Xref: utzoo sci.electronics:6573 rec.autos.tech:8573 Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!sunybcs!kitty!larry From: larry@kitty.UUCP (Larry Lippman) Newsgroups: sci.electronics,rec.autos.tech Subject: Re: parasitic anodes for rust prevention ??? Summary: Correcting more misinformation... Keywords: corrosion, causes, misconceptions Message-ID: <3221@kitty.UUCP> Date: 12 Jun 89 04:13:42 GMT References: <11854@bloom-beacon.MIT.EDU> <4345@druco.ATT.COM> <3220@kitty.UUCP> <3164@sunny3.che.clarkson.edu> Distribution: usa Organization: Recognition Research Corp., Clarence, NY Lines: 146 In article <3164@sunny3.che.clarkson.edu>, kweeder@sunny3.che.clarkson.edu (Jim Kweeder) writes: > I need to take issue with some of your intuitive opinions: Be my guest... > >(1) no significant amount of corrosion > >occurs as a result of potential difference between exposed metal areas; > > Potential differences are mandatory for corrosion. What makes corrosion > engineering fun is all the neat and unusual ways for the potential difference > to occur. First of all, you neatly omitted the phrase "in an automobile" which preceded and qualified the applicability of my statement to the conditions found in an automobile. Since the topic of this discussion appears to be corrosion mechanisms which may be mitigated through use of sacrificial anodes and/or anodic protection (i.e., the use of an external current), it is clear that the discussion applies to conditions involving galvanic corrosion where a "corrosion cell" exists. In order for this type of corrosion to occur, ALL elements of an electrochemical cell MUST be present: 1. An anode. 2. A cathode. 3. An internal circuit formed by an electrolyte between the anode and cathode. 4. An external circuit, which is a metallic connection between the anode and cathode. If ANY one of the above elements is missing, then a corrosion cell does not exist, and galvanic corrosion does not occur. A good example of galvanic corrosion is the union of two significantly dissimilar metals, such as copper and zinc, which would be found where a copper pipe fitting is connected to galvanized pipe. In this example, the anode which corrodes is the zinc (galvanized pipe), the cathode which does not corrode is the copper, the internal circuit is formed by the water (i.e., the electrolyte) inside the pipe, and the external circuit is the junction of the two metals where they are threaded together. 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. Since this article will be long enough as it is, I am not going to digress into crevice or intragranular corrosion mechnisms which also exist, but which are not readily amenable to anodic or cathodic protection in this application, anyhow. Stated another way, two pieces of metal, galvanized or bare, fastened together with zinc-plated or bare steel fasteners or by welding, will not BETWEEN THEMSELVES undergo galvanic corrosion in the environment of an automobile body. > >(2) the surface area beneath a vehicle is significantly "small" as compared > >to those situations where corrosion control methods are usually implemented; > > I'm not sure what you mean here. While area has a large impact on corrosion, > I don't see what you're talking about. I mean two things: 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. 2. The possible cathode area beneath an automobile is insignificant when compared to the anode area (i.e., the body steel). In order for significant galvanic corrosion to occur, the cathode area must approach or exceed the magnitude of the anode area. > >(3) the physical contours beneath a vehicle are so complex that any passive > >or active anode arrangement will be ineffective. > > Huh? As a matter of fact, cathodic protection is often used on the underside: > it's called galvanized steel. The primary protection mechanisms resulting from galvanizing steel are: (1) isolation of the steel through cladding with zinc; and (2) the passivation of the zinc surface through its own carbonate and hydroxide corrosion products. 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. > >> > I was wondering if there is any simple way to make a system that > >> > *really* works for protecting some of the parts that tend to collect > >> > water, such as on the bottom insides of the doors. > > > > Forget it, and don't waste your time. > > No, don't forget it. If you have a situation where water will consistently > collect, then sacraficial electrodes could be use successfully. Simply > bolt a piece of zinc or magnesium where it will get wet. You need to > make sure that you're getting good electrical contact between the steel > and the electrode. 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? > > Also, shipboard corrosion and its control is a totally different > >environment from that of an automobile (after all, the ship is continuously > >and uniformly immersed in the electrolyte!), and no parallels should be > >drawn between shipboard corrosion and the discussion about automobiles. > > Hogwash. Automotive and marine corrosion are parallel. True, having > the hull immersied in water makes implementation of cathodic protection > (sacrafical anodes or impressed potentials) very straight forward. Do you have any idea what important item ships have that automobiles don't have? This particular item is essentially the SOLE cause of galvanic corrosion of steel in ship hulls. This item also functions as a clearly identifiable cathode. In case you haven't guessed yet, the item is a bronze propeller! > However, cathodic protection is very much a viable alternative in > automobiles (eg: zinc coated steel). The automobile does present some > problems for cathodic schemes, however, if you understand how things > work, then you should be able to apply cathodic protection to certain > automotive corrosion problems. Since you have taken such vehement issue with my previous article, you clearly must claim to "understand how things work". Therefore, please help me to overcome my obvious ignorance by explaining: 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)? 2. What are a few "certain automotive corrosion problems", and how can cathodic protection be applied? Please do not talk about the trivial case of coating steel with zinc; I believe that most Net readers already know about the use of galvanized steel or zinc-rich paints in automobile construction. <> 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?"