The effect of dissolved ozone on the corrosion behaviour of some stainless steels

Abstract

The corrosion behaviour of some stainless steels and pure metals has been investigated in solutions with dissolved ozone. The pH of the test solution was 1, 2, 3 and neutral (adjusted by H2SO4). 2.3 and 23 g/l Na2SO4 was added to increase the conductivity in anodic and cathodic polarisation measurements. The temperature of the test solution was 20, 50 and 75 °C. Test materials were four different stainless steels: ferritic stainless steel Polarit 815 and austenitic stainless steels Polarit 720, Polarit 752 and Ralloy 654MO. Also Armco iron, pure Mo, Ni and Cr were tested. The general electrochemical behaviour of stainless steels and pure metals was studied with potentiodynamic cyclic polarisation experiments at scanning rates 1, 10 and 100 mV/min and pitting behaviour at 10 mV/min. Potentiostatic experiments were also used. Immersion tests were also carried out mainly to produce test samples for surface examination but also to measure the weight losses. The structure and thickness of the oxidized surfaces were examined and analysed using optical microscope, SEM, GDOS, ESCA and X-ray diffractometer methods.

Dissolved ozone increases the redox-potential of test solutions to about +1200 mV vs. SCE and the corrosion potential of stainless steel to the transpassive region, below the oxygen evolution potential. The current densities of stainless steels in this region increase as the amount of alloying elements increases. On the other hand, dissolved ozone increases current densities only slightly compared to oxygen bubbled solution. Higher ozone concentration obtained under high pressure has no effect on the corrosion behaviour of stainless steels. The corrosion of stainless steels is general but peeling of oxide layers was also observed. Shallow corrosion damages are formed in immersion. In cyclic polarisation experiments pits are formed in Ralloy 654MO. The pits formed are similar to those formed by chloride induced pitting. As the pH of the solution is decreased, the current density of higher alloyed steels increases at higher pH than lower alloyed steels. Dissolved ozone increases the thickness of the oxide layers greatly. In oxygen bubbled solution at pH 3 the thickness of the layers is a few nanometers and in ozonated solution 60 - 260 nm. The compositions of these oxide layers has been found to vary greatly. Cr concentration is negligible in the oxide layers especially for Ralloy 654MO and P752. Molybdenum concentrates in the outer layer, even more so with lower alloyed steels. Iron and alloying elements have oxidised to their highest valencies. The main components in the oxide layers are iron oxides and on the surfaces also MoO6 occurs.