Post-Dispatch reporter Nick Pistor broke a story about corrosion in the Gateway Arch. The gleaming stainless steel monument, at age 45, was rusting.

Pistor had driven by the Arch in January and noticed gray streaks on the legs. It didn’t look good, but it was raining so he attributed the streaks to the weather. In July, he was stopped at a light in front of the Arch and looked up to see whether the streaks were visible in the bright sunlight. They were, but they appeared orange.

The next day Pistor examined the Arch on foot. He asked a security guard what was going on with the surface and the security guard indicated that a study of it had been done a few years earlier, but he didn’t know what was found.

The reporter went online and found a reference in an engineer’s resume to an “Arch Corrosion investigation.” The National Park Service didn’t have a copy, but Pistor learned that the Arch has a document library. The corrosion report was on file there. The report detailed severe corrosion and st

aining. He reviewed it for 40 minutes before the library closed for the day.

When Pistor returned the next day, an assistant to the Arch superintendent said he would not be allowed to see the document again because of national security. He continued to lobby for the document’s release and wrote a story, “Gateway Arch Shows Its Age,” based on the notes he took during that initial visit.

Two weeks later, the National Park Service released the corrosion report and Pistor wrote a follow up. The first story made newscasts and newspapers across the country. In his follow-up story, he wrote about how the water had apparently emanated from inside the Arch (condensation?), not from the outside. He described how water had collected at the base of the north leg and workers used mops to wipe it up.

“It was a perfect example of a reporter noticing that something didn’t seem quite right and asking questions,’’ said Jean Buchanan, a Post editor in charge of investigative projects, in a memo to the Post staff.