How thin can it be and still hold pressure? #3 *PIC*

These photos show a welded construction locomotive boiler used in regular service. Unfortunately, boiler water chemistry was not a high priority for the railroad. While the crown sheet proper was not significantly corroded, the stays adjacent to the crown sheet were SEVERELY corroded. One stay, after aggressive descaling, was measured at 1/2" diameter. Original diameter of the crown stays were 1-1/4". Further investigation revealed the crown sheet sagged 3/8" from the right to left side.
This boiler, 6 weeks before these photos were taken, had been blessed by an insurance inspector for another year of service at 200 psi.