It doesn't take some brass cartridge cases long to become brittle. I had some Winchester .22 WMR ammunition that wasn't but about 15 years old that split lengthwise, as you show. It may be a function of the manufacturing processes that cause aging straight-walled cases to do this.
As a testament to how long black powder can stay functional I offer this account. A friend of mine has been a lifelong War Between The States artifact collector. He used the best metal detectors that could be found at the time in his searching. The brand was Nautilus, AIR. Anyway, he found a cannonball at a depth of several feet with the metal detector and unearthed it, then in cleaning it he found that it was an explosive round that obviously had not gone off on impact. He disassembled it somehow and removed the black powder inside, which he said was actually grayish in appearance. He was an avid flintlock shooter/hunter as well and loaded his rifle with it and it actually fired. This was in the 1980s, so the powder was at least 115-120 years old.
