system
Given the following post, respond as if you're commenting on Reddit:
human
Why are rockets/ spacecraft corrosion resistant, if there is no oxygen in space?

I was reading about the different types of alloys used in rockets, and many of them are labeled as 'corrosion resistant'; does this actually matter or is it just a useless byproduct of the alloys that rockets use? (btw, sorry if I used the wrong flair.)
gpt
There are 3 main aspects that are relevant there on why you would want corrosion resistant materials.

First while the goal is to send the thing to space it will spend a significant amount of time on the ground first. Even in a clean room environment you can get corrosion. Moreover contrary to a lot of other vehicles and systems you can't easily protect the materials with paint, oils or other rust inhibitor because they are typically not vacuum compatible. Rockets also often launched close to the coast where they can have to spend days in hot humid environment where there can be salt water spray.

The second thing for some parts is that a lot of propellants are very chemically agressive and corrosion resistant materials are usually also pretty resistant to chemical reactions.

The last one is that there is actually oxygen in space! It's a very tiny amount but at the very top of the atmosphere you get what is called "atomic oxygen", basically instead of O2 it's single atoms of oxygen. Those are way way more agressive than normal oxygen and it can be a significant problem after years in low earth orbit. Plastics might get attacked for example. Using stainless steel or passivated aluminium really helps cutting this effect.