What's titanium worth to you?

Discussion in 'Materials' started by Stumble, Nov 24, 2012.

  1. myark
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    myark Senior Member

    not worth answering
     
  2. El_Guero

    El_Guero Previous Member

    For all you old salts out there.

    All forms of titanium cause stainless and aluminum to corrode faster, especially if chlorides (salt water) are present. If there is an electrical or physical contact between them.

    Aircraft grades of titanium are susceptible to problems in the presence of salt water.

    ASTM grades 1, 2, 7, 11 and 12 alloys of titanium are considered better for use if you will expose them to sea water.

    Titanium used in your rigging

    ....

    If it contacts or grounds to other metals, you will eventually have problems .... I do not know the time frame, but titanium alloys in contact with SS or aluminum will cause metal fatigue sooner.

    Special requirements must be met for Mil-Spec in order for titanium alloys to be used with other metals - on those projects those engineers must show that they have taken care of this problem before they can use these metals together.

    As I noted above, aircraft grades do not do well with salt water, I read fast - when I read that page on galvanic corrosion of titanium, I just skipped that portion, because I assumed, aviation grade would be ok. That was a mistake on my part.

    Enough 'free consulting.' If you use titanium on your boat, use caution and keep it from contacting other metals.

    wayne
     
  3. El_Guero

    El_Guero Previous Member

    I answered for you .... :)
     
  4. myark
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    myark Senior Member

    you answered your self
     
  5. Stumble
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    Stumble Senior Member

    First as a primer;

    There are three major Grades (different alloys) used in the Marine environment (2,5, and 9), out of around 40 major commercial grades made. These are also the three major grades of titanium used total, between them accounting for about 95% of the metalic titanium used world wide.

    Grade 2 is a commercially pure alloy, that is 99.2% pure titanium, and is characterized by very high corrosion resistance, its light weight, and in relation to other titanium grades its low cost and easy of machining.

    Grade 5 is also called 6Al4V after its two alloying agents 6% Aluminium and 4% Vanadium. It is the strongest of the marine metals, with a yield strength roughly 5 times that of stainless steel, and an Ultimate Tesile Strength (UTS) roughly twice that of 316.

    Grade 9 is also called 3Al2V. Also named after its alloying metals, and as you might expect is 95% titanium, 3% Aluminium, and 2% vanadium. Grade 9 was developed specifically for structual tubing because Grade 5 tubing is very difficult to both make and form. With a yield roughly 2.5 times that of stainless, and a UTS about 1.5 times that of 316.

    Not to answer the specifics...





    Its worth avoiding any alloy except 2,5, and 9. Its pretty easy to do since other alloys will be much more expensive. No one would ever intentionally replace Grade 2 with Grade 7, since 7 contains palladium as an alloying agent, and is VERY expensive.

    Unlike SS which is only non-magnetic when it hasnt been cold worked, titanium is non-magnetic is all forms.



    The easy way? Ask for a cert on the alloy being used. A little more complicated, but still pretty easy is to figure out its density. Because 5 and 9 use aluminium as an alloying agent they are slightly lighter than Grade 2. But different enough that its easy to tell with an accurate volume measure (graded beaker) and scale.

    It depends on the application, there is no one best alloy. Again almost all titanium in the marine market is 2,5 or 9. But some highly specific industrial applications may use others.

    All metals corrode to some extent. The question is, if in the application contemplated something is going to occur that will corrode it noticeably.

    Of the alloying agents, Palladium is the best for high temprature salt water applications to the best of my knowledge. But I would recommend speaking with a materials engineer or mettalurgist on this. But again there is no one best, it depends on the chemical you need to protect against.

    Among the titanium alloys we are talking about, and at normal marine service temperatures (below the boiling point of water) corrosion frankly isn't a concern with any of the general use alloys. There are some application specific alloys with even higher corrosion resistance, but because they use very expensive alloying metals (like palladium) they tend to be reserved for only the most critical applications in very specialized applications.

    To put some specifics behind this:

    Crevice Corrosion Grade 2 titanium is completely immune to crevice corrosion in normal marine service. No matter what the sodium chloride concentration level is, titanium does not suffer from crevice corrosion until the temprature reaches at least 70 degrees Celsius (for crevice corrosion to occur at this temprature concentrations in excess of 25% sodium chloride are required). At normal salt water concentrations of 2.5% it takes a minimum tempreature of about 115 degrees celsius for crevice corrosion to take place. See http://www.azom.com/article.aspx?ArticleID=1336#_Hot_Salt_Stress Figure 2 (Included as an attachment below).

    Stress Crack Corrosion Grade 5 is the only alloy (of these three) that i know has been found to be subject to SCC. In labratory conditions it required tempratures in excess of 600 degrees celcius to force it to occur. At tempratures below this threshold there have been no reported cases of SCC to my knowledge, in any alloy.

    I would assume other alloys would also be susceptable to SCC at tempratures above 600C as well. But I haven't seen any reports of this being the case. And since I am NOT a materials engineer I would recommend consulting one in extreme cases.

    Galvanic Corrosion Titanium (both active and passivated) are at the top of the galvanic chart. So with the exception of carbon fiber there are no general marine materials that will cause titanium to corrode galvanically. However titanium will cause other metals be subject to galvanic attack.

    Generally titanium is slower at causeing damage than other metals, because the titanium oxide film that is created on the surface of the part acts as an electrical insulator. As an example, all white gel coat (and most colored ones as well) used titanium dioxide as a coloring pigment, but no one worries that just being in contact with it will cause galvanic corrosion. The science on this is pretty complicated, and frankly beyond my math skills, but i have references if someone wants to work out the numbers. However everyone I know of still recommends normal galvanic isolation procedures be followed (Loctite, rubber gaskets, ect).


    First I doubt the 10 year inspection cycle. I know of a number of stainless parts manufacturers that recommend inspection and replacement far more often than at ten years. That being said, I don't know what standards body you are refering to with a 10 year inspection cycle, so i can't respond what they recommend for titanium.

    Discussed above. I would also point out that Grade 2 is significantly less expensive than Grade 5 or 9. Which would generally be considered an advantage.

    I am not a welder but I have discussed this with them. All marine grades of titanium are relatively easy to weld, so long as appropriate argon shields are maintained.

    As discussed Grade 2 is more corrosion resistant than 5 or 9 alloys, but all titanium alloys far excede stainless steel's corrosion resistance.

    In order of least expensive to most:
    Grade 2
    Grade 9
    Grade 5

    This is based on billet cost, finished piece costs can vary significantly. For instance, since grade 9 is primarilly used for tubing trying to obtain small plate could be quite expensive.

    Because alloys have different properties than pure titanium. I reject the concept that one is better than the other, there are just different application needs that justify the use of one versus the other.


    We don't have engineers, we are a fabrication company.

    As I already pointed out, the material itself is immune to SCC, if you disagree with the material science guys at MatWeb, Azom, NASA, NOA, I would love to see the basis for your concern, because no one currently working in the Titanium industry can find it. Including the research guys in Universities, and the people that have been using it for years in the marine environment. As discussed above, if you are planning on using a titanium alloy in high temprature applications or with caustic chemicals. (Say a jet exhaust then you should contact a materials engineer for guidance).
     

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  6. Stumble
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    Stumble Senior Member

    I received an e-mail about this response, asking how much titanium actually cost to put into service. In the real world the actual cost is highly variable, and depends as much on the part and the manufacturing method as the cost of the raw materials.

    As an example:

    We currently make a 8" titanium horn cleat that runs about $250 where a stainless cleat this size would be $27. Obviously here ours is massively more expensive, which is why I wouldn't recommend them to anyone. But this is a good example of how different manufacturing methods can drastically alter the product cost.

    Right now our cleats are being milled from a solid block of Grade 5 titanium because it is cheaper to mill a one off than it is to pay to make a mold. This however requires a lot of machine time, and a lot of machinist work to make. It also results in a lot of waste because cleats don't have that much volume compared to the size of block required to cut them out.

    We currently are not casting these parts because there hasn't been a demand high enough to justify the cost of making a mold and setting up an investment casting run of them. To do so we would have to invest roughly $4,000 in a casting mold for the cleats. This cost is bearable if the number of parts ordered is large enough, but for now there is no retailer willing to place an order large enough to offset the cost of the mold.

    But for a moment lets assume that the mold were free. The actual cost of the investment casting process runs a little less than $100 a pound, with smaller parts being slightly more than large parts.

    Since a SeaDog 8" stainless cleat weighs in at .88lbs
    And titanium casting runs about $72/pound (this is rough retail cost btw)
    And titanium is roughly 1/2 the weight of steel

    Therefore we could sell this cleat for: .88 pounds * $72/pound * .5 = $31.68


    So our actual cost delivered out the door would be roughly $4 per cleat more than the stainless ones currently on the market, or a little more than 10%. By the way the same cleat in Bronze is $34.

    http://www.sailboatstuff.com/dk_Sea_Dog_cleats.html


    The reality is that stainless manufacturers have been casting this stuff for years, have a huge imbedded market, and have already paid off the cost of the tooling. While we are still in the early stages of doing so. It will come, and it is slowly getting there, but there is a huge up front cost that someone has to pay to get the ball rolling.
     
  7. myark
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    myark Senior Member

  8. pdwiley
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    pdwiley Senior Member

    That's the part I really don't like. I can do field expedient repairs on stainless using a basic stick welder and a handful of 2.5mm E309 and E316 rods.

    If a Ti fitting goes, I'm SOL.

    No problems provided they're very unlikely to fail so I wouldn't rule them out just because I couldn't weld them but saying Ti is easy to weld is a bit simplistic. Sure, easy to weld provided you've got a decent TIG setup, a bottle of Argon or Helium and a box or similar to flood with gas and do the welding in. Even I wouldn't carry all that stuff and I'm an unabashed tool junky & pack rat.

    PDW
     
  9. myark
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    myark Senior Member

  10. tomas
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    tomas Senior Member

    What are some practical examples of Galvanic corrosion involving Titanium with other metals?

    What are some worst case examples?

    Links please, if possible.
     
  11. tomas
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    tomas Senior Member

    Is it okay to mix (direct contact with each other) Grade 2 components with the 6-4 alloy parts in seawater?
     
  12. Stumble
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    Stumble Senior Member

    According to the general material use standards ( http://www.engineersedge.com/galvanic_capatability.htm) it is galvanically acceptable to use two different grades of titanium together. The recomendation for marine service is that to be considered galvanically compatible the metals need to be within .15v, while the main grades are all within about .1v.

    Note that 316 stainless (active) and 316 (passive) can range from .47 to .29v apart. Which is why drive shafts are such common galvanic problems.


    Tomas,

    I am not sure what you mean here. When discussing galvanic corrosion there are two major issues. The first is if two materials are galvanically compatible. Titanium is not compatible with many marine metals, but is fine with others. The three most common metals in the marine world would be 300 series stainless, bronze (of some alloy), and aluminium (of different alloys).

    The stainless alloys are tricky because they have three states. In its active state 300 series stainless actually is compatible with titanium, in its passive state it isn't, and in it bio-fouled state it isn't. The fun part is that none of these three states are galvanically comparable with each other either. And it is absolutely possible to have a stainless rod that exists in all three states at the same time. With part of it being active (typically underwater), part of it being passive (inside the boat), and part of it have growth on it. The most common example of this is a drive shaft that protrudes thru the boat.

    Aluminium is relavtive with all metals, and is just slightly more noble than zinc. Which is why it is used on fresh water boats for sacrificial anodes. Is is subject to galvanic corrosion from contact with all metals except for zinc (and magnesium actually).

    Bronze exists in a huge variety of alloys that each have their own spot on the galvanic chart. Some of the alloys can have as much as 40% zinc in them, and so are very susceptible to galvanic attack, other alloys are much more noble, like nickle-copper bronze that is above passive stainless on the chart.


    The second question in dealing with galvanic corrosion is the rate at which the corrosion occurs. This is a wholy different question that IF corrosion occurs.

    The rate of galvanic corrosion is dictated by a few things:

    First is the galvanic potential (which we dealt with above). The further apart the two materials are on the galvanic chart the faster corrosion will occur.

    Second is the strength of the electrolyte that is present. Typically in the marine world we worry about salt water as the electrolyte though there are exceptions. Sadly salt water is a pretty good medium which is why this is a problem at all.

    Third is the quality of the electrical contact between the two metals. The better the parts are insulated from each other the slower the reaction will be. Down to the point that if we stick a thin rubber gasket between two metals while there is technically corrosion still occurring, it is so slow we can ignore it.

    Finally there is the relative mass of the two parts. The larger the anode is relative to the cathode, the slower the corrosion will occur. Basically the damage done is spread out over the entire mass of the anode. This is why we don't normally worry about our aluminium masts with stainless rigging, because while the aluminium is the anode, it is massive compared to the stainless attached to it.


    When dealing with titanium specifically, it is high on the potential chart which is a problem, the electrolyte we will assume is the same, BUT because titanium created an electrically insulating layer of titanium oxide almost immediately when exposed to air or water the quality of contact is going to be low. Finally is the relative size, and this is just very application specific.

    So based on the above, a worst case scenario would be a large titanium part, a small zinc part, in a strong electrolyte (battery acid would be a favorite), in an anarobic environment where the titanium can't self oxidize.

    I don't know of any situations where this has occurred in practice, simply because no one would design an application like this outside of a battery research lab.


    I am not aware of any incidents where titanium lead galvanic failure of a structure (though I would argue it could have happened). However as discussed here (http://www.ncbi.nlm.nih.gov/pubmed/15246296 ) the titanium oxide coating was effective enough at slowing galvanic corrosion that there was actually less corrosion at the Ti-SS interface of parts than at the SS-SS interface.
     
  13. tomas
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    tomas Senior Member

    Greg,

    I really appreciate the timely, thoughtful and thorough response. Also, thank you for the links. Yes, you answered my questions.

    I am quite ignorant when it comes to nautical issues but in the design/fabrication work that I do, I've had to overcome quite a few technical challenges and I am concluding that the use of Titanium for seawater applications/fabrication requires reasonable, thorough, due-diligence and planning, but nothing extraordinary nor exotic, at least according to my experience.

    Over the years I've worked with various graphites (not fiber), molybdenum, tantalum, so-called machinable tungsten, various stainless steels, copper, brass, Hastealloy, Monel, Inconel 718, Beryllium, Lead-Lithium, along with multiple types of ceramics and glass, in numerous applications ranging from 300C to 2100C. Even the most pessimistic views expressed in this thread so far do not cause me concern. My only titanium experience has been a simple heat-flux sensor, which only required lathe turning and drilling.

    An extreme scenario that has not been mentioned here that I've read about elsewhere involved a shop lathe being destroyed by a Titanium fire. At and above "red" temperatures, it burns like Magnesium, in both oxygen and unique to any metal that I'm aware of, also in Nitrogen! (Reminds me of the "gold" titanium-nitride plating commonly used on watch cases. etc.) While not being privy to the details of that particular incident, I don't have to tell you that there is no normal machining procedure which would cause this, unless there was gross negligence/incompetence.


    Another question that just occurred to me, which I can also ask about elsewhere, involves the possibility of welding grade 2 to alloy 6-4 and if there are any particular details to be mindful of. Ideally, I would use the same material but it may vary depending on actual application, pricing and availability.

    Also, from my internet-browsing so far, I've noticed that there is not much stock inventory of thick-wall Ti-tubing. Depending on the outcome of my "design-spiral", if this is needed I can fabricate this if necessary. (I recently rolled and welded a 24" long cylinder, 10" diameter x 1/4" wall of stainless 316, because I was able to do it for a fifth of the commercial quoted price of $1200 !)


    Just like other technologies and materials, I think the use of Titanium for marine applications will become more and more part of the mainstream with time. Keep up the good work.

    Thanks again.
     
  14. Frosty

    Frosty Previous Member

    I just wondered if Titanium can be welded to other materials such alluminium.
     

  15. tomas
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    tomas Senior Member

    Yes, but probably not a good galvanic combination in seawater.

    Friction welding, inertial welding or stirring, sometimes allows the joining of materials that otherwise would be impossible.

    There is frictional welding of Titanium to aluminum, to copper, to stainless steel, to nickel, to niobium, to steel, and to tantalum.


    Here's a youtube video demonstrating the process:

    http://www.youtube.com/watch?v=-aEuAK8bsQg
     
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