Scale Inhibition and Corrosion Inhibition Performance of 1-Hydroxy Ethylidene-1,1-Diphosphonic Acid (HEDP)

1-Hydroxy Ethylidene-1,1-Diphosphonic Acid (HEDP) is a well-established organophosphonic acid that functions as both a scale inhibitor and a corrosion inhibitor. Its performance is characterized by excellent chemical stability, particularly under high temperature and harsh chemical conditions, with distinct dosage thresholds for achieving scale versus corrosion inhibition.

Scale Inhibition Performance

HEDP is effective against a range of scales, with its primary strength being the inhibition of calcium carbonate, alongside calcium sulfate and iron oxide deposits .

Effective Dosage and Temperature Limits: For boiler water applications, HEDP provides a good scale inhibition effect at a dosage of 5 to 7 mg/L when the temperature does not exceed 100°C. However, if the operating temperature exceeds 100°C, the dosage must be increased to maintain a high scale inhibition rate .

Mechanism: Its scale inhibition is attributed to its ability to form stable, six-membered ring chelates with metal ions in water, especially calcium ions (Ca²⁺). This chelation, combined with a significant threshold effect, prevents the formation of scale . The Chinese industry standard for HEDP even includes a calcium chelating value (≥ 500 mg/g as CaCO₃) as an indirect indicator of its calcium carbonate scale inhibition capacity .

General Dosage Range: For general water treatment as a scale inhibitor, typical concentrations range from 1 to 20 mg/L, depending on the specific system and water quality .

Corrosion Inhibition Performance

HEDP also provides corrosion protection, particularly for carbon steel, though this typically requires a higher concentration than scale inhibition.

Effective Concentration for Carbon Steel: Electrochemical and autoclave static experiments have demonstrated that HEDP has a good corrosion inhibition effect on 20# carbon steel at a concentration of 25 mg/L . One study noted that 25 mg/L was the optimal concentration for both scale and corrosion inhibition in their tested conditions .

General Dosage Range: As a corrosion inhibitor, HEDP is generally used at concentrations of 10 to 100 mg/L . It works by forming stable complexes with metal ions like iron, copper, and zinc, and can dissolve existing oxides on metal surfaces, thereby helping to protect the underlying metal .

Key Stability Advantages

A significant part of HEDP's utility comes from its chemical robustness. It possesses a very stable C-P bond, which gives it better resistance to oxidation compared to some other phosphonates like ATMP. It remains stable and effective at temperatures up to 250°C, at high pH, and shows good resistance to hydrolysis and chlorine oxidation . This makes it suitable for demanding environments such as industrial cooling water systems and low-to-medium pressure boilers .

HEDP is a versatile and durable water treatment chemical. For scale inhibition, it is highly effective at low dosages (5-7 mg/L) below 100°C, primarily targeting calcium carbonate. For corrosion inhibition, a higher dosage (around 25 mg/L or more) is required to protect carbon steel. Its excellent thermal and chemical stability makes it a reliable choice for industrial systems operating under challenging conditions.