From r16884-a-t-freescale.com Wed Jan 25 09:13:00 2012 Date: Wed, 25 Jan 2012 17:13:53 +0000 Subject: RE: amp gauge bypass John, this may not get to the list from this email, please forward. Could have used more detail about direction of current flow vs electron flow.... Link to the article: http://www.designnews.com/author.asp?section_id=1365&piddl_msgpage=2&doc_id=237963&page_number=2 When zinc-coated screws are used to fasten stainless steel plates, the screws will corrode rapidly. But when stainless screws are used to fasten zinc-coated steel plates, neither the screws nor the steel will corrode particularly fast. Why? To answer this question, we need to understand galvanic corrosion. Most engineers have heard something about it. They may have heard that certain materials shouldn't be used in combination with others, but the details of this are often only partially understood. After all, most mechanical engineering curriculums don't include classes on corrosion. Corrosion is an electrochemical reaction, and for it to take place, you need three things: an anode, a cathode, and an electrolyte (usually water). For any combination of two metals, the less noble of the two will be the anode, and the more noble of the two will be the cathode. The anode will corrode; the cathode will not. Which metal of a given pair is more noble? This can be determined by reference to something called the galvanic series. The most noble metals are at the top of the list, while the least noble metals are at the bottom. (Sometimes it is given in the opposite order, with the least noble metals on top.) The most noble metals include platinum, gold, silver, and titanium. The least noble metals include magnesium, zinc, cadmium, and aluminum. The galvanic series is for seawater. The order of some metals may be slightly different in other environments. However, in the absence of detailed information about a particular environment, the galvanic series in seawater is usually a good reference point. The farther apart two metals are in the galvanic series, the greater the tendency will be for the less noble of the two to corrode. For example, the corrosion current between titanium and aluminum will be greater than the corrosion current between aluminum and cadmium. Corrosion rates are related to the anode-to-cathode surface area ratio. The smaller the anode is relative to the cathode, the more quickly the anode will corrode. Going back to the question at the beginning of the article, we can now see why zinc-plated bolts used to fasten stainless steel plates will corrode rapidly. Zinc is less noble than stainless steel, so the zinc-plated screws will act as anodes. The screws are small compared to the steel plates. This means that the anodes are small compared to the cathode, leading to a fast corrosion rate. Here are a few design guidelines for dealing with galvanic corrosion: Where possible, use metals that are close together in the galvanic series. The closer two metals are in the galvanic series, the less the tendency for corrosion will be. If dissimilar metals are used, try to prevent electrical contact between them. In order for there to be corrosion, there needs to be a flow of current from the anode to the cathode. Using plastic or rubber spacers between dissimilar metals can prevent the flow of current. Keep water out. We've read about cathodes and anodes, but as I mentioned before, there is one more thing which is necessary for galvanic corrosion: an electrolyte. If no water can reach the junction between the two dissimilar metals, galvanic corrosion can't take place. Make sure the anode area is larger than the cathode area. If you have to use dissimilar metals, and you can't keep them dry and out of electrical contact, then make sure the larger part is made out of the less noble metal, and the smaller part is made out of the more noble metal. This will keep the corrosion rate low. Following these simple rules can help keep your designs safe from galvanic corrosion. Further reading: Connector Corrosion Protection http://www.designnews.com/document.asp?doc_id=229768 Corrosion Outbreak on Whirlpool Machines http://www.designnews.com/author.asp?section_id=1367&doc_id=210531 -----Original Message----- Sent: Sunday, January 22, 2012 8:57 PM To: full size jeep list Subject: fsj: amp gauge bypass this is from an article michel shared: http://www.madelectrical.com/electricaltech/images/amp-ga27.jpg http://www.madelectrical.com/electricaltech/amp-gauges2.shtml Decided to install 100A fuses in place of the fusible link and then also wire in relays for my headlights, add some interior lights, fix my backup lights and fuel gauge... john ----- ------------------------------------------------------------------------ -o|||||o- fahrvergnugen y'all Snohomish, Washington - where Jeeps and VWs don't rust, they mold http://AMSOIL.com/redirect.cgi?zo=283461 TRSTGZS http://JohnMeister.com HTTP://WAGONEERS.COM http://fotomeister.us ------------------------------------------------------------------------