Whenever the board of commissioners shall deem it expedient to issue refunding bonds under the provisions of this chapter, they shall by resolution duly adopted and entered upon the minutes of the district specify, the amount and date of the bonds to be refunded, the amount of the refunding bonds proposed to be issued, shall designate the denomination or denominations thereof, fix the date of issue, the rate of interest, which shall not exceed six per cent (6%) per annum.