Moving Forward Every Day — 2010.02.27

Answer: In addition to the characteristics of conventional fatigue, corrosion fatigue is a very complex phenomenon of material or component failure caused by exposure to a corrosive environment. There are numerous factors that influence it, including metallurgy, materials, environment, stress, time, temperature, etc.; any change in any of these factors can affect the corrosion fatigue performance. One of the characteristics of corrosion fatigue damage is the reduction in the fatigue resistance of the material or structure. At the same stress level, especially near the air fatigue limit, the corrosion fatigue life is much shorter than the normal fatigue life, often being reduced by many times. Therefore, under corrosion fatigue conditions, metal materials often do not have a distinct fatigue limit; instead, the conditional fatigue limit at a specified number of cycles is used. For some metal materials, no fatigue limit is observed at all. The condition fatigue limit for corrosion fatigue has no direct relationship with the mechanical properties of metal materials in the atmosphere. Some measures commonly employed to improve the fatigue strength of materials under normal fatigue conditions include increasing the material’s yield strength or ultimate strength. Corrosion fatigue has little effect on the conditional fatigue limit, and sometimes it even has the opposite effect. Another characteristic of corrosion fatigue is that its performance is strongly dependent on the frequency and waveform of the cyclic loading, whereas in conventional fatigue, the stress alternation frequency and waveform have little effect on fatigue performance. Therefore, during corrosion fatigue testing, the loading frequency cannot be too high, as corrosion fatigue performance depends significantly on the actual loading frequency. Generally speaking, the lower the frequency of cyclic loading, the longer the time during which stress acts in conjunction with the environment in each cycle, and the more severe the corrosion fatigue becomes. Furthermore, corrosive media make steel components less sensitive, or even insensitive, to surface micro-geometric properties and mechanical stress concentrations during corrosion fatigue, which is also an important characteristic of corrosion fatigue. Corrosion fatigue also exhibits macroscopic characteristics different from those of conventional fatigue. Under corrosion fatigue conditions, multiple fatigue cracks often form simultaneously and propagate in a direction perpendicular to the tensile stress. In the air, such fatigue cracks are often only one. Corrosion fatigue generates multiple cracks simultaneously, resulting in a multi-planar characteristic of the fatigue fracture surfaces of carbon steel and low-alloy steel in neutral corrosive media

The degradation of a material caused by the combined effect of corrosion in an corrosive environment and alternating stress is known as corrosion fatigue.

(1) Characteristics: Corrosion fatigue does not require a specific combination of material and environment. Under dynamic load stress, all the metal materials of electric control valves can undergo such phenomena in any medium, but it occurs more easily in media that tend to cause pitting. Fatigue cracks typically appear as short and thick groups of cracks; they usually originate from pits or surface defects and mostly propagate through the grains. The fracture pattern is brittle, with no significant macroscopic plastic deformation; most of the fracture surfaces are covered by corrosion products, while a small portion of them have a rough surface.

Feature: Corrosion fatigue does not require a specific combination of material and environment. Under dynamic load stress, all the metal materials of electric control valves can undergo such phenomena in any medium, but it occurs more easily in media that tend to cause pitting. Fatigue cracks typically appear as short and thick groups of cracks; they usually originate from pits or surface defects and mostly propagate through the grains. The fracture pattern is brittle, with no significant macroscopic plastic deformation; most of the fracture surfaces are covered by corrosion products, while a small portion of them have a rough surface.

Characteristics of corrosion fatigue: Corrosion fatigue does not require a specific combination of material and environment. Under dynamic load stress, all the metal materials of electric control valves can undergo such phenomena in any medium, but it occurs more easily in media that tend to cause pitting. Fatigue cracks typically appear as short and thick groups of cracks; they usually originate from pits or surface defects and mostly propagate through the grains. The fracture pattern is brittle, with no significant macroscopic plastic deformation; most of the fracture surfaces are covered by corrosion products, while a small portion of them have a rough surface.

Compared to stress corrosion, corrosion fatigue has the following main differences: (1) Stress corrosion occurs only under specific combinations of material and environment, whereas corrosion fatigue is not subject to such restrictions and can occur in any environment. (2) For stress corrosion, there is a critical stress intensity factor K1SCC, which is an inherent property of the material; when the applied stress intensity factor K1 is…

Corrosion fatigue does not require a specific combination of material and environment. Under dynamic load stress, all the metal materials of electric control valves can undergo such phenomena in any medium, but it occurs more easily in media that tend to cause pitting. Fatigue cracks typically appear as short and thick groups of cracks; they usually originate from pits or surface defects and mostly propagate through the grains. The fracture pattern is brittle, with no significant macroscopic plastic deformation; most of the fracture surfaces are covered by corrosion products, while a small portion of them have a rough surface.

The characteristic of corrosion fatigue is the presence of numerous deep corrosion pits; cracks extend through these pits, and there can be several such cracks, oriented perpendicular to the stress. It is a typical transgranular type, without branching cracks, and the edges of the cracks are serrated.

(1) Characteristics: Corrosion fatigue does not require a specific combination of material and environment. Under dynamic load stress, all the metal materials of electric control valves can undergo such phenomena in any medium, but it occurs more easily in media that tend to cause pitting. Fatigue cracks typically appear as short and thick groups of cracks; they usually originate from pits or surface defects and mostly propagate through the grains. The fracture pattern is brittle, with no significant macroscopic plastic deformation; most of the fracture surfaces are covered by corrosion products, while a small portion of them have a rough surface. (2) Mechanism: Under alternating stress, dislocations move back and forth across the grain boundaries without accumulating there. Over time, slip steps were formed, providing active sites for pitting. The formation of pits increased the stress concentration, leading to the initiation of cracks. The crack tips became anodic areas and suffered preferential corrosion; under alternating stress, this promoted the propagation of the cracks into macroscopic corrosion fatigue cracks.

Compared to stress corrosion, corrosion fatigue has the following main differences: (1) Stress corrosion occurs only under specific combinations of material and environment, whereas corrosion fatigue is not subject to such restrictions and can occur in any environment. (2) For stress corrosion, there is a critical stress intensity factor K1SCC, which is an inherent property of the material; when the applied stress intensity factor K1 is…