PASSIVATION

[rc_12321]

Would any explain me about passivation of the column?

[tom jupille]

Done to minimize corrosion of the stainless steel portions of the system.
Stainless steel can be passivated by washing with dilute nitric acid.

For more information, see: http://en.wikipedia.org/wiki/Passivation

[HW Mueller]

Using the word "column" would indicate a tube filled with stationary phase, most of which would rather not see 3-6M HNO3 for several hours.