IN-SERVICE CORROSION POTENTIAL MEASUREMENTS IN A SERIES OF FEEDWATER DEAERATOR SYSTEMS IN SMALL PEAT-FIRED STEAM RAISING UNITS

This paper gives an assessment of a series of studies aimed at establi shing the corrosion potentials prevalent in working deaerator systems of small 20-40 MW peat fired steam raising units. Two Ag/AgCl referenc e electrodes were inserted into each deaerator and their tips position ed at different feedwater storage vessel weld run locations. As well a s corrosion potential, water chemistry parameters, unit load and feedw ater temperature details were recorded. It was shown that the referenc e electrodes: (i) sensibly recorded the corrosion potential in deaerat or feedwater storage vessel weldments, (ii) the level of corrosion pot ential was similar at different weld locations, (iii) the agreement be tween both readings in any vessel was excellent, and (iv) exhibited a range in durability with the 0.1 m reference electrode being better th an the 3.0 m. An initial potential hysteresis, where corrosion potenti al (CP) values remained at higher levels than those predicted from pot ential-dissolved oxygen trends, was consistently observed in all three deaerators. Also, various sudden changes in corrosion potential were recorded and it was shown that these coincided with either normal work ing unit load fluctuations or off-load events. A significant influence of surface condition was evidenced, in that a bright clean weld surfa ce recorded a CP value which was consistently over 150 mV more positiv e than that shown by an oxidised weld surface. Finally, it was conclud ed that the present deaerator system corrosion potential data compared well with other reported potential-dissolved oxygen trends and exhibi ted reasonable agreement with modern day empirical and deterministic m odels which predicted crack growth behaviour in stainless and low allo y steels in hot aqueous environments. (C) 1998 Elsevier Science Limite d.