Seriously corroded Macintosh Color Classic
I suspect @Allan is right, and the board is too far gone to salvage. But if you want to gamble on further damage vs. possible resurrection, there at least one thing you can try.
It looks to me like that was area around the the battery that has most of the damage; the battery probably leaked (acid or alkaline, depending on the type) and corroded everything around it. The corrosion is basically irreversible, but you can at least try to remove the acid/alkali. Alcohol won't do a good job of this, it takes a polar solvent like water. The problem is that water can also accelerate the corrosion, so you have to do it carefully. Note: read all of these instructions before starting, and get all the tools ready in advance.
Tools: a sink, a old soft-bristle toothbrush, compressed air (something like this, not the high-pressure stuff you'd find in a machine shop), paper towels, a hair dryer or heat gun, and some uncooked rice for final drying.

Use running water. Turn a tap on low (you don't want it to splash). Hold the board with the damaged corner tilted down, so the water runs toward it and off the board, not toward the undamaged sections. Let the water run on where the battery was, so it runs across the damaged section. Move the board around a bit, so the whole damaged area gets washed.

Use a soft toothbrush to gently scrub the components and traces as the water runs across them. Try to get especially between the tiny wires on the components. Don't push too hard; the wires will be weak, and you don't want to tear anything loose! Rinse the toothbrush out periodically so you aren't grinding crud into the things you're trying to clean. When you're done with this process, throw the toothbrush out; you don't ever want it near your mouth again!

When you're done brushing, rinse that corner of the board for a little bit longer.

Now take the board out from under the water, and use the compressed air. Blow mostly across the board (not straight down), and toward the damaged corner. You need to blow out under the components (and maybe the corroded-looking end of the SIMM socket), and you don't want to blow any of the crud into the clean section of the board. If possible, blow air out through the connectors at the back of the board as well.

Now rinse the board again. Repeat the rinse &amp; blow cycle a couple of times to clean out better under components and inside the connectors.
When you're done washing, you want to dry the board as quickly and completely as possible. The water is both washing crud off, and accelerating corrosion while the board is wet. Once you're done rinsing, the water is hurting more than it helps!

Gently pat-dry it with a paper towel, then use the compressed air to blow-dry it a final time (same tactics as before, to blow water out from under things). Get the entire board this time, in case any water splashed onto the undamaged section. Similarly, get the bottom side of the board in case any water got there.

Now use a hair dryer or heat gun to continue drying the board. You probably want the low setting; you want to dry it, not cook it.

When you get bored with the hair dryer, switch to rice. You want to basically submerge the board in dry rice (see figure 2 here) and leave it there at least overnight. If there's any remaining battery gunk there, it'll hang onto the water longer than you expect, so be patient. When you're done with this, throw out the rice; like the toothbrush, it should be considered contaminated.

Finally, when you're confident it's fully dry, reinstall it any try powering it back up. Without a battery, it may not work right on the first try; IIRC those old Macs get a dead-looking black screen on first boot with a dead battery, but will come up if you turn them off &amp; right back on (assuming everything else is ok).


Do not apply power before you're confident it's all dry. Power will greatly accelerate the damage that water does, so it's better to be patient.