In many industries and in construction, technological methods involving gas mixtures are used. This can be, for example, processing parts under propane torches or the formation of protective media during welding to isolate the workpiece from oxygen. Under certain conditions, such processes can provoke gas corrosion - in particular, at elevated temperature or pressure. The chemical activity increases, which negatively affects the structure of metals and alloys. Therefore, special means are being developed to prevent such phenomena and to combat the resulting traces of corrosion of this kind.
Determination of gas corrosion
This type of corrosion damage is a chemical deformation of the surface of metals at high temperature. Typically, such phenomena are found in the metallurgical, petrochemical and chemical industries. For example, corrosion can occur during the production of sulfuric acid, in the synthesis of ammonia and the formation of hydrogen chloride. Also, gas corrosion of metals is an oxidative reaction process that proceeds under conditions with a certain coefficient of humidity in the surrounding air. Moreover, not every gas can provoke corrosion. The most active mixtures in this regard are nitrogen oxides, sulfur dioxide, oxygen, hydrogen and halogens. As for the targets, in this capacity reinforcing rods of furnaces and boilers, piping networks, surfaces of gas turbines, elements of internal combustion engines and alloys, which are subjected to heat treatment in metallurgy, are more often used.
Process features
At the first stage of the reaction, chemisorption of oxygen atoms occurs on a metal surface. It is in the specifics of the interaction of oxygen with metal that the main feature of this corrosion lies. The fact is that the reaction is in the nature of ionic interaction and this distinguishes it from typical chemical processes in dioxide. The bond is stronger, since the field of the underlying metal atoms acts on oxygen atoms. Further, oxygen adsorption processes take place, and under conditions of thermodynamic stability, the chemisorption layer is rapidly converted to an oxide film. Ultimately, gas corrosion can form salts, sulfides and oxides on the surface of a metal. The intensity of the processes of corrosion damage is affected by the properties of the oxidizing agent (gas environment), microclimatic parameters (temperature, pressure and humidity), as well as the current state of the chemical reaction object itself.
Alloy gas corrosion protection
One of the most common methods of protecting metal from various kinds of corrosive processes. This method is based on changing the properties of the structure of the corroding metal. Alloying per se involves modifying the alloy by introducing components that passivate its structure. In particular, tungsten, nickel, chromium, etc. can be used. Especially for gas corrosion protection , elements are used that increase the heat resistance and heat resistance of the metal. The alloying process can be performed both by applying special coatings and by immersing the workpiece in the gas phase of the modifying components. In both cases, the resistance of the metal to oxidative processes increases. For example, in order to halve the oxidation rate of an iron component at 900 ° C, it is necessary to alloy it with an A1 grade alloy of 3.5%, and for a fourfold reduction with an A1 modifier of 5.5%.
Protective atmosphere as a means of combating corrosion
Another technique for protecting metal billets and alloys from corrosion damage due to gas oxidation. Protective atmospheres can be formed by argon, nitrogen, and carbon. For each metal, specific gas mixtures are used. For example, cast iron is protected by argon or carbon dioxide compounds, and steel interacts well with hydrogen and nitrogen. In the maintenance of trunk pipelines, this kind of protection is used mainly when performing assembly welding activities. In constant operation, they often use the electrical protection of gas networks from corrosion, which is technically performed by semiconductors with cable loops. This is a type of electrochemical anticorrosive coating, which includes elements of the anode-protective galvanic protection in the structure.
The use of anti-corrosion heat-resistant coatings
This method also consists in reducing the speed of the corrosion process, but due to special heat-resistant coatings. A commonly used technique is the deposition of iron-aluminum thermodiffusion layers, which is known as thermochromization. Effective protection is provided by cermet processing of metal parts and structures. The advantages of such protection against gas corrosion include not only a reliable thermal and mechanical coating, but also the possibility of flexible modification of the physicochemical properties of the shell. The composition of the functional layer may involve both refractory oxides and metal components like molybdenum and tungsten.
Conclusion
Specialists are involved in the organization of control of anticorrosive protection agents, developing and approving projects for specific objects. In Russia, one of the largest departments for the protection of gas networks from corrosion is Mosgaz JSC. The employees of this structure are engaged in servicing gas facilities, maintaining the optimal state of the working infrastructure. In particular, the organization carries out such works as installation of electrochemical protection installations, hazard assessment of underground gas pipelines, analysis of the intensity of corrosion aggressiveness of materials, etc. For most of the work, modern metrological equipment is used that allows accurate and comprehensive investigation of target objects for corrosion and protection from her.