Formation of Phosphate Coatings on Steel and Corrosion Performance of Phosphated Specimens in Alkaline Solutions

Crystalline and amorphous phosphate coatings were formed on steel samples with the intention of using them for corrosion resistance enhancement of steel reinforcement in concrete. The physical properties of the phosphate coatings were studied using the following methods: the weights of phosphate coating and those of dissolved metal were calculated from gravimetric measurements, surface morphology-studies were carried out with an AFMand the phase composition of coatings was analysed by X-ray diffraction. An indicator of film porosity was considered as the maximum difference between potential values of growth completeness of phosphate film and those of metal dissolution. The corrosion behaviour of phosphated steel specimens was studied in 1 x 10 -5 and 0.1 M NaOH solutions (pH=8.5, 13) without/with 0.1 M Cl - ions. The phosphate coating protection abilities were related to its composition, thickness and porosity. All the crystalline phosphate coatings studied protect steel from corrosion in alkaline solutions in a wide range of pH even when Cl - ions are present. The best protective properties were exhibited by medium weight crystalline phosphate coatings. Amorphous phosphate coating is not chemically stable in strongly alkaline solutions because of the hydrolysis of iron phosphate present in it.