Corrosion inhibitors in common use can be classified from multiple dimensions including chemical composition, electrochemical mechanism, physic‑chemical mechanism and physical state. Below are the specific categories and typical representatives:
Classified By Chemical Composition1.Inorganic Corrosion Inhibitor
It was commonly used in the early days, but due to issues such as toxicity and low efficiency in acidic media, some of them have been replaced by organic corrosion inhibitors. Typical examples include:
• Oxide film type: Chromates (such as sodium chromate), nitrites (such as sodium nitrite), molybdates (such as sodium molybdate), tungstates, vanadates, borates, etc., form a dense oxide film on the metal surface through oxidation to inhibit corrosion.
• Precipitate film type: Zinc salts (such as zinc nitrate, zinc sulfate), phosphate salts (such as sodium orthophosphate), react with natural ions in water to form a precipitate film that covers the metal surface.
• Others: Arsenide compounds (due to their high toxicity, they are rarely used anymore), ammonium cyanide, etc.
2. Organic Corrosion Inhibitor
At present, it is the most widely used and has a rich variety. Typical representatives include:
• Copper corrosion inhibitors: Benzotriazole (BTA), Mercaptobenzothiazole (MBT), and Tolyltriazole (TTA). They are mainly applied in circulating cooling water systems to protect copper and copper alloy equipment, and can also be used as tarnish inhibitors and lubricant additives.
• Acid pickling corrosion inhibitors: imidazoline derivatives. They are used to inhibit steel corrosion during metal pickling with hydrochloric acid.
• Aliphatic chain organic amines: tallow amine, hexadecylamine, octadecylamine, etc. They form protective films by adsorbing onto the metal surface.
• Others: quaternary ammonium salts, rosin amine derivatives, acetylenic alcohols, organophosphorus compounds (e.g. phosphate esters, phosphonocarboxylic acids), organic acids (e.g. oxalic acid, acetic acid, citric acid), etc.
3. Polymer-Based Corrosion Inhibitors
Including polyethylene types, POCA (polycarboxylic acid), polyaspartic acid and other oligomeric polymers. They inhibit corrosion through adsorption or film formation and are commonly used in water treatment systems.
Classified According To Electrochemical Mechanisms
1. Anodic Corrosion Inhibitor
They achieve corrosion inhibition by suppressing anodic dissolution (passivation) of metals, mostly being inorganic strong oxidants (e.g. chromates, nitrites).
Note: Insufficient dosage may form corrosion cells of "small anode‑large cathode", which will aggravate pitting corrosion instead. These are classified as “dangerous inhibitors”.
2. Cathodic Inhibitor
They realize corrosion inhibition by suppressing cathodic reduction reactions (e.g. hydrogen evolution, oxygen absorption), mostly including zinc salts, phosphates, calcium salts, etc.
Features: Corrosion will not be accelerated under insufficient dosage; these belong to “safe inhibitors”.
3. Mixed Type Corrosion Inhibitor
They inhibit both anodic and cathodic reactions. Most are nitrogen‑containing, sulfur‑containing or hydroxyl‑containing organic corrosion inhibitors (e.g. mercaptobenzothiazole, benzotriazole, hexadecylamine). They form monomolecular films by adsorbing on metal surfaces, exhibiting both anodic and cathodic corrosion‑inhibiting effects.
Classified According To The Physical And Chemical Mechanism
1. Oxide Film Type Corrosion Inhibitor
It directly or indirectly oxidizes the metal surface to form a dense oxide film of 0.003~0.02 μm (e.g. chromates, nitrites). It delivers remarkable performance for passivable metals (iron, nickel, chromium, etc.), yet is ineffective for non‑passivable metals such as copper and zinc, and tends to lose efficacy in acidic media.
2. Precipitate Film Type Corrosion Inhibitor
It reacts with ions in water (e.g. Ca²⁺, CO₃²⁻, PO₄³⁻) to form precipitation films with thickness ranging from tens to one hundred nanometers (e.g. zinc salts, phosphates). The films feature relatively poor compactness and are prone to scaling, so they need to be compounded with antiscalants.
3. Adsorption Membrane Type Corrosion Inhibitor
Most are organic corrosion inhibitors. They adsorb onto metal surfaces via polar groups to form monomolecular films (e.g. amines, imidazolines, mercaptans). They exhibit excellent performance in acidic media, covering both anodic and cathodic zones to suppress corrosion reactions.
Classified By Physical State
1. Water-Soluble Corrosion Inhibitor
They can be directly dissolved in water for application, such as inorganic salts (sodium phosphate, sodium nitrite) and organic amines (imidazoline derivatives). They are widely applied in systems including circulating cooling water and boiler water.
2. Oil-Soluble Corrosion Inhibitor
Soluble in oils or organic solvents, they are commonly used in oil‑based media such as lubricating oils, fuel oils and cutting fluids to protect metal components against corrosion.
3. Gas-Phase Corrosion Inhibitor (Vci)
Volatilizable into gas at ambient temperature, they adsorb on metal surfaces to form protective films. Typical examples include dicyclohexylammonium nitrite and organic amines. They are widely used for rust prevention in metal packaging as well as storage and transportation protection.