Layer-by-layer (LbL) assembly has been used to produce nanoscale anticorrosion coatings for various metals, with the notable exception of carbon steel. In this work, we introduce the first LbL assembly that completely prevents carbon steel corrosion in deionized water by combining tannic acid (TA), a corrosion inhibitor, with polyethylenimine (PEI) via LbL assembly. A 60 nm thick TA/PEI multilayer coating can prevent carbon steel (1008–1010 and X65 grade) from corroding in deionized water for at least 1 month. This corrosion resistance is built upon the ionic-bonding-based water stability and coordination-bonding-based surface adhesion. Finally, the use of Cr­(VI)-free components makes the production of TA/PEI multilayer coating more environmentally friendly.