【Q&A Question 210】November 16, 2016

Attached to the inner wall of pipes to prevent corrosion

1. Phase-forming film theory: The phase-forming film theory involves artificial corrosion inhibitors forming a protective film on the metal surface that is difficult to dissolve, thereby preventing the medium from corroding the metal. This type of protective film includes oxide films and precipitation films. 2. Adsorption film theory: The adsorption film theory holds that certain corrosion inhibitors inhibit the corrosion of metals by the medium by forming an adsorption protective film through the physical or chemical adsorption of their molecules or ions on the metal surface. Some corrosion inhibitor molecules or ions undergo physical adsorption on the surface due to electrostatic attraction and intermolecular forces. Other corrosion inhibitors can undergo chemical adsorption by forming coordination bonds with the metal surface. Corrosion inhibitors adsorb onto the metal surface through their hydrophilic groups, while their hydrophobic groups remain away from the metal surface; this forms an adsorption layer that covers the active centers of the metal, thereby preventing the medium from eroding the metal. Such corrosion inhibitors are mainly organic corrosion inhibitors. 3. Electrochemical theory: The electrochemical theory suggests that corrosion inhibitors reduce the rate of metal corrosion by increasing the resistance of either the cathodic or anodic processes involved in corrosion. Therefore, they are classified into anode-inhibiting, cathode-inhibiting, and mixed-inhibiting corrosion inhibitors.

1) Mechanism of phase-forming film. Corrosion inhibitors form a protective film on the metal surface through oxidation or deposition, preventing contact between the medium and the metal ； 2) Adsorption membrane mechanism. Corrosion inhibitors bind to the active metal centers through physical or chemical adsorption, preventing the medium from coming into contact with these active metal centers, thereby protecting the metal ； 3) Electrochemical mechanism. Such corrosion inhibitors slow down the corrosion of metals by increasing the resistance at the anode or cathode of the corrosion process.

Corrosion inhibitors form a protective film on the inner wall of the pipes, reducing the corrosion of the pipe walls by the medium

Film formation theory: Film formation theory suggests that corrosion inhibitors are able to provide corrosion protection because they can form an insoluble protective film on the metal surface. This protective film can be an oxide film formed on the metal surface by the corrosion inhibitor, or it can be a precipitate film formed as a result of the reaction between the corrosion inhibitor and molecules or ions in the corrosive medium. For example, K2CrO4 in neutral water can oxidize the surface of iron to form an iron oxide passivation film. ZnSO4 can form a Zn(OH)2 precipitate film on the surface of iron in neutral water. According to electrochemical theory, corrosion inhibitors work by increasing the resistance of the cathodic or anodic processes involved in corrosion (i.e., polarization), thereby slowing down metal corrosion

Network and solubilization effects, lattice distortion effects, electrostatic repulsion effects.

The action mechanisms of corrosion inhibitors can be generally divided into two types, namely electrochemical mechanisms and physicochemical mechanisms. The electrochemical mechanism is based on the electrochemical processes that occur on the metal surface to explain the action of corrosion inhibitors. The physicochemical mechanism is based on the physicochemical changes that occur on the metal surface to explain the action of corrosion inhibitors. These two mechanisms approach problems in different ways, but they are not contradictory; rather, there is a certain causal relationship between them.

Corrosion inhibitors can slow down or prevent the rate of metal corrosion

The action mechanisms of corrosion inhibitors can be generally divided into two types, namely electrochemical mechanisms and physicochemical mechanisms. The electrochemical mechanism is based on the electrochemical processes that occur on the metal surface to explain the action of corrosion inhibitors. The physicochemical mechanism is based on the physicochemical changes that occur on the metal surface to explain the action of corrosion inhibitors. These two mechanisms approach problems in different ways, but they are not contradictory; rather, there is a certain causal relationship between them.

A protective film is formed on the surface layer. Slows down oxidation.