Corrosion fatigue influencing factors

It is often difficult to conduct laboratory tests in which both the environmental and stressing conditions approximate to those encountered in service. This applies particularly to the corrosive conditions, since it is necessary to find a means of applying cyclic stresses that will also permit maintenance around the stressed areas of a corrosive environment in which the factors that influence the initiation and growth of corrosion fatigue cracks may be controlled. Among these factors are electrolyte species and concentration, temperature, pressure, pH, flow rate, dissolved oxygen content and potential (free corrosion potential or applied). 

Cyclic load frequency is the most important factor that influences corrosion fatigue for most material environment and stress intensity conditions. The dominance of frequency is related directly to the time dependence of the mass transport and chemical reaction steps involved for brittle cracking. 

This paper focuses on how to model the deterioration of static pressurized process equipment to enable efficient inspection and maintenance planning. Such equipment tends to gradually deteriorate over time from erosion, corrosion, fatigue and other mechanisms, and at some point of time inspection, repair or replacement is expedient with respect to safety, production and costs. The deterioration of the equipment is influenced by many factors such as type of equipment, system design, operation and process service, material and environment. For hydrocarbon systems, the most critical deterioration mechanisms are corrosion due to CO2 and H2S, microbially influenced corrosion, sand erosion and external corrosion (DNV 2002). In general, CO2 is the most common factor causing corrosion in oil and gas system of low alloy steel (Singh et al. 2007). 

Although corrosion fatigue phenomena are diverse and specific to the environment, several variables are known to influence crack growth rate. The following factors must be considered in any study of corrosion fatigue ... 

N.A. Miller, Some Factors Influencing the Corrosion Fatigue Behaviour of a High-Strengdi Aluminum Alloy, New Zealand Jourrud of Science, Vol 12, 1969, p 346... 

What is important for this space truss problem depends on which of the various technical issues influence the design. Is stiffness an issue Is strength an issue If so, why Is buckling an issue Can fatigue be a problem Or corrosion Thermal expansion or joints Those factors are listed in Figure 7-23. 

Wear is the process of physical loss of material. In sliding contacts this can arise from a number of processes in order of relative importance they are adhesion, abrasion, corrosion and contact fatigue. Wear occurs because of local mechanical failure of highly stressed interfacial zones and the mode of failure is influenced by environmental factors. 

Part (b) of the Figure 11.48 shows the superposition of the behaviors of mechanical fatigue and stress corrosion cracking. In general, the influence of the latter shows up mostly at low strain rates (d /dy = 10 - 10 s ). In the graph that represents Aa/AN as a function of log K, a step appears at the point where the stress intensity factor that corresponds to the maximum value of the applied cyclic stress, reaches the value iscc. the threshold stress intensity for stress corrosion cracking. 

Stress is a requirement for SCC or fatigue, but can also influence the rate of general corrosion. The severity of corrosion is affected by time. Corrosion rates are expressed as a factor of time. Some corrosion rates are rapid and violent, while most are slow and almost imperceptible on a day-to-day basis. 

Loss of adhesion at the interface, in the interfacial (interphase) material, or in nearby material can occur as a result of a number of effects. These include mechanical stress, chemical corrosion, diffusion of material to or away from the interface, or fatigue effects. Sometimes several factors are involved at the same time, such as stress and corrosion. In some cases, film properties influence the failure mechanism. For example, residual film stress can add to the applied mechanical stress and can even stress the interface to such an extent that adhesion failure occurs without any externally applied stress. 

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