I apologize in advance if this question is off the topics answered on this forum. Some hours back my drone fell into some dirty water. I was able to rescue the device, unplug its battery, open up its circuit board, dry it thoroughly using a hairdryer and finally putting it in place. It worked fine afterwards. But my question is how I could prevent or rather reduce the chances of the circuit board been corroded soon?
You need to wash it.
Seriously, what will cause corrosion is any salts that were dissolved in the dirty water that have remained behind when you air dried it.
You need to buy some distilled water and repeatedly wash the board a few times in fresh distilled water to dissolve and remove as many of those salts as possible. Flushing under components may require the use of something like an ear syringe, or as Peufeu suggests, a dental water jet.
Then, once the thing has had sufficient time to dry, and if there is a risk of this happening again, you should coat the board in a urethane, or better.. epoxy, so this will not happen again.
TIP: It is an old trick to stick your board inside a bag with a desiccant for a few days to completely remove any moisture trapped inside hidden places on your board, wires and other components. Rice apparently, despite the rumours, is not a great choice and there are better desiccants just don't use a salt, and keep the desiccant itself from directly touching the parts.
NOTE: Salts + humidity = electrolyte. Not only will that corrode stuff, but it also conducts which can cause shorts, arcing, and function/part failure.
If there are still ions (salts) on your board, any humidity which condenses on your board will become conductive, and corrode the copper away through electrolysis.
My favorite tool for washing a board is a dental water jet. It's great for getting junk out from below SMD chips. If you don't own one, you can also use a spray with the nozzle set to make a jet, but it is a bit more exercise.
If you use tap water, there will still be a few ions on the board, though. So it would be best to use distilled or demineralized water. As an alternative, you can use compressed air (or canned air) to blow most of the water off. You can also spray some isopropyl alcohol to blow the water off, but this wastes a lot of it.
Make sure you also look at the connectors and motors in your drone, unplug all connectors so the contacts dry, etc.
The hair dryer is nice because it will blow the water off too, and dry the hard to reach places, but you don't need to dry things ASAP. A circuit can stay wet for a while without anything bad happening if it is not powered. So rushing to unplug the battery probably saved your board.
You can use spray-on conformal coating on the board to protect it from water. This is easiest to remove compared to epoxy, if need be. However, be careful about the connectors... the stuff is insulating, so if it gets into contacts, potentiometers and the like, you're in trouble.
It is so called conformal coating. High end electronics for industrial, automotive, ...use are coated with a varnish. Could be based on epoxy, silicone and other based upon the final use.
You should wash the circuit in isopropyl alcohol and then dry it, as the water can contain salts.
If you don't want to coat your stuff in a hard potting compound (epoxy) or conformal coating (acrylic) then you can smear/immerse it in dielectric grease.
Amazon has something like this readily available, But really, any "waterproof marine electronics grease" will work.