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Weight of Zinc and Iron Phosphate Conversion Coatings Determined with Proposed ASTM Standards

Two proposed standards that are currently being balloted will offer coil coaters a definitive chemistry to evaluate the performance of painting and finishing coatings on coil galvanized steel.

“These are meant to be precise and quantitative, to give a particular number to tell what their adhesion or corrosion performance will be,” said Duane Fudge, a chemical engineer with Betz Dearborn, Horsham, Pa., who prepared the draft standards with co-members of ASTM Subcommittee D01.53 on Coil Coated Metal who represent the steel, chemical, and coatings industries.

Standard Practice for Determination of Phosphate Coating Weight on Zinc Coated Steel will calculate the weight of phosphate conversion coatings on hot dip galvanized and electrogalvanized steel. An ammonium dichromate solution removes the phosphate, and weight loss is used to calculate coating weight.

Standard Practice for Determination of Phosphate Coating Weight on Cold Rolled Steel will determine the weight of iron phosphate conversion coatings on cold rolled steel. A chromic acid solution removes the phosphate compounds, and weight loss is used to calculate coating weight.

Both standards address required reagents, preparation of stripping solution, apparatus, test specimens, procedure, and evaluation. “These will be used primarily by a coil coater or a coil painting and finishing operation, that would clean and apply these conversion coatings. These methods are used to monitor the quality of that finishing operation,” Fudge said. Using the iron phosphate standard, for example, a coil coater can apply this surface treatment chemistry and via this method, periodically test the iron phosphate coating weight that has been deposited and ensure that it stays within the specification or range desired.

For further technical information, contact Duane Fudge, Engineering Manager, Betz Dearborn, 4636 Somerton Rd., Trevose, PA 19053 (215/942-3276; fax: 215/ 942-3311). Committee D-1 meets June 11-14 in Reno, Nev. For meeting or membership information, contact D-1 Staff Manager Tim Brooke, ASTM (610/ 832-9729). //