What is the corrosion inhibition mechanism of EDTMPS?

The corrosion inhibition mechanism of scale inhibition EDTMPS (sodium ethylenediaminetetra(methylenephosphonic acid)) is a typical multifunctional synergistic process. Unlike some single-function agents, it suppresses corrosion through multiple pathways acting in concert.

Its core corrosion inhibition mechanism can be summarized as: primarily “cathodic” inhibition, combined with “precipitation film” and “adsorption film” characteristics.

Below is a detailed step-by-step explanation:

1. Cathodic-Dominant Inhibition Mechanism (Primary Mechanism)

This represents EDTMPS's most significant and effective corrosion inhibition pathway.

Step 1: Complexing Dissolved Oxygen and Metal Ions

The EDTMPS molecular structure contains multiple phosphonic acid groups (-PO₃H₂), which act as potent chelating agents. These groups form stable complexes with dissolved oxygen in water and divalent metal ions (especially Ca²⁺ and Zn²⁺) present in the systa.

Step 2: Creating a Physical Barrier at the Cathode

During the electrochaical corrosion process, the cathode region is where the oxygen reduction reaction occurs (O₂ + 2H₂O + 4e⁻ → 4OH⁻). The complexes formed between EDTMPS and metal ions preferentially diffuse and deposit onto the cathode region of the metal surface.

Step 3: Inhibit Cathodic Reactions

This deposited layer forms a dense, tightly adherent “protective film” at the cathode. This film effectively:

Blocks oxygen diffusion to the cathode surface.

Impedes electron transfer.

By significantly inhibiting the cathodic reaction (oxygen reduction), the current in the entire corrosion circuit is interrupted, drastically reducing the metal's corrosion rate.

Simple Analogy: This is akin to covering the battery's “positive electrode” (cathode) with a blanket, preventing it from receiving “fuel” (oxygen and electrons), thereby rendering the battery inoperable.

2. Adsorption Film-Type Corrosion Inhibition Mechanism (Auxiliary Mechanism)

Beyond film formation in the cathode region, EDTMPS molecules can also directly act upon the entire metal surface.

Characteristics: The phosphonic acid group (-PO₃H₂) in EDTMPS molecules is a highly polar functional group.

Process: These phosphonic acid groups can chaically or physically adsorb, firmly attaching directly to the metal surface (regardless of whether it is in the anodic or cathodic region).

Effect: This forms an extraely thin “monomolecular adsorption film.” This film isolates the metal substrate from corrosive media in water (e.g., O₂, Cl⁻), providing fundamental, comprehensive protection.

3. Synergistic Effects

EDTMPS is rarely used alone as a corrosion inhibitor; it is typically blended with zinc salts, producing a synergistic “1+1>2” effect.

Synergy with Zinc Salts (Zn²⁺): This is the most classic combination.

EDTMPS forms a stable, water-soluble complex with Zn²⁺, preventing Zn²⁺ from forming zinc hydroxide precipitates under alkaline conditions.

When this complex reaches the metal surface, it decomposes and deposits, forming an EDTMPS-zinc composite protective film.

Zinc ions are highly effective cathodic corrosion inhibitors, capable of rapidly forming zinc hydroxide precipitation films independently in cathodic zones.

When combined, EDTMPS facilitates the stable transport and uniform deposition of zinc ions, while zinc ions significantly enhance EDTMPS's film formation rate and density in cathodic zones, resulting in faster and more robust protective films.

Summary

Mechanism Type Mode of Action Protective Effect

Primary: Cathodic Corrosion Inhibition Forms complexes with Ca²⁺, Zn²⁺, etc., depositing and forming films in the cathodic zone to inhibit oxygen diffusion and reduction reactions. Highly effective and durable, serving as the primary means of controlling corrosion rates.

Auxiliary: Adsorption Film Corrosion Inhibition Phosphonic acid groups directly adsorb across the entire metal surface, forming a monomolecular barrier layer. Provides fundamental, comprehensive protection.

Key: Synergistic Effect When blended with Zn²⁺ and other agents, it stabilizes zinc and synergistically promotes film formation, creating a denser, more efficient composite protective film. Significantly enhances corrosion inhibition efficiency, serving as standard practice in practical applications.

In summary, the core corrosion inhibition mechanism of EDTMPS lies in its potent chelating ability to form a dense protective film in the cathodic region of the metal surface, fundamentally suppressing the electrochaical corrosion process. Its multifunctional structure enables simultaneous adsorption, dispersion (preventing under-deposit corrosion), and synergistic enhancaent capabilities.