Metal finishing Q&As since 1989

-----

Rhodium plated glass mirror - binding layer required?

2006

We are seeking a solution for an underwater optical mirror that will resist corrosion over long-term immersions. We have tried e-nickel plated aluminum, and diamond-turned stainless, but both will have unacceptably short lifespans. Our current best-option is Rhodium (or platinum) coated zerodur glass, but we have a couple of unanswered questions:

- How thick can the rhodium layer be made without sacrificing surface quality?

- Other rhodium-plated glass mirrors use a thin Chromium "binding" layer between the two. Is this layer required? What are the consequences of leaving it out?

Thanks for any help,

Brendan Simons
Stern Laboratories (consulting engineers for the nuclear industry) - Hamilton, Ontario, Canada

---

Hello Brendan,
Rhodium can be plated at a thickness of .00025"-.00030" without sacrificing surface quality. A Plated thickness above that can add stress to the deposit. Mil-Spec R46085B includes deposit thickness classes and underplate requirements for substrates. Surfaces other than Ni, Ag, Au, Pt shall be Ni plated or Ni over Cu. It seems to me the reason other mirror mfg use chrome is for reflectivity, corrosion resistance, added durability, and a barrier metal between the substrate and the Rh. The absence of the chrome plate could affect the aforementioned properties. With adequate Rh thickness, would bright Ni be a viable underplate to consider? Good Luck!

Mark Baker
process engineer - Malone, New York
2006

---

Thanks for the help. Here's a follow up: Are rhodium mirrors usually electroplated, or vapour deposited? Are there any suppliers out there who could help us fabricate a rhodium/glass mirror?

Thanks,

Brendan Simons
Stern Laboratories (consulting engineers for the nuclear industry) - Hamilton, Ontario, Canda
2006