corrosion Fatigue

introduction: corrosion fatigue is a big and widely section. What this term paper is going to represent , First the definition of fatigue corrosion and what the most effective process on it , After that the process of corrosion fatigue in generally and how can the engineer know if there is a corrosion fatigue . Also, present the most important to understanding the fatigue life which is S-N diagram. Then presenting how the corrosion fatigue initiates and propagates . what happen to fatigue in corrosive environment will be discuss and showing some graphs about that. After that the evidence of corrosion fatigue and the difference between the corrosion fatigue and just fatigue . Also, presented some Characteristics of corrosion fatigue with a graph and the Variables affecting corrosion fatigue.

corrosion Fatigue: Corrosion Fatigue is process in which the failure of a metal by cracking under repeated cyclic loading at lower stress level and in a corrosive environment . A corrosive environment plays a significant role in the fatigue of high-strength structural materials like steel, aluminium and titanium alloys. Also The corrosive environment can cause a faster crack growth. fatigue can be in ships, aircraft, heat exchangers tubes , pump shaft and steel equipment. Fig(1): Corrosion Fatigue Cracks in Brass

Process of corrosion fatigue : This is a process where the fractures will happened under condition of corrosion. The cracking begins at the surface defect. Hole and irregularities and propagate trans granularly . Failure occurs by crack propagation . The crack are unbranched and propagate perpendicular to the surface of the metal. But before all that We have two stages first one Is about the initiation of corrosion fatigue crack then the propagation of corrosion fatigue crack.

Fig(2): trans granular cracks in corrosion fatigue diagram N-S: A fatigue curve is commonly known as S-N diagram . is obtained by plotting the number of cyclic required to Couse failure against the maximum applied cyclic stress . Fatigue occurs when a material fails under a cyclical stress which is below the yield stress of the material and on each repeated stress cycle, the crack grows. The repeated cyclic will continue repeating until the cracking is happened . in the fig there is two lines the first one is about the fatigue curve in the air as you can see the endurance limit . endurance limit is the maximum stress at which a material will never exhibit fatigue. Some metals dont have endurance limit. As long as the cyclic increase and the stress decrease the fatigue happened. the second one is about the fatigue corrosion which is about the aluminium alloys in tap water. As you can see aluminium dont have endurance limit. Fig(4): fatigue and corrosion fatigue curves for an aluminium alloy

In figure (4) there are three conditions for the same metal . As shown in the figure fatigue behaviour of the metal in air stops at almost 300 N/mm*2 this is the endurance limit for the metal in air. The second and third lines the metal is in corrosive environment For water and sodium chloride solution as the graph represented they are under the endurance limit for the metal . Fig(4): fatigue and fatigue corrosion for some metal in air-water and sodium chloride solution.

fig(5):low cyclic fatigue

Initiation of corrosion fatigue cracks: The a fatigue crack has two parts, initiation and propagation . There are several possible for starting the initiation of fatigue such as scratch on the metal ,twin boundaries, and slip band. The cracks are initiated start often at the surface of the metal. But it could also start from under surface in the presence of surface defect . during the loading part of the cycle , slip bands will occurs . the surface created by slip may oxidize during the unloading period . After that the first step cyclic make the intrusion or extrusion . Then by continued deformation in the next cyclic the intrusion may grow and form a crack.

Fig(6):slip band intrusion and extrusion

Propagation of corrosion fatigue: After the crack is happened now the dissection the phenomena of propagation of corrosion fatigue . The crack start to propagate in the direction of the stress axis. The stress concentration cusses local deformation in a zone at the crack tip . because of the increasing in plastic zone the crack will growth. The zone continue growing until it reach the thickness of the sample. After the zone became as the thickness of the sample there is no stress plain. The crack propagates in a direction perpendicular to the tensile stress. The crack undergoes rotation. After that on each cyclic the crack start to growth.

Corrosion fatigue evidence: The difference between the fatigue and the corrosion fatigue is that in the fatigue rarely there is any evidence that conform that in the fatigue there is more than one crack while in the corrosion fatigue have more than one crack its usually easy to determine whether the failure was because of the corrosion fatigue or by fatigue . two things that special the corrosion fatigue the surface of the metal is pitted and the cracks are often are in the bottom of the pits. Fig(7): corrosion fatigue failure from the origin

Corrosive Environment : Fatigue crack rate depends very strongly on the environments as well as on the metal . In an Aggressive environment the rate of the metal cracking is higher than in the inert environment and the two environment have the same condition. The lowest crack rate was observed in the inert environment . there is two type of environment liquids and gaseous , generally the most aggressive rate the cracking could increase in is in the liquid. figure(8): influence of corrosive environment on fatigue crack rate This figure is about corrosive and the inert environment . As the graph presented the in the y-axis there is the log dl/dn which is about the time and I the x-axis log ka which is threshold value. As you can see the the the inert environment are less than corrosive environment in the kc (fracture toughness) , So based on the graph you can know that rate of fatigue cracking in corrosive environmental more than the inert environment.

Characteristics of corrosion fatigue:1- The main thing that unique the fatigue corrosion is the cracks near the fracture , in a normal fatigue usually there is one crack near the fracture.2- The corrosion fatigue cracks in certain steels are trans granular . figure (2).3- The crack generally initiated at the surface where the stress is maximum. 4- The striations in corrosion fatigue is less clearer than in fatigue failure. Figure (9).

Fig(9): fatigue striations

Variable affecting corrosion fatigue: 1- Mechanical : Stress intensity . Cyclic load frequency. Stress ratio 2- Metallurgical : Microstructure. Composition . Heat treatment. Fracture toughness.3- Environmental : Temperature. Species. Ph. Potential.

Conclusion: What I came up to after this report is knowing what is the corrosion fatigue and the most affective on it is the corrosive environment and it reduce the metal life , the difference between the corrosion fatigue and fatigue, learning and studying the most important diagram for corrosion fatigue which is S-N diagram and knowing the endurance limit , the process of fatigue and there is to things that the corrosion fatigue depend on them before the failure of the metal which is initiation and propagation of corrosion fatigue , the evidence of whiter or not there is corrosion fatigue or just fatigue by knowing that there is in corrosion fatigue more than one cracking and pits in it.

Reference:1- Philippe Marcus, corrosion mechanisms in theory and practice .2- Herbert H. Uhlig , the corrosion handbook. 3- Zaki ahmed , principles of corrosion engineering and corrosion control.4- Mirko Klesnil and Petr Lukas, Fatigue for metallic material.5- Pierre R.roberge, corrosion inspection and monitoring.