Question:

Must one immerse a peeler in the Mikveh?

Answer:              

Yes. If it will be also used for peeling foods that are edible raw, such as an Apple or carrot and the like, then it is to be immersed with a blessing. However, if it will only be used for peeling foods which can only be eaten after cooking, such as potatoes, then it is to be immersed without a blessing.

Explanation: Only vessels which come in contact with foods while they are in an edible state are obligated in immersion according to all opinions, and hence can have a blessing said upon immersing them. However, vessels which only come into contact with foods while still in their inedible state, is disputed amongst the Poskim as to whether they are to be immersed and practically Ashkenazim immerse them without a blessing [while Sephardim do not require them to be immersed at all]. Now, if the vessel is used both for edible and inedible food then the Poskim agree that it is to be immersed with a blessing. Accordingly, if the peeler will also be used for peeling foods that are edible raw, such as an Apple or carrot and the like, then it is to be immersed with a blessing. However, if it will only be used for peeling foods which can only be eaten after cooking, such as potatoes, then it is to be immersed without a blessing.

Sources: See Michaber and Rama Y.D. 120:5 regarding Shechita knife; P”M  451 M”Z 6, brought in Pischeiy Teshuvah 120:7; Aruch Hashulchan 120:30; Darkei Teshuvah 120:47; Pesakim Uteshuvos 120:21