An association may invest its funds in the valid warrants or bonds of any city, county, school district, port district, or other municipal corporation in the United States having a population of not less than fifty thousand inhabitants as determined by the last federal census, which municipal corporation has not defaulted in the payment of interest or principal upon any general obligation, including those for which its credit was pledged, within ten years last past, and for the payment of which the faith and credit of such municipal corporation is pledged and taxes are leviable upon all taxable property within its limits. No such investment shall be made unless the warrants or bonds for purchase are rated not less than BAA by Moody’s Investors’ Service, or have equivalent rating of another standard rating bureau, and the aggregate of the investments of an association in any issue of such warrants or bonds shall at no time exceed five percent of the amount of its savings accounts.