Reinforced concrete is widely used in construction projects due to its high strength, convenient construction, energy saving and economy. However, various environmental factors can cause premature failure. Protection measures are particularly important.
Currently commonly used methods include: using special steel bars, coating protection, cathodic protection, steel bar rust inhibitors, etc. The application of steel rust inhibitors in reinforced concrete structures has been generally recognized and widely recognized. The method is simple to apply and has been proven to be economical and effective by engineering practice.
Rust inhibition mechanism of rust inhibitor
Rebar corrosion is mainly due to the corrosion of chlorine salts When an electrochemical corrosion reaction occurs, the rust inhibitor can inhibit or slow down electrochemical corrosion by increasing the corrosion potential, reducing the ability of the iron matrix cathode and anode to gain and lose electrons, or increasing the ohmic resistance of the electron transfer channel and reducing the corrosion current density.
Increasing the corrosion potential can enhance the corrosion resistance of the steel bar and make the steel bar less susceptible to corrosion. Increasing the ohmic resistance can reduce the corrosion current density and reduce the corrosion rate. Through incorporation or migration, the rust inhibitor reacts at the interface of steel bars in concrete to form an adsorption film or passivation film, thereby effectively inhibiting the electrochemical corrosion reaction process.
Therefore, steel bar rust inhibitor is defined as: adding a small amount of chemical substances to concrete that can effectively inhibit or delay the occurrence of corrosion and reduce the corrosion rate of steel bars.
There are various types of rust inhibitors, and there are many classification methods. They can be divided into water-based and powder-based rust inhibitors according to their forms; they can be divided into main chemical components It is an organic, inorganic and mixed type rust inhibitor; it can be divided into migration type and incorporation type rust inhibitor according to the adding method; it can be divided into cathode type, anode type and composite type rust inhibitor according to the mechanism of action. The following mainly introduces the action mechanisms and related applications of these three types of rust inhibitors.
Anodic rust inhibitor
1. Anodic rust inhibitors inhibit steel corrosion by preventing or slowing down the electron loss process of the electrochemical anode. They are mainly inorganic salts such as chromate, molybdate and nitrite. Such substances are usually oxidizing and can react on the metal surface to form a dense passivation film that increases the film resistance and slows down the electron gain and loss rate of the anode, thereby inhibiting the overall electrochemical corrosion reaction process.
For example, calcium nitrite rust inhibitor, the ferrous ions that have lost their electrons chemically react with nitrite ions in an alkaline environment to produce Precipitate and form a passivation film Fe2O3 or γ-FeOOH on the surface of the steel bar, which inhibits the anode process and reacts as follows:
2Fe2++2OH-+2NO2-→2NO ↑+Fe2O3+H2O
Fe2++OH-+NNO2-→ON↑+γ-FeOOH
The above reaction can only be carried out in an alkaline environment. With the erosion of chlorine ions and the consumption of OH- in the reaction, the pH value decreases. This type of rust inhibitor loses its rust inhibitory effect. Research shows that alkali when the pH value is greater than 6.0 Nitrite only shows good rust-inhibiting effect in a non-toxic environment. The corrosion of chloride ions can cause the pH value to decrease and affect the rust-inhibiting effect of nitrite. Therefore, the ratio of [Cl-]/[NO2-] is closely related to the rust-inhibiting effect.
In marine environments or conditions with high chlorine salts, sufficient dosage of this type of rust inhibitor should be ensured, otherwise it may cause accelerated corrosion. Therefore, this type of rust inhibitor is also called a “dangerous” rust inhibitor, and nitrite is a carcinogen and its use is subject to certain restrictions. The use of nitrite rust inhibitors has been banned in Germany and Switzerland.
Cathode rust inhibitor
2. Cathode rust inhibitors inhibit steel corrosion by preventing or slowing down the electron gain of the electrochemical cathode. The main surfactants include phosphates, zincates, and higher fatty acid ammonium salts.
This type of rust inhibitor mainly reacts with certain ions in the liquid phase of concrete to form insoluble salts, which are effective in forming a film or adsorbing on the surface of the cathode area. Isolate the intrusion of water, gas and harmful ions to protect the steel bars. For example, sodium monofluorophosphate, Na2PO3F reacts with Ca(OH)2 to form insoluble apatite Ca5(PO4)F, which covers the cathode surface to slow down the dissolution of oxygen and inhibit the corrosive cathode reaction. The reaction that inhibits the cathode process is as follows:
5Ca(OH)2+3Na2PO3F+3H2O→Ca5(PO4)3F+2NaF+4NaOH+6H2O
Cathode rust inhibitor is non-toxic and non-hazardous, but if it is to achieve obvious rust inhibition effect, it must have enough insoluble salt to form a film or be adsorbed on the surface of the cathode area to prevent rust. The dosage of the agent is relatively large and the price is relatively expensive. Cathodic rust inhibitors have poor rust inhibitory effects when used alone, which is not conducive to market application and promotion.
Composite rust inhibitor
3. Composite rust inhibitors mainly inhibit corrosion of steel bars by preventing or slowing down the ability of electrochemical cathodes and anodes to gain and lose electrons. They are mainly composed of several oxidizing types, types that can generate insoluble salts, types that inhibit electron transfer and other substances that are properly matched. Compounded.
The rust inhibitory effect of the composite rust inhibitor is related to the composition of each component, which hasThe ingredients can be firmly adsorbed on the metal surface to form a dense molecular layer protective film, which not only inhibits the dissolution of the anode, but also provides a protective barrier for the cathode, which can effectively prevent the corrosion of steel bars.
For example, the active ingredients of migrating rust inhibitor (MCI) migrate to the surface of steel bars through diffusion, and their N-containing hydrophilic groups form chelates with Fe ions. The protective film of the compound molecular layer is adsorbed on the surface of the steel bar, and the non-polar groups form a hydrophobic barrier to isolate harmful ions, water, and oxygen from the matrix.
Compound rust inhibitor has the advantages of a single component, but overcomes the shortcomings of a single rust inhibitor, and its rust inhibitor effect is better than that of a single rust inhibitor. The agent effect is more excellent and has been promoted and applied in engineering practice. In particular, MCI type rust inhibitors are widely used in repair projects because of their simplicity, economy and effectiveness. However, the mechanism of action of composite rust inhibitors is relatively complex, and there is relatively little basic research on molecular component design. Research and development of environmentally friendly, high-efficiency composite rust inhibitors is still the main direction of future research and development. </p
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