Two aerial firefighting jets, including one based in Missoula, have been grounded because of corrosion apparently caused by a new fire retardant the U.S. Forest Service approved for use beginning last year. Two large air tankers — passenger jets converted to carry 3,000 gallons of retardant each — used a magnesium chloride fire retardant product while fighting wildfires last year. Both are grounded pending a joint investigation by the National Transportation Safety Board, National Institute of Standards and Technology and the Forest Service. The magnesium chloride retardant was in limited use last fire season, loaded into the two large air tankers and some smaller, single-engine aircraft also in Montana. After the discovery this winter of corrosion in areas of tankers where the retardant accumulated, the Forest Service decided not to use it this year. Instead, the agency will continue its widespread use of ammonium phosphate fire retardant that has been the go-to retardant nationwide for years.