Method for removing scale from metallic substrates

The method of removing magnetite scale from a metallic substrate comprising:

• contacting the scale-bearing substrate for a period of at least one hour and less than about 6 hours, at a temperature of from about 200.degree. F to about 300.degree. F, and a pH of from about 4 to about 10.25, with an aqueous solution consisting essentially of water, from about 0.25 to about 0.5 weight percent of ammonium bifluoride, and from about 0.5 weight percent to about 2.5 weight percent of at least one compound selected from the class consisting of polyphosphonic acids (1) and (2), the amine salts of said polyphosphonic acids and the alkali metal salts of said polyphosphonic acids, acids (1) and (2) being structurally defined as follows:
• ##STR14## where R.sub.1 is selected from the class consisting of C.sub.1 -C.sub.12 alkyl groups, and the group ##STR15## and ##STR16## where R.sub.2 is selected from the class consisting of --H--,--PO.sub.3 H.sub.2, C.sub.1 -C.sub.12 alkyl groups, and the group ##STR17## and in the course of said contacting, circulating the aqueous solution to attain a total effective volumetric quantity thereof in contact with the substrate to maintain maximum iron concentration of said solution at less than about 0.5 weight percent.

In the method of removing iron-containing scale from a metallic substrate which comprises contacting the substrate for a period of at least one hour, at a temperature of from about 175.degree. F to about 300.degree. F, and a pH of from about 4 to about 10.25 with an aqueous solution consisting essentially of water and at least 0.5 weight percent of at least one chelant compound selected from the class consisting of polyphosphonic acids (1) and (2), the amine salts of said polyphosphonic acids and the alkali metal salts of said polyphosphonic acids, acids (1) and (2) being structurally defined as follows:

• ##STR22## where R.sub.1 is selected from the class consisting of C.sub.1 -C.sub.12 alkyl groups, and the group ##STR23## and ##STR24## where R.sub.2 is selected from the class consisting of --H--,--PO.sub.3 H.sub.2, C.sub.1 -C.sub.12 alkyl groups, and the group ##STR25## the improvement for retarding the formation of an insoluble, iron-containing polyphosphonate complex which comprises adding to said aqueous solution before contacting the substrate, an amount of from about 0.25 weight percent to about 0.5 weight percent of ammonium bifluoride.