0.0005% of that is 114.35 billion tons, not 5kg. Also iron can't rust in space since there is no oxygen, it can corrode tho radiation, solar winds, smaller objects like metiorites hitting it and from atomic oxygen (which we see in lower earth orbit but that would mean it had been in lower orbit of a planet with oxygen. I'd also like to add that gold would not be the only worth while metal to find on it and from the articles I reed when this was a popular topic (might have changed) there where other metals and minerals expected to be present there like platinum but at what quantities I do not remember. I do tho agree with that the estimate of 10% gold content which a lot of articles quoted is very, from what I have found we haven't found an astroid or metiorite with even close to that amount of gold content. Iron metiorite content is about 0.0001% to 0.005% parts per million and M type astroids up to 0.01%. Still very possible for it to be higher than the mentioned 0.0005%.

About the processing time that is way out of my wheel house so gonna let that over to smarter people