Distinguishing Between Types of Chemical Inhibitors: Antiscalants vs. Anticorrosives

In the world of industrial water treatment, two main enemies that constantly lurk are scale and corrosion. Both can reduce efficiency, increase energy costs, and cause expensive equipment damage. To combat these threats, two main chemical weapons are used: antiscalant and anticorrosive.

Although both are called ‘inhibitors’, their modes of action and targets are very different. Understanding these fundamental differences is key to selecting the right protection strategy for your water system.

What Are Inhibitors? Active Protection for Your Water System

Chemical inhibitors are chemicals added to water in small doses with the aim of inhibiting or preventing undesirable processes, such as scale formation and corrosion. They act as active ‘protectors’ working at the interface between water and metal surfaces.

Antiscalant: The Smart Scale Inhibitor

1. Definition and Objectives

Antiscalants are inhibitors specifically designed to prevent the formation of hard deposits (scale) on the surfaces of equipment such as pipes, heat exchangers, and RO membranes. Scale usually forms from compounds such as Calcium Carbonate (CaCO₃), Calcium Sulphate (CaSO₄), and Silica (SiO₂).

2. How it works: Inhibiting the crystallisation process

Antiscalant does not remove minerals from water, but rather disrupts the crystal formation process through several intelligent mechanisms:

• Threshold Inhibition: Holds scale-forming ions (such as Ca²⁺ and SO₄²⁻) in concentrations that exceed the normal saturation point (solubility limit) without forming deposits.

• Crystal Modification: Changes the shape and structure of the crystals that form. Instead of forming hard crystals that easily adhere, antiscalants make them irregular, soft, and easily suspended so that they are carried away by the water flow.

• Dispersion: Gives a negative charge to the crystal particles that begin to form, so that they repel each other and cannot combine to form a solid deposit.

3. Common Types of Antiscalants:

• Phosphonates: Such as ATMP and HEDP. Very effective and commonly used.

• Polymers: Such as polyacrylates and polymaleates. Good for dispersing particles.

• Polyphosphates: Simple and economical, but can be hydrolysed in hot water.

Anticorrosive: Forms a Protective Shield for Metals

1. Definition and Objectives

Anticorrosive is an inhibitor that aims to protect metal surfaces (iron, copper) from corrosion by forming a protective layer.

2. How it Works: Forms a Protective Layer (Film)

Anticorrosives work by forming a thin layer that prevents direct contact between the metal and water and oxygen, the two key elements that cause corrosion.

• Passivation (Anodic Inhibitor): This type of inhibitor (e.g., nitrite, molybdate) forms a very thin, hard, impermeable oxide layer (such as γ-Fe₂O₃) on the metal surface. This layer ‘massifies’ the metal, changing it from an active state to an inert one.

• Forming a Barrier (Cathodic Inhibitor): This type of inhibitor (e.g. Phosphate, Silicate, Zinc) forms an impermeable physical layer on the cathode, thereby inhibiting the oxygen reduction reaction that drives corrosion.

• Adsorption (Film-Forming Amines): Inhibitor molecules (such as film-forming amines) adsorb and form a monomolecular, hydrophobic (water-repellent) layer over the entire metal surface.

3. Common Types of Anticorrosives:

• Anodic Inhibitor: Sodium Nitrite, Sodium Molybdate. Very effective but risky if the dosage is insufficient (can cause pitting corrosion).

• Cathodic Inhibitors: Phosphates, Polyphosphates, Zinc Phosphates.

• Mixed Inhibitors: Silicates, which work on both electrodes (anode and cathode).

• Volatile Corrosion Inhibitors (VCI): For steam and condensate systems.

How to Choose the Right Combination?

It is very rare for an industrial system to face only one problem. Cooling systems, boilers, or RO systems usually require protection from both scale AND corrosion. Here is a general guide:

1. Water Quality Analysis: This is an absolute first step. Parameters such as hardness, alkalinity, pH, TDS, and chloride content will determine which threat is more dominant.

2. System Identification:
   

   • Reverse Osmosis (RO) System: The threat of scale on the membrane is the top priority. Antiscalant is the primary inhibitor, sometimes supplemented with anticorrosive if there are metal components.

   • Boiler System: A combination is essential. Here, an oxygen scavenger (a type of anticorrosive) is primary for preventing corrosion, while a phosphonate-based antiscalant is used to control scale.

   • Cooling Tower System: Both are equally important. Treatment programmes typically use blended inhibitors containing scale inhibitor components (such as polymers) and corrosion inhibitor components (such as phosphates, zinc, or molybdates) in a single formulation.

3. Ready-to-use Chemical Considerations: At PT. Jaya Sakti Internusa, with our BOWENCO brand, we provide ready-to-use chemicals for boilers, cooling towers, chillers, and reverse osmosis systems, formulated according to the problems and needs in the field.

4. Operating Conditions: Temperature, pressure, and flow rate affect inhibitor performance. Choose products designed for the specific conditions of your system.

Conclusion:

Both antiscalants and anticorrosives play complementary roles in the water system protection ecosystem. Choosing the right combination is not about which is better, but about understanding your system's risk profile. Consult with us to conduct a comprehensive assessment and recommend an integrated inhibitor programme to achieve maximum operational efficiency and extended equipment lifespan.