Corrosion-Fatigue

Fatigue life can be slightly lengthened by anodic protection or by passivation. In acids even passive stainless CrNi steels suffer corrosion fatigue [104]. Resistance can occur if the passive film itself has a fatigue strength (e.g., in neutral waters [105]). 

Fatigue is the failure of a metal by cracking when it is subjected to cyclic stress. The usual case involves rapidly fluctuating stresses that may be well below the tensile strength. As stress is increased, the number of cycles required to cause fracture decreases. For steels, there is usually a stress level below which no failure will occur, even with an infinite number of cycles, and this is called the endurance limit. In practice, the endurance limit is defined as the stress level below which no failure occurs in one million cycles. A typical S-N curve fatigue curve, commonly known as an S-N curve, is obtained by plotting the number of cycles required to cause failure against the maximum applied cyclic stress. 

When a metal is subjected to cyclic stress in a corrosive environment, the number of cycles required to cause failure at a given stress may be reduced well below the dotted line obtained for the same metal in air shown in Fig. 6.48. This acceleration of fatigue called corrosion fatigue , is revealed by comparing the solid line in Fig. 6.48 with the dotted line reference. The solid curve indicates that metal life under such conditions can be much lower than the reference curve established in air. The S-N curve with corrosion tends to keep dropping, even at low stresses, and thus does not level off, as will the ordinary fatigue curve. 

A marked drop in or elimination of the endurance limit may occur even in a mildly corrosive environment, especially in the case of a film-protected alloy. For example, deionized water, which ordinarily 

Failures that occur on vibrating structures (e.g., taut wires or stranded cables) exposed to the weather under stresses below the endurance limit are usually caused by corrosion fatigue. Corrosion fatigue also has been observed in steam boilers, due to alternating stresses caused by thermal cycling (Fig. 6.49). 

The petroleum industry regularly encounters major trouble with corrosion fatigue in the production of oil. The exposure of drill pipe and of sucker rods to brines and sour crudes encountered in many producing areas results in failures which are expensive both from the standpoint of replacing equipment and from loss of production during the time required for fishing and rerigging. 

Corrosion fatigue testing with non-precracked specimens 

A positive sign is assigned to tensile stress. The figure shows two other parameters that are often used the average stress, = (Cmax + and the maximum stress 

F ure 11.43 Different parameters characterizing the fatigne corrosion test. 

In fatigue tests with non-precracked specimens one measures the time to failure tf, which is usually expressed in number of cycles  

Corrosion reduces the time to failure and eliminates the fatigue limit. Its detrimental effect is particularly important in low cycle fatigue. In this kind of fatigue, the applied stress is sufficiently high to cause plastic deformation and as a consequence the time to failure, even in the absence of corrosion, is usually less than 10 cycles. 

As the name implies, corrosion fatigue is affected by both the severity of corrosive conditions and mechanical, cyclical stress factors. Stress raisers such as notches, holes, weld defects, or corrosion pits can initiate fatigue cracks and a corrosive environment can reduce crack initiation time. For many materials, the stress range required to cause fatigue failm-e diminishes progressively with increasing time and with the number of cycles of applied stress. 

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Doyle, J. L., Wood, G. R., and Bondurant, P. D. Using Laser-Based Profilometry to Locate and Measure Corrosion Fatigue Cracking in Boiler Tubes, Materials Evaluation, D The American Society of Nondestructive Testing, Inc., Vol. 51, No. 5, pp. 556-560 (1993). 

Corrosion fatigue is a type of failure (cracking) which occurs when a metal component is subjected to cyclic stress in a corrosive medium. In many cases, relatively mild environments (e.g., atmospheric moisture) can greatly enhance fatigue cracking without producing visible corrosion. 

Crooker T Wand Leis B N (eds) 1983 Corrosion Fatigue Mechanics, Metaiiurgy, Eiectrochemistry and Engineering STP 801 (ASTM)... 

McEvily A J (ed) 1990 Atlas of Stress-Corrosion and Corrosion Fatigue Curves (Materials Park, OH ASM)... 

Speidel M O, Denk J and Scarlin B 1991 Stress Corrosion Craoking and Corrosion Fatigue of Steam-Turbine Rotor and Blade Materials (Luxembourg Commission of the European Communities)... 

Pitting corrosion may occur generaHy over an entire aHoy surface or be localized in a specific area. The latter is the more serious circumstance. Such attack occurs usuaHy at surfaces on which incomplete protective films exist or at external surface contaminants such as dirt. PotentiaHy serious types of corrosion that have clearly defined causes include stress—corrosion cracking, deaHoying, and corrosion fatigue (27—34). 

Corrosion Fatigue Corrosion fatigue is a reduction by corrosion of the abihty of a metal to withstand cyclic or repeatea stresses. 

Slime masses or any biofilm may substantially reduce heat transfer and increase flow resistance. The thermal conductivity of a biofilm and water are identical (Table 6.1). For a 0.004-in. (lOO-pm)-thick biofilm, the thermal conductivity is only about one-fourth as great as for calcium carbonate and only about half that of analcite. In critical cooling applications such as continuous caster molds and blast furnace tuyeres, decreased thermal conductivity may lead to large transient thermal stresses. Such stresses can produce corrosion-fatigue cracking. Increased scaling and disastrous process failures may also occur if heat transfer is materially reduced. 

Most cracking problems in cooling water systems result from one of two distinct cracking mechanisms stress-corrosion cracking (SCC) or corrosion fatigue. 

SCC and corrosion fatigue differ in the nature of the stresses and corrodents that cause them. SCC occurs under static... 

Several theories have been proposed to explain the corrosion-fatigue phenomena. One is that cyclic stressing causes repeated rupture of protective coatings. Corrosion-fatigue cracks propagate as the coating is successively reformed and ruptured along a plane. 

No common industrial metal is immune to corrosion fatigue since some reduction of the metal s resistance to cyclic stressing is observed if the metal is corroded, even mildly, by the environment in which the stressing occurs. Corrosion fatigue produces fine-to-broad cracks with little or no branching. They are typically filled with dense corrosion product. The cracks may occur singly but commonly appear as families of parallel cracks (Fig. 10.2). They are frequently associated with pits, grooves, or other forms of stress concentrators. Like other forms of... 

Numerous factors can have a potentially significant effect on corrosion-fatigue cracking. Most of these relate to stress and the corrosiveness of... 

Figure 10.2 A family of short, transverse corrosion-fatigue cracks originating on the external surface.

Perhaps the most important stress factor affecting corrosion fatigue is the frequency of the cyclic stress. Since corrosion is an essential component of the failure mechanism and since corrosion processes typically require time for the interaction between the metal and its environment, the corrosion-fatigue life of a metal depends on the frequency of the cyclic stress. Relatively low-stress frequencies permit adequate time for corrosion to occur high-stress frequencies may not allow sufficient time for the corrosion processes necessary for corrosion... 

Figure 10.3 Blunt fracture edges typical of corrosion-fatigue cracks.

Corrosion-fatigue cracks can be detected by nondestructive testing techniques such as magnetic particle inspection, radiography, ultrasonics, and dye penetrant. Corrosion-fatigue cracks may occur in numerous tubes simultaneously. Nondestructive testing of tubes at locations similar to those in which cracks are observed can be useftil. 

Mitigation or elimination of corrosion-fatigue cracks involves gaining control of the critical factors that govern the mechanism. 

Alter the environment to render it less eorrosive. This approach may be as simple as maintaining clean metal surfaces. It is well known that the chemistry of the environment beneath deposits can become substantially different than that of the bulk environment. This difference can lead to localized, underdeposit corrosion (see Chap. 4, Underdeposit Corrosion ). The pit sites produced may then induce corrosion fatigue when cyclic stresses are present. The specific steps taken to reduce corrosivity vary with the metal under consideration. In general, appropriate adjustments to pH and reduction or elimination of aggressive ions should be considered. 

Alter the metal to achieve greater corrosion resistance. Since a metal s susceptibility to corrosion fatigue depends largely on its corrosion resistance in a particular environment, improving corrosion... 

Separate the metal from the environment with a physical barrier. Many corrosion inhibitors make use of this principal to protect metals. Proper use of an appropriate inhibitor may reduce or eliminate pitting. Pits are frequently initiation sites for corrosion-fatigue cracks. The effectiveness of inhibitors depends upon their application to clean metal surfaces. An example of this method is the use of zinc coatings on steel to stifle pit formation. 

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