terms invented by marketing,,,) enclosures in the past, I saw no real challenge to the task. 

 

While laying out the preliminary product concept, I specified TPE (Thermoplastic Elastomer) as the gasket material between the housings.  I had used that solution previously, and its sealing capability to prevent water -- even pressurized and/or blowing water -- was proven.  TPEs are highly compressible and provide excellent dimensional compliance when gasketing between harder plastics.

 

Luckily, a peer who had been down a similar road looked over my shoulder and quickly pointed out a problem:  remember, the product must pass not only IP64 but also temperature extremes.  His past experience taught that a TPE, when compressed per design intent, would take a permanent compression-set at extreme cold.  Later, as the product cycled back to higher temperature, the TPE would no longer be compliant as a gasket and allowed significant water intrusion after only one cold cycle.    The solution was to use silicone rubber, which has true spring-back to original geometry, even after hundreds of temperature cycles.

 

While injection-molded Liquid Silicone Rubber (LSR) resolved that particular problem, it was not without a long list of other engineering challenges; but I'll save those for a subsequent post.  The points to remember are, (1) TPEs take a "set" at extreme cold, and (2) don't be afraid to listen to your peers.

Ann R. Thryft
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Blogger
Can naval tech help washdown?
Ann R. Thryft   3/9/2012 12:12:09 PM
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TJ, thanks for a clear explanation of what can go wrong with water ingress.  Your mention of both the tugboat and industrial washdown environments made me wonder how much cross-over there might be from technology developed for naval environments to industrial contexts?


TJ McDermott
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Re: Can naval tech help washdown?
TJ McDermott   3/9/2012 12:24:38 PM
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I would guess less than you might think. Take the junction box design, for instance. Any commercially available industrial J-box has a flange all the way around the door. It adds stiffness, and prevents directed jets from impinging directly on the door gasket. Shipboard means of corrosion protection include layer after layer of heavy paint (assuredly not latex house paint, either). I don't see that sort of anticorrosion coating making a comeback in factories.

Ann R. Thryft
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Blogger
Re: Can naval tech help washdown?
Ann R. Thryft   3/9/2012 12:37:14 PM
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Thanks for the feedback. I can see where the corrosion protection would be different in the two environments. I guess I was thinking more about the water aspect of the environment, but it looks like the two are inseparable.


sensor pro
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Gold
Re: Can naval tech help washdown?
sensor pro   3/12/2012 10:23:33 AM
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In my experience the scheduled cleaning of the service equipment was the biggest issue with corresion.I noticed that seals were not holdig with time and the quarterly cleaning let some moisture inside. This was the main cause for corrosion on power terminals and transformers, especially where the thermal cycling was evident.

Aldo Agraz
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Iron
Water intrusion is a hard enemy for instrumentation purposes
Aldo Agraz   3/11/2012 10:06:00 PM
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Currently I am working on the characterization and performance evaluation of solar collectors, we are using thermocouples for temperatures measurment in a closed loop of solar collectors in order to read temperature elevation of a recirculation fluid. One of our headaches is water in the terminals of the sensors, it produces oxidation in the ferrite metal of the thermocouple and therefore lecture errors, if anyone can suggest an efective way to isolate thermocouple terminals from water it will be really appreciated. 

bob from maine
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Platinum
Re: Water intrusion is a hard enemy for instrumentation purposes
bob from maine   3/12/2012 11:26:49 AM
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Something to think about: If you make a sealed box with wires entering it and you daily heat this box to temperatures significantly above ambient, then at night cool it, you have made an efficient pump. If the box is vented, the vent needs to be in a location that draws in dry air. If it is not vented, the box will find a way to breathe expelling and replacing the air from somewhere.

Walter
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Silver
Re: Water intrusion is a hard enemy for instrumentation purposes
Walter   3/12/2012 12:45:32 PM
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Why do you use thermocouples?  RTDs (Resistance Temperature Detectors) are more accurate, more reliable, cost less and are more corrosion resistant.  Other than that there's no reason to use them.

Battar
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Platinum
Capillary
Battar   3/12/2012 10:16:18 AM
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Capillary action can draw water between the strands inside a cable from the connector outside the watertight box. a gastight gland is no protection against this.

It's worth remembering that a 1$ plastic bottle of soda has a perfect gastight screw-top seal with no o-rings or RTV.

bob from maine
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Platinum
Re: Capillary
bob from maine   3/12/2012 11:18:08 AM
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I've had radar lines on a 90' sailboat mast dump water into a junction box that was sealed. Every fall when the mast was removed, the terminals in the box would be severely corroded and the wire needed to be stripped-back to find bare wire. The wire was sealed in the radar dome and sealed in the junciton box, however in winter the mast was stored horizontally with the dome removed and the wire exposed. Somehow water got into the cable and until it was replaced, caused corosion in the sealed box. Once you see this, you can find the same symptom in other areas. It makes for interesting trouble-shooting.

Battar
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Platinum
RTV
Battar   3/12/2012 11:36:19 AM
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Watch out for the RTV. It outgasses acidic fumes when curing, and that can accelerate corrosion. If you seal warm damp air in a gas tught box, the humidity will condense when you cool it at night.

Critic
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Platinum
Re: RTV
Critic   3/12/2012 12:10:41 PM
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Not all RTVs outgas significantly!  This is something to consider when selecting an RTV, though.

Critic
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Platinum
Condensation
Critic   3/12/2012 12:03:29 PM
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It is important, if you want to keep water out of enclosures, to consider that the water entering the enclosure may not be in liquid form.  Some seals are great for keeping out liquid water, but may not prevent humid air from entering the enclosure.  When the temperature drops, the moisture in the air condenses and ends up as liquid water inside the enclosure.  Solutions include pressurizing the enclosure with dry air or an inert gas, making the enclosure air tight (as well as liquid-tight), or adding a condensate drain if high humidity inside the enclosure can be tolerated.

William K.
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Platinum
Corrosion and water entry.
William K.   3/12/2012 8:15:54 PM
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I have seen thermocouples hose water 50 feet, I have seen the shield braid on RG8/U cable hose wate farteher than that. BUt thermocouples are cheap and easy and adequate for many applications, that's why folks use them. I also had a situation on a house that we purchased where water ran out of the entry panel when it rained, because the cheap person who installed the entry cable did not use an extra foot for the drip loop.

Water Proof encloseures will fill with water brought in by cable and conduit problems, so the very best choice is to have a drain in the enclosure bottom. But be sure that water can't enter by the drain opening. Another method that will prevent water entry is to have a pressurized enclosure, with sufficient air pressure to prevent water entry. This has been used on communication cables for many years. It is an expensive option, but much cheaper than fixing the problems caused by moisture entry. Of course, you must use dry air or dry nitrogen for pressurizing. That might have worked on the tug boat problem.

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