[sean8862]

Good Morning All,

I have a (very) limited background in chemistry, and work as a mechanical engineer.  Recently a supplier of our Type J thermocouples modified their process such that they are now silver brazing the ends of the wires to the plug.  One wire is iron, the other is constantan, each to be brazed individually to unique connectors.  Based on what I recall, the anodic index explains how likely a reaction/corrosion is to occur with dissimilar metals.  From reference tables, for a standard manufacturing environment, up to .25V difference is acceptable.  Silver (.15V) and Iron (.85V) far exceed that. 

Can someone confirm that there is likely to be an issue here, and possibly explain in layman's terms why? 

Going forward, I'm thinking that a tin-based solder (.65V) with the iron wire would be less likely to have issues once in the field.  Something like a 95% Tin / 5% Antimony.  Does this sound about right?  How likely is it that a rosin-core flux would accelerate corrosion in the iron wire? 

Finally, I'm thinking of spec'ing a conformal coating of a urethane or enamel to reduce the likelihood of moisture/oxygen reaching the joint in the first place. 

Thanks for your assistance with this.  I'm certainly interested in learning what I need to, to ensure historical problems we've had with the product are minimized in the future.  I've attached a photo of one of the silver brazed joints for reference. This one was manufactured 20 hours before I received it. It then sat in a saltwater solution for 15 minutes to attempt to accelerate any reaction before being photographed.  It will be misted over the weekend. 

[Borek]

Lack of answers doesn't mean nobody has read your question.

I don't have exact experience with this type of connection, all I can say is that I would be suspicious about possibility of the galvanic corrosion just like you are.

[sean8862]

Thanks Borek,

I figured it was one of those topics that a bit more unusual than your standard textbook questions.  After letting the samples sit over the weekend, I'm a bit more confident that there is indeed a reaction between the two materials.  I've attached a photo for reference.



Best,
-Sean

[Enthalpy]

Galvanic corrosion pops up to mind but is not the only possible explanation. Metals can also make eutectics with a remarkable oxide colour.

"Silver brazing" is not pure silver. Wiki
https://en.wikipedia.org/wiki/Brazing#Silver_brazing
tells "such as copper, zinc and cadmium" where Zn and Cd have a potential very far from silver.

Corrosion potentials work... more or less, because some alloys are heterogenous, and other reasons. If your dirt appears in few days without remarkable humidity, it's not necessarily galvanic corrosion.

Corrosion having no excellent theory, I wouldn't stick permanently to the potentials story. This must be experimented anyway.

A different explanation would be that the brazing flux used to deoxidate the metals is still present and corrodes them. This flux can differ from one solder to an other: boric acid, rosin... and varied alloys react differently to the flux. Then the answer would be instead to brush away all traces of flux, not to cover them with a waterproof layer.

Rosin is rather less aggressive to metals, but it's good practice to brush it away. Boric acid is corrosive and frequently used with silver brazing.

Urethane: I'm not enthusiastic. I once overmolded an electronics circuit in polyurethane foam, and all copper corroded brutally - iron can only go worse. Epoxy may be less aggressive, or maybe glue from a hot glue pistol. If your temperature permits organic coatings, you could consider paints and lacquers, which are designed non-corrosive; car lacquers are hard and waterproof. Or use thermo-shrinking sleeve, if you can let it fit your complex shape: waterproof, chemically inert as a polyolefin.

If you know someone who makes or repairs brass instruments, ask him: they know very well brazing, corrosion and all around it.