Investigation of Atmospheric Intergranular Corrosion Behavior of Sensitized SUS 430

Abstract

In this work, atmospheric intergranular corrosion behavior of sensitized SUS 430 was investigated by SEM observation and the electrochemical reactivation method. SUS 430·BA (bright annealed) was sensitized by heating for 2 minutes in the siliconit furnace set at the temperature range from 900°C to 1, 100°C and air cooling. Atmospheric corrosion test was done for 35 days, accelerating the corrosion reaction by spraying the artificial sea water provided by ASTM D-1141-52 on the sensitized SUS430 twice a day. By the SEM observation, intergranular corrosion was detected around the pit on the surface of SUS 430 sensitized at 1, 050°C and 1, 100°C, and the tensile strength decreased within the range of 4-6kgf·mm^-2. From the measurement of electrochemical reactivation method using the sensitized SUS 430 and standard Fe-Cr alloys, it was found that the atmospheric intergranular corrosion of SUS 430 was generated when the Cr concentration near the grain boundary decreased under 13%, and the reactivation ratio increased over 15%. Referring to the reports about the sensitization of Fe-Cr alloys, the cause of these experimental results was considered as follows.
(1) Spherical Cr carbide and nitride in SUS430·BA were hold between 900°C and 1, 000°C.
(2) Although a little free C and N were generated between 1, 000°C and 1, 050°C, these were absorbed into austenitic phase, then the progress of sensitization was restrained.
(3) A great deal of free C and N were generated between 1, 050°C and 1, 100°C, and the sensitization progressed to generate atmospheric intergranular corrosion.