Corrode Movie

[Chris Domino]

Corrodecomes from Latin corrodere ("to gnaw to pieces"), a combination of the prefix "cor-" (used here as an intensifier with the meaning of "completely") and the verb "rodere" ("to gnaw"). At one time, "corrode" was used to literally indicate the action of gnawing away, as in "woodworms corroded the wood." But it is the more figurative senses from the action of gnawing or eating away that have persisted, as in "salt water corroded the iron" or "graphic content on television and the Internet is corroding the moral fabric of society."

I've found that carbon, which can be harvested from pencils and old batteries does not corrode contrary to my expectations (I'd expect it to produce $\ceCO2$). But soldering on carbon is completely impossible. Also, I need some bigger electrodes.

As mentioned before, you can use Platinum and that works great, though it is super expensive. I have used graphite electrodes in the past with either metal fittings or alligator clips. Could that work?

I'm a glaciologist working in Patagonia where everything is always wet. Often water makes its way into instruments, radios and other devices and I feel that always the positive terminals of batteries (and the circuitry connected to it) corrodes more effectively than the negative side.After not finding a explanation for this, I thought it might only be my impression. But then I searched for some batteries and instruments that spent some time underwater and indeed the positive terminals are corroded and the negative ones are like new. The example below show some Li-Ion batteries that display that situation (of course the picture shows two batteries, but believe me that positive/negative terminals in both batteries looks pretty much the same).

I'm considering this question because I'm now designing an instrument that uses MOSFETs as switches for peripherals, and I'm struggling to find P-Channel MOSFETs to switch the positive side. As N-Channel MOSFETs are much more common and cheaper.

NOTE: One line of thought that I've explored is that the battery will produce water electrolysis. Although, my chemistry knowledge is very limited, I think in that case the highly reactive acid H+ (that I think is how hydrogen stays in aqueous solution) should go to the negative side, so I would expect corrosion there. The negative side gets O2, which could be the reason of the oxidation of the positive terminal, but as there is plenty of dissolved O2 in water and even more in air, I would still think that the negative side should get the worse corrosion, so this doesn't makes much sense to me either.

Probably better asked on a chemistry site than electronics. At any rate, electrolysis does happen but it's mainly electrolysis of dissolved salts since pure water requires quite more energy to tear apart.

Now if you do electrolysis on, say, table salt, you get chlorine and sodium which are both aggressive substances. However, your electrodes are metal, and chlorine reacts aggressively with metal while sodium is a metal and react aggressively with water which does not affect the electrode: in fact, the sodium competes (and wins) in who gets corroded first.

Now the chlorine is produced at the positive electrode where \$\mathrmCl^-\$ ions are turned into highly reactive \$\mathrmCl\$ radicals or at least \$\mathrmCl_2\$ gas. So that is where the corrosion happens.

It's not just sodium chloride in solution, of course, but the vast majority of salts are made from positively charged metallic ions and negatively charged substances that will, once you invest the energy to discharge them, act aggressively towards metals in order to regain the missing electrons, ripping out positively charged ions. Which the positive electrode does not mind all that much since it actually was short of electrons anyway.

Normally one would expect the negative electrode to get smaller and the positive to get bigger, as the technical defined direction of current from plus to negative is wrong. In reality its via versa, charge is phyically transported from minus to plus - so ions are transported from the negative towards the positive electrode.

profile register preferences faq search SSFFriend: Email This Page to Someone! next newest topic next oldest topicAuthorTopic: Does silver corrode?fidda

Posts: 45

Registered: Nov 2004posted 01-24-2005 11:26 AM Can silver actually corrode like iron? I am not quite sure about this yet. I know some certain rubbers and plastics can cause corrosion but can silver corrode by itself in contact with air (like iron)? I think tarnish is its way of corrosion but then again i am not sure since tarnish does not actually break down the surface like rust does to iron... or does it? Do the very stubborn black spots on silver that don't go away (unless heavy polishing is applied) corrode the surface of silver or does it simply stain it without effecting the metals strength? Please help me out here since I have been pondering on this for the last few months and can't seem to find an answer anywhere.IP: Logged

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Registered: May 2003posted 01-24-2005 01:48 PM The following is from my experience only, and others' experiences may differ. Silver is almost a noble metal like gold and platinum, but with an Achilles heel, which is sulfur. When chemically attacked by sulfur, it can both form a very thin, even brown-black patina that may be polished away with slight metal loss, and it can form black fissures and pits. Strong acid and salt conditions appear to increase the pitting attack, such as seen in salt dishes and mustard pots, hence the gilding often applied. Note that rubber bands contain sulfur, and so tend to leave black stripes on banded flatware. The black sulfur corrosion product is a chemical conversion of the base silver metal and not an extraneous surface stain, and its removal by means of polishing removes what used to be base silver metal. Conversion products like Tarn-X appear to reduce the silver sulfide back to elemental silver, but they leave a whitish texture because the reduced silver is microscopically spongy instead of solid. Polishing will restore the shine, but mostly by removing the spongy reduced silver layer, and so the result may not be much more conservative than skipping the chemical reducing agent and simply polishing in the first place. Finally, each piece seems to have its own personality. Some pieces seem to tarnish more easily than others, and on some pieces, the tarnish seems to be much more difficult to remove than on others. For heavily tarnished pieces that are very resistant to polish, I recommend using Tarn-X or the like (more than one application may be necessary, wash thoroughly afterwards), and polishing away the resultant whitish patina later. An exception is three-dimensional pieces, such as repousse, where the chemical reducing agent should NOT be introduced into the crevices. FWIW.IP: Logged

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Registered: Mar 2003posted 01-24-2005 02:06 PM Corrode? Yes.

Like Iron? No.Iron corrosion is mainly the result of its chemical reaction with oxygen in the air or water resulting in the creation of iron oxide which is commonly known as rust.Silver is a fairly stable metal and under normal circumstances does not corrode quickly. Silver corrosion is mainly the result of its chemical reaction to sulfur in the air or water resulting in the creation of silver sulfide which is commonly called tarnish. Silver can also react with chlorine or bromine in a similar way. Silver does not react at the same speed to these as iron reacts to oxygen and so tarnish typically does not wind up converting large percentages of the metal into the corrosion products unless subjected to high concentrations of sulfur, chlorine or bromine over a long time. Sulfur can be sorbed down below the surface forming a "stain" that can only be removed by polishing the metal away until you get down to "unstained" metal. The best way to avoid all of this is to keep silver away from any possible sources of sulfur, chlorine or bromine. One of the biggest sources of sulfur in a typical house is from cooking or heating gas. As such, the worst place to store silver would be in or near a kitchen with a gas stove or in or near a basement with a gas heater.Silver objects are rarely made of pure silver. They are typically alloys of silver plus copper plus other metals, or they are plated over these other metals. These other metals tend to be more aggresively reactive to elements found in air or water, and the higher percentage of these other metals, the more likely the item is to have these additional reactions.

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Registered: Nov 2004posted 01-24-2005 02:41 PM hmm ok ok I see thanks a lot.

2 more questions left -will tarnish spread? What I mean is if there is a spot of tarnish that is not polished out but the silver is maintained properly by washing etc. will the spot of tarnish still grow?and Will a stubborn spot of tarnish ''eat away'' the silver underneath by time and as a result pit the silver? in which case better removed and if harmless not worth the bother of heavy polishing to remove a few small spots at the back of a piece.Thanks for all the help.IP: Logged

Posts: 289

Registered: May 2003posted 01-24-2005 05:00 PM Again, my opinion only.Yes and no. If the item is kept completely clean, dry, and unexposed to sulfur (and, as mentioned by another, chlorine/chloride and bromine/bromide), then the spot won't grow. If the storage is less than ideal, then the tarnished spot may tend to grow before clean metal tarnishes, because the spot provides a favored place for condensation and the collection of aggravating impurities such as salts and salt contaminated skin oils.Yes, the tarnish spot may result in a deep pit if not removed, as indicated above. Any spot of tarnish is already a "pit" in that sulfide (most typically) attack has eaten into the metal at least a little bit. Often the "pit" is rather shallow and not noticeable when the sulfide layer is polished away. It is probably best to try to polish tarnish spots out if they are not deep. OTOH, in some cases, as often happens with salt dishes, the pitting is so deep that too much metal would have to be removed to get the pits out. You have to eyeball it when you polish and determine how deep the pit really is.IP: Logged

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