Do aluminum polespears need anodizing?

    I thought this might be a good thread.


    I've seen divers and manufacturers take both sides of this issue. Spotted one Pole Spear Manufacturer in Florida that had a write up to why it is NOT needed.


    His poles spears are expensive and are NOT anodized.


    From a non Technical perspective, if it is not need for corrosion protection and just a trick way to add color, why are there so many anodizers across the world.? Why are there clear coat anodizing processes.


    I think it is an add advantage for reduced corrosion possibilities, what say you? -John

    Aluminum will form a very thin layer of oxide at its surface that will protect the rest of the metal underneath. Anodizing is a controlled method of increasing the thickness of this layer. Technically it is not needed to protect the metal but anodizing gives a much more uniform finish, allows for colors and provides better abrasion resistance as aluminum oxide is harder than the pure aluminum metal.


    For a finished product I would say the anodizing will look better and produce a higher quality piece in the end. For areas of the spear that are likely to com in contact with rocks or other hard objects it will help protect the spear. For internals or parts that wont be seen it is probably not necessary if you are trying to cut costs but nice for longevity of the item.

    Quote from Wishihadgills

    Aluminum will form a very thin layer of oxide at its surface that will protect the rest of the metal underneath. Anodizing is a controlled method of increasing the thickness of this layer. Technically it is not needed to protect the metal but anodizing gives a much more uniform finish, allows for colors and provides better abrasion resistance as aluminum oxide is harder than the pure aluminum metal.


    For a finished product I would say the anodizing will look better and produce a higher quality piece in the end. For areas of the spear that are likely to com in contact with rocks or other hard objects it will help protect the spear. For internals or parts that wont be seen it is probably not necessary if you are trying to cut costs but nice for longevity of the item.


    Well said. I would add that you will see more corrosion from using stainless fasteners (galvanic corrosion) than you will from not anodizing. I use aluminum fasteners wherever I can and avoid mixing aluminum and stainless whenever possible. From a practical standpoint it's unlikely either will affect the polespear's performance, as long as you rinse in fresh water after using, since the actual time wet is a fairly small percentage, and for galvanic corrosion to occur the ss/ aluminum has to have an electrolyte, which is the salt water. Rinse in fresh water after using, and the galvanic corrosion all but stops. Use aluminum fasteners, and it doesn't get started.

    Wood Guy, Very good.


    I kind of Cheated on this Question / Thread: I wanted to get a thread going for all to read and get an idea on some of the material issues we have with Spear guns, Pole spears and the such.


    Material science is a big part of what we do and design around.


    The Anodizing questions was a "Starting point". There is also different types of Anodizing to consider.


    I got a good friend that is a great resource on Technical issues of all sorts. I'm an Electrical Engineer and my other Buddy a Mechanical Engineer (JoeP) call him our own " Carnac the Magnificent" . Our term of endearment for him.


    Anyway Carnac commented on this topic for this forum. Next Post: Comments and dialog welcome. -John

    Edited once, last by Linghunt ().

    CARNAC SAYS


    "John,


    I'll give my two cents on anodizing and corrosion.


    1. Anodizing provides a very thick saphire (Al2O3) layer that provides abrasion resistance. I doubt it really does much for corrosion enhancement in low wear applications as it's a porous layer. Just ask the makers of wear resistant cookware why they anodize. It makes things easy to clean and allows cleaning/cooking with things not made of soft plastic.


    2. Corrosion resistance is a fairly complicated thing. As described in the link, it would sound like SS is corrosion proof because of the Cr2O3 passivation layer. But put it in a marine environment without oxygen, such as under a bolt or encased in epoxy, and it will corrode just fine. The passivation layer makes things corrosion resistant, not corrosion proof. They key for passivating layers to work is that there is oxygen around to keep corroding any freshly exposed layer, and that the oxidized molecule is smaller than the metal molecule it replaces so it doesn't flake off. The small molecule is why both Cr and Al can form passivating oxide layers while large Fe oxides will flake away.


    The second half of passivation requires oxidation in the chemical sense meaning the oxidizing atom steals electrons from the metal atom. In that sense, turning Ca into CaO or CaF2 is an oxidation reaction in both cases. This broader type of oxidation reaction provides both another avenue for protection and an avenue for attack. To prevent flaking of a thin film layer, one can pre-oxidize the bottom of the layer with an adhesion layer. For example, Ni has a high work function which allows it to steal electrons (ie oxidize) other metal atoms which makes it a good candidate for a glue layer for metals. Once it steals the electrons, that top layer is both electrostatically bonded to it giving good adhesion and it makes the other metal inert to further oxidation, so if one tries to attack the boundary with oxygen, oxygen has to steal electrons from nickel which is both hard and makes a fairly small oxide. The point of this being that protection of the grain boundaries from attack by things like chlorine and sulfur is also a key part of corrosion prevention. I suspect an anodization layer will help here because it may be that one doesn't clean all the microscopic crevices of things like salt. Anodizing might help here by protecting the underlying metal grain boundaries from full exposure to all the nasty stuff in sea water.


    I'm not readily seeing anything that says anodizing greatly improves aluminum corrosion resistance in sea water, but I'm also not seeing much talking about repeated dip then dry cycles. This review article does say that salt facilitates the galvanic corrosion of aluminum structures near the ocean, so one does need to be a little careful if the spear has parts made from other metals http://www.sheetpileeurope.com…s)/aluminum_corrosion.pdf


    By the way, the superhydrophilic coating people advertise marine protection against corrosion because their coating keeps water away from the surface. I know they advertise marine paint for hulls, but I've yet to find any place selling it, nor can I think of anyone providing a durable coating. "

    Edited once, last by Linghunt ().

    When I asked Carnac permission to Post his work I got this comment from him. Carnac is not a Diver like us, but he does know material Science... Carnac wanted me to make sure you all knew some other items when he realized I wanted to Post to SpearDiver.


    CARNAC Says


    " John,


    Feel free to post provided you put my response in the proper context. Namely, the comments about saltwater corrosion are off the top of my head and not a well researched answer. For example, calling an anodized coating porous is a relative term encompassing things like pinholes and the manufacturing process. See for example the comments on "barrier anodization" here Yahoo! Groups or 7075 Al here Pin hole corrosion in 7075 Aluminum I've got anodized products laying around that don't even stop a volt meter reading the underlying aluminum, so there's definitely a difference in the galvanic protection of the different processes. Any weak spots in the coating will simply fall back on the protection of the natural oxidation process.


    This also brings to mind a comment I heard a few weeks ago that the carbon fiber/Al honeycomb construction used in the America's cup yachts will quickly fail from galvanic corrosion if seawater gets into the honeycomb. This might be something to watch if aluminum tips are put on carbon fiber shafts.


    The comments on tight binding oxides providing corrosion protection are pretty standard, and one can look up sources for that, but I can't give a source for Ni being a good glue layer. That's a model I came up with during work on protection of PVD metal films that were failing due to atmospheric sulfur leaking through holes in a thick protective overcoat and splitting the metal from the substrate by attacking the bond with the underlying substrate. Adding a thin metal layer (a few atoms thick) of the right type under the metal layer had a dramatic effect on the protection and also changed the growth of the metal layer making the crystal size much smaller which showed up in measurements of the metal conductivity. This also fit very nicely with later work done on surface roughness which showed a sharp maximum when the metal layer was deposited on top of a metal whose work function matched the deposited metal. Changing the work function of the glue layer to either higher or lower than the deposited metal produced smoother films with smaller crystal size. The theory being that the deposited metal atoms would hit and stick due to oxidation from the glue metal which prevented lateral movement on the surface leading to island growth and large crystals. Presumably a low work function metal like Mg would also work as a glue in that sense, but data from other metals suggest it would actually make the joint easier to attack because the top layer gets more electrons which are then gobbled up by oxygen. The role of MgO or MgF2 min preventing that issue muddles things. I suppose one could look at the corrosion of the superplastic Al alloys containing Mg to get some sense of that issue.


    Another thought popped to mind when you mentioned cleaning Al with fine steel wool. I would have thought one would want a thick, clear anodized layer there for the same abrasion resistance in cookware, but more importantly, that steel wool leaves a surface contaminated with iron. That type of surface contamination from tools is one of the arguments I've heard for needing to passivate SS. Have you seen the current citric acid passivation products? Stellar Solutions - Citric Acid Passivation I went looking for a paper the other day (unsuccessfully) that compared the pitting from both citric acid and nitric acid passivation. The nitric acid surface looked much more etched which I suspect is generally a bad thing. The reviews seem to be swinging to citric acid passivation which is something to consider if your audience is cleaning their stainless with steel wool. The stuff can be found at places like the rust store The Rust Store - Solutions for All Rust Problems "

    CLEANING with Steel wool In CARNAC comments: That references the many customers, I have had that requested a NON anodized Pole spears and they just wanted to clean up the dings and scratches with some 0000 Steel wool.


    That's what they want, that is what I made. Called custom, made your way. -John

    John,


    I think you were talking about my spears. Our spears are aluminum and built for performance and durability cosmetic appearance was not a priority. My engineer and I both felt comfortable with not anodizing them. I am not an expert, but felt that the normal oxidation provides outstanding corrosion resistance. Our spears would go from a high polish to a duller gray after a few uses. Protected but not shiny. As we have become more successful and moving from just direct sales to add retail sales, the retail market has demanded that we start to anodize. In my opinion, it does nothing but drive up costs and keep it shiny. Again not an expert, just giving my opinions. Good luck!


    John, as always, I enjoy your discussions and appreciated the guidance.


    Thanks,


    Karl

    karl @ kellerbizz .com


    Palmbeachfreedivers.com

    Karl, You are one of many.


    I anodize my poles, but I do have diver requesting poles without it, they like to polish the poles with a fine steel wool for the "new look". I don't think they consider the normal oxidation of Aluminum a hardened finish.


    There are different classes of Anodizing as well.


    Handling raw aluminum does get your hands black from the ALO2.


    I started this post, so I could refer customers here when they ask the question. CARNAC did add a lot of detail.


    I think I will send this link to my local Anodizers and see what they say.


    If any of you have any Material Questions for CARNAC, he would be happy to answer them. He's a PhD Physicist btw.

    PM me and I will sent you his contact info.


    Don't expect him to just do your homework, but If you are stuck on a particular problem, I'm sure he would being willing to help.


    If you are doing electrical, I can help with that. If you are in beginning class I guess you are doing F=ma, and ballistics. Maybe some Energy Conservation and momentum.


    Carnac's expertise is with thin films, materials, and light. He does seem to know how to answer all of my questions.

    Thanks for the reply ,but i was only joking about the homework thing, i won't bother you guys with homework . Although i love physics , its my joy subject , lets me relax from my organic chem and bio studies . These kinds of threads are awesome, lets us share info and apply sciences to a great sport . Keep up the good work :thumbsup2:

    Be safe ... Happy hunting .

    LOL, I figured it was a tongue and cheek deal.


    You got a big advantage today in school with Web access. My options were to dig thru text book in the Library. It was actually a great resource, just took time to track stuff down.


    Organic Chemistry, Not my deal at all.... I just had to take the two basic Chemistry classes for my degree. Pretty basic looking back at it.

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