Traditional methods of protecting metals from corrosion are less and less meeting the technical requirements that are imposed on the performance properties of critical structures and materials. Bearing beams in house frames, pipelines and metal claddings cannot do without mechanical rust protection, if we are talking about long-term use of the product. A more effective approach to corrosion protection is the electrochemical method and, in particular, passivation. This is one way of using active solutions that form a protective insulating film on the surface of the workpiece.
Technology Overview
Passivation should be understood as the process of forming a thin film on a metal surface, the structure of which is characterized by increased resistance. Moreover, the functions of this coating may be different - for example, in battery electrolytes, it not only prolongs the life of the electrodes, but also reduces the intensity of self-discharge. From the point of view of corrosion protection, passivation is a way to increase the resistance of the material to an aggressive environment that provokes the development of rust. The very mechanism of formation of a protective-insulating coating may be different. The electrochemical and chemical methods are fundamentally different, but in both cases the final result will be the transition of the external structure of the workpiece to a chemically inactive state.
The principle of electrochemical corrosion protection
A key factor in electrochemical passivation is the effect of external current on the target surface. At the moment the cathode current passes through the corroding metal structure, its potential changes in the negative direction, which also changes the nature of the ionization process of the workpiece molecules. Under conditions of anodic action from the side of an external polarizer (typical for acidic media), an increase in current may be required. This is necessary to suppress the polarizer and the subsequent achievement of complete corrosion protection. However, with enhanced surface passivation due to an external current, hydrogen evolution increases, which leads to hydrogenation of the metal. As a result, the process of dissolving hydrogen in a metal structure begins, followed by a deterioration in the physical properties of the workpiece.
Cathode protection method
This is a type of electrochemical anti-corrosion insulation, in which the cathodic current deposition technique is used. But this method can be implemented by different methods. For example, in some cases in production, a sufficient potential shift is provided by connecting the part to an external current source as a cathode. The anode is an inert auxiliary electrode. This method performs passivation of welds after welding, protects metal platforms of drilling structures and underground pipelines. The advantages of the cathodic passivation method include efficiency in suppressing various types of corrosion processes.
In addition to general rust damage, pitting and intergranular corrosion are prevented. Practiced and such methods of cathodic electrochemical effects, such as tread and galvanic. The main feature of these approaches is the use of a more electronegative metal as a polarizer. This element is in contact with the protected product and performs the function of the anode, collapsing during the operation. Similar methods are usually used in the insulation of small structures, parts of buildings and structures.
Anode protection method
When anode insulation of metal parts occurs, the potential is shifted to the positive side, which also contributes to the surface resistance before corrosion processes. Part of the energy of the applied anode current is spent on the ionization of metal molecules, and another part is on the suppression of the cathodic reaction.
Among the negative factors of this approach is the high dissolution rate of the metal, which is not comparable with the rate of reduction of the corrosion reaction. On the other hand, much will depend on the metal to which passivation is applied. These can be both actively dissolving materials and parts with incomplete electronic layers, the structure of which in a passive state also contributes to inhibition and the destruction reaction. But in any case, to achieve a significant effect of corrosion protection, the use of large anode currents is required.
From this point of view, this method is impractical to use when maintaining insulation for a short time, but the low energy costs of maintaining the applied current fully justify anode passivation. By the way, the formed protection system in the future requires a current strength of only 10-3 A / m 2 .
The use of chemical inhibitors
An alternative technological approach to increasing the resistance of metals when used in aggressive environments. Inhibitors provide chemical passivation, which reduces the rate of dissolution of metals and, to varying degrees, eliminates the harmful effects of a corrosion attack.
In itself, an inhibitor is, in a sense, an analogue of the applied current, but with a combined chemical or electrochemical effect. The activators of the protective film are organic and inorganic substances, and more often their specially selected complex compounds. The introduction of an inhibitor into an aggressive environment causes changes in the structure of the metal surface, affecting the kinetic electrode reactions.
The effectiveness of the protection will depend on the type of metal, environmental conditions and the duration of the whole process. Thus, in the long run, the passivation of stainless steel will require more energy resources to counter an aggressive environment than in cases with brass or iron. But a key role will still be played by the mechanism of action of the inhibitor itself.
Passive Inhibitors
Active corrosion protection according to the principles of passive resistance formation can be formed by different inhibitors. So, adsorption compounds in the form of anions, cations and neutral molecules are widely used, which can have a chemical and electrostatic effect on a metal surface. These are universal corrosion protection agents, but their effect is reduced in environments where oxygen polarization dominates. For example, a special inhibitor with oxidizing properties should be used to passivate stainless steel. These include molybdates, nitrites and chromates, which create an oxide film with a positive polarization shift sufficiently to isolate oxygen molecules. Chemisorption of the formed oxygen atoms occurs on the metal surface, blocking the most active coating zones and creating additional potential for slowing down the dissolution of the metal structure.
Using Passivation to Protect Semiconductors
The operation of semiconductor elements under high voltage requires a special approach to corrosion protection. In such cases, passivation of the metal is expressed in the circular insulation of the active region of the part. An electrical edge protection is formed using diodes and bipolar transistors. Planar passivation involves the creation of a protective ring, as well as coating the crystal surface with glass. Another method of mesa-passivation involves the formation of a groove in order to increase the maximum permissible level of stress on the surface of a structural metal crystal.
Modification of anti-corrosion film
The coating formed as a result of passivation allows a variety of means of additional reinforcement. This can be cladding, chrome plating, staining and creating a conservation film. Methods of supporting reinforcing anti-corrosion protection as such are also used. For zinc coatings, special solutions based on polymer and chromium components are being developed. For a conventional galvanized bucket, flushing non-reactive additives can be used.
Conclusion
Corrosion is a destructive process that can manifest itself in different ways, but in each case it contributes to the deterioration of certain operational properties of the metal. The occurrence of such processes can be eliminated in various ways, as well as by the use of noble metals, which are initially distinguished by a reduced sensitivity to rust. However, due to certain financial and technological reasons, the use of standard corrosion protection agents or the use of metals with high corrosion resistance is not always possible.
In such cases, passivation becomes the optimal solution - this is a relatively affordable and effective method of protection of metals of various types. According to some estimates, a single electrode with a correctly selected inhibitor may be enough to protect the 8-kilometer line of an underground pipeline from corrosion. As for the shortcomings, they are expressed in the technical complexity of the use of electrochemical passivation methods in principle.