All interest which becomes due and is paid on securities pledged to secure public deposits must be paid over to the depository financial institution until such time as it defaults in the repayment of the funds of the public corporation deposited as provided herein. After thirty days from such default, upon demand in writing made by the public corporation involved, the custodian shall deliver the securities to the public corporation with which pledged, and such securities may be sold as in the case of other pledges, and the proceeds thereof, or so much thereof as may be necessary, must be applied to the repayment of the public deposit.