ST. LOUIS -- The Gateway Arch in St. Louis is showing corrosion, and newly released documents indicate there has been no significant effort to fix it or even keep records of it, the St. Louis Post-Dispatch reported Thursday.

And while there are no short-term safety concerns, repairs could be expensive.

The newspaper first reported last month about the corrosion problems in the 630-foot monument to westward expansion that was completed in 1965.

On Thursday, the Post-Dispatch reported that the National Park Service provided new photos of carbon steel inside the north leg of the Arch, showing orange and red rust that has been evident in other pictures for at least five years.

"Corrosion is much like a cancer," said R. Craig Jerner, president of Dallas-based JEI Metallurgical Inc. and an expert metallurgist who reviewed the documents for the Post-Dispatch. "If you leave it alone, it will eat the steel up."

Jerner said he saw "distinct potential for a very big and expensive problem."

Park service officials said a corrosion investigation expected to make recommendations on addressing the problem was only recently funded and has not begun.

The 2006 report cited by the Post-Dispatch said possible corrosion within the steel walls is bleeding through failed welds and staining the outside surface, with some corrosion taking place aggressively.

But measuring just how aggressive is difficult because of a lack of maintenance records, the report stated. The report had recommended periodic photos to document deterioration. More than four years later, the park service has not taken pictures.

"We are not photographing because the recommendations are preliminary and do not provide sufficient detail as to location, method, frequency, etc.," said an e-mail from Frank Mares, deputy superintendent of the Jefferson National Expansion Memorial, which oversees the Arch.

Mares said maintenance workers use mops and a makeshift system of wicks and barrels to collect water in the interior legs of the Arch. No other repair work or cleaning has been done, he said.

One of the park service reports speculated that it could take a "long time" before corrosion would "induce any integrity concern."

The report also recommended an "expensive" cleaning within 10 years but did not give a cost estimate. The cost would be difficult to estimate because there is no clear way to access the upper reaches of the structure's exterior.

"The designers of the Arch provided no means of exterior access for future maintenance," according to a 2010 Historic Structure Report. It mentioned the possible use of cranes, scaffolds, ropes or helicopters.