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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'.
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