[Words_of_Truth]

Corax wasn't scared of facing Curze, the only Primarch he said he couldn't imagine beating was Angron. Corax fled the battle because A) he'd been fighting from the onset of the battle and Curze was fresh and B) He still had Lorgar to deal with and to take both of them would of cost him his own life and that of his legion which he wasn't willing to sacrifice.

 

[Words_of_Truth]

He didn't use the whip till A Raven's Flight, he was wielding a per of claws during the actual battle I believe.

 

[Words_of_Truth]

He's not using the whip in the main battle he's using his pair of lightning claws, one which gets destroyed during the battle and I'm assuming the second he puts aside or discards so he can use the whip and heavy bolter in A Raven's Flight.

 

[Words_of_Truth]

I could be misremembering...but did Corax somehow craft the power whip after the destruction of his lightning claw

The whip was made on Kiavahr so no I don't believe so, I think it was just at his waist through the major battle.