Plumber, I have seen pinholes fixed in large sectional cast iron boilers with 10 pounds of salt.
When you spend a month assembling it, the man hours involved to take it apart to change out a single section that has a pin hole leak can be more than the assembly time. the salt treatment saves a time and money.
The salt corrodes the pinhole shut and forms a plug in it. We tore out one very large boiler that had maybe three sections that had leaked over the years and had been fixed with salt as a sealer.
You flush the salt out of the system after the leaks are sealed up.
Of course this does not work on high pressure boilers but does work on lower pressure systems.