Environmentally favorable magnesium phosphate anti-corrosive coating on carbon steel and protective mechanisms

Polymer coatings are commonly used to protect carbon steels from corrosion but they are susceptible to weathering and many of them have environmental concerns. Therefore, we studied the possibility of an environmentally favorable inorganic magnesium phosphate cement (MPC) coating for protecting mild steel. A formulation suitable for coating steel was developed by compositional modification [i.e., incremental replacement of dead-burned magnesia (MgO) with magnesium hydroxide (Mg(OH)(2))] to a road-repair MPC. This modification yielded an acceptable working time and prevented pore formation at the coating-steel interface. Corrosion monitoring by linear polarization and electrochemical impedance spectroscopy for 14 days found that, the MPC coating substantially increased the linear polarization resistance (R-p) [e.g., day 1: (8.2 +/- 1.7)x10(3) (nadir value) vs. 495 +/- 55 Omega cm(-2)] and charge transfer resistance (R-ct) (e.g., day 1: 9.3x10(3) vs. 3.8x10(2) Omega cm(-2)). The coated steel underwent neutral sodium chloride (NaCl) salt spray for 2400 h without visible rusting. Immersion for 24 h in liquids simulating the pore fluid indicated that, passivation by the excess MgO in the coating was a major contributor to its anti-corrosive property. Tafel polarization in the liquids found that, corrosion current density (I-corr) followed the rank: 3.5% NaCl solution (6.0 mu A cm(-2))>3.5% NaCl solution containing MgO (3.6 mu A cm(-2))>3.5% NaCl solution containing fragmented MPC (1.7 mu A cm(-2)), suggesting that a physical barrier effect and dissolved phosphate ions improved its protection. This study shows that, MPC coating is a promising durable and environmentally favorable anti-corrosive material for protecting steel structures in some applications.

Automated summary

By incrementally replacing components, environmentally friendly magnesium phosphate cement coatings can effectively protect steel from corrosion and demonstrate good protective performance in corrosion monitoring experiments. The anticorrosive properties of magnesium phosphate cement coatings in simulated liquids indicate their potential for protecting steel structures.