The stock jumped 5% to US$62.137 in extended trading.
Net income fell to US$150mln, or US$0.77 a share, from US$225mln, or US$1.21 a share, a year earlier, the company said in a statement. That beat estimates of adjusted earnings per share of US$0.68.
The aluminium maker also updated its full-year outlook for adjusted earnings before interest, taxes, depreciation and amortisation (EBITDA), excluding special items, to a range of US$3.5bln to US$3.7bln.
Last month, US President Donald Trump moved to impose tariffs on steel and aluminium imports, with initial exemptions for Canada and Mexico. The US has since granted temporary exemptions to the European Union and six other allies.