Corrosion and "Salt Air" ??

Can someone point me to a discussion / information on the way salt can be carried in air to cause corrosion in a marine environment? Surprisingly I didn't find much on Google or searching here. It seems to be common knowledge that "Salt Air" (as compared to Salt Spray) is a common corrosion factor.

What I thought I knew was that salt molecules are carried in the air as aerosols or tiny particles, then deposited on a surface. But I can't document that.

I'm trying to get it right for the stuff I'm writing on the Wiki about Onboard Computers. Thanks!

 

Here in Louisiana near the Gulf of Mexico, we have a huge problem with corrosion. Cars don't last as long, lawnmowers, bikes, etc. This is because of the humid Gulf air carries salt in the water molecules. Often when I go to the offshore oil rigs, when I run my hand down a stair handrail I have a film of salt on my hand. (these stairs are way above the water, so the salt must be carried by the humid air.)

I'm having trouble finding documentation, but I was always told to keep cars, lawnmowers and other misc. things in the garage because of the "Salty Air" that we have in Southeast Louisiana. See if this site helps though
http://library.kcc.hawaii.edu/external/chemistry/everyday_corrosion.html

 

I found this information at this site.

http://www.glenair.com/qwikconnect/vol7num4/coverstory.htm

Water is the most common chemical on the face of the earth. Water consists mostly of water molecules, but it also has low concentrations of H+ ions and OH- ions. The humid, salty air of the Gulf contains more water molecules and ion elements than the desert setting, and is thus the better electrolyte. When electricity is passed through this electrolyte, a chemical reaction called “electrolysis” occurs—which is another of the many forms of corrosion. And as we already know, when electricity flows in the presence of an electrolyte, chemical changes can occur to the metal materials connected by the current.

 

How much 'salt' inside typical cabin??

What I'm really wondering about is how much salt/corrosive air actually gets inside the typical closed cruising boat cabin? Do consumer electronics die after a year? 5 Years? A Month? What about other typical objects / fittings? What are the variables and considerations?

I've been mostly a Freshie for 50 years, about to go Salt...

 

This is really a great question.

I, myself, like to breathe in that “salt air.” Probably many of us here do.

I also blow the science off rather easily, and just enjoy it.

I can tell you from growing up on a house on a lake, and spending summer’s at “the beach house”, (really more of a shanty) that there is definitely more salt in the air, wreaking havoc for the reasons described above so nicely by mudman. Things at the fresh water lake, like a screw for instance, might be rusted somewhat and difficult to remove, but iron screws in the salt atmosphere would crumble, and might be partially welded into its seat by rust.

But I’m not really sure how the salt is suspended in the air, and I think that is what you are getting at. It certainly is not hot enough to vaporize sodium chloride! When water vaporizes off the surface of the ocean, does it pull some salt with it?

I spent some time “googling” with more scientific terms, like “solute” and “sodium chloride” and got many interesting hits. Your concerns are both valid and of interest to many people.


Here is a technical article that shows your concern is very real. Its about the noxious compounds formed from combustion that occurs “in atmospheric air of high sodium chloride content.” I ran into quite a few publications about this, as it is seen as a health hazard to folks who work in marine environments.

http://cat.inist.fr/?aModele=afficheN&cpsidt=3658956

Here is an article (from Haavid, as we like to call it), that looks at the shape of NaCl crystals given the relative humidity. Lots of research on “deliquescence and efflorescence.”

http://adsabs.harvard.edu/abs/1999JGR...10421275W


That was exhausting. I have read much more than these two. Keep “googling” terms like “atmospheric sodium chloride”, and there is a lifetime of reading. I have read only enough to convinced myself that salt is aerosolized (this is the big word) with water, and that can remain suspended in air at least in part by its buoyancy in the atmosphere. Kind of interesting to think about water buoyant in air, when we all like to think about how things are buoyant in water. Incidentally, I think bouyancy at this level allowed Millikan to perform his famous "Millikan Oil Drop" experiment.

It really makes me mad to think that some salt is efflorescencing itself right out of solution and onto my boat while I type this.

Keep up the great questions (and some answers!)

Mike

 

Salt in Air

Those references are interesting; I never thought about how different the salt might be depending on RH. I'm surprised with the search you did and what I've done that there isn't more clear info out there. There's LOTS of Salt-Spray and testing standards and all that, but nothing I found that talked specifically about airborne salt and it's effects. Although there are lots of references that say it exists! Hawaii is supposed to be bad even a ways inland...

I've been reading a lot on "Marinized" computers and the opinions there go from "Salt atmosphere itself is not a problem" in a major publication, to lots of "Conformal coating on Circuit Boards is a necessity...". I'm trying to come up with a design for a computer system unit that would have a high degree of reliability for long duration cruising, when mounted in a location without a direct seawater splash. The at-the-helm display(s) and control(s) would have to be capable of withstanding a direct splash. There are testing standards for that kind of stuff, which most all GPS units say they pass...

I participated in the design of the IBM Industrial PC's years ago, but they were designed for cold, hot, dust-particulate, and EMI, but NOT for "Highly Corrosive Atmospheres". Looking at "Industrial Computers" today, the system units are not designed for Salt Spray either, just the Displays and some Mice and keyboards.

Sony says their Marine-Line radios and CD players have special coatings on the circuit boards.

One of my kids is the designer of NRG Wind-Energy data logging systems, and I hope to get one of these nice blue boxes with the gasket, from him

OK, there are still 1000 new things I don't know...

 

I think salts need a substrate like dust to become airborne. Filtration would allow humidity to enter a house, for instance, while removing the salt along with the particulate. Salt air makes all the cedar shingled houses on Cape Cod turn silver quickly. Just guessing, I'd bet salt air is more prevailant near the coast than out to sea where there is less dust in the air. Just guessing though.

A.

 

Terry,

I have experience with microprocessor controlled marine reverse osmosis systems, as well as systems in a variety of industrial atmospheres. There is a standard for watertight electrical enclosures, called NEMA 4X, which would give the protection you want, but the enclosures are for industrial applications.

Panasonic's Toughbook series laptops might be just what you're looking for. They claim to be tested to MIL SPEC 810 F, which includes exposure to rain, splashing, salt fog, dust, dirt, impact shock, and other extremes.

http://panasonic.com/business/toughbook/tough_customers.asp

Another alternative would be a waterproof case for the laptop, take it out only inside the cabin when needed, then back in the dry and secure case. Otter Products, the makers of Otterbox waterproof ipod cases (I own one, it works as described), makes the Otterbox 7000, designed to keep a laptop dry and secure even when submerged to a depth of 1 m. Here's a review, including submergence test: http://www.macnn.com/reviews/review.php?id=268 . There is a similar product, called a Pelican 1490, although I don't know anything about the mfr.

A Toughbook kept in an Otterbox might be overkill, but it should do everything you want.

Good luck in your quest,

Charlie

 

The secret to keeping the corrosion factor out is controlling the relative humidity. No moisture, no corrosion. I have worked in a few places that have had very corrosive environments, be it concentrated salt air, concentrated caustic air or acidic air. The methods used by us to control corrosive degredation was to control the humidity in the electronic equipment enclosures. Drying the recirculated air with an air drying filtering system seemed to work the best.In some places we had tried the epoxy coated pc boards which were ok if they had no connectors, but they were hell to work on when there were component failures. There are seal tight pvc enclosures that would do the job nicely, but install heating pads in them to drive off moisture due to condensation. They won't be great for looks however.

 

It's absolute humidity, not relative humidity, that we want to remove. Relative humidity is just a simplified way of measuring water vapor content relative to temperature. 95 deg F air with 15% RH has more moisture content than 70 deg F air with 30 % RH. Anyway, the easiest way to remove moisture from air is to cool the air; cooler air can't hold as much moisture. Heaters won't remove moisture, they just revaporize the moisture that condensed out as liquid. Desiccant packs will absorb moisture, that's why most electronics are packed with little bags of silica for shipment. Silica is a natural desiccant and absorbs moisture, reducing humidity. Eventually the desiccant must be discarded or regenerated. Regenerate it by heating it, the moisture it holds will revaporize into the surrounding air. Dehumidifiers have porous wheels that rotate in an airstream, and heaters that regenerate the desiccant in an adjoining airstream.

Putting some desiccant bags into a waterproof enclosure like the Otterbox will remove any humidity from the air inside the box. Might only have to replace the silica bag 2-3 times a year.

 

Why do we use electric driers on boats, which are just a low level heater. May not fit your theory but it works. They pack electronics with silica gel packs because using a heater would not make sense for shipping. You could also use a corrosion inhibitor which we used in many enclosures that we couldn't heat.http://www.actonrust.com/electrical.htm

 

Heating a surface will vaporize any liquid. Liquid water will turn into water vapor. Unless the air circulates, the moisture is still there and will condense back into liquid form when the temperature drops. You're right about the silica gel packs; thay are used because they absorb moisture from the air in the container without needing any energy input. The electronics are manufactured and packed and the silica packs inserted in temperature and humidity controlled rooms.

The emitting corrosion inhibitors you mention have a chemical that is emitted as a vapor and condenses on the surface of the electrical or electronic component, coating it with a plastic-like coating that seals and protects it, preventing corrosion. In HVAC systems with aluminum finned cooling coils that are to be installed within a few miles of a body of salt water, mfrs offer an option of a coating applied by dipping or spraying and curing. Same principal; a thin but impermeable coating over the parts at risk, which will prevent or postpone corrosion. It works, and something like that might be able to protect circuit boards and drive components in a laptop. I don't know enough about microchips to know if that might cause problems on the chip itself. I've read that today's microchips have etched paths separated by the width of a molecue, which is why they have to use ultrapure water to rinse the chips, to avoid any dissolved mineral molecues from "shorting out" a chip by bridging the extremely narrow gaps between paths.

Sorry about the length of this post. I design and sell dehumidifier systems, so what I'm passing on is stuff that works.

 

Controlling Humidity

OK, these last posts are bringing this together... Thanks!

Keeping humidity low on electronics circuit boards by 'almost sealing' them and using a dessicant seems workable for cruising.

I have to ask my Son whether they use dessicant routinely in the NEMA4 boxes for his loggers (See my previous post). I need a box big enough for a Mini-ITX board 170 mm (6.7 inches) square and a small 12 Volt input power supply.

A question is how well 'sealed' the box needs to be. Typical DB type connectors (serial ports and VGA Video), USB and Network connectors are not 'sealed' but do have very little leakage of air. Hmmmm. maybe THEY need to be sealed better, with RTV or etc. Or maybe the cables should all go thru a resilient sealing bar and the connectors stay "inside".

Again, for a protected-in-the-cabin location, maybe absolute sealing isn't needed..

What have you seen on other shipboard stuff??

 

some electronics on military
aircraft are encased in a hard gell to help keepfrom vibration and impact damage,,,drawback is it is difficult to repair,,,,to overcome moistier issues on our fishing boats ,we simply left the electronics turned on ,,the heat kept the moistier out ,no real problems over 10 yrs,longliner

 

Longliner, and others, are "regular" electronics (say typical 12V Car Radios, CD players etc) able to survive in a 'cabin' environment for a year or two?? I know SONY and others make (expensive) Marine models...

If we 'seal' the computer system box, then an (optional) CD/DVD player drive, for those music albums and Movies played at anchor, needs separate protection. I'm thinking of a small gasketed box that can be opened to insert a disc.