Why does corrosion occur?

Pros and cons of metal corrosion

Many people at least sometimes admire the greenish coating on copper, or are upset by the darkening of silver jewelry, or are indignant at the appearance of rust on the surface of a car. But not everyone understands that these color changes are caused by the same phenomenon - corrosion. What is it?

What is metal corrosion?

Metal corrosion is the destruction of its surface due to the chemical or electrochemical influence of the external (corrosive) environment.

Iron and its alloys react with oxygen and water. As a result, oxides Fe2O3 x nH2O and iron metahydroxides FeO(OH) and Fe(OH)3 , which are called rust . The process is irreversible and penetrates deeper into the metal, continuing until the product is completely destroyed.

Other metals and their alloys also corrode. But unlike a layer of rust, the oxides formed on the surface of these materials prevent further damage.

There are 2 types of corrosion: chemical , electrochemical . The first occurs under the influence of dry gases and non-electrolytes. The second is that due to contact with electrolyte solutions containing ions, it causes an electric current. Temperature speeds up the process.

pros

• Permanent dye (E 172). Used for coloring ceramics, cement and food products (baked goods, pates, sweets, fish caviar). By treating with alkalis, calcination or steam, yellow, red and black powders are obtained. By mixing, orange and brown colors are obtained.
• Use of superconductivity . When air with a low oxygen content interacts with iron, a slow reaction occurs, forming black rust Fe3O4. It is a ferromagnet that has the property of being magnetized without an external magnetic field, which is used for the production of superconductors.
• Application in current sources . Electrolytic corrosion is used. A metal with a negative potential (anode) is placed in an aqueous solution of an electrolyte (acid, salt, alkali). It gives off positive ions to the liquid and gradually dissolves. At the cathode, which has a positive charge, electrons are collected and then transferred to the external environment. The lower the electrode potential of a metal, the faster it corrodes, releasing ions into the solution. Precious metals, due to their resistance to corrosion, are applied to the contacts of electrical circuits; platinum is used in spacecraft.
• Alloying . A metal that has a lower electrode potential (such as aluminum or zinc) is sacrificed to protect the iron, steel, or other metal. The “sacrifice” is applied on top of another material, and it prevents destruction until it itself is completely corroded.
• Savings on disposal costs . The gradual transformation of iron debris into powder occurs naturally. No additional effort is required for disposal.
• Promotes the birth and application of inventions . People are coming up with new tools and means to get rid of rust. For example, sandblasting and shot blasting guns for mechanical cleaning, industrial spray cleaners, and a hydrosandblasting ejector have appeared.

Minuses

• Unaesthetic appearance . The sight of rusty pipelines is disgusting, and rusty tools are uncomfortable to hold in your hands. Having been in practice in a house or in a “hot” workshop with severely neglected communications, some students become disillusioned with their future profession. The country is losing failed mechanics, electricians, assemblers and representatives of other blue-collar professions who would have to work with hardware.
• Declining sales . Buyers refuse to buy cars if, upon inspection, they notice “traces” of corrosion. It is unpleasant to get into a taxi with rust stains on its body. Therefore, there are cases of passengers leaving for other taxi companies, where the cars look not only cleaner, but also without corrosion.
• Loss of strength . Buildings and structures for chemical production are often constructed from reinforced concrete. Metal reinforcement in the form of bars, mesh or frame is inserted into concrete to increase the strength and other characteristics of the composition. But the walls outside and inside such rooms experience elevated temperatures, moisture vapor and active chemicals in the air, which significantly accelerates corrosion processes. Such buildings require increased protection, so paint or polymer materials, or an alloying metal are applied to the reinforcement. In this case, alloying can be enhanced by applying a film or paint, lining or facing layer.
• Environmental harm and accidents . The destruction of pipelines leads to the leakage of gas and oil products and other chemicals. Consequently, the natural environment is polluted and emergency situations arise. Therefore, it is important to promptly find defects in equipment and prevent negative effects.
• Deterioration of housing and communal services water pipes . Experts say that steel pipes last 3-4 times less than the standard period, which is due to the lack of reliable waterproofing. The catastrophic condition of underground pipelines affects the deterioration of the properties of drinking water and the epidemiological situation (foci of severe gastric and intestinal diseases, hepatitis). Poor quality of coatings and insufficient control over the condition of heating radiators leads to leaks, which cause burns to the population, damage to floor coverings and furniture, and flooding of the rooms below.
• Human sacrifices . In different countries, due to rusted metal structures, the stability of bridges is significantly reduced, which is one of the reasons for their collapse. As a result of the complete or partial collapse of the crossings, people are injured and killed.
• Economic damage . According to experts, losses from corrosion and the costs of combating it in economically developed countries are estimated on average at 3% of GNP.

Conclusion

Corrosion has its positive aspects. For example, there is a reason to buy a new stove or other household appliances, as well as kitchen utensils, when they rust. Because if there are no signs of corrosion, then many people feel sorry for throwing away old things. But the damage from corrosion is so catastrophic that it is necessary to protect against it.

Source: https://plusiminusi.ru/plyusy-i-minusy-korrozii-metallov/

Steel corrosion: how and why

Steel corrosion and the repair of damage associated with it is a worldwide problem worth hundreds of billions of dollars per year. However, the cost of corrosion is even more than just a financial cost. These include loss of natural resources, terrible disasters and many other indirect losses.

Reduce losses from corrosion

Corrosion is a natural phenomenon that can never be completely avoided. However, the fact that nothing can be done is also not true. Applying adequate steel corrosion protection at the outset of a project, such as hot-dip galvanizing, can significantly reduce these enormous annual corrosion losses.

Protecting steel from corrosion

The simplest way to protect steel from corrosion is to hot-dip galvanize it. Hot-dip galvanizing of steel has been widely used on a huge industrial scale for over 100 years to protect steel from corrosion. However, even now new markets for its use are emerging. At first, hot-dip galvanizing was considered only as a means of corrosion protection. Now it is used for many other reasons:

• low initial cost,
• strength,
• durability,
• availability,
• manifold,
• environmental friendliness and even
• aesthetics.

Metal corrosion process

Corrosion, often simply called rusting for steel, is the tendency of all metals to revert to their natural state—the original ore, which is a lower energy state.

Metal corrosion is an electrochemical process in the sense that it involves both chemical reactions and the flow of electrons.

The basic electrochemical process that drives the corrosion process is galvanic action, in which current is generated internally by physical and chemical reactions occurring between the components of a galvanic cell.

Galvanic corrosion

There are two main types of galvanic cells that lead to corrosion:

• bimetallic pair and
• concentration cell.

A bimetallic pair (Figure 1) is similar to a battery, consisting of two different metals immersed in an electrolytic solution. Electric current (electron flow) occurs when two electrodes are connected by an external continuous circuit.

Figure 1 – Bimetallic galvanic couple

The concentration cell consists of an anode and a cathode of identical metals or alloys, as well as a reverse electrical circuit. The electromotive force is provided by the difference in the concentration of solutions in contact with metals.

For corrosion to occur, a galvanic cell must have the following four necessary elements.

• Anode . An electrode at which negative ions are discharged and positive ions are formed. Other oxidative reactions may also occur. Corrosion occurs at the anode.
• Cathode . An electrode at which positive ions are discharged and negative ions are formed. Other reducing reactions may also occur. The cathode is protected from corrosion.
• Electrolyte . A conducting medium in which the flow of electrons is accompanied by the movement of matter. Electrolytes include aqueous solutions of acids, alkalis and salts.
• Reverse electrical circuit . A metal chain connecting the anode and cathode. Often passes under the coating over the base metal.

Eliminating any of these elements stops the flow of electrical current and corrosion does not occur.

In the so-called galvanic series, metals and alloys are arranged in order of decreasing electrical activity. The metals at the top of this series are "less noble" and have a greater tendency to lose electrons than the metals at the bottom of the series. Hot zinc coating takes advantage of this phenomenon: zinc (anode) is sacrificed to protect the underlying steel (cathode).

Steel corrosion

The corrosion process that occurs on unprotected carbon steel can be very complex due to differences in steel chemical composition and structural state, the presence of impurities, the presence of residual stresses, as well as varying environmental influences.

Figures 2a, 2b and 2c show the process of formation and replacement of cathode and anodic regions of steel rusting in air.

Figure 2a - A “mosaic” of microscopic areas - cathode and anodic - is formed on the surface of the steel. They connect electrically to each other through the steel underneath.

Figure 2b - Moisture in the air closes the electrical circuit between the anodes and cathodes. Due to differences in potentials, electric current begins to flow and destroys the anode sections. The iron ions that form at the anodes react with oxygen in the air to form flake-like iron oxide known as rust.

Figure 2c - When the anodic section corrodes, a new material of a different composition and structure appears on it. This leads to a change in electrical potentials and a change in the location of the cathode and anode sections. Over time, areas that were previously untouched by corrosion also begin to corrode, and the entire surface of the steel becomes rusty. This continues until all the steel has completely turned to rust.

The rate of corrosion of metals depends on factors such as temperature, humidity, pH of the electrolyte, the electrical potential of the metal and the electrical resistance of the anodic and cathodic sites.

Source: https://steel-guide.ru/zashhita-stali-ot-korrozii/korroziya-stali-kak-i-pochemu.html

Copper corrosion: causes of rusting and methods of protection

Copper products have been used by humans for several centuries. In pre-revolutionary times, the price of this metal was equal to the cost of gold, its production was so expensive. Now copper has become much cheaper, so in addition to jewelry, it is used to make dishes, interior accessories and other items.

Corrosion of copper, unlike iron, develops slowly due to its resistance to this phenomenon, and yet sometimes it is necessary to take measures to clean products from unsightly plaque.

Corrosion refers to the process of destruction of metal under the influence of aggressive environmental factors. To one degree or another, all metals and alloys rust, as a result of which rust and areas of integrity damage (holes) appear on them. Non-metals can also deteriorate over time: an example is the aging of rubber or plastic from interaction with oxygen, frequent contact with water, and temperature changes.

The main cause of corrosion is considered to be the thermodynamic instability of the metal to the influence of physical factors or chemical substances that are present in the contact medium. Compared to iron, copper oxidizes much less, but with increasing temperature this process accelerates significantly. When regularly exposed to an environment with temperatures above +100 degrees, any metal rusts several times faster.

Corrosive properties of copper

Copper is a metal with high plastic properties, having a red-golden color, and after removing the oxide film - slightly pinkish. In terms of electrical conductivity, it is second only to silver, and is also characterized by high thermal conductivity. Due to its low resistivity, copper is used in electrical engineering: it is used to make copper plates, wires, and motor windings.

Due to its high anti-corrosion qualities, the metal is included in alloys to improve their technical characteristics (bronze, brass and others). In a galvanic environment, copper becomes a cathode, enters into electrochemical processes and causes accelerated rusting of other metals.

Copper is an inactive chemical element, therefore it practically does not interact with air or water (fresh, sea). If the air is dry, an oxide film up to 50 million thick is formed on the surface of the material.

The copper product darkens, becomes brown or greenish, this is called patina. In some cases, patina is perceived as a decorative coating.

The intensity of corrosion is low upon contact with dilute hydrochloric acid, but when reacting with a number of other acids, with halogens, and “regia vodka,” the metal is oxidized to form copper carbonate.

Material failure conditions

Despite their resistance to damage, even copper products can rust under certain conditions. Such phenomena are least expressed in humid air, water, soil, and more pronounced in an acidic environment.

Corrosion can be seriously reduced by tinning - coating copper with a layer of tin. High-quality tinning provides reliable protection from damage, increases corrosion resistance, and makes the material not susceptible to high temperatures, rain, hail, and snow. The service life of tinned products is more than 100 years without loss of original properties.

Water influence

The rate of copper corrosion in water strongly depends on the presence of an oxide film on its surface, as well as on the degree of oxygen saturation of the water. The higher the content of the latter, the more intense the destruction of the material occurs. In general, copper is considered resistant to the harmful effects of salt and fresh water, and is only adversely affected by dissolved chlorine ions and low pH levels. Strength and resistance to rust allows the material to be used for the manufacture of pipelines.

If there is a brown or green oxide crust on the surface of a copper-plated product, destructive substances penetrate inside to a small extent. Typically, the oxide layer forms after 60 days of the metal being in water. The green crust (carbonate) is considered more durable, while the black (sulfate) crust is loose and less strong.

In sea water the level of corrosion is almost the same as in fresh water. Only when the movement of the liquid accelerates does corrosion become impact, and therefore more intense. Copper is a material that is not capable of being overgrown by marine microorganisms, because its ions are destructive to mollusks and algae. This property of the metal is used in shipping and fishing.

Exposure to acids and alkalis

Copper does not deteriorate in alkalis, because the material itself is alkaline, but acids are the most harmful in terms of their effects. The most significant and rapid corrosion occurs upon contact with sulfur and its acidic compounds, and nitric acid completely destroys the structure of the material.

Copper dissolves in concentrated acids, so it requires additional protection when manufacturing equipment for the oil and gas industry. For this purpose, inhibitors are used to slow down chemical reactions:

1. Shielding – form a film that prevents acids from reaching the copper surface.
2. Oxidizing - convert the top layer into an oxide, which will react with acids without harm to the metal itself.
3. Cathode - increase the overvoltage of the cathodes, thereby slowing down the reaction.

Corrosion in soil and moist air

The soil contains many microorganisms that produce hydrogen sulfide, so the environment here is acidic, and the rate of copper corrosion increases. The more the pH value deviates towards acidification, the faster the destruction processes occur.

If the soil is saturated with oxygen, the metal oxidizes, but rusts less. When copper products are left in the ground for a long time, they turn green, become loose and may even crumble.

Brief exposure to soil causes a patina that can be removed from the item.

Humid air has a bad effect on the condition of the material only with prolonged contact, and initially also causes the appearance of a patina (oxide layer). The exception is steam saturated with chlorides, sulfides, carbon dioxide - corrosion develops more rapidly in it.

Why copper products need to be cleaned regularly

Copper pots, ladles, and samovars are distinguished by high thermal conductivity, so heating in them proceeds evenly and food cooks faster. This determines the high popularity of products in everyday life. The need to clean copper objects is due to their loss of visual appeal over time. Products that are exposed to air or frequently heated up fade and lose their natural color especially quickly.

Oxide film - patina - is popular only where it is required to give things a vintage look, antique stylization. Otherwise, it spoils the appearance of dishes, utensils, jewelry and figurines. To remove oxide deposits, darkening elements and restore shine, you will have to periodically clean the items. Cleaning is also required to prevent harmful compounds that may be present in the black or green layer from getting into the food.

Effective Copper Cleaning Methods

Cleaning copper items is easy and does not require expensive products. Here are the most popular techniques used at home:

1. Ketchup. Take a little tomato ketchup, lubricate the product with it, and leave for two minutes. Then rinse with running water.
2. Dishwashing solution. Lather a household sponge with regular dish soap, wipe the surface thoroughly, and rinse with water. This method is best for items that are only slightly faded.
3. Lemon. Rub the copper surface with a slice of lemon, then go over it with a stiff bristled brush and wash with water.
4. Vinegar and flour. Pour a little vinegar into a cup, add flour until you get a medium-thick dough. Lubricate the copper with the dough, leave until dry, then remove the residue and rub the product with a soft cloth.
5. Vinegar and salt. Pour 9% vinegar into a stainless steel pan, add a little salt, and let it boil. Turn off the heat, put a copper object into the solution, do not remove it until the liquid cools. This method is suitable for heavily soiled surfaces.

Cleaning Copper Coins

Copper coins are antiques and are not issued in modern times. They often have to be cleaned to restore their attractive appearance. If the coin has been in contact with lead, the coating may be yellowish. In this case, it can be easily cleaned with table vinegar (9%). Green plaque is removed with a solution of citric acid (10%) or lemon juice, brown plaque is removed with ammonia or ammonium carbonate.

It must be remembered that sometimes a layer of patina gives coins a more noble and vintage look, so it is not always advisable to remove it. Some, on the contrary, try to artificially age money at home.

To do this, you need to take a liter of distilled water, 5 g of pharmaceutical potassium permanganate, 50 g of copper sulfate. Heat the solution without boiling, throw coins into it, and leave until the desired shade is achieved. To consolidate the effect, treat dried money with a mixture of benzene and alcohol (1:1).

Afterwards, the coins will acquire a beautiful aged appearance and will be able to decorate any collection of antiques.

Source: https://kraska.guru/specmaterialy/korroziya/rzhavchina-na-medi.html

Types of metal corrosion. A professional's view

Types of metal corrosion. A professional's view: features of the process.

As with any other phenomenon, in order to develop effective methods for combating corrosion, it is necessary to first determine the essence of this process, as well as the types of corrosion of metals.

In this case, a professional’s opinion will be more useful than ever, including in order to choose the optimal means of metal protection.

The main types of corrosion, the division of which is based on the mechanism of the process, are chemical and electrochemical corrosion, but there are many other classification criteria.

Types of corrosion depending on the corrosive environment

Gas corrosion, as the name implies, occurs in a gas environment with a minimum moisture content (optionally, at high temperatures). This type of corrosion is often found in the petrochemical industry, but in everyday life it is quite difficult to encounter it, just like with radiation corrosion caused by radiation. But atmospheric corrosion, which occurs in any moist gas, including air, is already more common.

Underground corrosion is a process that occurs when metal is in the soil or soil. Biocorrosion is formed as a result of the vital activity of microorganisms, and contact corrosion occurs when metals come into contact with different standard potentials.

Corrosion by current is divided into a process that occurs as a result of exposure to current from an external source and corrosion from stray current.

This type is typical for industry, but is not as dangerous as stress corrosion, the formation of which is influenced by two factors at once - the presence of a corrosive environment and mechanical stress. As a result, this can lead to a decrease in the fatigue limit of the metal.

Corrosion cavitation is close to this type, but in this case the corrosive influence of the external environment is combined with impact. Finally, fretting corrosion is a combination of a corrosive environment and vibration.

Types of corrosion depending on location

Continuous corrosion covers the entire surface of the metal, and it can spread at the same speed or be stronger in some areas (uniform and uneven corrosion). But alloys are often characterized by selective corrosion, which affects one component or structural component of the alloy. An example in this case is dezincification of brass.

Local corrosion often occurs, affecting individual areas of the metal. At the same time, there can also be different types of metal corrosion. The look of a professional will immediately determine which of the local damages is ulcers, having the appearance of a shell deeply buried in the thickness of the metal, or is pitting corrosion.

Subsurface corrosion causes delamination of metal products, quickly penetrating deep from the surface. Intergranular corrosion determines the destruction of metal along the grain boundary and is dangerous because it practically does not change the appearance of the metal, however, significantly reducing its strength. Finally, crevice corrosion depends on the design features of the product and occurs in places of fastenings, gaps, etc.

The Docker Chemical GmbH Rus company offers a large selection of corrosion control products. NITTRON - Neutral rust converter with enhanced inhibitors

Source: http://DockerSpb.ru/udalenie-rzhavchiny/vidy-korrozii-metallov-vzglyad-professionala

Which metal is not susceptible to corrosion?

The definition of metal corrosion should be understood as the process of its destruction as a result of the negative impact of environmental factors - oxygen, water, carbon oxides and sulfur oxides in the air, water-salt solutions (for example, sea water).

In general, today there are two types of corrosion:

• chemical (metal destruction occurs under the influence of non-electrolytes (oxidizing agents) even at normal temperatures. For example, oxygen, chlorine, other types of gas);
• electrochemical (destruction of metal occurs as a result of exposure to electrolyte solutions, for example, humid air).

Technical iron is the most susceptible to corrosion. At the same time, there are metals and alloys that are resistant to such negative effects.

Ways to combat corrosion

Today, there are several ways to combat corrosion. The main list includes the protective coating of a metal surface with paints or a layer of another metal (silver, chromium, tin, zinc), as well as the addition of metal impurities to create alloys. The choice between them determines the purpose and scope of application. For example, in the food industry, metals coated with paints and varnishes to combat corrosion cannot be used.

Corrosion-resistant metals and alloys

So, it was said above that technical hardware is most vulnerable to the negative influences of the environment. It is important to note that chemically pure iron practically does not rust (as evidenced by many examples of well-preserved historical iron artifacts).

The question arises: why shouldn't modern society use pure iron? The fact is that without impurities, iron loses a number of its properties, and therefore is not suitable for use in a number of areas.

In their pure form, aluminum, copper, tin, zinc, and brass are not subject to corrosion.

Sometimes they are used to coat metal in order to prevent its direct contact with the environment. Thus, metal products are chrome-plated, nickel-plated, galvanized, etc. For example, coating iron with tin (tinning) produces tinplate, which is often used in the production of canning containers.

Light stainless alloys traditionally contain impurities of the same corrosion-resistant metals - aluminum, titanium, cupronickel, brass, bronze.

Areas of application of corrosion-resistant alloys

The resulting alloys with an admixture of titanium are actively used in the aerospace industry and recreational sectors, nickel silver in medicine, and brass in the chemical industry and mechanical engineering.

In general, by combining various metals in different ratios, you can obtain a huge number of corrosion-resistant alloys suitable for use in a number of areas of modern human activity.

We, sellers of rolled metal, are faced with this obsession - rust, like no one else. And we know for sure the harm caused by corrosion. In this article we will say a few words about this problem, its manifestations, its scale.

Damage, damage

Everyone has seen those orange-brown or yellowish rust spots on metal parts. The economic damage from metal corrosion is enormous. In the USA and Germany, the estimated damage from corrosion and the costs of combating it amount to approximately 3% of GDP. At the same time, metal losses, including due to the failure of structures, products, and equipment, amount to up to 20% of the total steel production per year. For Russia, exact data on losses from corrosion have not been calculated.

It is known for certain that it was rusted metal structures that caused the collapse of several bridges in the United States, including with numerous casualties. Environmental damage is also extremely unpleasant: leakage of gas and oil due to the destruction of pipelines leads to environmental pollution.

Source: https://moreremonta.info/strojka/kakoj-metall-ne-podverzhen-korrozii/

Stress Corrosion Cracking

Stress corrosion cracking (SCCC) is cracking caused by the combined effects of tensile stress and environmental corrosivity. The effects of SCR usually manifest themselves on the properties of the metal in the form of (so-called) “dry” cracking or in the form of a decrease in the fatigue threshold of the material. Tensile stresses can be either in the form of directly applied stress or in the form of residual stress.

Stress corrosion cracking (SCCC) is characterized by cracks that propagate either transcrystalline or intergranularly (along grain boundaries). There are several types of stress corrosion cracking (SCC), such as: chloride-induced SCC and hydrogen sulfide (H2S)-induced SCC.

Stress corrosion cracking (SCCC) is the result of the combined action of three factors:

• Tensile stress in metal
• Aggressive environments - especially chloride-containing or hydrogen sulfide-containing (H2S) environments. Chloride-induced CRPN usually occurs at temperatures above 60 °C (140 °F)
• Use of materials susceptible to stress corrosion cracking (SCCC)

Stress corrosion cracking of stainless steel products in a chloride-containing environment is preceded by pitting, which occurs when the stainless steel does not have sufficient resistance to pitting corrosion. Thus, cracks appear in the metal, which increase over time. This ultimately leads to loss of contact between the metal grains.

Causes of Stress Corrosion Cracking (SCCC)

The formation of cracks in metal occurs in places of stress.

This process involves accelerated corrosion along a path of increased corrosion susceptibility, with the bulk of the material generally not undergoing corrosion degradation. Most often, the active path is along the grain boundary, where impurities can impede passivation.

Thus, some form of crevice corrosion may develop, causing the grain boundary to corrode while the surface around the crack remains intact. This process can occur in the absence of load and lead to intergranular corrosion, which will spread evenly over the surface of the material.

The effect of the applied stress is likely to be mainly to open the cracks, and therefore to facilitate the process of spreading corrosion products away from the crack tip, which, in turn, will also lead to an acceleration of the corrosion process.

Corrosion processes along the active path are inherently limited by the corrosion rate of the metal at the crack tip: this limits the maximum crack growth rate to 10-2 mm/s, but crack growth rates are often much lower, approximately around 10-8 mm/s (about 1 mm within 3 years) or even less.

Cold deformation and forming, welding, heat treating, machining and grinding can cause residual stresses. The magnitude and importance of such stresses is often underestimated.

Residual stress resulting from welding work tends, as a rule, to the (conditional) yield strength. The increase in corrosion products in enclosed spaces can also cause significant stress and this aspect should not be overlooked.

SCC typically occurs as a result of a combination of three factors: alloy sensitivity, adverse environmental conditions, and loading.

As a rule, most of the surface is not subject to the destructive forces of corrosion, but small cracks can still penetrate into the material. Based on their microstructure, these cracks can have intercrystalline or transcrystalline morphology. Cracks under the influence of CRPN have a macroscopically fragile appearance.

CRF is classified as a catastrophic type of corrosion due to the fact that detection of such small cracks can be very difficult, and the damage from their occurrence is very difficult to predict. Experimental statistics on CRF are notoriously variable.

Horrible destruction can occur completely unexpectedly even with minimal overall loss of material.

Micrograph (X500) illustrates intercrystalline SICC in a heat exchanger tube with a grain boundary crack.

Photomicrograph (X300) illustrates SCC in an AISI 316 stainless steel chemical treatment piping system. Chloride stress corrosion cracks in austenitic stainless steel are characterized by multiple branched zippers.

CRPN under the influence of chloride

This is one of the most important forms of stress corrosion: it is related to chloride corrosion in the nuclear industry.

Chloride corrosion is a type of intergranular corrosion that occurs in austenitic stainless steel under tensile stress in the presence of oxygen, chloride ions and high temperature.

It is believed that it begins with chromium carbide accumulating along boundaries that leave the metal exposed to corrosion. This form of corrosion is controlled by maintaining low levels of chloride and oxygen ions in the environment and by using mild steel.

H2S-Induced Stress Corrosion Cracking
Process fluids used in the oil and gas industry for wetting and oxidation applications often contain some amount of hydrogen sulfide (H2S). When considering the risk of corrosion caused by acidic process fluids, it is necessary to take into account not only the pH value, but also the partial pressure of H2S. In addition, it is worth paying attention to the temperature, oxygen and chlorine content, as well as the presence of any solid particles (such as sand)

H2S stress corrosion cracking has been confirmed to occur most frequently at temperatures around 80°C (176°F), but cracking can occur at temperatures below 60°C (140°F).

How to reduce the risk of stress corrosion cracking (SCCC)

The risk of stress corrosion cracking (SCCC) can be minimized through good design of equipment and fixtures. It is especially important to avoid concentration of mechanical tensile stress, which appears on sharp edges and cutouts. In many cases, stress corrosion cracking (SCCC) problems can be solved by properly selecting the appropriate material.

The most effective means to prevent CRPN are:

1. proper use of appropriate metals;
2. voltage reduction;
3. elimination of critical elements from the environment: hydroxides, chlorides and oxygen;
4. avoiding stagnant zones and crevices in heat exchangers where chloride and hydroxide can concentrate. Low alloy steel is less susceptible than high alloy steel, but it can be susceptible to SRC due to water containing chloride ions.

Austenitic steels like ASTM304 and 316 have limited resistance to stress corrosion cracking (SCCC) even at very low chlorine content and low temperatures.

Summary of Stress Corrosion Cracking (SCCC)

Source: http://stroy-metall.ru/stati/korrozionnoe-rastreskivanie-pod-napryazheniem