CRITICAL-EVALUATION OF A COLD PHOSPHATING BATH

The usefulness of control parameters normally practiced in on-line monitoring of phosphating baths is well established. Their applicability to evaluate cold phosphating baths was studied with respect to a formulated cold zinc phosphating bath. The evaluation was made by determining the control parameter values during continuous operation and by studying the changes in the open circuit potential with time during phosphating for three sets of successive operations. The study reveals the ineffectiveness of the control parameters in quantifying the amount of phosphoric acid generated at the metal/solution interface and the use of potential-time measurements in successfully characterizing the critical changes in acidity at the interface.