How long must two surfaces be in contact for vacuum weld/cementing to occur?

Question

Vacuum cementing apparently is far more likely in space than on a planetary surface in atmosphere. How long must two surfaces be kept in contact with each other in a vacuum for vacuum weld/cementing to 'take'?

I came to complain that "vacuum weld" (my preferred form) is more common that "vacuum cement", and Google Fight backs me up, but the difference is not statistically significant. — dmckee --- ex-moderator kitten, Jul 19 '12 at 17:40
tic Were one to equate vacuum weld = vacuum cement ; after simplification the equation would be weld = cement.
Google Fight puts the ratio cement:weld = 10600000: 4150000 or approximately more than 250% — Everyone, Jul 19 '12 at 17:52

score 2 · Answer 1 · answered Sep 15 '15 at 17:54

Instantaneously given the initial conditions are right, such as the materials:

Being properly oriented (consider their crystal lattices).
Are capable of bonding on the atomic scale (usually the same materials are considered).
Not being "passive". The surface of non-noble metals would have already chemically reacted with the air, forming a passive layer which would prevent chemical bonding [1]. This is the reason the welding you mention doesn't work in our atmosphere.

Temperature would affect the rate of bonding. Perhaps insignificantly.

Consider also the case of the machining lathe, where a cold weld can occur between the metal and tool, because the metal had not interacted with the air yet to become passive, there is no lubricant, and the tool being spun too slow [2].

[1] https://en.wikipedia.org/wiki/Passivation_(chemistry)

[2] https://www.reddit.com/r/askscience/comments/1t8fxv/if_you_can_cold_weld_metals_in_a_vacuum_because/ce5qr03

How long must two surfaces be in contact for vacuum weld/cementing to occur?

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