Corrugated, Sheet-Like Architectures in Layered Alkaline-Earth Metal R,S-Hydroxyphosphonoacetate Frameworks: Applications for Anticorrosion Protection of Metal Surfaces

Reactions of M2+ (M = Mg2+ (1), Ca2+ (2), Sr2+ (3), or Ba2+ (4)) salts with R,S-hydroxyphosphonoacetic acid (HPAA) in aqueous solutions of pH 2.0−2.7 at a 1:1 ratio yield hydrated M−HPAA layered or three-dimensional coordination polymers with varying degrees of hydration (metal-coordinated water or lattice water). The crystal structures of 3 (two different phases, 3a and 3b, formed at slightly different pH) and 4 have been determined by single-crystal X-ray crystallography. Both enantiomers (R and S) of HPAA are incorporated in these metal−HPAA materials. Compounds were also characterized by a plethora of other techniques (ATR-IR, SEM, TGA, elemental analyses, powder XRD). Corrosion experiments were carried out at pH 2.0 and 7.3 to study the effect of combinations of externally added Sr2+ or Ba2+ and HPAA (1:1 ratio) on corrosion rates of carbon steel. It was found that at pH 2.0 Sr/HPAA or Ba/HPAA 1:1 combinations are not able to inhibit corrosion. However, at pH 7.3 quantitative corrosion inhibition is achieved. Anticorrosion films were studied by FT-IR, EDS, and XRF. These amorphous (by XRD) inorganic−organic protective coatings are composed of Sr2+ or Ba2+ and HPAA in a 1:1 ratio and are identical to Sr−HPAA or Ba−HPAA materials synthesized at pH 7.3 and different from those synthesized at pH 2.0−2.7. Structure elucidation was not possible because they are amorphous.