Nitric acid causes problems with Boeing's Starliner

Interesting chemical problem with Boeing's rocket: 

Boeing’s Starliner spacecraft has been recalled to the factory because of sticky valves...

In the days of troubleshooting that followed, engineers were able to get nine of the 13 valves working but four remained stuck.

“If we were able to free them all, we would have been in a good operational condition,” said John Vollmer, Boeing’s vice president and program manager for Starliner. “That’s what we were shooting for, but obviously, with not getting all the valves, we made the decision that we were just out of runway and we had to come back to the factory.”

Mr. Vollmer said the problem occurred among 24 valves that control the flow of nitrogen tetroxide, a propellant used by Starliner’s thrusters. Some of the nitrogen tetroxide appears to have permeated through Teflon seals and interacted with moisture on the other side to produce nitric acid, Mr. Vollmer said.

“That nitric acid resulted in some corrosion which resulted in the stiction of those valves,” Mr. Vollmer said.

Stiction is a new word to me.