When does US Federal Reserve redeem the corporate bonds it bought?
When do the corporates pay back?

These are bonds.  It doesn't matter who owns them.  They get &quot;paid back&quot; -- dividends interest and principal -- according to the terms of the bonds.

What is the impact on economy when they have to payback?

None, because the central bank makes the money disappear.  On the plus side, it slightly reduces the money supply.

where does the Fed get money from for this corporate bond buying?

They &quot;printed money&quot;.
A better question (which has also been answered, if you search for it) is why the trillions of printed dollars (aka Quantitative Easing) hasn't produced inflation.