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This thread seems to be covering Dissimilar Metal Corrosion. So here are couple of thoughts.\ You could simply use a cadium plated bolt and overcome the dissimilar metal corrosion factors. However scratch it and it will rust. Perhaps to simply purchase some Aviation fasteners (al) will help ease things up by reducing the equation to a simple routine maintenance. (Changing Fasteners) Another choice would be to to use a Zinc primer and an epoxy coating. Expensive?? Perhaps. The right materials for the right application. One other soloution would be to dip the fastener into a sealant and insert. This would provide a barrier. Hell even a GE 35 clear would possibly work and be cost effective too. I would do this with the Cad plate fasteners into the Al. Bear in mind what the application is and how critical the component mating factor is as well. Charlie FAA G&AF 517575 BTW Nice Post At 10:04 AM 8/29/2001 -0400, you wrote: >A few questions have been asked about using aluminum for seawater >applications WRT corrosion. First we need to understand that aluminum comes >in pure form and in alloys. Most of what we use are alloys because of their >enhanced structural properties (e.g. strength, fatigue resistance, etc.) and >corrosion resistance. Aluminum gains much of its corrosion resistance from >the fact that it corrodes forming an oxide layer on the surface which then >works to minimize corrosion. Events that diminish the ability of AL to form >this oxide layer typically increases corrosion. Oxygen (O2) for the most >part is required to form this layer; however once formed it may not be as >important in maintaining it (depending on the environmental conditions). >Differential aeration cells can form causing O2 not to be uniformly >distributed with the result being increased corrosion. This generally occurs >in regions of stagnant flow (in submerged applications) such as in thread >regions, under components etc. The point being that corrosion in aluminum >may be from several processes and not just one. > >Acidity/alkalinity (PH) levels are critical for corrosion in aluminum. To >either side of the "ideal" ph level, corrosion in aluminum increases. >Furthermore certain methods to reduce corrosion, such as using zinc as a >sacrificial anode to corrode preferentially rather than the AL base metal, >may work effectively when in a neutral or acidic environment, but may >actually tend to increase corrosion when the environment turns alkaline. >Again this can point to stagnant flow conditions. When aluminum is >cathodically protected (i.e. using zinc or magnesium anodes, or impressed >current) it is imperative that over-protection (excessive electrical >current) does not occur as alkalies may accumulate at the region of the >cathode (aluminum) and cause corrosion. Damage do to excessive alkalies can >also be caused by contact of aluminum with concrete (especially wet >concrete). As a note cathodic chalking (such as from excessive >overprotection using sacrificial anodes or impressed current) forming on the >metal surface can cause delamination of rubber bonded to the metal (such as >an electrical connector/penetrator with a molded cable). Its amazing >anything works in seawater!!! > >Fluid or entrapped moisture is one of the worst environments for aluminum >(outside of direct chemical attack). When aluminum was first used to build >superstructures on ships with steel hulls to reduce the overall weight and >improve stability (lower CG - greater metacentric height), it was discovered >that even in a marine atmosphere (not submerged) that severe corrosion would >occur at the steel/al joint due to the moisture in the joint region (same >galvanic battery effect we see with SS screws and al). Efforts to >electrically isolate the two metals were attempted but were awkward, >ineffective, structurally weakened, and high in cost. A much better solution >was devised by explosively bonding aluminum strip to steel strip thereby >fusing the two metals together without a gap in which moisture could invade >and create this galvanic battery couple. Then the steel side of the strip >was welded to the hull and the aluminum side of the strip was welded to the >superstructure components. An effective solution and the key being the >elimination of the gap between the metals which entrains the mositure >(electrolyte). I investigated a similar process of explosively bonding >titanium to carbon steel for use as tubesheets in seawater-cooled heat >exchangers. The titanium cladding provided the corrosion resistance to the >seawater (although no fouling resistance) and the carbon steel provided the >low-cost and the mechanical stiffness (titanium is almost 1/2 as stiff as >steel). The explosive bonding eliminate the crevice between the two >dissimilar metals thereby eliminiting the galvanic corrosion potential (no >electrolyte). > >Traces of copper present in seawater or freshwater can cause corrosion in >aluminum. Water containing high copper ions (perhaps harbors from >anti-fouling paints) can lead to increased corrosion in aluminum. The copper >ions react with the aluminum and deposite metallic copper. These copper >sites are effective cathodes which react locally in a galvanic battery >couple with the more anodic aluminum base metal initiating and propagating >pitting which can be quite pronounced. > >Years ago during a project fabricating mid-depth ocean drifting buoys, I >discovered that selecting the form (i.e. extruded bar, pipe, flat plate, >strip, etc.) of the aluminum prior to fabrication was important with regard >to corrosion in seawater. Typical alloys for marine construction like boat >hulls, for example, are 5000 grade al alloys. However most non-construction >applications use a 6061-Tx grade because of the availability in many forms, >the cost, the material properties from temper, etc. However 6061-T6 alloys >contain higher percentages of copper than some of the marine al alloys. It >was noted that when endcaps, for example, were machined from solid 6061-T6 >round barstock, they exhibited more pitting corrosion than when they were >made from flat stock. An investigation pointed to the "grain pattern" in the >aluminum. 'That bar stock is extruded and the grain pattern tends to align >axially with the stock. It appears that pitting corrosion is more likely to >form on the "end grain" part of the stock then on the sides. An analogy >would be like the end grain on a piece of wood. I suspect the corrosion >mechanism may be an intergrannular corrosion between the copper and the >aluminum in the alloy? Don't know for sure as the relatively deep pitting >was eliminated when the part was machined from flat plate where the exposed >surface was not endwise to the "grain" developed from the rolling process >for fabricating the plate. The bottom line is use flat plate to make endcaps >whenever possible. I get round disks saw-cut out of plate so the machinist >can chuck the plate in the lathe more easily (and the saw-cutting eliminates >the heat if otherwise burned out that would diminish the temper in the alloy >and may cause precipitation at grain boundries leading to intergrannular >corrosion). > >With regard to stainless steel, I was at the LeQue Center for Corrosion in >N.C. a number of years ago and the chief engineer took me around and showed >much of their operation (they are a premium test center for corrosion >studies). He showed various methods to simulate corrosive conditions in >materials (I was interested in stainless steel at the time). One method of >simulating crevice corrosion conditions on a stainless steel tube was to >simply slip a tight-fitting o-ring over the tube. The tight joint between >the o-ring and the tube wall was sufficient to creat the conditions >necessary for accelerated crevice corrosion. One sample was quite pronounced >as once the pit forms, it can accelerate rapidly through the material in a >tunneling fashion. Another method was to sandwich the metallic test specimen >(plate) between two washers made from Delrin or nylon. The compressibility >of the plastic washer created the tight crevice. Patterns that were machined >on the faces of some of the washers were duplicated in corrosion on the >surface of the metal. Eak! 'And artistic. > >Aluminum corrosion can be a complex issue; however we, in scuba diving >applications, must keep it all in perspective. Our applications are not too >critical and we tend to spend more time maintaining our equipment - or at >least we should be. > >-- >Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. >Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'. > -- Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'.
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