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Stress Corrosion Cracking In SDA Systems
The subject of the last Atomizer Update was the eight classical forms of corrosion and their affect on SDA systems. Stress corrosion was identified as the most insidious corrosion form because no other corrosion attack need be obvious before cracks appear. Serious consideration must be given to retire major components where stress corrosion cracks are identified. Complete understanding of stress corrosion is critical in design and manufacturing to avoid process errors and apply preventive measures as possible. Knowledge of SCC is helpful to the user in establishing appropriate inspection criteria that can identify stress corrosion cracks and possibly avoid catastrophic failures.
THE STRESS
Stress corrosion cracking (SCC) requires very specific mechanical, metallurgical and chemical reactions to be in place for a crack to initiate and propagate. As the name implies, stress is one factor and this will be tensile stress. Tensile stress is the kind of force that pulls things apart, like stretching a rubber band. We usually consider the tensile stress that is applied to the system from operation, but there may also be residual tensile stress left over from manufacturing. There are many occasions in industry where the residual stress is the major source of tensile stress that causes an SCC failure. Tensile stress is also a component of bending stress or shear stress or compressive stress. Newton says for every action there is an equal and opposite reaction. If we bend a bar, there will be tensile stress on the top and compressive stress on the bottom. If we twist the bar, the outer fibers will be in shear, but one component of the shear stress is tensile stress. If we push something together in compression, there must be an equal force pulling on it. That pulling force may be mostly internal, but it is there. In all such examples, it is the resultant tensile stress component that drives crack initiation by SCC.
THE MATERIAL AND CORROSION PAIR
Another ingredient for stress corrosion cracking is the corrosion environment. This gets very specific because the alloy system and the corrodant must be matched for SCC. The acid gases that can be a problem in the stainless steels used in SDA systems will not cause SCC in copper alloys. However, ammonia compounds will cause SCC in many copper alloys and have no such affect on stainless steels. Caustic solutions can be a problem in plain carbon and alloy steels but, seldom produces SCC in stainless steels. Other examples of the paired relationship for SCC include nickel alloys like inconels and monels in caustic environments, and under extreme conditions, some titanium alloys can experience SCC in seawater.
THE TIME ELEMENT
The last ingredient of SCC is time. SCC failures can be produced in a laboratory environment in a matter of seconds. In most industrial applications, SCC does not become a factor for months or years. This is the usual case in SDA systems. There is a threshold of corrodant concentration where SCC can begin. In some cases, it may take considerable time to concentrate enough corrodant to pass this threshold. There is also a minimum stress threshold. This can be surprisingly low and the greater the stress beyond the minimum, the less time elapses for crack initiation. The strict definition of SCC does not consider other types of corrosion acting in concert with SCC. But, this occurs in these SDA systems and other applications and also compresses elapse time to failure.
An understanding of each of the four ingredients needed for stress corrosion to occur the material and corrosion pair and stress and time can aid in an effort to control damage from SCC. Time mostly ends up a function of one or more of the other factors, so we really only affect this ingredient by control of the others. We have little control of combustion and the contaminants in the coal or other fuel that determines the corrosion component. Any reasonable effort to control SCC must address the material or the stress.
MATERIAL SELECTION AND MANUFACTURING PROCESSES
Material selection is made considering material strength, corrosion resistance and value. There are alloys available that can eliminate stress corrosion cracking in these systems, but they have other shortcomings, cost being primary among these. The trash burners in this country require the use of hastelloy from a general corrosion standpoint. The hastelloy is also resistant to SCC, but is not as erosion resistant. Stainless steel is used in some European waste to energy plants, but uniform corrosion attack is extreme and SCC is very evident. Considering performance and cost, the stainless steels used today seem the best value for coal burners in this country.
The design of the component attempts to manage the magnitude of applied stress and the attention to manufacturing detail should consider minimizing residual stress. The design engineer does not have much latitude about the applied stress. The operational parameters are not going to change. The package size does not change either, so the engineer is limited in approach to reduce stress. Good practice includes providing generous radii at section changes to reduce stress concentration.
Traditional manufacturing processes to complete components from these alloys can have a negative effect on SCC resistance. For example, these alloys can be made very strong and hard for strength and erosion resistance by heat treatment. However, the high strength heat treatment makes these alloys less resistant to SCC. The challenge then is to get the best from hardness and strength while minimizing SCC susceptibility.
BENEFICIAL COMPRESSIVE STRESS
Since tensile stress is required for stress corrosion cracking and the corrodant is present on the surface of the material only, compressive stress on the surface of the part can be beneficial. There are a few ways to accomplish this, but the most adaptable is shot peening. Shot peening is widely used in the aircraft industry for fatigue resistance, panel forming and stress corrosion cracking resistance'. If shot peening can be designed and applied properly to the surfaces exposed to the corrodant, one of the stress corrosion cracking ingredients tensile stress is effectively removed. Like most good things, there are some limits. It is not always possible to get the necessary compressive stress profile from the shot peening process given the configuration of the part. Also, shot peening is a thin layer of compressive stress and if there is much uniform corrosion attack in the system, the compressive layer will be defeated and the benefit lost.
Still, the presence of compressive stress on the surface can be dramatic in mitigation of SCC and Omega Atomizers employs this technology for SCC resistance on some components. In fact, one unit with extensive use of shot peening has been fielded at one powerplant burning an eastern coal with more aggressive SCC corrodants. Omega Atomizers will use shot peening as appropriate and continue to search for added value by using other creative measures to improve the performance of SDA components in stress corrosion cracking environments. LET OUR EXPERIENCE AND CREATIVITY WORK FOR YOU.
IMAGES
FEA Element Map
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FEA Deformation Graph
White Isobars - At Rest
Red Isobars - At Speed
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Stress Corrosion Crack (100X)
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Stress Corrosion Cracking - Visible with the naked eye
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