Abstract:
Duplex stainless steels are finding increasing use in seawater
applications, but are prone to attack by crevice corrosion. A mechanism of
deaeration and acidification is thought to produce locally aggressive conditions
within the crevice. A variety of experimental techniques were therefore used
to investigate the crevice corrosion resistance of some commercial duplex
stainless steels in seawater and other, more aggressive Cr-containing
environments. The established marine grade 316L austenitic stainless steel
was used for reference purposes. The investigations provided both
quantitative data regarding alloy performance, and qualitative observations
regarding the attack mechanism.
The crevice corrosion resistance of the duplex stainless steels was
shown to be superior to that of 316L, and to increase with (Cr + N) content.
Electrochemical polarisation scans demonstrated the importance of
dissolved 02 in the seawater in maintaining a protective passive film on these
alloys. Active peak current density, a measure of the severity of attack, was
shown to increase with decreasing pH. In a simulated crevice solution, the
duplex alloys were shown to exhibit both a narrower active range and lower
peak current density than 316L.
Potentiostatic tests showed weight loss to be an order of magnitude
lower for the duplex alloys. Measured weight losses were in agreement with
those predicted from polarisation data. The technique suggested that cathodic
protection potentials as noble as -600 mV (SCE) may be sufficient to protect
duplex stainless steels in seawater. Metallographic observation and electron
beam analysis showed that the attack mode is potential dependent, with
ferrite and austenite preferentially attacked at more active and noble potentials
respectively.
Real time tests showed that the internal crevice pH of austenitic
stainless steel could fall to as low as 1.3. The rise in corrosion current was
found to be a reliable indicator of attack, and was consistent with measured
weight loss. Potential shifts were found to be relatively insensitive indicators
of breakdown.