Erosion corrosion fretting

Erosion —corrosion, fretting corrosion, impingement attack, cavitation damage stress corrosion cracking, hydrogen cracking, corrosion fatigue... 

Another special case of erosion-corrosion, fretting corrosion, occurs when two heavily loaded metals rub rapidly together, causing damage to one or both metals. Vibration is usually responsible for the damage, but corrosion is also a factor because the frictional heat increases oxidation. In addition, mechanical removal of protective corrosion products continually exposes fresh metal. Fretting corrosion occurs more frequently in air than in water. 

Erosion-corrosion in the widest sense of the term will include impingement attack, cavitation damage and fretting corrosion, but since the latter two are dealt with in separate sections (see Sections 8.7 and 8.8) they will not be considered here. 

Fretting corrosion is a specialized form of erosion corrosion where two metal surfaces are in contact and experience very slight relative motion that causes damage to one or both surfaces. Again, in the presence of a corrodent, the movement causes mechanical damage of the protective film, leading to localized corrosion. 

Any action in mitigating flow maldistribution must be preceded by an identification of possible reasons that may cause the performance deterioration and/or may affect mechanical characteristics of the heat exchanger. The possible reasons that affect the performance are [131,147] (1) deterioration in the heat exchanger effectiveness and pressure drop characteristics, (2) fluid freezing, as in viscous flow coolers, (3) fluid deterioration, (4) enhanced fouling, and (5) mechanical and tube vibration problems (flow-induced vibrations as a consequence of flow instabilities, wear, fretting, erosion, corrosion, and mechanical failure). 

Mechanically assisted degradation can consist of the following types of corrosion erosion-corrosion, water drop impingement corrosion, cavitation erosion, erosive and corrosive wear, fretting corrosion, and corrosion fatigue (CF) (Fig. 1.14). Erosion-corrosion consists of the corrosion process enhanced by erosion or wear. Fretting corrosion consists of the wear process enhanced by corrosion. CF consists of the combined action of fluctuating or cyclic stress and a corrosive environment. 

Erosion corrosion caused by high velocity flow, fretting caused by vibration of two surfaces in close contact, and cavitation caused by very high flow... 

Leaving cavitation corrosion and fretting aside as separate corrosion forms (see Sections 7.10 and 7.11), erosion and abrasion corrosion can be divided into three types, a), b) and c), as described below. The first two types are erosion corrosion, while type c) is to be considered as abrasion corrosion. The three types may overlap each other and partly occur simultaneously in the same system. 

There are some corrosion cases where other factors, i.e. stress, or other mechanical factors are involved. For example, stress corrosion cracking, corrosion fatigue, hydrogen embrittlement, erosion corrosion, cavitation corrosion, fretting corrosion, etc. can be mentioned. 

Corrosion is the destructive attack of a metal by chemical or electrochemical reaction with its environment. Deterioration by physical causes is not called corrosion, but is described as erosion, galling, or wear. In some instances, chemical attack accompanies physical deterioration, as described by the following terms corrosion-erosion, corrosive wear, or fretting corrosion. Nonmetals are not included in this definition of corrosion. Plastics may swell or crack, wood may split or decay, granite may erode, and Portland cement may leach away, but the term corrosion, in this book, is restricted to chemical attack of metals. 

Switches and relays are also required to maintain conductive contact surfaces as well as thousands or millions of cycles of operation. The same corrosion mechanisms, such as fretting, exist with these devices as observed with connectors and contacts. However, the mechanical requirements can lead to additional wear and erosion corrosion problems. 

The movement of solutions above a certain threshold velocity level can result in another form of attack that is the result of the interaction of fluid-induced mechanical wear or abrasion plus corrosion. The general term erosion corrosion (E/C) includes all forms of accelerated attack in which protective surface films and/or the metal surface itself are removed by this combination of solution velocity and corrosion such as impingement attack, cavitation damage and fretting corrosion. 

The term "erosion" applies to deterioration due to mechanical force. When the factors contributing to erosion accelerate the rate of corrosion of a metal, the attack is called "erosion corrosion." Erosion corrosion is usually caused by an aqueous or gaseous corrodent flowing over the metal surface or impinging on it. The mechanical deterioration may be aggravated by the presence of a corrodent, as in the case of fretting or corrosive wear. 

Velocity effects include erosion-corrosion, a form of attack caused by high velocity flow cavitation caused at even higher flow by the collapse of bubbles formed at areas of low pressure in a flowing stream and fretting that is caused by vibratory relative motion of two surfaces in close contact under load erosion-corrosion, cavitation, fretting). 

Fig. 10.16 Types of localized corrosion initially associated with the environment, (a) Crevice corrosion, (b) Deposit corrosion, (c) Waterline attack, (d) Filiform corrosion (e) Erosion corrosion, (f) Drop corrosion, (g) Turbulent-flow corrosion, (h) Fretting.

Erosion-corrosion (as impingement or water drop corrosion) is a serious problem since it keeps the active state of magnesium at low pH values or prevents the formation and stability of the passive layer in alkaline solutions. Preventive measurements of erosion-corrosion and fretting fatigue corrosion include better passive surfaces that can be achieved by inhibitors, surface treatments and selected coating to improve wear resistance and wear corrosion of Mg alloys. 

Erosion-corrosion, including cavitation erosion and fretting corrosion Intergranular corrosion, including sensitization and exfoliation Dealloying... 

Leakaae Modes -Across seats -Lowers overall plant efficiency -Could cause reverse rotation of Brayton unit -Erosion -Corrosion -Scoring, spalling, fretting, wear, cracks -Particle interference -Valve seat distortion -Improperly seating disc (misalignment) -Improper contact or contact loads... 

FYom the multitude of intricate corrosion processes in the presence of mechanical action (friction, erosion, vibration, cavitation, fretting and so on) it is justified to touch upon corrosion types joined under a single failure mode induced by mechanical stresses. These are the stresses that govern the corrosion wear rate of metals during friction. Such processes are usually called corrosion stress-induced cracking in the case that the mechanical action is effective only in one definite direction, or otherwise termed corrosion fatigue in the case that compressive and tensile stresses alternate within cycles. In spite of the differences between the appearance of these corrosion types, they have much in common, e.g. fundamental mechanisms, the causes, and they overlap to a certain degree [19]. 

Being less plentiful and more expensive than nickel, cobalt is usually alloyed with chromium for applications where the alloys have practical advantages over similar nickel- or iron-base alloys. The cobalt-base alloys, for example, are better resistant to fretting corrosion, to erosion by high-velocity hquids, and to cavitation damage. 

Standard Test Method for Measuring Fretting Corrosion of Osteosynthesis Plates and Screws Standard Test Method for Cavitation Erosion Using Vibratory Apparatus Standard Terminology Relating to Wear and Erosion... 

Fretting of the fuel rods has been considered a failure mode. It results from erosion by debris collecting at the interference between the fuel rods and grid spacers or flow induced vibratimi (HV) of the fuel rods and grid spacers. Corrosion, hydriding and embrittlement of cladding at LOCA have also been regarded as fuel failure modes in LWRs. 

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