What is the proposed mechanism for HEDP\'s corrosion inhibition on mild steel?

The corrosion inhibition mechanism of HEDP on mild steel is a classic example of mixed-type inhibition with a h3 anodic preference, facilitated by the formation of a protective chemisorbed film. It does not act as a simple passivator but rather as a film-forming inhibitor.

Here is a detailed breakdown of the proposed, widely accepted mechanism:

Core Principle: Formation of a Protective "HEDP-Fe-Oxide" Composite Film

HEDP does not merely adsorb; it participates in the corrosion electrochemistry to build a dense, adherent barrier layer on the steel surface.

Step-by-Step Mechanism:

1. Initial Physiosorption and Chelation

HEDP molecules in the water, primarily in their deprotonated anionic forms (e.g., H₂L³⁻, HL⁴⁻), are attracted to the positively charged sites on the steel surface. These sites are typically anodic areas where Fe is oxidizing to Fe²⁺.

The oxygen atoms in the phosphonate groups (–PO₃²⁻) act as