Three Types of Corrosion Inhibitors Physicochemical Mechanism
From the point of view of physicochemistry, the role of corrosion inhibitors can be divided into three types. Oxide film formation, precipitation film, and adsorption film. Therefore, corrosion inhibitors are also classified into oxide film type corrosion inhibitors, precipitation film type corrosion inhibitors, and adsorption film type corrosion inhibitors.
1. Oxide film type corrosion inhibitor
The oxide film type corrosion inhibitor itself is an oxidizing agent. It can interact with metals. Or it is not oxidizing by itself. Instead, the dissolved oxygen in the medium is used as an oxidant to form a tight oxide film on the surface of the metal. This can cause the metal ionization process to be blocked. Thereby slowing down the corrosion of the metal. This corrosion inhibitor is also known as a passivating agent.
The inhibition effect of potassium dichromate, potassium chromate and potassium permanganate on aluminum and magnesium in an aqueous solution containing oxygen belongs to this category.
The oxide film type corrosion inhibitor has high corrosion inhibition efficiency. It has been widely used. It should be noted that if the amount is insufficient, a large cathode small anode may be formed on the metal surface to cause pitting corrosion. Therefore, this type of corrosion inhibitor is also called “hazardous corrosion inhibitor”.
2. Precipitation film type corrosion inhibitor
The precipitated film type corrosion inhibitor forms a precipitated film on the metal surface. The precipitated film can be formed by the interaction between the corrosion inhibitor molecules. It can also be formed by the action of corrosion inhibitors and metal ions in corrosive media.
In most cases, a precipitated film is formed in the cathode region and covers the surface of the cathode. It separates the metal from the corrosive medium. This is used to suppress the cathodic process of electrochemical corrosion of metals. This is the cathodic inhibition type.
Sometimes the precipitated film covers the entire surface of the metal. At the same time, it inhibits the anode process and the cathode process of electrochemical corrosion of metals. This type is called a mixed inhibition type.
3. Adsorption film type corrosion inhibitor
Adsorbent film type corrosion inhibitors are mostly organic corrosion inhibitors. They have good adsorptivity to metal surfaces in corrosive media. This adsorption changes the properties of the metal surface. Corrosion of metal is inhibited.
The molecular structure of such corrosion inhibitors has an asymmetry. The molecule consists of a polar group and a non-polar group. The non-polar group is a hydrocarbon group and is lipophilic. The polar groups such as -COOH, -S03H, etc. are hydrophilic. It also has an affinity for the metal surface.
When the polar group of the corrosion inhibitor molecule is adsorbed on the metal surface. Its longer non-polar groups are also closely aligned under the action of van der Waals forces. Thereby a strong adsorption film is formed.
On the one hand, surface adsorption changes the charge state and interfacial properties of the metal surface. This tends to stabilize the energy state of the metal surface. Increase the activation energy of the corrosion reaction. Slow down the corrosion rate.
On the other hand, the isolation of the non-polar groups separates the metal surface from the corrosive medium. Impeding the transfer of charge or matter associated with an electrochemical reaction. Thereby slowing down the corrosion.
The above is a detailed guide to the physicochemical mechanism of corrosion inhibitors by IRO. Please feel free to contact us if you have any questions about water treatment chemicals. Our official website is www.irowater.com