khf 2 powder having a water content of 10 and nahf 2 flakes in a ratio of two molecules one molecule was added thereto and while a replenishing solution was also added thereto so as to maintain concentrations of other components afterward the performance of each specimen was measured the results are set forth in table 2 the same procedure as in example 1 was repeated with the exception that the mixed solution of khf 2 and nahf 2 was replaced with a 5 nahf 2 the results are set forth in table 2 the same procedure as in example 1 was repeated with the exception that the concentration of free fluoride was maintained at about 0 g l the results are set forth in table 2 in this case piping systems are more liable to clog than in example 1 the same procedure as in example 1 was repeated with the exception that the mixed solution of khf 2 and nahf 2 was replaced with a 5 khf 2 solution the results are set forth in table 2 the same procedure as in example 1 was conducted except that mn 2 was eliminated from the treatment solution the same procedure as in example 1 was conducted except that ni 2 was eliminated from the treatment solution table 2__________________________________________________________________________ plate example comparative exampletest item for test 1 1 2 3 4 5__________________________________________________________________________wet aluminum plate 100 100 81 100 28 100 76 100 95 100 90 100adhesion zinc plated steel plate 100 100 100 100 36 100 100 100 98 100 90 100 steel plate 100 100 100 100 62 100 100 100 100 100 100.