INTRODUCTION
An open circuit reference electrode (OCRE) has been incorporated in a potential ? control circuit to measure passive current (corrosion rate) during the usual noble drift of corrosion potential. This technique permits current measurements in the early stage of passive film formation and allows subsequent continuous ? time measurements of film growth kinetics in the absence of artificial constant potential control. Corrosion potentials and corrosion rates are presented for type 304 austenitic stainless steel (304 SS) and nickel base alloy 22 in sulfuric acid solution for comparison with prior work. Mott- Schottky measurements showed that passive films on each alloy have similar semiconducting properties whether formed by dissolved oxygen or maintaining a constant passive potential using a potentiostat..
Passive current density, proportional to passive corrosion rate, is often accurately measured by applying a constant potential in the passive potential region using a potentiostat without any dissolved oxidizers in the electrolyte. The continuously recorded decay of passive current at a constant potential in the passive region measures kinetics of the passive film growth with time. For example, Jones and Greene1 measured passive current decay on 304L stainless steel in deaerated 1-N H2SO4. They observed that the passive current density i followed the relation,
i = io tn, (1) with time t.
In log i-log t plots, the slope n was observed to be about ?1 for all potentials tested between 100 and 300 mV vs. the saturated calomel electrode (SCE). Similar constant-potential passive current decay kinetics have been reported for iron and nickel base alloys in sodium chloride solutions.2-5
Potentiostatic tests presumably simulate open-circuit passivation in the presence of dissolved oxidizers. However, potential is not constant but often increases in the noble direction with time during open circuit passivation with dissolved oxidizers.1,6 It is not known expressly whether passive current is affected by a continuously changing corrosion potential, although it is almost universally assumed that there is no effect. Furthermore, the passive film formed at open-circuit potential with dissolved oxidizers may be inherently different than one formed at a constant or variable potential applied with a potentiostat.
The objectives of the present investigation were to:
1. Design and experimentally verify a method whereby passive current can be measured under a controlled potential, which follows the variable open-circuit corrosion potential of a passive electrode in the presence of a dissolved oxidizer; 2. Compare the passive corrosion rates thus measured using dissolved oxygen, with those measured without dissolved oxygen under constant potential on representative stainless alloys, and
3. Characterize and compare the semiconducting properties of passive films formed by dissolved oxygen and formed by potentiostatic control at comparable passive potentials on the same alloys.
EXPERIMENTAL PROCEDURES
Electrode and Solution Preparation:
Two alloys were investigated, AISI 304 (UNS S30400) stainless steel (304 SS) and Ni-Cr-Mo- W Alloy-22 (UNS N06022), both received from a commercial supplier as discs, 16 mm diameter and 6 mm thick in the as-received mill-annealed condition. The compositions are given in Table 1. Insulated copper wire was soldered to the back of the specimen for electrical connection, using silver-containing solder and acid flux. The melting point of the solder was <200º C; the soldering time was about 3 sec.; and the maximum temperature was 68.5ºC as measured by a thermocouple placed 3 mm away from the solder. Thus, we conclude
