Here's my deal, it is a case to case basis when we talked about corrosion. As long as the end user of the device does not try to power the phone on and from day one it will be subject for microscopic inspection for corrosion, 80% of chances you can save the phone from further damage.
I warn you not to try powering it "on" unless you do the following:
1. Ask the owner how long it was damaged. From my experience less people are being truthful. So, you have to do ocular inspection. When "rust" already present on the board, chances are very thin for recovery/restoration. This means it is in advanced staged of oxidation.
2. Ask the owner what position it was soaked in (angle). If it is only on the sides, top, under or the whole phone. This will give you an idea where to begin. Again, ocular inspection is the best way to do it.
3. After questions above are done., you can begin by taking out the baseband/bga chip "metal cover". This will exposed all visible corrosion (using microscope).
4. Detailed and Record all affected corroded parts and cross-reference it to the schematic diagram. This procedure will give you idea which components to concentrate when the ultrasonic cleaning is done.
5. I used the following specs (below) of ultrasonic cleaner. With heat/power set into "200 watts" and timer set in 30 minutes (depending on corrosion). Cleaning solution specs below.
Ultrasonic Cleaner''italic text''
Inside Tank Dimensions L x W x H (Inches/Millimeters) : 19.5 x 11.75 x 8 / 495 x 297 x 200
Voltage : 115v/230v
Tank Capacity (Gallon/Liter) : 7 / 26.1
Available Frequency : 45 kHz - 132 kHz
Sonic Power (average wattage) : (45 kHz) (132 kHz) / 300 - 240
Heat/Power (wattage): 1,000
Ultrasonic Solution/Liquid''italic text''
CAS 7732-18-5, Lactic Acid CAS 79-33-4, Gluconic Acid CAS 526-95-4, Lauramine Oxide CAS 1643-20-5, Tripropylene Glycol N-Butyl Ether CAS 55934-93-5, Blue 1 CAS 3844-45-9, Yellow 5 CAS 1934-21-0.
6. After done with the ultrasonic cleaning, wash it running water and do the ultrasonic cleaning again, this time use a 100% industrial alcohol. Heat/Power set to 200 watts and timer set to 5 minutes.
7. Dry the mother/logic board with ultrasonic dryer (specs below).
Utrasonic Dryer''italic text''
AIR: 2SCRM AT 60 PS/G - 125PS/G
VENT: 3" COLLAR
8. Make sure mother/logic board is corrosion free and properly dry. Then, you may test the mother board with other working/good cosmetic components (e.g. LCD/Digitizer, Side Key (power, vol, etc), charging port, etc.).
9. If above procedure will give you luck, congratulations. If not, you may start the micro soldering and bga repair works starting with the power management block (if it doesn't power on/does not charge, etc.), RF block (if you have signal issues. wi-fi) and other related problems.
10. Goodluck. You will learn by experience. All you need is patience and time. :)