Aminophosphonate corrosion inhibitors for steel

Abstract

The protective properties of aminophosphonic acids and their magnesium and calcium complexes were studied on the example of rotating steel cylinders specimens in soft water. Only two of the seven acids studied, namely 1,1-hydroxycarboxypropane-3-amino-di(methylenephosphonic) and hexamethylenediamine-N,N-tetra(methylenephosphonic) acids, can suppress the corrosion of steel in water completely. As a rule, complexonates are much more effective them the corresponding acids; if the complexing agent is fixed, then the stability constants (K s) of the complexes become the major factor. For phosphonates of Mg2+ and Ca2+, which are usually less stable than the corresponding iron complexonates, the dependence of the protective concentration on K s passes through a maximum; the complexonates of those acids whose own protective properties are weaker are more effective. Imino-N,N-diacetic-N-methylenephosphonic acid can serve as an example. By contrast, the relatively more stable complexonates, for example calcium nitrilotri(methylenephosphonate), are much less effective than the acid itself.