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

Abstract

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 (Rp) [e.g., day 1: (8.2 ± 1.7) × 103 (nadir value) vs. 495 ± 55 Ω cm−2] and charge transfer resistance (Rct) (e.g., day 1: 9.3 × 103 vs. 3.8 × 102 Ω 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 (Icorr) followed the rank: 3.5% NaCl solution (6.0 µA cm−2) > 3.5% NaCl solution containing MgO (3.6 µA cm−2) > 3.5% NaCl solution containing fragmented MPC (1.7 µA 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.

A Review of Recent Modification, Optimization, and Functionalization Methods for Organic Anticorrosive Coatings

“…Yin et al [68] studied an environmentally friendly inorganic magnesium phosphate coating (MPC), which can effectively prevent the formation of pores between the coating and the substrate. The research results show that the coating has no obvious corrosion in a neutral NaCl salt spray for 2400 h. After immersing the MPC-modified coating in a 3.5 wt.% NaCl solution for 30 min, the electrochemical impedance is (1.8 ± 0.6) × 10 4 Ω•cm 2 .…”

Chloride-induced depassivation and corrosion of mild steel in magnesium potassium phosphate cement

“…In this case, the former can decrease the resistance to chloride-induced attack while the latter may benefit. It has been found by limited research that the resistance of reinforcing steel to chloride-induced attack in MPC systems is higher than what has been anticipated [17,29,30], which was attributed to the formation of iron phosphate and hydration products on the surface of mild steel embedded in MPC exposed to chloride ions [29]. However, the combined role of phosphate ions and pH played in the depassivation and corrosion of mild steel is still unclear.…”