Inhibitors for magnesium corrosion: Metal organic frameworks

Electrochemical measurements demonstrate that magnesium surfaces can be protected by alkyl carboxylate. In a nearly neutral pH solution of sodium decanoate, the reduced corrosion rate and a passivation behaviour are attributed to the formation of Mg(C10H19O2)2(H2O)3 (Mg(C10)2) at the magnesium surface whereas heptanoate Mg(C7H13O2)2(H2O)3 (Mg(C7)2) is not efficient in such media. The crystal structures of the two metal carboxylates Mg(C7)2 and Mg(C10)2 are determined by X-ray diffraction. Single crystal data: Mg(C7)2, P21/a, a = 9.130(5) Å, b = 8.152(5) Å, c = 24.195(5) Å, β = 91.476(5)°, V = 1800.3(15) Å3, Dx = 1.242 g cm−3, Z = 4. Synchrotron powder data: Mg(C10)2, P21/a, a = 9.070(3) Å, b = 8.165(1) Å, c = 32.124(1) Å, β = 98.39(1)°, V = 2353.85(8) Å3, Dx = 1.188 g cm−3, Z = 4. Their layered structures are quite similar and differ mainly by the length of the hydrophobic chains. They consist of two planes of O-octahedra centred by Mg atoms, parallel to (001). The distorted octahedra are constituted by three oxygen atoms from carboxylate groups and by three oxygen atoms coming from water molecules. The layers are connected by hydrogen bonds. The carboxylate chains are located perpendicularly and on both sides of these planes. One carboxylate chain is bridging the Mg atom along [010] while the other is monodendate. The presence of structural water is confirmed by thermal analyses.

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