From: "Barry L. Ornitz" <ornitzz@dpnet.net> Newsgroups: rec.crafts.metalworking Subject: Re: Stainless/ Aluminum reaction? Date: Thu, 14 Oct 1999 22:30:36 -0400 Dave Carlson wrote in message <38057BE1.DFBF94C3@newportnet.com>... >Can anyone tell me if I ought to avoid using stainless screws >through aluminum in a roof repair I am about to make? >I'm hoping to avoid the reaction/ galvanic corrosion that I know >relates to where each resides on the periodic table of the >elements. Aluminum is anodic to stainless steel so the aluminum will corrode preferentially. [Stainless can be both anodic and cathodic to itself in certain situations.] If you want to avoid galvanic corrosion, use aluminum screws or completely insulate the screw from the aluminum with a plastic shoulder washer and insert. Barry L. Ornitz ornitzNOSPAM@NOSPAMdpnet.net From: "Barry L. Ornitz" <ornitzz@dpnet.net> Newsgroups: rec.crafts.metalworking Subject: Galvanic Corrosion, was RE: Stainless/ Aluminum reaction? Keywords: corrosion, galvanic series, electromotive series, dissimilar metals Date: Sun, 17 Oct 1999 22:11:05 -0400 I sent this out last Friday night, but never saw it posted to the group. Another post to the sci.engr.joining.welding did not show up either but it was archived by DejaNews. This one was not. Strange things happen with Usenet's distribution floods! For those that might be interested, here it is again - I hope! Barry Several people have touched on the subject of dissimilar metal or galvanic corrosion. The subject can be extremely complicated, but there are several good generalities that can be used as a guide when picking compatible metals. First - you need a chart or table of the galvanic series. You really need one that lists the alloys you are contemplating using. Just saying aluminum or stainless is not enough if you really want good results. Different alloys behave quite differently. Understand how the tables work too (see below) Two of the better tables I have found are listed below. Second - pick compatible metals based on how close they are in the galvanic series. The closer the galvanic potentials are, the less corrosion that will occur. Third - the amount of corrosion is proportional to the ratio of cathode to anode area ratio. This leads to a number of methods to minimize corrosion. Make the anodic metal piece much larger than the cathodic one. The corrosion will be spread out over a larger piece of metal and thus the loss of some atoms from the surface will likely not result in much weakening. If you have to use a different metal as a fastener, use one more cathodic than the metal in which it is to be used. Fourth - remember that sometimes the above rules must be bent a little for other reasons, and that corrosion can be far more complex than just galvanic corrosion. Look at the stainless steels for example. Most can be both cathodic and anodic to themselves! Corrosion while immersed in sea water can be quite different than corrosion from exposure to pollutants in the atmosphere. So before presenting the table, let's look at the original question - stainless fasteners on aluminum. First we need to know what alloy of aluminum and stainless we are talking about. With no other information all we can do is note that aluminum is generally more anodic than any of the stainless steels. This is good from the standpoint of having a small fastener. But note that zinc or cadmium plating is likely better as they are closer to aluminum in the galvanic series. I would probably pick a hot- dip galvanized screw over a cadmium plated one because of the thicker coating, but I would pick a cadmium plated one over a simple zinc plated one (which is likely to be a rather thin plating). But I would certainly not pick brass, copper, or bronze. This brings up the point of why I listed two tables. The shorter one gives a better idea of how far apart the metals tend to be in the series. The longer list gives many alloys, but if you view these with their electromotive potentials (not shown), you will find many alloy series differ little in their actual placement in the series. Thus with aluminum, you might notice little difference between the corrosion with either 304 or 17-7PH stainless steel. Corrosion can be a nearly exact science if everything you work with is exceptionally pure. But in the real world, this is not the case and an experienced metallurgist is a wonderful friend to have. I first learned this in a corrosion class, but it really sank home after I learned it by experience too. [And on one or two occasions, I have seen my metallurgist friend scratch his head in confusion too!] When your choice of materials is limited, sometimes you have to accept that some corrosion is inevitable and design accordingly. Barry L. Ornitz ornitzNOSPAM@NOSPAMdpnet.net Take the NOSPAM out before replying directly. Thanks. ********************************************************* First Table: A Galvanic Series of Certain Metals and Alloys Arranged In Order of Corrosivity ANODIC (Least Noble) End Material Magnesium Magnesium alloys Zinc Aluminum 25 Cadmium Aluminum 17ST Steel or iron Cast iron Chromium-iron (active) Ni-Resist 18-8 Chromium-nickel-iron (active) 18-8-3 Chromium-nickel-molybdenum-iron (active) Lead-tin solders Lead Tin Nickel (active) Inconel (active) Hastelloy C (active) Brasses Copper Bronzes Copper-nickel alloys Monel Silver Solder Nickel (passive) Inconel (passive) Chromium-iron (passive) 18-8 Chromium-nickel iron (passive) 18-8-3 Chromium-nickel-molybdenum-iron (passive) Hastelloy C (passive) Silver Carbon and graphite Platinum Gold CATHODIC (Most Noble) End material Second Table from MIL-STD-889: Active (Anodic) End ------------------------------------ Magnesium Mg alloy AZ-31B Mg alloy HK-31A Zinc (hot-dip, die cast, or plated) Beryllium (hot pressed) Al 7072 clad on 7075 Al 2014-T3 Al 1160-H14 Al 7079-T6 Cadmium (plated) Uranium Al 218 (die cast) Al 5052-0 Al 5052-H12 Al 5456-0, H353 Al 5052-H32 Al 1100-0 Al 3003-H25 Al 6061-T6 Al A360 (die cast) Al 7075-T6 Al 6061-0 Indium Al 2014-0 Al 2024-T4 Al 5052-H16 Tin (plated) Stainless steel 430 (active) Lead Steel 1010 Iron (cast) Copper (plated, cast, or wrought) Nickel (plated) Chromium (Plated) Tantalum AM350 (active) Stainless steel 310 (active) Stainless steel 301 (active) Stainless steel 304 (active) Stainless steel 430 (active) Stainless steel 410 (active) Stainless steel 17-7PH (active) Tungsten Niobium (columbium) 1% Zr Brass, Yellow, 268 Uranium 8% Mo. Brass, Naval, 464 Yellow Brass Muntz Metal 280 Brass (plated) Nickel-silver (18% Ni) Stainless steel 316L (active) Bronze 220 Copper 110 Red Brass Stainless steel 347 (active) Molybdenum, Commercial pure Copper-nickel 715 Admiralty brass Stainless steel 202 (active) Bronze, Phosphor 534 (B-1) Monel 400 Stainless steel 201 (active) Carpenter 20 (active) Stainless steel 321 (active) Stainless steel 316 (active) Stainless steel 309 (active) Stainless steel 17-7PH (passive) Silicone Bronze 655 Stainless steel 304 (passive) Stainless steel 301 (passive) Stainless steel 321 (passive) Stainless steel 201 (passive) Stainless steel 286 (passive) Stainless steel 316L (passive) AM355 (active) Stainless steel 202 (passive) Carpenter 20 (passive) AM355 (passive) A286 (passive) Titanium 5A1, 2.5 Sn Titanium 13V, 11Cr, 3Al (annealed) Titanium 6Al, 4V (solution treated and aged) Titanium 6Al, 4V (anneal) Titanium 8Mn Titanium 13V, 11Cr 3Al (solution heat treated and aged) Titanium 75A AM350 (passive) Silver Gold Graphite ---------------------------------- Passive (Cathodic) End From: "Barry L. Ornitz" <ornitzz@dpnet.net> Newsgroups: rec.crafts.metalworking Subject: Re: Galvanic corrosion - marine battery terminals Date: Tue, 19 Oct 1999 01:05:28 -0400 Mike Graham wrote in message +ADwAXw-8LO3.6088+ACQ-K95.235600@newscontent-01.sprint.ca>... > Something just caught me as odd. I believe that cranking battery >terminals are generally lead. If this is the case then wouldn't the best >material to avoid electrolytic corrosion be lead terminal clamps? Marine >specs call for brass clamps, though, right? I'm just curious why this is. >Is it so the terminal is sacrificial and the clamp stays clean? Hi Mike, I think this is a case of having to ignore galvanic corrosion in favor of other constraints. Lead is not a particularly good conductor of electricity. Copper, brass or bronze are pretty good in this regard. They are also stronger mechanically. One common approach is to have the copper or brass clamp lead plated or tinned with lead-tin solder. With cranking currents often in the hundreds of amperes, a little resistance goes a long way in reducing the voltage to the starter. Barry |
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