Does passivation help against Chromium Carbide precipitation?

Carbide precipitation is a heat issue. The material needs to be annealed and rapidly quenched to prevent carbide precipitation. Surface treatments will not correct carbide precipitation.

1) After a very long period of time. But SS with carbon content less than 0.020% is immune for all practical purposes. Add Nitrogen and you may have Cr2N precipitation, but I haven't encountered that in any literature. Process this alloy with due attention to time at temperature and this steel's corrosion resistance will be alright (and because it is L grade, unlikely to be specified for high temperature service where damage can happen).
2) No. The two phenomena are unrelated.

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