Any county electing to designate a specific portion of the levy authorized by this chapter to industrial development within such county may also pledge to continue to levy an assessment, not in excess of three-fourths (3/4) mill, against all of the assessed property in said county for a period of time no longer than twenty (20) years, in order to further secure any indebtedness of the council incurred pursuant to the granted authority under Section 57-32-5. Once the board of supervisors of any county has so pledged to levy such millage, not in excess of three-fourths (3/4) mill, the county shall be obligated to continue such levy until the indebtedness of the council for which the levy was pledged under the authority of this chapter has been paid in full, but in no case shall such levy continue in force longer than twenty (20) years. No county shall be obligated to continue any levy to pay off any indebtedness of the council the board of supervisors of such county, by appropriate order, shall have approved the borrowing of such money by the council for a specific purpose and shall have pledged to levy such assessment, not in excess of three-fourths (3/4) mill, for such specific indebtedness of the council.