Corrosion is an electrochemical process. A piece of steel in water does not rust uniformly — it sets up small cells, with an anodic area where the metal dissolves and a cathodic area where the reduction happens. The two areas are connected through the metal and the water, and the current that flows between them is what carries the metal away. An inhibitor works by interrupting one of those steps, and which step it interrupts decides what kind of inhibitor it is.

Innovacorp India manufactures and supplies corrosion inhibitors in Agartala for industrial boilers, cooling circuits and process pipelines. This page is about the corrosion side specifically — anodic and cathodic inhibition, pitting versus general corrosion, multi-metal systems and how the corrosion rate is monitored. The boiler and cooling programmes that use these inhibitors are covered on separate pages, because the programme design is a different decision from the inhibitor selection.

How an Electrochemical Corrosion Cell Works

The corrosion cell has three parts: an anode, a cathode, and a conductive path between them. At the anode, the metal loses electrons and dissolves into the water as an ion. At the cathode, those electrons are consumed — in neutral water, usually by dissolved oxygen. The anode and cathode can be millimetres apart on the same piece of steel, or metres apart in a mixed-metal system.

The corrosion rate depends on how fast the electrons can flow between the two. Anything that slows the flow at either end slows the corrosion. An anodic inhibitor slows the anodic reaction; a cathodic inhibitor slows the cathodic reaction; a mixed inhibitor slows both. This is the framework that the rest of the inhibitor selection sits inside.

Anodic, Cathodic and Mixed Inhibition

The three inhibitor types behave differently, and the choice depends on the metal and the water chemistry:

• Anodic inhibitors form a passivating film on the anode, which slows the metal dissolution. They are effective at low dose, but an anodic inhibitor dosed too low can make pitting worse, because it protects part of the surface and concentrates the attack on the rest. The dose has to be above the critical minimum for the water chemistry.
• Cathodic inhibitors slow the cathodic reaction, usually by forming a film that restricts the access of oxygen or water to the cathode. They are less prone to the "too low dose makes it worse" problem than anodic inhibitors, but they are also less effective at low dose.
• Mixed inhibitors act on both the anode and the cathode, and they are the usual choice where the corrosion is not dominated by one mechanism. Film-forming amines and multi-metal blends fall into this category.

The choice between the three is not a preference. It depends on the specific metal, the water chemistry and the type of corrosion present, and the wrong choice can be worse than no inhibitor at all.

Pitting vs General Corrosion

The two corrosion types need different responses, and they are often confused:

• General corrosion attacks the surface more or less evenly, so the metal thins over a wide area. The rate is measurable by weight loss, and the inhibitor programme can be set to keep the rate within an acceptable range.
• Pitting corrosion attacks a small area deeply, so the metal can fail while the rest of the surface looks intact. It is usually caused by chlorides, dissolved oxygen or under-deposit conditions, and it is harder to detect because it does not show up as uniform thickness loss.

A programme that controls general corrosion can still leave a pitting problem, because the two have different mechanisms. Pitting is the corrosion type that causes unexpected tube failures, and it is the one that the inhibitor selection has to address explicitly rather than as part of a general corrosion rate target.

Multi-Metal Systems: Why One Inhibitor Is Not Enough

Many industrial circuits contain more than one metal — carbon steel tubes, copper alloy heat exchangers, brass valves, stainless steel fittings. A single inhibitor designed for carbon steel can be aggressive towards copper alloys, and a copper-specific inhibitor can be ineffective on carbon steel.

Multi-metal inhibitors use a combination of components: a film-forming component that protects the ferrous surfaces, and a copper-specific component (usually a triazole or thiadiazole) that protects the copper alloys. The two work together, and the ratio between them depends on the metallurgy of the circuit. A "multi-metal" claim on a datasheet does not tell you whether the product is right for your specific mix of alloys — the formulation details do.

Under-Deposit and Microbially Influenced Corrosion

Two corrosion types are often missed because they develop under something else:

• Under-deposit corrosion — happens under a layer of scale, sludge or biofilm. The deposit creates a local chemistry that is different from the bulk water, and the metal under the deposit corrodes faster than the rest of the surface. The inhibitor has to reach the surface to work, which is why deposit control and corrosion control are linked.
• Microbially influenced corrosion (MIC) — caused by bacteria that produce corrosive metabolites, such as sulphide from sulphate-reducing bacteria or organic acids from acid-producing bacteria. The inhibitor programme alone does not control MIC; a biocide programme is needed alongside it.

Both are reasons why corrosion control is a programme rather than a single product. An inhibitor that protects the clean surface does not necessarily protect the surface under a deposit.

Product Data

Parameter	Details
Inhibitor types	Anodic, cathodic, mixed, multi-metal
Form	Liquid (product dependent)
Application	Boilers, cooling circuits, process pipelines
Active content	TDS on request (per product)
pH (as supplied)	TDS on request
Density	TDS on request
Shelf life	TDS on request

How the Dose Is Set

The dose is expressed in ppm of the treated water, and it is set from the metal, the water chemistry and the type of corrosion present. A fixed ppm is not reliable across systems, because the same product at the same ppm behaves differently on carbon steel and on copper alloy, and differently in a low-chloride water and a high-chloride one. We share a starting range from the TDS and adjust it as the corrosion monitoring data comes in.

The critical point for an anodic inhibitor is the minimum dose below which pitting gets worse. That minimum depends on the water chemistry, and it is one of the reasons a dose set by rule of thumb can be worse than no dose at all.

Storage and Handling

Standard industrial packing; pack sizes on request. Store in a cool, dry place away from direct sunlight, and keep the drum closed when not in use. Corrosion inhibitors are corrosive in concentrate form, so handling should follow the SDS.

Working with Your Plant Team

We help with inhibitor selection, dose setting and interpretation of your corrosion monitoring data. Where a number cannot be confirmed without a trial, we say so. Final specifications are confirmed after a trial in your system.

Plant Teams Ask

What is the difference between anodic and cathodic corrosion inhibition?
An anodic inhibitor slows the corrosion reaction at the anode, where the metal dissolves, usually by forming a passivating film on the surface. A cathodic inhibitor slows the reaction at the cathode, where the reduction happens, usually by forming a film that restricts the access of oxygen or water. Some inhibitors act on both, and the choice depends on the metal and the water chemistry. An anodic inhibitor dosed too low can make pitting worse, because it protects part of the surface and concentrates the attack on the rest.

How are multi-metal systems protected against corrosion?
A system with copper, brass, stainless steel and carbon steel in the same circuit needs an inhibitor that protects all four, because a product designed for carbon steel can be aggressive towards copper alloys. Multi-metal inhibitors use a combination of film-forming and copper-specific components to protect the mixed metallurgy. The choice depends on the specific alloys in the circuit, not on a general multi-metal claim.

What is the difference between pitting corrosion and general corrosion?
General corrosion attacks the whole surface more or less evenly, so the metal thins over a wide area. Pitting corrosion attacks a small area deeply, so the metal can fail while the rest of the surface looks intact. Pitting is usually caused by chlorides, dissolved oxygen or under-deposit conditions, and it is harder to detect because it does not show up as a uniform thickness loss. The inhibitor programme has to be selected for the type of corrosion present.

How is corrosion rate monitored in an industrial system?
The common methods are corrosion coupons, which are weighed before and after exposure, and electrochemical probes, which measure the corrosion rate in real time. Coupons give an average rate over the exposure period, while probes show the trend. The choice depends on the system and the level of monitoring the plant can support. A programme that is not monitored is a programme that cannot be adjusted when the water chemistry changes.

Get in Touch

For inhibitor selection, dose setting or a corrosion monitoring discussion for your plant, call +91-9312871070. We will ask about your metallurgy, water chemistry and current programme first, because that is what decides the right inhibitor type.

Related reading: boiler chemicals for scale, oxygen and pH control and cooling water treatment for chillers and heat exchangers.