Define high critical flange. Regardless, what is your own thought when looking at the picture? If that amount of rust would be under your car at a certain critical load bearing joint, would you be surprised if your car didnt meet the annual MOTT (or vehicle inspection)?
A proactive action would be to apply coating.

Can we apply coating after surface preparation while the operation temperature 20C

Check with your coating specialist

Get away! That is superficial surface rust on the flanges.

More of a concern is that you will have a lot of trouble getting the bolts off and the dangerous bit is the bit you can't see - i.e. the condition of the bolts inside the flange.

What you should do is remove the flange, replace the bolts and grease them well and blast clean the flanges. Then paint them or coat them in something suitable before re-assembly. Power wire brushing might get rid of the rust, but tends to smooth the surface leaving nothing for the paint or coating to grip onto.

You Might get away with changing one bolt at a time but "high critical service" is a vague term without meaning in piping design terminology.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

maybe we can ask more about what service that flange into,
is it hot fluid? or cold fluid? it's different type of coating or paint
is it 24/7 mainline service pipe?

i agree to blast the rust with sandblast or anything and repaint, and renew bolt and nut.
but must consider another way if it needs to shutdown the process.. maybe you can arrange the plans while periodically shutdown station.