Hysteresis effects in response of mild steel circuits to changes in water chemistry

The concentration of iron building up in solutions flowing through a mild steel coil at feed heater temperatures has been measured as a function of concentration of ammonia and hydrazine in the water. It is shown that the dominant influence upon the concentration of iron reached is the previous operating history of the circuit. Thus whilst iron levels are reduced both by increasing ammonia in the range 0 to 2 mg kg-1 and by keeping hydrazine levels low the influence of aging the oxides on the steel surfaces is very much stronger than either of these two factors. The variation in oxide condition as a function of its history has an influence on many of the possible surface reactions. This has been demonstrated by measurements of the electrochemical potential of steel surfaces, and of the currents which arise at an inert electrode when hydrogen, or oxygen or hydrazine are present in the solution. The change in condition of the steel surfaces is reflected in a change in their electrochemical potential. This in turn influences both the thermodynamic solubility of the oxides on the surface and the kinetics of their dissolution. These interactions are considered in detail. (author)