316 SS HARDWARE ON TITANIUM 2

[Tofflemire]

Hi All

I'm new to Titanium, and it's do's and don'ts. I have read not to have direct contact of Aluminum to titanium.

My question is: Can you use 316 SS hardward to fasten a end plate to a flange? I also have a pressure switch which has a SS NPT fitting is this OK?

Thanks in advance for any Info...

Tofflemire

 

[TVP]

What is the environment to which this assembly will be subjected? Chemicals? Temperature? Titanium is the more noble metal, so similar to aluminum, the potential exists for the stainless steel to be preferentially corroded.

The following is a brief summary from ASM HANDBOOK Volume 13 Corrosion, one of the foremost references available on the subject:

If the stainless steel is passive in the environment, galvanic coupling with a more noble metal is unlikely to produce significant corrosion. If the stainless steel is active or only marginally passive, galvanic interaction with a more noble metal will probably produce sustained rapid corrosion of the stainless steel without repassivation.

 

[Tofflemire]

Hi TVP

Thanks for your response. The environment is in the ocean submereged to 1000 meters, lets argue the temperature is 5 degrees C.

So to be safe, I should use titanium hardware when ever possible.

Thanks again

Tofflemire

 

[kenvlach]

For this environment, SS and titanium are incompatible.

For determining the galvanic compatibility of various metals in sea water, marine atmosphere and industrial atmospheres, see Tables 1 and 2 of MIL-STD-889B and updates (Notes 1 & 3). Table 1 shows that SS and Ti are incompatible in seawater, but compatible in atmospheric applications.

Also, Appendix A of MIL-STD-889B gives protective measures for dissimilar metal combinations, and Appendix B gives a thorough explanation of galvanic corrosion (including effects of area, pH, aeration, etc.).

This MIL spec. is a free download from the DoD’s ASSIST site:

http://assist2.daps.dla.mil/eAccess/index.cfm?ident_number=36032

 

[TVP]

I can only agree with kenvlach's excellent post, and recommend that you not use titanium and stainless steel together in this type of environment.

 

[kenvlach]

Thanks TVP,
Your answer and eliciting the necessary info for a more specific answer also earns a star.

Tofflemire,
Even though SS and Ti are generally incompatible in seawater, people do do sometime find it necessary to sometimes use the combination. Passivated 316 is near Ti in the seawater galvanic series, see The Boatowner's Guide to Corrosion, p. 21-22 (2001) reproduced from American Boat and Yacht Council, Standard E-2.
If you take proper precautions, e.g., use a good thread sealant (CRC Corrosion Inhibitor, Boeshield T-7, Corrosion Block, Corrosion X, LanoCote, etc.) and a plastic washer under the fastener head, this combination can survive. There are also special fasteners with pre-attached washers, etc.