Two US astronauts successfully dodged highly toxic ammonia flakes during a risky spacewalk Friday to repair a cooling system at the International Space Station.

Scott Kelly and Kjell Lindgren floated out of the space station almost an hour ahead of schedule but encountered a small leak early on and soon fell behind.

The goal for Friday's spacewalk was to complete the final repairs to a system that broke down about three years ago, by restoring the external ammonia cooling system to its original configuration, the space agency said.

While the duo finished most of their plumbing tasks outside the orbiting lab, they were unable to complete a key part of the job involving the retraction of a backup radiator.

They folded up the 44-foot (13-meter) accordion-style radiator with the intent of stowing it, and then extended it again because they ran short of time to cinch it in place.

- Ammonia leak -

Early in Friday's outing, the spacewalkers reported seeing flakes of ammonia when disconnecting some of the equipment, but the crew was never in any danger, said NASA commentator Rob Navias.

Hazardous ammonia is used to cool electronics at the orbiting outpost, and the thermal system has been plagued by problems.

Just after midway into the spacewalk, they finished topping off ammonia in the system with about 7.5 pounds (3.4 kilograms) each into a primary and a backup tank, NASA said.

The operation was a "perfect fill," Navias said.

Later, the pair worked together to vent off residual ammonia from a plumbing line.

After opening the valve, they moved out of the way and inspected each other's suits for any signs of ammonia flakes.

They found none.

Had any of the ammonia made its way onto the astronauts' spacesuits, they would have had to stay outside the airlock longer than planned to allow it to bake off.

In the past, following ammonia leaks outside the space station, astronauts have stayed in the sun for about an extra half hour to allow any frozen crystals to vaporize from their spacesuits so as not to bring the toxic substance inside the station.

In 2012, NASA detected a major leak in the ammonia cooling system. Astronauts replaced the ammonia pump on the station's truss in May 2013.

- Marathon mission -

Friday's spacewalk lasted seven hours and 48 minutes, far longer than the initial 6.5 hours that mission control had anticipated.

It was the 190th spacewalk in support of the International Space Station, which just marked 15 continuous years of human habitation.

Nine days ago, Kelly and Lindgren went on their first-ever career spacewalk to perform maintenance and upgrades outside the ISS -- one that also ran long at seven hours 16 minutes.

Kelly is more than halfway through a year-long mission at the space station that aims to help NASA study the effects of long-term space flight on the body and mind.

He is scheduled to return to Earth in March 2016.