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Corrosion of structural materials

Updated: 06.02.2023
Article author : Enex

Corrosion problem

corrosion of structural materials

One of the main problems in the design and further operation of products made of non-corrosion-resistant materials is corrosion. So what is corrosion? Let's try to sort it out with you in this process.

Corrosion is the spontaneous destruction of materials due to their physicochemical interaction with the environment (aggressive atmosphere, sea water, solutions of acids, alkalis, salts, various gases, etc.).

Under the influence of an aggressive environment, most metals that have thermodynamic instability in real operating conditions are capable of spontaneously collapsing, passing into an oxidized state.

In some cases, the course of the corrosion process leads to more serious consequences than the loss of metal mass. The most dangerous consequences caused by corrosion include

loss of important technological and physico-mechanical properties by metal: mechanical strength, plasticity, hardness, reflectivity, etc. In this regard, when assessing corrosion losses, a comprehensive approach is needed, including consideration of all possible consequences caused by it.

Corrosion losses can be divided into direct and indirect.

Direct losses are the cost of replaced corroded products (machines, mechanisms, pipelines, roofing materials, etc.), the cost of protective measures (electroplating and paint coatings, the use of inhibitors, the construction of warehouses for storing equipment, etc.) and irretrievable metal losses (spraying it due to corrosion). According to experts, irretrievable metal losses account for about 10-15% of global steel production.

Indirect losses are much more difficult to calculate, but even by an approximate estimate they amount to billions of dollars. Here are examples of indirect losses.

  • Downtime. Replacing a corroded pipe of an oil refinery costs several hundred dollars, but underworking products during downtime can bring a loss of up to 20 thousand dollars per hour. Replacing a corroded boiler or condenser at a large power plant can cause an under-production of electricity by 50 thousand dollars per day. The total cost of under-generation of electricity in the United States due to corrosion downtime is tens of millions of dollars per year. 

  • Losses of finished products. During the overhaul period, oil, gas and water leaks occur due to corrosion damage to technical systems; corrosion of the car radiator leads to the loss of antifreeze, and gas leakage from a damaged pipe can lead to an explosion. 

  • Loss of power. Due to the deposition of corrosion products, the thermal conductivity of the heat exchange surfaces deteriorates. Reducing the flow sections of pipelines due to rust deposition requires an increase in pump power. It is estimated that increasing the capacity of pumps of water supply systems costs millions of dollars a year. In automotive internal combustion engines, where piston rings and cylinder walls are constantly corroded by the action of gaseous combustion products and condensates, losses from increased consumption of gasoline and oil are comparable to losses from mechanical wear, and sometimes exceed them. Potential losses of this type in energy conversion systems are estimated at several billion dollars per year. 

  • Contamination of products. A small amount of copper entering the system as a result of corrosion of copper piping or brass equipment can spoil an entire batch of soap. Copper salts accelerate the aging and spoilage of soap and thereby reduce its shelf life. Metal impurities can change the color of dyes. Lead equipment cannot be used for the preparation and storage of food products due to the toxicity of lead salts. Soft water passing through lead pipelines is not safe to drink. The same group of losses includes food spoilage due to rusting of metal containers. One of the plants where fruits and vegetables are canned suffered losses of about a million dollars a year until the factors that led to local corrosion were identified and eliminated. Another company that used metal lids on glass cans lost $ 0.5 million a year due to pitting corrosion of the lids, which led to bacterial contamination of products. 

  • Corrosion tolerances. This factor is common in the design of reactors, steam boilers, condensers, pumps, underground pipelines, water tanks and marine structures. In cases where the rate of corrosion is unknown and the methods of combating it are unclear, the design of such structures becomes much more complicated. Reliable data on the corrosion rate allows you to more accurately estimate the service life of the equipment and simplify its design. A typical example of corrosion tolerances is the choice of wall thickness of underground oil pipelines. The estimated wall thickness of the pipeline with a diameter of 200 mm and a length of 362 km is 8.18 mm (including corrosion), and the use of appropriate corrosion protection reduces it to 6.35 mm, which leads to savings of 3,700 tons of steel and an increase in the useful volume of the pipeline by 5%.;

It is obvious that indirect losses make up a significant part of the total corrosion losses. However, the calculation of indirect losses is a difficult task even within a single industry.

In some cases, losses cannot be expressed in monetary units at all. Such cases include accidents related to explosions, destruction of chemical equipment, or corrosion-caused accidents of airplanes, trains, cars and other vehicles, leading to loss of health or death of people.

With the development of industrial potential in all countries, the growth rate of corrosion losses began to exceed the growth rate of the metal stock. This is due to two main reasons:

  • changing the structure of the areas of use of metal. Previously, metal was consumed mainly by rail transport, utilities and machine tool construction. Now the share of metals in industries using them in aggressive environments has increased (chemical, petrochemical, pulp and paper industry, energy, automotive, aviation, navy, etc.); 

  • a significant increase in the aggressiveness of the atmosphere and natural waters due to their pollution by industrial emissions. 


PATTERNS OF CORROSION AND COMBINATION OF CORROSION AND FATIGUE CRACKING

Corrosion processes of structural materials operating in nuclear reactors are always more or less intense. 
The kinetics of corrosion oxidation of most structural materials, in particular metals and alloys, can be represented by a parabolic relationship between the loss of mass per unit surface W and the duration of the corrosion effect t at ambient temperature T.
In general, the rate of corrosion of structural materials in a nuclear reactor depends primarily on the duration of operation, operating temperature, the type of corrosive medium and impurities in it and the radiation environment (radiation intensity) that stimulates corrosion processes. When cyclic loads are imposed due to the kinetics of the reactor operation, combined damage to structural materials occurs due to the mechanisms of stress corrosion cracking and fatigue failure. Therefore, corrosion and stress cycling in NPP structures are more serious problems than the problems that arise in the case of conventional thermal power plants.


CORROSION OF REACTOR STRUCTURAL MATERIALS

Despite the fact that metals, ceramics and cermets are included in this category of materials, corrosion effects in metals and alloys are of practical interest and will primarily be considered. The corrosion processes of ceramic materials and cermets usually proceed much more sluggishly than those of metals.
The corrosion process includes: 1) oxidative (major) corrosion and 2) radiation (minor) corrosion, however, the latter can increase the speed of the former.

  1. Corrosion of beryllium. Beryllium is easily oxidized in air or in polluted water. The formation of oxides on it is greatly facilitated as it approaches the reactor core. The film of beryllium oxide VeO perfectly protects the metal at temperatures up to about 650 ° C. It has good corrosion resistance in Ne and CO2 used as heat carriers in gas-cooled reactors, as well as in liquid metals (Na or NaK) used in fast breeder reactors. In the reactors of these two types, beryllium or its oxide is also used as a structural material and a neutron reflector. 

  2. Corrosion of magnesium and its alloys. As already noted, magnesium alloys, for example magnox A-12, are the main structural material of gas-cooled reactors with graphite moderator of the Calder Hall reactor type. These alloys have good corrosion resistance in CO2 at temperatures up to 400 °C. At higher temperatures, the protective oxide film on the surface begins to crack and collapse, which leads to a sharp increase in the rate of corrosion. Small Be additives, as in the magnox A-12 alloy, increase the corrosion resistance of magnesium alloys. Magnox-type alloys are very well compatible with uranium fuel and CO2 coolant used in British gas-cooled reactors. The corrosion resistance of magnesium and its alloys in water and water vapor is low. The presence of a small amount of water vapor in CO2 can increase the rate of corrosion of the material of the shells of fuel rods made of magnesium alloys. 

  3. Corrosion of aluminum and its alloys. Aluminum and alloys based on it, which are the structural material of the shells of fuel rods and other components of research and training reactors on thermal neutrons, have high corrosion resistance in air, in clean water and water vapor. Uranium-aluminum fuel rods (flat or curved plate type), after several years of operation in reactors, practically did not succumb to corrosion. 

The reason for the high resistance of Ai and its alloys to oxidative corrosion is the high chemical affinity of aluminum and oxygen and the formation of a protective film of Al2O3 oxide, tightly bonded to the metal and protecting it from further interaction with free oxygen available in most aqueous media.
At temperatures up to 220 ° C, aluminum corrodes evenly in the water coolant. At higher temperatures as a result of radiolysis of water and corrosion reactions.


atomic hydrogen is formed, which, having penetrated into the metal, turns into molecular hydrogen. This increases the rate of formation of corrosion products and leads to the spread of gas blisters on the metal surface.At relatively low temperatures (220-250 °C), the corrosion rate is low. Above 400 ° C, the rate of corrosion increases with temperature and duration of exposure and there is a tendency to corrosion breakdown, i.e. a sharp increase in the rate of oxidative corrosion. However, the addition of a small (about 1%) amount of Ni increases the resistance to water corrosion at high temperatures of aluminum alloys such as alloy 1100.

Corrosion of zirconium and its alloys.One of the main reasons that zirconium alloys are chosen as the main structural material of light-water and heavy-water reactors is their high corrosion resistance in water. The alloys of zircaloy-2 and zircaloy-4 are widely used as materials for the shells of fuel rods of boiling reactors and cooling channels of power reactors, respectively. In addition, pressure pipes with uranium fuel for the active zones of heavy-water reactors are made from zircaloy-2. 

Zirconium and its alloys exhibit high corrosion resistance not only in water, but also in many environments found in the chemical industry, for example acidic and alkaline. Compared with tantalum, zirconium practically does not interact with fairly concentrated (about 50%) hydrochloric and nitric acids, caustic soda and sulfuric acid at temperatures of about 100 ° C, characteristic of fuel processing technology.

The corrosion rate of zirconium can increase dramatically as a result of the diffusion of oxygen ions by the vacancy mechanism from the water—oxide interface to the oxide—metal interface and the diffusion of hydrogen formed as a result of corrosion reactions through the oxide layer into the metal with the formation of zirconium hydride:

Corrosion of austenitic stainless steels and nickel alloys. This class of materials is widely used in fast breeder reactors, including those with liquid metal coolants. Both steels and nickel alloys exhibit high corrosion resistance in liquid metals Na, NaK, etc. This ability is due to the presence of chromium and nickel in the materials (as alloying elements or as a base). 

The reason for the corrosion resistance of austenitic stainless steels is the formation of an insoluble protective oxide film that evenly covers the surface of metals. At high temperatures, stainless steels begin to succumb to the corrosive effect of the coolant and methods of suppressing this tendency of steels are ineffective.

In fast reactors with liquid metal heat carriers, stainless steels — the material of fuel element shells, pipeline systems and other structures and equipment — are usually in contact with the Na or NaK coolant. At temperatures above 650 ° C and a sufficiently long time of forced circulation of the coolant, significant mass transfer (or mass loss) and deterioration of the corrosion properties of stainless steel occur. Therefore, the operating temperature of fuel element shells, piping systems and other stainless steel equipment in contact with Na is chosen below 650 ° C.

At temperatures below 540 °C, the phenomenon of decarburization of ferritic steels and carburization of austenitic stainless steels is observed in materials in contact with Na. Consequently, reactor systems containing both carbon and stainless steels must be designed in such a way as to avoid carbon transfer between these two types of steels at relatively low temperatures. Alloys based on niobium, nickel, titanium and vanadium are considered as an alternative to stainless steel for operation at higher temperatures. Thus, the effect of corrosion on structural materials can be weakened.

The presence of oxygen in sodium in the form of oxides invariably increases the corrosion rate of stainless steel, therefore it is necessary to take certain measures to maintain the concentration of oxygen in sodium at a very low level. Usually systems made of stainless steels include so—called cold traps - devices that are placed in the bypass part of the main coolant circuit. The temperature in them is maintained lower than in the main circuit (at about 150 ° C). Since the solubility of sodium oxide in Na at low temperatures is very low, it precipitates in the trap and is removed from it from time to time. Thus, the cause of oxidative corrosion of stainless steel is practically eliminated

As for nickel alloys, for example, the inconel-800 alloy used in the FFTF reactor (Fust Flux Test Facility) has good heat resistance and excellent corrosion resistance.

Corrosion and heat-resistant cracking. Corrosion cracking is a consequence of the corrosive effect of heat carriers on the stressed fuel element shells, pipelines and other components and systems of a nuclear reactor during its long-term operation. The cause of this type of corrosion may be stresses of microstructural, intergrain and macroscopic nature. 

The experience of reactor operation shows that corrosion cracking sometimes occurs in the area of the end parts of fuel rods, in piping systems with heat carriers, at the junctions of pipelines with the reactor vessel, in welded joints. The development and spread of the process of corrosion cracking leads to a violation of the tightness of fuel rods, pipelines, welded joints and other structures and components of the reactor

Violation of the integrity of various reactor components can occur not only as a result of corrosion, but also thermal fatigue cracking, usually caused by the cyclical thermal stresses that occur in materials under real reactor operating conditions.

As a result, the combination of corrosion and heat—fatigue cracking processes with vibration arising from the movement of the coolant leads to a breach of tightness or even destruction of reactor structures - fuel element shells, pipelines, places of their interface with the body, welded joints and fasteners.


Thus, the problem of corrosion is the problem of increasing the operational and technical reliability and durability of metals and other structural materials, the cost-effective use of natural resources and material resources. It has a global character.

Mankind has been dealing with the problem of corrosion for a long time. Even in ancient Egypt, metals were covered with mineral paints, and in China and Japan, lacquer coatings were used. The first substantiated (and, from modern positions, correct) theory of corrosion was proposed by A. de la Rive in 1830 (the theory of trace elements). T.P. Hoar, G.V. Akimov and others made a significant contribution to the development of this theory. The kinetics of electrochemical processes and their mechanism were studied by Yu . Tafel, A.N. Frumkin, I.A. Izgaryshev, M. Folmer, V.A. Kistyakovsky, N.D. Tomashov, Ya.M. Kolotyrkin and many others. A broad program of international cooperation is being implemented in the field of corrosion control.




Sources:

  • leg.co.ua
  • studref.com
  • Wikipedia




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