Modified Zinc Phosphate Coatings: A Promising Approach to Enhance the Anti-Corrosion Properties of Reinforcing Steel

Reinforcing steel embedded in fresh concrete develops a protective passive layer on its surface. This layer, which is formed as result of high alkalinity of the concrete pore solution (pH~13), consists of γ-Fe2O3 adhering tightly to the steel. As long as that oxide film is present, the steel remains intact [1]. However, chloride ions attack and concrete carbonation can destroy the film and, in the presence of H2O and O2 reinforcement corrosion takes place. Chloride ions, which were described by Verbeck [2] as the specific and unique destroyer, can be present in concrete either through the use of contaminated aggregate, sea water, brackish water or admixtures containing chlorides [1]. In the literature, there are state of the art review studies on numerous methods of protection against reinforcement corrosion [3-8]. Applying zinc phosphate coating on reinforcing steel will probably be a vital approach in the future [9-14]. The phospahting process has been known for over a century [15]. It was extensively used in the automobile and appliance industries [16,17]. This treatment primarily provides an inexpensive [17,18], non-toxic [19,20], reasonably hard, highly adherent and electronically non-conducting phosphate coating [15]. The insulation properties make an important contribution to the prevention of reinforcement corrosion [14]. Further, the phosphate conversion coatings are currently considered ecofriendly [21]. However, Phosphate coatings usually contain pores which are pathways for the corrosive electrolyte diffusion into the metal substrate. As a result, the corrosion resistance of the conventional phosphate coating is not high enough. Additives such as Ni2+, Ca2+, Mo2+, Cu2+ and Mn2+ and nanoparticles have been extensively used to produce phosphate coatings with uniform structure, lower porosity and improved corrosion resistance and adhesion properties [22,13,12,23,11,17,24] Being a newly adapted method in the field of reinforcement corrosion protection [25-27,13]this paper tries to review some important aspects related to the improvement of corrosion resistance of reinforcement using zinc phosphate coatings modified by metal cations or nanoparticles.