Stress Corrosion Cracking: Definition, Causes, Types, Prevention and Resistant Materials

Stress corrosion cracking (SCC) is a dangerous mode of material failure that happens when a susceptible material is exposed to both tensile stress and a specific corrosive environment. This combination of factors can lead to sudden and unexpected failure, even in materials that seem sound. For manufacturers and engineers, understanding stress corrosion cracking is very important. This is especially true for cast components, as scc in castings can be a significant problem.

Cast parts have a unique internal structure and can contain stresses left over from the casting process, which makes them more prone to this type of damage. This article will define stress corrosion cracking in castings, explain its causes, discuss different types, and show you how to prevent it by choosing the right materials.

What is Stress Corrosion Cracking (SCC)?

Stress corrosion cracking is a type of material degradation that occurs when a material susceptible to corrosion is subjected to tensile stress. The failure is not caused by corrosion or stress alone, but by their combined action. This effect is a common reason for the sudden and brittle failure of components that are normally considered ductile. The process often leaves a component looking undamaged on the outside, hiding the deep cracks within its structure.

The Mechanism

The basic mechanism of SCC begins with the corrosive environment causing very small, localized pits or fissures on the surface of the material. This initial damage acts as a point of weakness.

When a tensile stress is present, it becomes concentrated at the tip of this small crack. The concentrated stress helps to break the atomic bonds at the crack tip, allowing the crack to grow deeper into the material. This process repeats itself: the corrosion creates a new point of weakness at the crack tip, and the stress widens it, causing the crack to grow.

This cycle of corrosive attack and mechanical stress leads to the slow, steady propagation of the crack until the remaining material can no longer support the load and fails suddenly.

Microscopic View

When viewed under a microscope, the path that a stress corrosion crack takes can provide clues about the failure. The cracking can follow one of two paths. Intergranular cracking happens when the crack follows the boundaries between the material’s grains.

This is common in castings because the grain boundaries can be areas where certain elements have gathered, making them more susceptible to corrosive attack. The second path is transgranular cracking, where the crack cuts straight through the grains themselves. The type of cracking path depends on the specific material, the corrosive environment, and the amount of stress.

For stress corrosion cracking in castings, the presence of internal residual stresses and the non-uniform grain structure can make the material more prone to intergranular crack growth.

Types of Stress Corrosion Cracking

There are several distinct types of stress corrosion cracking, each with a unique way of damaging a material. The specific type of cracking depends on the combination of the material, the type of stress, and the corrosive environment.

Intergranular Stress Corrosion Cracking

As the name suggests, this type of SCC happens when the cracks follow the grain boundaries of a material. In many metals and alloys, grain boundaries are chemically different from the main body of the grains.

This can happen due to the segregation of certain elements or the formation of different phases during the cooling of a casting. These differences make the grain boundaries more vulnerable to corrosive attack. When stress is applied, the corrosion takes advantage of these weaker pathways, causing the crack to grow along the boundaries.

This kind of stress corrosion cracking in castings is common in certain stainless steel alloys that have been improperly heat treated, which can lead to chromium depletion at the grain boundaries.

Transgranular Stress Corrosion Cracking

In this type of SCC, the cracks do not follow the grain boundaries but instead cut straight through the grains themselves. The path of the crack is usually straight and can look like a series of interconnected lines. The exact mechanism for this type of cracking is complex, but it is often connected to a combination of corrosion and mechanical stress that breaks the atomic bonds within the grains. A classic example of transgranular SCC is the failure of some brass alloys when exposed to ammonia environments.

Hydrogen-Induced Stress Corrosion Cracking

This type of SCC involves hydrogen atoms, which can enter the metal from a corrosive environment. For example, hydrogen can be created when certain acids react with the metal. The hydrogen atoms then move into the material’s crystal structure, where they can collect at areas of high stress, such as at the tip of an existing flaw. The presence of hydrogen in these areas reduces the material’s ability to resist fracture.

When a tensile stress is present, the material becomes more brittle and the crack can grow very quickly. Hydrogen-induced cracking is a significant problem in many high-strength alloys and can be a concern for certain cast components used in hydrogen-rich environments.

Chloride Stress Corrosion Cracking

Chloride SCC is a widespread problem that mostly affects stainless steels and some other nickel alloys. It is triggered by the presence of chloride ions, which are common in saltwater, industrial coolants, and de-icing salts.

These ions can damage the protective oxide layer on the surface of the metal, leading to localized pitting corrosion. These pits then serve as starting points for stress corrosion cracking. This is a major concern for scc in castings used in marine or chemical processing environments.

Ammonia Stress Corrosion Cracking

Ammonia SCC, sometimes called “season cracking,” is a form of stress corrosion cracking that affects copper alloys, especially brass. The name comes from early failures of brass ammunition casings that cracked in storage during the rainy seasons in tropical climates due to the presence of moisture and traces of ammonia from nearby animal waste. The ammonia corrodes the brass, and when combined with tensile stress, it causes intergranular cracking.

Caustic Stress Corrosion Cracking

Caustic SCC, also known as “caustic embrittlement”, happens when carbon and low-alloy steels are exposed to hot, concentrated solutions of sodium hydroxide (NaOH) or other strong alkalis. The corrosion happens at a fast rate in these environments and can cause a rapid crack to form.

The cracking often follows the grain boundaries, making it a type of intergranular stress corrosion cracking. This type of damage can happen in boilers, reactors, and pipelines, and it is a known issue for cast steel components used in these conditions.

Sulfide Stress Corrosion Cracking

Sulfide SCC is a significant problem in the oil and gas industry. It affects steels that are exposed to environments containing hydrogen sulfide (H2​S). The hydrogen sulfide reacts with the steel, creating hydrogen atoms that enter the metal.

As explained earlier, this hydrogen makes the material more brittle, leading to a much higher chance of fracture when a tensile stress is present. This is a very serious form of stress corrosion cracking in castings used for oilfield equipment, where both high stress and H2​S are often present.

The Root Causes of SCC in Castings

Stress corrosion cracking requires three conditions to be met at the same time: a tensile stress, a specific corrosive environment, and a susceptible material. Castings are particularly vulnerable to these conditions due to their unique properties and manufacturing process.

Tensile Stress

Tensile stress can be either applied or residual. Applied stress is the load put on the component during its use, such as a cast pump housing that is under pressure. This stress can cause the metal’s internal structure to deform and become more reactive to the environment. Residual stress is the stress that remains within a material after it has been shaped or manufactured.

Castings often have high levels of residual stress due to uneven cooling after they are poured. As some parts of the casting cool and shrink faster than others, internal stresses can build up. This can make the component more susceptible to stress corrosion cracking without any external load.

Corrosive Environment

A corrosive environment is the second key ingredient for SCC. The specific type of corrosion needed to cause stress corrosion cracking depends on the metal. For example, stainless steels are very sensitive to chloride ions, while copper alloys are sensitive to ammonia.

Castings used in industrial settings, marine environments, or chemical plants are often exposed to environments that contain these specific corrosive agents. Even small amounts of these agents can be enough to start the cracking process, especially if the metal is already under stress.

Material Susceptibility

The susceptibility of a material is the third factor. A material’s internal structure and chemical composition determine how it will react to stress and a corrosive environment.

For castings, the microstructure can be a significant factor. The grain boundaries in cast materials can be a location where impurities or certain elements gather during solidification. These areas can be more reactive to corrosion than the rest of the material, which can lead to intergranular scc in castings. The presence of certain elements in the alloy can also increase susceptibility. For instance, low-nickel stainless steel castings are more prone to chloride SCC than those with higher nickel content.

Prevention and Mitigation Strategies for Castings

Since stress corrosion cracking requires the presence of stress, a corrosive environment, and a susceptible material, prevention strategies focus on reducing or eliminating at least one of these factors.

Material Selection

Choosing the right material is the first and most direct way to prevent SCC. The goal is to select an alloy that is resistant to the specific corrosive environment the component will encounter. For example, for parts that will be in contact with chloride solutions, a duplex stainless steel, such as grade 2507, with a higher nickel content is a better choice because nickel helps to make the material more resistant to chloride attack. Other materials like some aluminum and titanium alloys are naturally more resistant to stress corrosion cracking.

Stress Reduction

Controlling the amount of tensile stress is another way to prevent SCC. This can be done by changing the design of the component to spread out the load, which lowers the applied stress. To address residual stresses that are left over from manufacturing, castings can be put through a post-casting heat treatment. Processes like annealing or stress relieving heat the casting to a specific temperature and then allow it to cool slowly. This controlled process helps to rearrange the internal grain structure and relieve the locked-in stresses, making the component much less likely to suffer from stress corrosion cracking in castings.

Environment Control

If you cannot change the material or the stress, you can try to control the environment. This might involve adding chemicals called inhibitors to the environment. Inhibitors can reduce the corrosive nature of the fluid that the casting is in contact with. Other methods include lowering the temperature or pressure of the system, which can slow down the rate of corrosion. Controlling the environment is a common strategy for preventing scc in castings in large industrial systems where replacing components is difficult.

Protective Coatings

Applying a coating to the surface of the casting is a way to separate the metal from the corrosive environment. A properly applied coating, such as a special paint, a polymer layer, or a metallic plating, acts as a barrier. This barrier stops the corrosive agent from reaching the metal’s surface, so the first step of the stress corrosion cracking process cannot begin. It is important that the coating has no defects, like scratches or pinholes, because even a small break in the barrier can give the corrosive agent a way to attack the material underneath.

Materials that Resist SCC in Castings

Selecting a material that has a natural resistance to a corrosive environment is a very effective way to avoid stress corrosion cracking. Different alloys have different properties that make them more or less susceptible to this kind of damage. Below I have listed some typical corrosion resistant material types, particularly useful for resisting stress corrosion cracking.

Duplex Stainless Steels

Duplex stainless steels are a good choice for resisting stress corrosion cracking, especially in environments that contain chlorides. These alloys have a microstructure that is made up of both austenitic and ferritic phases in a roughly equal amount. The ferritic phase gives the material good resistance to pitting and chloride attack, while the austenitic phase gives it strength and toughness.

The combination of these two phases makes duplex stainless steel castings a popular choice for marine applications, as they have a much higher resistance to scc in castings than conventional austenitic stainless steels like 304. A common example is grade 2205, which is widely used in chemical tankers and desalination plants.

High-Nickel Alloys

High-nickel alloys, or nickel-based superalloys, are very resistant to a wide range of corrosive environments. Nickel helps to stabilize the protective oxide layer on the metal’s surface and makes it more resistant to localized pitting and crack initiation. Alloys with a high nickel content, such as Inconel and Hastelloy, are used in very aggressive chemical processing environments where the risk of stress corrosion cracking is high. These castings can handle conditions of high temperature and high pressure without falling victim to SCC.

Precipitation Hardening Stainless Steels

Martensitic stainless steels, which are strengthened through a heat treatment process called precipitation hardening, can also be used to resist SCC. While they are not as resistant as duplex or high-nickel alloys, their resistance can be improved by a specific heat treatment. For example, the martensitic stainless steel 17-4PH can be heat treated to a higher tempering temperature, which lowers its strength slightly but greatly improves its resistance to SCC. This is a common strategy for cast components that need high strength and some resistance to stress corrosion cracking, such as valve bodies and pump impellers.

Titanium and Its Alloys

Titanium and its alloys are known for their very high resistance to corrosion, including stress corrosion cracking, in a wide range of environments. They form a very stable and tough oxide layer on their surface, which makes them highly resistant to many corrosive agents. Titanium castings are used in aerospace, marine, and chemical applications where resistance to chlorides is a main concern. For example, titanium parts are used in offshore oil and gas equipment due to their ability to withstand the harsh saltwater environment without suffering from scc in castings.

Conclusion

Stress corrosion cracking is a complex form of material failure that can be very damaging to cast components. The process begins with a material that is susceptible to a specific corrosive environment while also being under tensile stress. The interaction of these three factors can lead to cracks that are not always visible on the surface.

Understanding the different types of SCC, from intergranular to those caused by chlorides or sulfides, is important for avoiding unexpected failure. The best approach to stopping stress corrosion cracking in castings is to control these three factors. This can be done by changing the design of a component to reduce stress, altering the environment to be less corrosive, or by using a protective coating. The most reliable method, however, is to select a material that is naturally resistant to the conditions it will be exposed to.

Materials like duplex stainless steels, high-nickel alloys, and titanium and its alloys are all excellent choices for preventing scc in castings. By taking a detailed approach to material selection and component design, engineers and manufacturers can help to stop this type of failure from happening.

Taiyuan SIMIS Investment Casting Co., Ltd is a leading metal casting foundry in China, with over 40 years of metalworking experience. At our facilities, we offer investment casting, sand casting, and die casting to produce castings from a wide range of materials. Thanks to our expertise in the foundry industry, we can help clients choose the most appropriate materials for different use cases, including material grades that resist stress corrosion cracking in corrosive settings. Contact us today to request a quote.