Short answer

• Metallic Ni or Co do not appreciably “react” (form bulk compounds) with Rb at room temperature in UHV; Rb atoms simply chemisorb on clean Ni/Co surfaces and can desorb upon heating. Bulk alloying or intermetallic formation requires much higher temperatures.
• What Rb does react with very readily is the surface oxide that is essential for a glass-to-metal (Kovar–glass) seal. Rb reduces Fe/Ni/Co oxides to the metals, forming Rb2O. That chemical reduction can undercut or embrittle the glass-to-metal bond and is the most likely cause of your seal degradation.

Why your Kovar viewport is uniquely vulnerable

• A Kovar–glass seal relies on a thin, adherent oxide layer on the Kovar for the glass to wet and bond. If that oxide is chemically reduced, adhesion is lost.
• In contrast, elsewhere in the chamber (stainless parts), the oxide is not structurally required for a bond to glass; reducing it only exposes clean metal and does not immediately compromise a seal.

Thermodynamics (driving force)

• Representative reaction (for NiO): 2 Rb(g) + NiO(s) → Ni(s) + Rb2O(s)
• At 298 K this (and analogous reactions with FeO/CoO) is strongly exergonic. Rb is an extremely strong reductant; formation of Rb2O provides a large driving force. Thus, if Rb atoms reach the oxide, reduction is thermodynamically favored even at room temperature.

Rates: what you can estimate In UHV you usually won’t find published “rate constants” for Rb + Ni/Co metal at room temperature because no net chemical reaction occurs beyond adsorption. For the oxide reduction, the overall rate at room temperature is effectively limited by how fast Rb atoms arrive (and by any passivation that accumulates).

The arrival (impingement) rate of Rb atoms at your stated pressure gives a useful upper bound:

• Impingement rate Z = P / sqrt(2π m k T)
• For P = 1×10^−7 mbar (≈7.5×10^−8 Torr), T = 300 K, and Rb (M ≈ 85 g/mol), Z ≈ 1.6×10^13 atoms cm^−2 s^−1.
• A close-packed “monolayer” is ~1×10^15 sites cm^−2. If the sticking coefficient S ≈ 1 on clean oxide, you accumulate roughly 0.016 monolayers s^−1.
• Stoichiometrically, reducing one NiO surface “monolayer” consumes ~2 Rb atoms per NiO unit. So, in the absence of passivation, one NiO monolayer could be reduced in roughly 1–2 minutes at 10^−7 mbar Rb. A few to a few tens of monolayers (a few nm) could be removed in tens of minutes to hours.

Important caveats

• Real seals are not uniformly exposed; only the vacuum-side edge of the glass-to-metal interface is accessible. Once Rb2O forms, it can partially passivate further reduction locally.
• When the chamber is vented, Rb2O rapidly converts to RbOH/Rb2CO3, which are hygroscopic and voluminous; this can mechanically stress and crack the seal edge. Repeated exposure/bake cycles accelerate visible “degradation.”
• On bare Ni/Co metal, the situation is different: at room temperature Rb mainly forms an adsorbed overlayer. Surface-science studies (e.g., Rb on Ni(111)) show strong chemisorption and desorption activation energies on the order of ~1.5–2 eV, with no bulk compound formation at room temperature. Alloying/interdiffusion requires substantially elevated temperatures (hundreds of °C).

What to do

• Keep Rb off the glass-to-metal interface:
  • Add a line-of-sight shield or shroud so the seal edge cannot “see” the Rb source.
  • Place a cold trap or sacrificial baffle at lower temperature in the line of sight to capture Rb.
• Use alkali-resistant viewports:
  • Sapphire windows brazed with Ti-containing active braze alloys to stainless steel (no Kovar oxide needed) are much more resistant. TiO2 is not reduced by Rb under these conditions.
  • Alternatively, use all-glass (fused silica/aluminosilicate) windows with no exposed glass-to-metal joint to the vapor.
• Consider protective coatings at the exposed seal edge:
  • Thin, dense oxides such as Al2O3 (e.g., ALD) are thermodynamically stable against Rb reduction and can act as barriers, provided UHV compatibility is maintained.
• Reduce Rb partial pressure near the viewport (move source, baffles, temperature management).

Bottom line

• Reaction rates of elemental Ni/Co with Rb at room temperature are negligible in the sense of bulk chemical reaction; what you are observing is almost certainly rapid, flux-limited reduction of the Kovar oxide (and potential attack of the nearby glass/AR coatings) by Rb atoms. At ~10^−7 mbar Rb, the incident flux is ~1.6×10^13 cm^−2 s^−1, sufficient to strip the accessible oxide at the seal edge on laboratory timescales unless shielded.