Microelectrode investigation of iron and copper surfaces aged in presence of monochloramine

Abstract

Ductile iron and copper coupons were aged 137–189 days and 2 days, respectively, with 2 mg Cl₂ L⁻¹ monochloramine under four water chemistries (pH 7 or 9 and 0 or 3 mg L⁻¹ orthophosphate). Subsequently, microelectrode profiles of monochloramine concentration, oxygen concentration, and pH were measured from the bulk water to near the coupon reactive surface, allowing estimation of flux and apparent surface reaction rate constants for monochloramine and oxygen. Both metals showed similar trends with orthophosphate where orthophosphate decreased metal reactivity with monochloramine (pH 9) and oxygen (pH 7). Comparing iron and copper coupons, apparent surface reaction rate constants for monochloramine and oxygen were one and two orders of magnitude greater, respectively, for iron coupons under all conditions. Overall, this research provides the first insights into monochloramine concentration, oxygen concentration, and pH by direct measurement near ductile iron and copper reactive surfaces aged in the presence of monochloramine.