Shares of aluminum producers Century Aluminum Co. (NASDAQ: CENX) and Alcoa Corp. (NYSE: AA) tumbled on Thursday, falling more than 10% and 13%, respectively, by 2:30 p.m. EDT. Driving the decline was Alcoa's second-quarter earnings report, which detailed a "significant" impact from aluminum tariffs instituted this year.On the one hand, Alcoa's second-quarter results were solid.