Nickel or Other Plating on Copper + Stainless Steel for Service at 600 °C

A. I did not think that high temp silver solder was functional at 600 °C. That is high. The differences in coefficient of thermal expansion should make this joint under considerable stress (impending failure) or be relatively tiny tubes. The problem with the colors of the nickel is mostly dirty parts. If the EN is thin, copper will migrate (alloy) and change color. Your adhesion problem is caused by the preparation steps for copper and SS are vastly different, requiring a method that is not great for either. There is an outside chance that you will have to use an electrolytic nickel strike. So, where did it blister, on the copper or on the SS or on the solder. Finally, why are you plating the part. To look pretty or ? James Watts - Navarre, Florida A. A. Electroless Ni is an alloy (normally 7-12% P) that shrinks when heated and has a moderate melting point of about 880oC. Sulphamate nickel (+99.9% Ni, melting point +1,400 °C) is usually plated very thick (sometimes over 0.100") on copper mold plates that are used for continuous casting of steel at 1,500 °C (mold is water cooled but nickel surface has been found to reach above 800 °C). But remember, anything may fail if adhesion is not superior. Guillermo Marrufo Monterrey, NL, Mexico

A. Solder melts < 800 °F (427 °C). Maybe actually brazing?

Rapidly (?) heating as-plated electroless nickel (EN) in air sounds like sabotage. Done properly, EN can be heat treated to 800 °C. However, I don't see any advantage to electroless nickel (EN) vs. electroplated nickel w.r.t. properties for your application. Maybe uniformity of plating? Also, a low-phosphorus EN may be more suitable than a mid- or high-P EN.**

Adhesion seems poor; use a Wood's nickel strike and follow either EN or electrolytic nickel plating with an adhesion bake.* Slowly heat and hold 1-4 hours at 190 °C to improve adhesion (via Ni interdiffusion with Cu & SS) while minimizing CTE problems. Then, slowly heat to 600 °C (even to 750 °C), followed by slow cooling to < 200 °C. Thereafter, rapid temperature changes can be made.

* ASTM B656 [withdrawn 2000] & ASTM B733: "Inert or reducing atmospheres or vacuum sufficient to prevent oxidation are recommended for heat treatment..." An Ar-5%H2 atmosphere is suitable.

** Mid- & high-P EN deposit with amorphous microstructures. During initial heating, amorphous EN crystallizes at ~300 °C, resulting in a 0.1-1.3% volume shrinkage. This causes microcracking, which reduces ductility and corrosion resistance. Properties can be largely restored by prolonged annealing at 600-750 °C -- cracks may heal [if not oxidized!], although hardness will be less than if heated to only 400 °C.

References:
• ASTM B656 & ASTM B733;
&null; W. H. Metzger, Jr., in Symposium on Electroless Nickel Plating, ASTM STP No. 265 (1959);
• Chapter 4 in "Electroless Plating", Glenn O. Mallory & Juan B. Haidu (eds.) (1990) [on eBay , Amazon, or AbeBooks] .
Electrodeposition [affil link on Amazon or on AbeBooks], Jack W. Dini, pp. 153-4 (1993).