Xref: utzoo sci.electronics:6590 rec.autos.tech:8619 Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!cornell!batcomputer!sunny2.che.clarkson.edu!kweeder From: kweeder@sunny2.che.clarkson.edu (Jim Kweeder) Newsgroups: sci.electronics,rec.autos.tech Subject: Re: parasitic anodes for rust prevention ??? Summary: Industrial Strength Flames Keywords: corrosion, causes, misconceptions Message-ID: <3167@sunny2.che.clarkson.edu> Date: 13 Jun 89 19:26:12 GMT References: <11854@bloom-beacon.MIT.EDU> <4345@druco.ATT.COM> <3220@kitty.UUCP> <3166@sunny3.che.clarkson.edu> <3223@kitty.UUCP> Reply-To: kweeder@sun.soe.clarkson.edu (Jim Kweeder) Distribution: usa Organization: Clarkson University, Potsdam, NY Lines: 87 Ok, Larry. You want flames, you got them. Note: if you're tired of this or if you're not interested enough in corrosion to get the straight word, hit 'n' now and avoid the following: In article <3223@kitty.UUCP> larry@kitty.UUCP (Larry Lippman) writes: [Quoting Larry from an earlier article] >> > The important point to bear in mind is that there is no significant >> >presence of dissimilar metallic junctions beneath an automobile which lend >> >themselves to the formation of galvanic corrosion cells which affect any >> >significant area of the automobile body [Material Deleted] > I already mentioned this in my previous two articles, in addition >to the above. The point to bear in mind is that these other corrosion >machanisms are NOT SIGNIFICANT in the automobile environment! If we read these statements closely enough, we will find that automobiles never corrode. Isn't that nice. However, Larry, there are a number of upstate NY rust buckets in the parking lot of this building. How did they get that way? Gemlins? Divine intervention? Sorry, Larry, but you can't have it *both* ways. I'm afraid that you're going to find out if you bother to look at any texts or design literature that concentration cells, and other non-dissimilar metal corrosion mechanisms are recognized as a problem in mild steel. > So what? It's not going to be done with either the same chemistry >or metallurgy found in an automobile environment, so it's a non-applicable >example. OH, you want a specific example. Ok, first I buzz down to your house with my nibblers, cut a hunk of sheet metal out of your fender, bring it back here and put it upright in a largish beaker. Next, I pour about 75 g of 10 mesh calcium chloride/sodium chloride (aka road salt) into the bottom. Finally, I carefully pour some water (tap or distilled) into the beaker. Viola, one steel-steel concentration cell using materials common on our nation's highways. Happy? [Quoting me] >> You greatly underestimate the need for mixing. I can >> easily see a difference in electrolyte concentration on a car. > > So tell me about it. Well, without going into the gorey details, the fact is that plain diffusion is very slllooowww. If I take a perfectly still beaker of liquid (at a uniform temperature to avoid bouyancy convection) and add a second liquid extremely carefully, and keep it where the temperature will remain uniform and as little vibrations as possible will shake it, it will take weeks, if not months, for the solution to come to chemical equilibrium. Yes, this experiment has been performed several times. Mixing is such an important aspect of chemical engineering that books are devoted to it. It is not a trivial operation. [My description of putting magnesium in your trunk deleted] > Oh, balls. I am trying to get across a message of whether or not it >is PRACTICABLE and REASONABLE to implement any type of cathodic protection >in an automobile environment. You really should proof read, Larry. Galvanizing is cathodic protection. So, that's example one. What I proposed is all very reasonable and practical. I wouldn't say it's the best idea (I would just indulge in some Rust-Oleum), but it would work. Further, galvanizing works only because it is cathodic protection. If you were stupid enough to use a more noble metal for a coating, the first scratch in the coating would quickly corrode. Unfortunately, you get much the same result from paint. It's not nearly as bad since we aren't plastering the small scratch with a hugh cathode, but the paint is permeable and does render the scratch slightly anodic. >Then tell me about what will happen after the magnesium gets wet the >first time, and a solid layer of magnesium oxide forms on its surface. Larry, this is your best example of leaping without looking. Magnesium is frequently used for sacraficial anodes on ships and inside water heaters. Do you think elves come along and scrape off the oxide? Wrong, the oxide layer is not sufficient to prevent further corrosion of the magnesium (this is why we don't use aluminum as a sacrifical anode, it does form an excellent oxide layer). So, Larry, sorry I had to bust your bubble; but, I just can't let your brand of dis-information go by. Jim Kweeder kweeder@sun.soe.clarkson.edu