US citizens living abroad full-time are taxed on worldwide income so your US Social Security benefits will be taxed in the same manner as a US citizen living in the US.
IRS says: If you are a U.S. citizen or resident alien, the rules for filing income, estate, and gift tax returns and paying estimated tax are generally the same whether you are in the United States or abroad. You are subject to tax on worldwide income from all sources and must report all taxable income and pay taxes according to the Internal Revenue Code.
U.S. Citizens and Resident Aliens Abroad