Titanium vs Stainless Steel

Discussion in 'Materials' started by mojounwin, Nov 10, 2017.

  1. mojounwin
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    mojounwin Junior Member

    Hi,
    I'm considering using titanium grade 5 bolts to mount 316 stainless steel chainplates.
    I'm ideally wanting to avoid stainless steel bolts as they are prone to corrosion when starved of oxygen.
    Is titanium a superior option?
    Is there much of a galvanic corrossion issue when against the stainless chainplate?
    How does it compare re crevice corrosion?

    Cheers
     
  2. gonzo
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    gonzo Senior Member

    You can use 316S to avoid crevice corrosion. They are passivated.
     
  3. Angélique
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    Angélique aka Angel (only by name)

    Agree with Gonzo, in Europe for the application in a chlorine and also in the marine environment I use SS bolts with the A4 grade mark on the head, and not A2.


    SS Stainless Steel hex cap screw head markings​ A2 class 50 70 and A4 class 70 80.gif
    British Stainless Steel Association ---> PDF Stainless Steel Fasteners to BS EN ISO 3506 | Grades A1, A2 (A3) & A4 (A5)
     
    Last edited: Nov 10, 2017
  4. Barry
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    Barry Senior Member

    Hydrogen Damage
    Titanium is resistant to hydrogen damage in a wide range of applications including galvanic couples and impressed current systems. The naturally occurring oxide film on titanium protects the base metal from hydrogen absorption which would result in reduced ductility of the metal. Factors required for hydrogen damage to titanium are: mechanism for generating nascent hydrogen; metal temperature > 80°C; solution pH <3 or >12

    Galvanic corrosion is not normally a concern for titanium due to the noble nature of the metal. Coupling with dissimilar metals will not result in corrosion issues as long as the entire system remains passive. If active corrosion is occurring in the system then potential for hydrogen damage to titanium is possible. Factors which influence galvanic corrosion are the cathode to anode surface area ratio, the solution chemistry and temperature as indicated in the section on hydrogen damage. Avoiding galvanic corrosion can be accomplished by coupling with a more compatible metal, electrical insulation of the connection or designing the system in 100-percent titanium.

    General Corrosion
    The corrosion resistance of titanium is the result of a tenacious surface oxide composed of titanium dioxide that autogenously repairs itself when damaged in the presence of even very low levels of oxygen or water. The ceramic-like corrosion resistance of titanium can be relied upon to resist corrosion in seawater.

    Commercially pure titanium is immune to general corrosion in seawater and brackish water to temperatures as high as 130°C. Low levels of alloying additions such as palladium in the case of Grades 7, 11, 16 and 17 or nickel or molybdenum in the case of Grade 12 will extend general corrosion resistance to temperatures in excess of 260 °C. Commercially pure titanium (Grades 1, 2, and 3) is immune to crevice corrosion in aerated seawater to temperatures of at least 70°C. In deaerated seawater, commercially pure titanium will resist crevice corrosion to temperatures as high as 94°C. When higher service temperatures are required or crevices cannot be engineered out of the process equipment titanium grades containing alloy addition can be applied to provide protection from crevice corrosion.

    Pitting is the localized attack of the exposed metal surface in the absence of crevices. Titanium is highly resistant to pitting attack in seawater unless impressed currents higher than plus-5 volts are applied. Titanium is routinely used in impressed current systems as the anodic breakdown potential exceeds that of most common engineering materials.

    The above from the internet

    On the Galvanic Scale Of Metals in Seawater with reference to a Silver Chloride Reference Cell (ABYC Table)
    Passive 18-8, and 316 Passive are immediately adjacent in the table of titanium with the Stainless. Ie a small variance in voltage, minimal chance of galvanic corrosion in seawater

    On the other hand, 18-8 active and 316 active even with 3% Mo, are at the top end the table (titanium 6 up from the bottom)

    The difference it appears between active and passive is that both active stainless and (active) titanium will self heal and become passive when exposed to oxygen.
    If you are particular, a large surface titanium flat washer under the titanium bolt head, and only the nut turned when tightening will reduce any chance of friction impacting the thin passive surface of the stainless.
     
  5. Angélique
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    Angélique aka Angel (only by name)

    Hi Barry,

    As far as I understand it, that goes for the titanium itself when in contact with other metals.

    But what will the titanium bolts do to the 316 stainless steel chainplates, like the OP asks ?

    I assume the use is at sea in an salt water environment.
     
  6. Barry
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    Barry Senior Member

    As per the ABYC Galvanic Series

    Passive 18-8, 316 Stainless is slightly anodic wrt to the titanium

    0.0 to -100 millivolts 18-8, 316 passive stainless

    -50 to +60 millivolts for Titanium, so a MAX possible differential of -100 to + 60 or 160 millivolt

    If ACTIVE,
    18-8, 316 -430 to -580 millivolt active stainless

    compared to -50 to +60 for Titanium, so a MAX possible differential of -580 to +60 or 640 millivolts

    So the stainless would corrode if any corrosion will happen. If passive the difference even at max is small and the ranges overlap.
    It will not be much of an issue.

    A solution, make the chain plates large to keep current density low, try not to scratch the surface of the stainless or tungsten for than matter when installing. Hence my suggestion of a large tungsten washer under the head of the tungsten bolt, and turn the nut only if possible.
     
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  7. Angélique
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    Angélique aka Angel (only by name)

    Thanks for the info Barry !

    If the chain plates still have to be made then I would suggest to make them of titanium as well when wanting to use titanium bolts, and so avoid any of the galvanic corrosion issues involved with metal contact mixing.

    If you can get them I'll guess stainless steel bolts grade A5 class 80 (see A5-80 in post #3) are at least as durable as the 316 stainless steel chain plates itself, even when the chain plates are new, and this would also avoid any of the galvanic corrosion issues involved with metal contact mixing.

    If this is about existing (so maybe old) 316 stainless steel chain plates then I would suggest to have a look at the recommendations at the cruisers forum on the topic replacing chain plates ?, I'll quote post #2 and #4.
    Good luck !
     
    Last edited: Nov 11, 2017
  8. gonzo
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    gonzo Senior Member

    Titanium, like SS alloys requires an oxygen rich environment to create a passive surface. I am not sure that titanium will be much better than stainless in a reducing environment that will remove the oxide.
     
  9. Angélique
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    Angélique aka Angel (only by name)

    Hi Barry, . . the quote is from post #6

    I assume the above ‘‘tungsten’’ (3×) is a slip for ‘‘titanium’’ (3×), and the below was meant . . ? ?
    If ‘‘titanium’’ (3×) was meant here, then I wonder about the advice for the large titanium washers under the heads of the titanium bolts on the stainless steel 316 chain plates, since I thought it was best in the contact mix to keep the surface area of the most noble metal as small as possible to keep the galvanic corrosion burden as low as possible for the least noble metal.

    From this my thinking was that it would be best to have large stainless steel 316 washers under the heads of the titanium bolts on the stainless steel 316 chain plates . . ? ?
     
  10. Barry
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    Barry Senior Member

    Good catch, yes of course I meant to type titanium,
     
  11. Angélique
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    Angélique aka Angel (only by name)

    OK, thanks, but then the main question, in the last sentence in the bottom quote, which keeps crossing my mind . . . . . . (quotes: post #6 and #9)
     
    Last edited: Nov 12, 2017
  12. Barry
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    Barry Senior Member

    The reason that I introduced the large titanium washer concept under the head of the titanium bolt was to increase the contact area of titanium washer as compared to just the bolt head contact area.
    The potential difference (voltage) between the two components, in this instance, the stainless chain plate and the titanium bolt are MINIMAL so long as the stainless is passive, ie not impacted by scratching upon installation where it might not be able to create its own passive surface. (like a lock washer might)
    For a set of parameters, temperature, salinity of water etc, a FIXED Current will flow across the dissimilar metals interface but by increasing the area of the interface by the larger Titanium flat washer, the current density (the amount of current per unit area) will be lower. With lower current density at the contact area, the anode, stainless, should not exhibit localized accelerated corrosion.

    While a stainless flat washer, would leave the same lower area, higher current density,under the titanium bolt head.
     
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  13. Angélique
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    Angélique aka Angel (only by name)

    Thanks a lot Barry, that explains to me why large titanium flat washers are better than SS∙316 here.

    Then the following question popped up for me . . . .

    Would it be even better to have two washers under each titanium bolt head, first a large titanium flat washer under the titanium bolt head, and then below the titanium washer a large SS∙316 flat washer on the SS∙316 chain plate, which would give the same low current density at the contact area as with only the large titanium flat washer, but now the contact area of the dissimilar metals would be on top of the large SS∙316 flat washer, and not on the SS∙316 chain plate, so the large SS∙316 flat washer would be sacrificial, is this correct . . ? ?

    While of course keeping in mind that if all the to be covered surface areas are kept passive (= unscratched) there will be hardly a galvanic corrosion issue at all for the dissimilar metals mix at hand.
     
    Last edited: Nov 13, 2017
  14. Angélique
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    Angélique aka Angel (only by name)

    When making new SS∙316 chain plates, how long would it take before a good passive layer is formed on the machined surfaces, such as the internal surface of the drilled holes, and these surfaces may be safely covered during assembly . . ? ?

    And what are the optimal conditions to form a good passive layer on SS∙316 . . ? ?
     
    Last edited: Nov 13, 2017

  15. Barry
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    Barry Senior Member

    An interesting set of parameters of which I would only be able to guess at an answer. My guess would be that the SS washer being in contact with the plate would not cause ONLY the SS washer to be sacrificed as it is in contact with the chain plate and have the same contact area as the titanium washer.
     
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