SELECTIVE PASSIVATING FILMS FOR LEAD SOLDER JOINTS

LEAD IS TOXIC TO HUMANS AND SUBSTANTIAL, UNSAFE AMOUNTS CAN BE INGESTED FROM DRINKING WATER SUPPLIES UTILIZING LEAD PIPES OR LEAD SOLDERED JOINTS IN COPPER DISTRIBUTION SYSTEMS. SILICATES AND PHOSPHATES ARE CORROSION INHIBITORS THAT ARE KNOWN TO PREVENT THE DISSOLUTION OF LEAD INTO WATER BY THE FORMATION OF A PASSIVATING FILM ON THE SURFACE OF THE METAL. TO SUCCESSFULLY APPLY THIS TECHNIQUE, INFORMATION IS REQUIRED ON THE KINETICS OF FORMATION OF THE PASSIVATING FILM, PARTICULARLY AS FUNCTION OF THE WATER CHEMISTRY. OUR PRELIMINARY LITERATURE SEARCH HAS SHOWN THAT SURPRISINGLY LITTLE QUANTITATIVE INFORMATION ON FILM FORMATION ON LEAD SOLDER JOINTS IS AVAILABLE. THIS PROPOSAL ADDRESSES THE IN SITU DEVELOPMENT OF PASSIVATING COATINGS UTILIZING SILICATE AND PHOSPHATES WITH ADDED THRID COMPONENT LEAD COMPLEXING AGENTS THAT WILL SELECTIVELY COAT THE LEAD RESIDUE FROM THE LEAD SOLDER JOINTS. CONDITIONS FAVORABLE FOR THE RAPID FORMATION OF THESE PASSIVATING FILMS WILL BE IDENTIFIED FOR EACH ADDITIVE USING A TEST LOOP THAT SIMULATES A WATER DISTRIBUTION SYSTEM. WE WILL ALSO DETERMINE WHETHER COMBINING THE THE SILICATE AND JPHOSPHATE ADDITIVES HAS A SYNERGISTIC EFFECT ON PASSIVATION, AND WHETHER LEAD CHELATING AGENTS, EITHER ALONE OR IN COMBINATION WITH THE CORROSION INHIBITORS, CAN BE BENEFICIALLY USED TO SELECTIVELY PASSIVATE LEAD SURFACES.