Stress Corrosion Cracking and Grain Boundary Widmanstatten Carbides
Abstract
Stress corrosion cracking (SCC) is a problem that many materials face when exposed to certain environmental conditions, such as high temperatures or harsh chemicals. This type of materials failure is most common in metals and alloys, particularly stainless steels and high temperature alloys. As the name implies, SCC occurs when stress and corrosion combine forces to produce a crack in the material. A unique form of SCC, referred to as Widmanstätten Grain Boundary Carbides (WGBC) occurs when grain boundary carbides form in the material, depositing along and perpendicular to the grain boundaries. WGBC are created when the surface of the material is exposed to high temperatures and corrosive solutions, resulting in a thick layer of carbide along the grain boundaries. In this paper, we discuss the formation of WGBC in detail, and analyze how the process affects the mechanical strength and durability of the material. Additionally, we will analyze how SCC affects the performance of WGBC in various environmental conditions and how it can be prevented.
1 Introduction
Stress corrosion cracking (SCC) is a type of corrosion failure that occurs when metal experiences tensile stress and corrosive environments simultaneously over a period of time. It is a slow process but is progressive and can cause extensive damage to components, leading to structural failure and even catastrophic events. SCC can occur in a variety of materials, but is most common in stainless steels, high-strength steels, and aluminum alloys.
SCC is not a new phenomenon and has been studied for many years. It is well accepted that the mechanism behind SCC is the combined effect of tensile stress and a corrosive environment, with any type of metal exposed to such conditions being susceptible. However, some unique and intriguing forms of SCC have been studied over the past few decades. One such form, referred to as Widmanstätten Grain Boundary Carbides (WGBC) occurs when grain boundary carbides form in materials, depositing along and perpendicular to the material grain boundaries.
2 Formation of WGBC
WGBC occurs when a material containing grains of carbide-forming elements is exposed to a corrosive medium or high temperatures. In this situation, the carbide-forming elements, such as silicon and titanium, dissolve from the material and are incorporated into the grain boundary, forming carbides which deposit along and perpendicular to the grain boundary. These carbides are then further depleted from the material, resulting in a layer of carbide along the grain boundary.
The resulting carbide layer is referred to as Widmanstätten grain boundary carbides (WGBC). WGBC exhibit varying numbers of layers depending on the material and environment it is exposed to. Generally, these layers are less than 10 µm thick.
3 Effects of WGBC on Material Performance
WGBC affects the mechanical strength and durability of the material it is found in, making it weaker and more susceptible to stress corrosion cracking. One explanation for this is that WGBC is brittle and can act as a source of stress concentration when exposed to tensile stress. Additionally, the thinner grain size associated with WGBC decreased toughness, thereby reducing the material’s ability to resist corrosion.
The formation of WGBC can also decrease the corrosion rate of the material, as carbides can act as a diffusion barrier, preventing the corrosive environment from reaching the grain boundaries and reducing the severity of SCC.
4 SCC and WGBC
Stress corrosion cracking is the most common form of failure in materials containing WGBC, as the combined effect of tensile stress and a corrosive environment exacerbates the damage caused by the carbide depositing layer. The type of SCC seen in materials containing WGBC is fracture-initiated SCC, which occurs in the area of the Widmanstätten grain boundary and then propagates across the material in the form of a crack. This type of SCC is a more severe form of SCC and can lead to complete structural failure if not prevented.
The severity of SCC in materials containing WGBC can also be increased by environmental factors such as elevated temperatures and elevated levels of chloride ions. Elevated temperatures reduce the material’s toughness, making it easier for cracks to form and propagate. Chlorides decrease the material’s resistance to corrosion, making it more susceptible to cracking.
5 Prevention of SCC in Materials with WGBC
There are various steps that can be taken to prevent SCC in materials with WGBC, such as:
• Eliminate or reduce tensile stress by using proper design techniques
• Ensure proper welding techniques when joining components
• Use proper corrosion-resistant materials for construction
• Incorporate stress relief techniques into components and parts
• Select materials with good SCC resistance
• Avoid exposing materials to corrosive environments and elevated temperatures
• Conduct regular inspections to detect and mitigate SCC
Conclusion
Stress corrosion cracking in materials containing Widmanstätten grain boundary carbides (WGBC) can have devastating consequences and cause catastrophic failure if not prevented. The Severity of SCC in materials containing WGBC can be decreased by controlling the environment, using proper design and fabrication techniques, reducing tensile stress, and selecting materials with good SCC resistance.
