Structural Integrity and Fitness-for-Service: Degradation Report

This report provides an overview of structural integrity and fitness for service, focusing on common degradation mechanisms that impact the strength and service life of structural members. It details various processes, including hydrogen attack (both low and high temperature), structural embrittlement (including liquid metal and metal-induced embrittlement), and mechanical distortion (such as twisting, bulking, and shrinkage). The report also covers fatigue and high-temperature creep, explaining how these processes lead to permanent deformation. Furthermore, it explores corrosion, outlining its different types (general attack, galvanic, environmental cracking, and localized corrosion) and discussing methods to mitigate its effects. The report references key literature on these topics, providing a comprehensive understanding of structural degradation and its implications. This report is based on the assignment brief provided.

STRUCTURAL INTEGRITY AND FITNESSS FOR SERVICE.
Common degradation mechanisms of structural members cause change in the strengths and service life
of the structures. There are various types of these degradation processes. Hydrogen attack is a process
of metal degradation that occurs due to interaction with hydrogen. Metal affected by hydrogen are
zirconium, aluminum, steel and titanium. The hydrogen may come from the environment in which the
structure is operating in. it may be categorized into two. Low temperature hydrogen attack (LTHA) or
high temperature hydrogen attack (HTHA). HTHA happens on metals operating in hydrogen
environment and above 400 OC such as in petroleum refineries, high pressure steam boilers and
chemical mixers. The hydrogen reacts with the carbon used for alloying the steel to form organic gases
such as methane. This results to Surface decarburizing which leads to a decrease in hardness. Ductility of
the steels is increased due to removal of alloying carbon. Members affected by hydrogen attack have
rough and irregular surfaces. (Nelson, 2015).
Structural embrittlement is the loss of material ductility caused by change in temperatures, change in
material composition, change in grain boundaries or change in stresses within the structure. The
material becomes brittle. They are of various types. The embrittlement which is caused by liquid metals
is called Liquid Metal Embrittlement while those caused by solids is known as Metal Induced
Embrittlement. Other common types of embrittlement are stress corrosion cracking, sulfide stress
cracking and hydrogen embrittlement. When embrittlement happens, the structural member becomes
more brittle. (Gillespie, 2012).
Mechanical distortion of structures happens when there are non-uniform stresses occurring at a point
on the members. This non uniform stresses cause the material to experience different expansion and
contraction rates. Mechanical distortion is commonly associated in welding. During welding,
compressive stresses are introduced in the parent material but upon cooling, tensile stresses occurs.
These expansion and contraction of the metals causes mechanical distortion. It can happen in six forms:
twisting, bulking, angular distortion, transverse shrinkage and longitudinal shrinkage. If the stresses
generated in the material by mechanical distortion go beyond the yielding strength, then the structural
member fails. Mechanical distortion is detected by fluorescence imaging. (Shimozawa, 2012).
Fatigue and High temperature creep are degradation process where the structural members
permanently deform slowly due to temperature and exposure to high levels of stresses for a long time.
Creep happening at high temperatures is called high temperature creep. Creep happens in forms such as
climb, bulk diffusion, thermally activated glide, grain boundary diffusion, and climb assisted glide (Cabet,
2013). Bulk diffusion occurs members are at low stresses but working at elevated temperatures.
Structural members should be prevented from creep by selecting the best grain size and crystal
structure that will reduce the rate of diffusivity. Fatigue happens in members that experience repetitive
stresses for a long period such as shafts, bearings and cranks. (Wang, 2018).
Corrosion is a process of metal deterioration caused by chemical or electrochemical reaction between
metal and the surrounding. Corrosion is of four types. General attack corrosion, galvanic corrosion,
environmental cracking and localized corrosion. General attack corrosion happens when chemical
reactions occur on the surface of the metal structure and “eating it away” thus reducing the thickness of
the structures. Corrosion happening only in some portions of the structure are known as localized
corrosion. Pitting is a form of localized corrosion where there is creation of pores on the structure

surface. Crevice corrosion happens when corrosive substance stagnates at a point, such on gaskets.
When water gets trapped under a coating, filiform corrosion can occur. Structures working in corrosive
environment should be made of corrosive resistant materials such as aluminum and stainless steel.
Painting, coating, electroplating and proper surface treatment of these structures should also be done.
(Bell, 2019).
REFERENCES
Nelson, Beachem, 2015, American Society for Metals, Metals Park, Ohio. Retrieved from
https://link.springer.com/article/10.1007/BF02643487. Retrieved on 18 July 18, 2019.
Gillespie, LaRoux K. 2012, Deburring and edge finishing handbook, SME, pp. 196–198, ISBN 978-0-87263-
501-2. Retrieved from: https://books.google.com/books?id=gdq6qajigqoC. Retrieved on 18 July
2019.
Shimozawa T, 2012, “Mechanical distortion of single acting filaments induced by external force”,
Semantic Scholar, Retrieved from: https://www.semanticscholar.org/paper/Mechanical-
distortion-of-single-actin-filaments-by-Shimozawa-Ishiwata/
b3dbb40e746ba135c2f489f5f3dc15fea9895c29. Retrieved on 18 July 18, 2019.
Cabet, Carroll, Wright, 2013, Low cycle fatigue and creep-fatigue behavior of Alloy 617 at high
temperature. J. Press. Vessel Technol. Retrieved from:
http://pressurevesseltech.asmedigitalcollection.asme.org/article.aspx?articleid=1750066.
Retrieved on 18 July 18, 2019.
Wang, Zhu, Zhang, Tu, 2018, “High temperature fatigue and creep-fatigue behaviors in a Ni-based
superalloy: Damage mechanisms and life assessment”. Retrieved from
https://www.sciencedirect.com/science/article/pii/S0142112318301798. Retrieved on 8 July 18,
2019.
Bell T, 2019, “What is Corrosion?”, The Balance, Retrieved from: https://www.thebalance.com/what-is-
corrosion-2339700. Retrieved on 8 July 18, 2019.