OXidation/reduction of water or oxygen (1 Viewer)

[roosterman]

hey guys

im getting confused when determining what equation the reduction for water and/or oxygen is in corrosion and electrolysis.

For example, the text book says in cathodic protection (sacraficial anode- zinc and iron)
The reduction equation is:
2H2O + 2e- --> H2 + 2OH-
why isnt it this one: O2 + 2H2O + 4e- ---> 4OH-

Also when using electrolysis to remove salts what water is oxidised:
40H- --> 2H20 + O2 + 4e
2H20- --> 2H20 + O2 + 4e

i hope that make sense...