Xref: utzoo sci.electronics:6582 rec.autos.tech:8602 Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!csd4.milw.wisc.edu!leah!bingvaxu!sunybcs!kitty!larry From: larry@kitty.UUCP (Larry Lippman) Newsgroups: sci.electronics,rec.autos.tech Subject: Re: parasitic anodes for rust prevention ??? Summary: Arrggghhh! Keywords: corrosion, causes, misconceptions Message-ID: <3223@kitty.UUCP> Date: 13 Jun 89 02:16:38 GMT References: <11854@bloom-beacon.MIT.EDU> <4345@druco.ATT.COM> <3220@kitty.UUCP> <3166@sunny3.che.clarkson.edu> Distribution: usa Organization: Recognition Research Corp., Clarence, NY Lines: 126 I am going to try one more time to get a few points across, and then I'll probably give up because I've got too much work to do to continue this argument. In article <3166@sunny3.che.clarkson.edu>, kweeder@sunny3.che.clarkson.edu (Jim Kweeder) writes: > > 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, and which are amenable to anodic > >or cathodic protection. . . . > > In your original article you said potential difference, not dissimilar > metals. You're right; I said potential difference originally, which apparently caused you to misinterpret my statement. Since differential concentration cells (oxygen, hydrogen or other ionic concentrations) involving the same metal, in addition to other less common effects do not contribute to corrosion in the automobile environment, the only effect even worthy of mention is that of dissimilar metal junctions. > You don't need dissimilar metals to form galvanic cells or > haven't you noticed that cars do rust. Different steel alloys, different > heat treatments, differing amounts of cold work all cause changes in the > electromotive potential. Thus, it's very easy to set-up a steel-steel > galvanic cell. 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! > Also, any corrosion is amenable to cathodic protection (although other > factors may make the implementation difficult). You're right; it is the "other factors" here which preclude the effective use of cathodic protection in the automobile environment. > >1. An autombile is not a "large" enough object for significant potential > > differences to exist between one end and the other, with one cause > > of such potential difference being, say, differential oxygen > > or other ion concentrations in clinging surface water. > > Excuse me, but I can set-up a pretty nasty concentration gradient in a > 100 ml beaker. I bet I can do it on 10 ml. :-) 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. > You greatly underestimate the need for mixing. I can > easily see a difference in electrolyte concentration on a car. So tell me about it. > >The mechanism of cathodic protection ONLY comes into > >play where there is a DEFECT IN THE ZINC CLADDING, resulting in exposure > >of bare steel, at which point the surrounding zinc coating will function as > >a sacrificial anode for COMPARATIVELY SMALL AREAS OF EXPOSED STEEL. > > Too bad you haven't had the opportunity to work in an automotive press shop > (I have). Pressing often leaves scores and other damage on parts. Thus, > if zinc didn't provide cathodic protection, then the coating would be > ineffective. Did I say that zinc coating would NOT result in cathodic protection? What I did say is that there are LIMITS to the area of protection to exposed steel that results from zinc coating. Galvanizing does wonders for the cathodic protection of small scores and other damage which you mention; however, in this situation the surface area of the zinc coating is orders of magnitude greater than the exposed steel areas to be protected. As a somewhat trivial example, one square inch piece of zinc is going to afford damn little cathodic protection to bare metal say, one foot away. > > Oh, really? It's that simple? How many square inches of zinc or > >magnesium are necessary to protect say, one square foot of body metal? > >And will there even BE any protection through such a seemingly simple method? > > Yep, just that simple. If the original poster wants some numbers, I'll > be glad to open my corrosion engineering book and work them up for him. I'd really like to see those numbers. > >1. Where and what are the cathodes which form the galvanic corrosion > > cells in an automobile? What is the ratio of surface area of > > these cathodes to the body metal (the anode)? > > As I pointed out earlier, there are numerous ways to create a potential > difference, dissimilar metals being only one method. I'll forgo an > attempt at a complete listing since it would be incomplete. This is the > whole problem with corrosion engineering: being able to predict or identify > galvanic cells. If you just think dissimilar metals, you'll miss quite > a bit. I don't just "think dissimilar metals", thank you. If you are going to claim a concentration cell or other mechanism, however, then tell me about it! > >2. What are a few "certain automotive corrosion problems", and how > > can cathodic protection be applied? > > Well, here's a quick example. Say that you have a trunk leak you can't fix. > Thus, water will collect at the bottom of the spare tire well. To prevent > rusting out, you can do three things: (1) apply a protective coating (ie > paint), (2) provide drainage to minimize electrolyte accumulation, or (3) > attach a few small magnesium blocks around the bottom of the well. In reality > I would suggest a combination of methods to cover contingencies. 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. And now you want to bolt magnesium blocks to the spare tire wells inside a trunk! Tell me about how you propose to fasten the magnesium block to the body metal, the effect of drilling holes in the auto body, and about how you intend to deal with the problems attendant in maintaining that junction. 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 Lippman @ Recognition Research Corp. - Uniquex Corp. - Viatran Corp. <> UUCP {allegra|boulder|decvax|rutgers|watmath}!sunybcs!kitty!larry <> TEL 716/688-1231 | 716/773-1700 {hplabs|utzoo|uunet}!/ \uniquex!larry <> FAX 716/741-9635 | 716/773-2488 "Have you hugged your cat today?"