Protective coating on Invar 36

[2626716]

Dear sir
We are using invar 36 material for the manufacture of fiber optic couplers.It is directly exposed to the atmosphere in the final product.For our product the operating temperature range is
-40 Deg C to 90 Deg C.
Please advice me whether Invar 36 material requires any protective coating to prevent from corrosion.If required, which is the suitable one.
Awating your feedback.
Thanks & Best Reagrds
Mehaboob

 

[JimMetalsCeramics]

Invar will rust. It's customary to electroplate with nickel to prevent this.

 

[2626716]

Dear sir,
I would like to know whether we need any coating on kovar and stainless steel 304 material to prevent from corrosion.
These materials are used in the manufacture of fiber optic couplers and operating in the specified temperature range as -40 Deg C to 90 Deg C.
Awaiting your feedback
Thanks & Best Regards
Mehaboob

 

[kenvlach]

Agree with JimMetalsCeramics except suggest high-phosphorus (9-12 wt% P) electroless nickel for better corrosion resistance than electroplated. Plating per ASTM B733, SC3 (SC4 if strong acids or saltwater spray). Use on both Invar and Kovar.

For 304 SS, follow mcquire's advice in thread367-65287.

 

[JimMetalsCeramics]

You might want to avoid electroless nickel. Most plating houses don't do it well and it should really be baked on for best results. If you like Ni-P, this can be electrodeposited.

Stainless doesn't require plating but you can Ni plate to make it more robust. I've used Kovar with and without plating. I like Ni then Au flash on Kovar.

 

[kenvlach]

JimMetalsCeramics, I respectfully disagree with your opinion on EN -- perhaps you need to find a plating shop that truly plates to spec. ASTM B733 and/or MIL-C-26074.
The bond strength of EN (unbaked) to most steels and easy-to-plate Ni alloys should be ~60,000 psi. It is less on alloys that self-passivate readily, e.g., Al-, Cr- and Ti-alloys.

http://www.macindustrialproducts.com/mechanical.html

Electroless nickel, especially high-P, has far better corrosion resistance than electroplated nickel due to its amorphous structure. This advantage is reduced by baking. Also, high-P has a lower CTE for less mismatch with Invar and Kovar.

http://www.macindustrialproducts.com/physical.html

A gold flash over Ni is good if within budget.

 

[JimMetalsCeramics]

Amorphous structure is a help. Some compsanies really know what they are doing (e. g., Electroless Nickel Specialties
- Santa Clara, CA).

A fiber optic coupler will see ambient conditions. A group that approached us with a new one maintained that ambient moisture was the only concern. I assume this is worse in, say, Hawaii. Unless the corrosion is particularly harsh, I personally would probably use electrolytic Ni. Kenvlach's points are correct but a lot of companies add EN to their list of capabilities and are not reliable. I have see it flake a lot. A good litmus test is to solder a copper wire to the plating and yank on it. A simpler one is to rub and peel Scotch tape. I would be concerned about bond integrity on steel, particularly if there is no "bake on".

If you really want EN, find a house that specifically does it on steel all day long and in their sleep.

 

[kenvlach]

JimMetalsCeramics,
Any PVD coatings that you would recommend (of comparable cost to Ni plating)?

 

[JimMetalsCeramics]

Unfortunately, PVD will never be in the price range of metal plating. The cheapest would be from any cutting tool coating house which might offer PVD. On Invar the expansion might not be an issue. Parylene is also PVD. Of course, with plating you can coat a whole body. PVD is strictly line-of-sight.