(Full Article with concluding part)
Introduction
Chemical plants like natural gas-based fertilizer industries operating at high temperature and handling corrosive and hazardous substances at higher pressure are subjected to ageing and its equipment and piping tend to degrade over time. There are many areas which are prone to aging but can go unnoticed, leading to safety hazards for the plant and people. Replacement of whole plant and even sub-systems are not always economically feasible which give rise to the need of assessment of systems and its components for risk associated with failure. With a view to assess the same, it calls for understanding the mechanism of degradation so that remaining life assessment of equipment can be carried out more precisely.
Degradation Mechanism
Materials degrades with passage of time due to many factors, out of which following major mechanisms are creep, microstructural degradation, high temperature fatigue, carburization, hydrogen damage and embrittlement, graphitization, thermal shock, liquid erosion, dual phase erosion, corrosion, erosion-corrosion mechanism, liquid metal embrittlement, Stress corrosion cracking, caustic gauging, metal dusting etc.
Creep
Equipment and piping under high temperature and stress are subjected to progressive degradation called creep. Creep is one of the most important degradation mechanisms which involves time-dependent deformation which can lead to rupture of the material. The failure of materials by creep damage depends on temperature and the extent of stress the material is subjected to. The degradation once starts gradually progresses and develops microcracks and propagates through grain boundaries leading to rupture.
High temperature equipment like catalyst tube in fired furnace, Boiler, tubes and headers in steam superheater, piping inside waste heat section of duct, tubes in BFW preheater, reactor, catalyst basket screen of Ammonia reactor, tubes TTS weld of heat exchangers handling hot process gas, intercoolers in syn gas compressor system, gas turbine components etc. are always subjected to continuous exposure of high temperature where stresses like internal pressure centrifugal force, static load are associated causing the combine degradation of creep.
At higher temperatures, creep can occur uniformly or with localized deformation having symptoms of large plastic strains, local wall thinning, elongation etc. The combination of lower and higher temp and stresses at higher or lower level produces different types of fractures and characterization of this cracks under various conditions can be known from characteristic map developed by Ashby, Mohamed and Langdon.
Qualitative approach of evaluating defects like creep void, microcracks can be done with Neubauer and Wedel based model. This model helps understand damage stages in high temperature steam network with the help of in-situ metallography. Collecting replica in in-situ metallography is easy method with damaging the component, but not suitable for many metals like some Cr-Mo steel, where voids may get created during polishing and etching.
The degree of spheroidization of carbides in bainitic and pearlitic structure tells about extent of thermal exposure and consequential damage.
Sometime, measurement of hardness is required to substantiate loss of tensile strength and assessment of remaining life in material under creep damage mechanism.
Microstructural degradation
This type of damage mechanism is caused by other damaging processes like creep, for which continual degradation in metallographic structures are seen. Most predominantly the metal structures with grains gradually degraded with precipitation in grain boundary, developing micro cracks and sometimes trans-granular cracks are being propagated leading failure.
Spherodization:
This is the aging phenomena, when certain type of carbon steel and low alloy steel are subjected to a specific range of temp (440 degC to 760deg C), carbide phases become unstable and start to form cluster resulting in loss of strength. The higher the temp close to higher end the degradation is faster even can happen in hours, however the process significantly slows down with the fall in temperature.
Although, the threat to failure due to spherodization is quite less, except for a few pressure vessels or piping operating in the susceptible temperature range or in a fired furnace where conditions tend to fluctuate widely which may include furnace catalyst tubes, exothermic reactors (e.g. Ammonia converter), and other pressure vessel with fluctuating operating temperatures which are exposed to uncontrolled exothermic reaction with possibilities of runaway reaction.
Designing these pressure vessels and choosing the materials for construction is tricky thing; as some steels are more susceptible to this degradation, including fine-grained normalized steels. Therefore, choosing the correct ASTM grain size range is key factor for ensuring long life of the components.
Likelihood of spheroidization damage can be evaluated with standard hardness testing and can be confirmed by metallography.
Softening and Decarburization
It is process of depleting carbons in carbon and low alloy steel when subjected to high temperatures in a oxidizing or reducing environment. It generally happens metals are heated beyond 700 deg C when the metal reacts with gases containing oxygen and hydrogen. The carbons which resides in the form of metal carbides, gets removed in the form of CO, CO2 or CH4. The depletion of carbon results in softening of metal, primarily on the external surface where decarburizing gas reacts with metal.
The steps of decarburization mechanism can be three distinct events: the reaction at the steel surface, the interstitial diffusion of carbon atoms and the disbanding of carbides within the steel. Decarburization can be utilized for softening of metal where it is desired.
Embrittlement and carburization
This phenomenon is reverse of Softening and decarburization as described above. This can happen in several different ways.
Austenitic stainless steels: Sigma phase formation in high temperature or in critical temperature range (approximately 565 to 980 °C) which causes loss of ductility and induce embrittlement.
Ferritic stainless steels: Embrittlement is observed when held at or cooled over in the temperature range 550 to 400 °C. If service conditions are close to this temperature range, metallographic checks are recommended after extended exposure to avoid unexpected rupture.
In addition to the embrittlement, carburization can lead to brittle structure it is exposed to a high temperature carburizing atmosphere for long time.
Fig D shows extensive carbide formation in grain boundary of SS321 pipe in flue gas service.
Hydrogen damage can occur in carbon steels under hydrogen atmosphere particularly in high temp and pressure, results in diffusion of atomic hydrogen to form methane reacting with carbon present in iron carbide.
Hydrogen damage or embrittlement can be categorized into two broad categories HAC (hydrogen assisted cracking) and HIC hydrogen induced cracking.
It has been revealed that steels with lower UTS (less than 140KSI) or hardness less than 32 HRC are not susceptible to hydrogen embrittlement phenomenon. Steels like precipitation hardened steel (like 17-4PH) severe loss of ductility is observed when exposed high pressure hydrogen.
Hydrogen damages are most common in carbon steel, aluminium and titanium, however austempered steels are resistant to embrittlement caused by hydrogen. Austenitic stainless steel, High nickel alloys, beryllium copper are ale resistant to hydrogen embrittlement.
The following three parameters are considered responsible for hydrogen damage:
Presence and partial pressure of hydrogen
a susceptible material
Pressure or stress
Temperature
When assisted by a concentration gradient where there is significantly more hydrogen outside the metal than inside, hydrogen diffusion can occur even at lower temperatures. Hydrogen damage mechanism is consisting of following:
Internal pressure within the metal: Hydrogen atoms get absorbed in surface and get diffused and may recombine to form hydrogen molecules creating pressure within the metal. This pressure ultimately led to opening of cracks when metal lose ductility and toughness. (HIC) Metal hydride formation: Hydrogen reacts with metal to form brittle hydrides which allows cracks to propagate.
Hydrogen enhanced decohesion: Hydrogen can induce decohesion where the strength of the atomic bonds of the parent material are reduced.
Hydrogen enhanced localized plasticity (HELP): The mechanism of HELP was first proposed by C. Beachem. The process illustrates enhancement of mobility of dislocations in the presence of hydrogen atoms, results in propagation of defects /cracks even at lower stresses required.
Hydrogen enhanced dislocation emission: This process tells about the enhancement of dislocation due to adsorption of hydrogen onto metal surface. Hydrogen enhances the mobility of dislocations in FCC, BCC and HCP ordered and disordered materials. It occurs for all dislocation types including partial and perfect lattice dislocations and grain boundary dislocations.
Graphitization: The process of microstructural degradation takes place in ferritic steels if is exposed to high temperature for long time. Since graphite phase is more stable the phases gradually change from cementite to pearlite and ultimately to graphite. During the service graphite particles tend to align and form the crack. With the development of Cr-Mo Steel this degradation could be avoided.
Thermal shock
Thermal shock is associated with rapid changes in temperature due to external and internal reasons. As result of steep temperature gradient, thermal fatigue is produced to initiate grain separation leading to generation of crack and further propagation. Such problems are being faced in chemical, refineries, thermal power, and fertilizer plant, wherein situation may arise due to process emergencies and run-away reaction in reactors in presence of catalyst. Failures are generally happened in brittle materials particularly in cast grade materials. Catalyst tubes which face extreme heat from radiant burner is susceptible to such type of failure.
(Crack getting initiated due to thermal shock, image source www.reasearchgate.net)
Prevention of such failure mechanism is adhering to manufacturing protocol or if there in nothing available, 20 to 30 deg C per hour heating and cooling rate is considered safe to avoid thermal shock in any component subjected to extreme heat.
Erosion
Erosion happens when fluid contains liquid or solid suspended particles. These particles cause scratch on fluid contact surface, particularly when flow velocity is higher. This is abundantly happening in flue gas containing fly-ash and other dusts leading to heat transfer tubes in boilers. When fluid reached to dual phases (like saturated steam), some water droplets are formed inside the steam piping causing erosion in piping and turbine blades. Thinning of components pose a great safety hazard as sometimes thickness of the pipe wall is very difficult to measure due to heat and when it happens in localized part.
(Erosion in boiler tube, source:www.davidnfrench.com)
The control of this erosion depends on content of particles in the fluid and velocity of it. The usual solution to decrease the flow velocity is by increasing the pipe size. Usually, installation of strainer is not feasible if particle size distribution is mostly submicronic. However, water droplets cannot addressed by effective use of impingement and coalescent type filter. The solution to fly ash erosion can be performed by improving boiler flue gas distribution, and further reducing high gas velocities.
Liquid metal embrittlement (LME)
Liquid metal embrittlement is a corrosion mechanism which can occur in a number of combinations of liquid, solid and metal. We have noticed a very serious consequences prevalent in fertilizer, chemical and refineries is LME of austenitic stainless steel in presence of zinc.
In addition to above, it can happen in several metal embrittling couples.
The possible occurrence of liquid metal embrittlement might be due to following:
Less or negligible solubility between the solid metals and interacting liquid.
No formation of intermetallic compound between the solid-liquid couple.
In 1975, the disastrous explosion at a plant in UK killed 28 people. The cause of failure was attributed to liquid metal embrittlement of a stainless-steel pipe in contact with molten zinc. Steels get embrittled by zinc in more than 400°C temperature. If stresses are present, it leads to fast rates of crack propagation.
Nickel alloys get embrittled by zinc, Mercury or bismuth, but to a lesser extent than carbon and stainless steels.
The presence of stress and higher temperature compound the effect on the severity of liquid metal embrittlement.
The classic example of liquid copper metal embrittlement of steel is shown in figure below where the Cu has penetrated along the austenite grain boundaries when the carbon steel was at a temperature of 1100 °C.
Cracking can be extremely rapid (>=0.05 m/s) and stress levels can be as low as 20 MPa for such cracking to take place.
Two types of embrittlement attack are thought to occur in austenitic stainless steel in presence if zinc.
Type 1 embrittlement is a relatively slow process, which depends on diffusion along austenite grain boundaries, which have Ni-depleted zones along it. The FCC austenite structure turns into to BCC ferrite, creating stress due to expansion leading to cracking.
Type 2 embrittlement occurs at a much faster rate than type-1. This requires an external stress to initiate crack. Protective layer of oxide film prevent such cracking unless it has local defects.
LME Couples
There are a specific liquid metals combinations called as LME couples which leads to catastrophic intergranular cracking.
For example,
carbon steels are susceptible to LME by copper.
Stainless steels are susceptible to LME by zinc respectively.
Nickel and its high alloys are affected by Bismuth.
Aluminum is susceptible to LME by mercury and gallium.
Copper and its alloys are susceptible to liquid metal cracking by mercury and lithium.
Cracking usually propagates in rapid pace, at a rate of 5-25 cm/s.
Sulfidation-oxidation in Austenitic Stainless steel
Sulfidation corrosion in elevated temperature is one of the most well-known damage mechanisms in chemicals and oil refining industry. It is prevalent in refinery industries as it has natural presence in fossil fuel. The corrosion can lead to thinning of pressure parts, like piping walls, shell wall of container and pressure vessels.
Sulfidation depends on many factors, such as sulfur content, service temperatures, presence of erosion mechanism like high flow, and H2 concentration.
This type of corrosion in high temperature is prevalent in super-heaters, coal gasification systems, Naptha or NG heater, gas turbines and fired furnaces. Sulfidation affects in two distinct mechanisms:
Condition where H2 is present in addition to sulfur, such as hydro-cracking.
Hydrogen free atmosphere in processing units.
However, both the mechanisms are diffusion-based that occur at elevated temperature.
Sulfidation in Hydrogen-Hydrogen Sulphide Environments:
The mechanism of Corrosion
Iron Sulphide scales are essentially formed in both the mechanisms, i.e. hydrogen free as well as H2-H2S environment. Cr alloy has shown better corrosion resistance in Hydrogen free environment than most of other steels, however, in H2-H2S environment, the resistance is only slightly better than carbon steel.
It is presumed that in presence of hydrogen sulphur or its compound forms H2S in elevated temperature and if H2S concentration is more corrosion rate is observed higher.
In Hydrogen free corrosion mechanism follows four steps:
(1) Adsorption of the sulfur compounds.
(2) Catalyzed decomposition of the sulfur compounds leading cation vacancies.
(3) Migration and diffusion of cation vacancies to the FeSx/Fe interface.
(4) Fe getting oxidized to form scale reduction of of cation vacancies.
As per API 571 a threshold of 500 °F (260 °C) is considered for hydrogen-free services while in H2-environment the threshold temperature is considered at 450 °F (230 °C). Rapid degradation takes place when oxygen partial pressure is low in gas mixture. It is because protective layer of Cr2 O3 is inadequately form on exposed surface. Under this condition sulfidation corrosion are very rapid.
Ref: Sciencedirect.com,
http://www.researchgate.net,
http://www.inspectionengineering.com,
https://inspectioneering.com/
DISLOCATION MOBILITY AND HYDROGEN – A BRIEF REVIEW by I. M. Robertson and H. K. Birnbaum (University of Illinois)
Image courtesy: Machinedesign.com