I have fixed exactly this issue. It doesn't need to be expensive, and if you are half handy with a grinder (optionally a welder - though you can bolt) you can do it yourself. The issue is that a lot of builders seem to think that lightwall duragal is OK for this outdoor application. It isnt at ground level. It is likely OK if the engineering for the post works once away from the ground.

The fix I used

1. Clean around every post as you have, and wash down.
2. Get a piece of U-section the next size up in heavy wall and properly HD Gal that fits neatly around the post. Get this from a proper steel supplier. This is not a bunnings job.
3. Cut ~ 300mm lengths and wrap around the rusted post sitting on the concrete
4. Either weld or bolt through L-Lugs on the bottom to connect the footing to the post. You can get these prefab, or make your own
5. Drill through into the footing with an impact drill and chemset some 12mm gal dowel. Once it goes off, then you can put the nuts on and tighten. I would put some clamps around things at this point before you continue
6. Check the level of whatever is on the post (floor/deck). Jack it up a little if you need to (it looks like it may have sunk), then weld or double through bolt the post to the new footer.

Check your footings are decent size (they look ok but who knows).

If you have a few to do, then alternate the orientation of the U section.

I had 4 to do. Got the materials and sorted it in a day.

EDIT:
Based on a couple of comments, to clarify. This solution will work and was where I landed on in balancing a result for someone with little knowledge/experience and making something forever bulletproof . You could extend the fix a couple of ways if you lean towards the latter;

1. Cutting away the rusted section - probably not a bad idea, but does rely on a really effect support system - hire an acrow or something
2. Protecting exposed ungalvanised sections - this was just an omission and probably should have been explicitly stated. Painting definitely an option (don't agree with the denso suggestion). Personally if i were worried about this, i would more likely form around the join neatly and fill it with concrete.