I am pretty sure engineered corrosive effect does nothing against Thargoids. It's been a time but I am 99,9% sure I tried to shoot some with this. I also had mining-lances to try if they are vulnarable with this
Nothing. They are not.
You need AX-weapons or Guardian weapons.
The exception are scouts of course. You can kill scouts with normal weapons.
I wouldn't expect a standard multi-cannon to actually do any damage. Whether the secondary effect applies or not is my question. Here's my line of thinking. The description for the corrosive secondary effect states that it "temporarily weaken
Now, it may be that the effect is so small as to be negligible vs. Thargoids, but then again maybe not.