Evolutions of diamagnetic magnetization (Mdia), critical current density (Jc) and irreversibility field (Hirr) for MgB2 have been investigated by changing the water-exposure time. The evolutions are found to follow a universal exponential law of AN(tR) = AN(0) exp(-tR/tN), where AN represents Mdia, Jc and Hirr for N = 1, 2 and 3, respectively, and tN is the corresponding time constant. When Mdia shows the most rapid decay with exposure time, Hirr exhibits the slowest degradation, with a difference between the decay-time constants of more than two orders of magnitude. For the case of Ti-doped MgB2, the exponential decay law can be still applied, but with a much longer decay-time constant. Microstructure analyses reveal that the grain boundaries of MgB2 are corroded by water when the exposing time is long enough. However, Ti doping significantly enhances the resistance to the corrosion of water.
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