Is my intention correct if I want to learn Surah Al-Baqarah so that it defends me on the day of judgement? What ruling applies to similar things like reading Qur'an to keep away the devil instead of reading it to get Ajr?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

Chapter Al-Baqarah (the Cow) will shade its reader on the Day of Judgement and defend him as reported in a narration by Muslim and others that Abu Umaamah Al-Baahili narrated that he heard the Prophet saying:  "Read the Qur'an, for it will come as an intercessor for its reciters on the Day of Resurrection. Read chapter Al-Baqarah (the Cow) and Aal-'Imraan (the family of 'Imraan) as on the Day of Resurrection they will come as two huge clouds or as two flocks of flying birds and argue in favour of their reader (who frequently recites them or memorizes them). Read chapter Al-Baqarah (the Cow) as it is a blessing and abandoning reading is a sorrow, and the magicians are but helpless about it."

Chapter Al-Baqarah also drives Satan away as the Prophet said: "Satan flees from a house where chapter Al-Baqarah is being read." [Muslim] Therefore, there is no harm on the person who intends to get the reward that has been mentioned in the two  narrations, but restricting the intention to that without having the intention of getting the reward of reading them is an apparent neglect of intention, as one should have more than one intention hoping to get more reward, like having the intention of seeking knowledge, being cured from all types of pains, softening the heart, and so on.
Allaah knows best.