Stress corrosion cracking.
Source: Stress Corrosion Cracking/Industrial Metallurgists, LLC.
The source of the tensile stress which causes SCC may be externally applied stress or residual stresses. Externally applied stresses arise from applied mechanical loads such as tensile or bending loads. Residual stress is an internal stress that exists in a metal without an external load being applied. Residual stresses can result from cold working, heat treating, or welding. Increasing the yield strength of a metal is one way to improve its resistance to SCC because the threshold stress for SCC increases as the yield strength increases. The yield strength can be increased through alloying, heat treating, cold‐working, and combination of these approaches. There is one very important consideration when increasing the yield strength. The increase in strength must not be accompanied by a significant reduction of the metal’s toughness, because decreasing the toughness will have a detrimental effect on a metal’s resistance to SCC and on its fracture toughness.
The environmental factors, such as pH and temperature, also influence the severity of SCC. By controlling the environmental factors, SCC can be controlled.
Chloride SCC
One of the most important forms of stress corrosion that concerns the nuclear industry is chloride stress corrosion. Chloride stress corrosion is a type of intergranular corrosion and occurs in austenitic stainless steel under tensile stress in the presence of oxygen, chloride ions, and high temperature. It is thought to start with chromium carbide deposits along grain boundaries that leave the metal open to corrosion. This form of corrosion is controlled by maintaining low chloride ion and oxygen content in the environment and use of low carbon steels.
Hydrogen Damage
Hydrogen can diffuse into metals and alloys from several sources during processing and subsequent service. These sources include the dissociation of moisture during casting and welding, thermal decomposition of gases, and pickling and plating operations. Hydrogen can also be generated from cathodic reactions during corrosion in service and from cathodic protection measures by sacrificial anodes and impressed current.
The effects of hydrogen are well known in ferritic and martensitic steels, where it can diffuse to suitable sites in the microstructure and develop local internal pressure resulting in the characteristic form of hydrogen embrittlement. In low carbon steels, which have inherent ductility, hydrogen may not give rise to cracking but will cause blisters to develop at inclusions. This can lead to delamination in‐plate due to the directional nature of the inclusions. Steels for sour gas service, where the environment contains wet hydrogen sulfide, must have very low sulfur levels or have been treated with additions to control the shape of the inclusions during deoxidation to minimize the danger of hydrogen embrittlement and blistering.
Failure Due to Hydrogen Damage
Failure is time‐dependent and occurs at low rates of strain as the load‐bearing cross section is reduced during slow crack growth in the embrittled region. Susceptibility for embrittlement is higher in alloys with higher yield strengths, i.e. those that are cold‐worked, age‐hardened or in their martensitic form. The sites at which hydrogen is trapped include the original austenite grain boundaries and the interfaces between the matrix and non‐metallic inclusions, for example, manganese sulfides. These then result in both intergranular cracking (with separation at the prior austenite boundaries) and transgranular cracking (flaking or quasi‐cleavage) which is associated with the inclusions. Hydrogen can assist in the propagation of corrosion fatigue cracks and can also cause sulfide stress corrosion cracking in ferritic and martensitic steels, including the stainless grades.
Addressing Hydrogen Damage
The first and foremost method for preventing hydrogen damage is the obvious option of preventing direct contact between a metal and the hydrogen‐containing agent. Controlling the environment during operations such as casting and melting will allow for the exposure to hydrogen to be moderated. Other than preventing exposure, it is also possible to give the metal or alloy a metallurgical treatment, which would serve to reduce the susceptibility of the material to damage caused by hydrogen, chemical means, or otherwise.
Corrosion Damage
Corrosion damage can be apparent in many different ways, including loss of material, surface pitting, and the buildup of corrosion deposits, but it is convenient to classify corrosion by visual observation of the corroded material before any cleaning is conducted. There are generally considered to be eight basic forms of corrosion.
General attack (uniform corrosion)
Galvanic corrosion
Crevice corrosion
Pitting
Intergranular corrosion
Selective leaching
Stress corrosion
Erosion‐corrosion
Although the distinctions between the eight basic categories of corrosive attack have become blurred, particularly when fundamental mechanisms are considered, this classification may help (at least in the first instance) to simplify the analysis. The identification of the factors associated with the forms of corrosion can guide failure investigators. A listing of the most important factors would ensure that engineers with little or no corrosion training are made aware of the complexity and multitude of variables involved.
Temperature can affect the corrosion behavior of materials in different ways. If the corrosion rate is only controlled by the metal oxidation process, the corrosion rate will increase exponentially with an increase in temperature. The higher the fluid temperature the faster the rate of oxidation. Experience shows that corrosion is more pronounced in hot water lines. Galvanic corrosion, also known as electrolysis, occurs when different metals come into contact with each other. Chemical composition of the fluid may have differing effects on the corrosive forces at play. When water velocities exceed 4 ft/s in oversized circulation pumps, installation of undersized distribution lines, multiple or abrupt changes in the direction of the pipe, corrosion may take place. The pH of a solution is also an important factor in the corrosion of materials.
How to Control Corrosion
There are many ways to organize and operate successful corrosion management systems, each of which is asset specific depending on factors such as Design, Stage in life cycle, Process conditions, and Operational history. The corrosion policy provides a structured framework for identification of risks associated with corrosion, and the development and operation of suitable risk control measures.
Corrosion is caused by a chemical reaction between the metal and gases in the surrounding environment. By taking measures to control the environment, these unwanted reactions can be minimized. Sacrificial coating involves coating the metal with an additional metal type that is more likely to oxidize, hence the term “sacrificial coating.” There are two main techniques for achieving sacrificial coating: cathodic protection and anodic protection. The most common example of cathodic protection is the coating of iron alloy steel with zinc, a process known as galvanizing. Anodic protection involves coating the iron alloy steel with a less active metal, such as tin. Tin will not corrode, so the steel will be protected as long as the tin coating is in place.
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