It will spread . Any surface treatment or coating has been compromised . Can you stop the corrosion ? Yes , but that would require removal of the barrel and complete clean and wiping the corrosion with Corrosion X or some other rust remover typically phosphoric acid . Keep in mind that it might also remove or compromise any parkerization . At the end of the day , any salt or improper care that could have led to that degree of corrosion , might have also caused corrosion elsewhere , just not as visibly . Until you completely strip that upper and check , you can ' t be sure that there isn ' t any other problem . Is it worth it ? No . I would return that upper ASAP . If you took care of your upper , you ' ll find someone else to trade with .