The county commission of the county issuing any warrants or certificates pursuant to this article may pledge and use or cause to be used, for the payment of the principal of and interest on such warrants and certificates, so much as may be necessary for such purpose of the general annual ad valorem tax of one half of one percent which the county is authorized to levy without reference to the purpose thereof under the provisions of Section 215 of the Constitution of Alabama of 1901. If more than one such pledge shall be made of the tax, then such pledges shall take precedence in the order in which they are made unless the proceedings making such pledge shall expressly provide that such pledge shall be on a parity with or subordinate to a subsequent pledge of the tax. All warrants and certificates for which the pledge authorized in this section may be made shall constitute preferred claims against the tax, and shall have preference over claims incurred in carrying on the governmental function of the county.