Industrial Corrosion and Protection--14 Fatigue Corrosion

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Immerse yourself in reading with a novel reader. Basic concept: Under the combined action of alternating stress and corrosive media, the fatigue strength or fatigue life of metals decreases compared to when there is no corrosion present; this phenomenon is known as corrosion fatigue. The so-called “no corrosive effect” here generally refers to the fatigue behavior of metals in air. Mechanism of corrosion fatigue (1): Cracks originate from pitting or other forms of local corrosion. Pitting or other local corrosion creates notches and gaps, which lead to stress concentration and serve as starting points for the formation of cracks on smooth surfaces under alternating stresses. However, it cannot be explained solely by the notch effect, as experiments have shown that conducting fatigue tests under conditions that lead to pitting does indeed promote crack formation; whereas when the potential is kept in a range where pitting does not occur, corrosion fatigue can still take place under the same alternating stress. Therefore, the chemical state specificity of the inner surface of the pit (active dissolution corrosion) should be explained in conjunction with stress concentration. (2) Slip promotes corrosion, and corrosion in turn affects slip. (3) Since the preferential dissolution of piled dislocations promotes slip roughening, the conditions for corrosion fatigue arise. It should be emphasized that the relationship between corrosion fatigue and stress corrosion cracking is such that alternating stress must act together with the corrosive medium in order to cause corrosion fatigue. Corrosion fatigue can occur in any metal in any corrosive medium, and no specific combination of material and medium is required. In materials susceptible to stress corrosion cracking, when subjected to alternating stresses, if the stress amplitude is below the critical value at which stress corrosion cracking can occur, only corrosion fatigue will result; if it is above the critical stress for stress corrosion cracking, then as the rate of stress variation decreases, a combination of stress corrosion cracking and corrosion fatigue may occur. Factors affecting corrosion fatigue (1) Mechanical factors 1. Influence of frequency. In all cases, corrosion increases as the frequency decreases. This is due to the decrease in frequency, which increases the time for the corrosion medium to react with the specimen. 2. Influence of stress waveforms. The effect of stress waveforms on corrosion fatigue: positive pulse waves and negative sawtooth waves cause less damage to corrosion fatigue resistance, whereas triangular waves, sine waves, and positive sawtooth waves cause greater damage. Overlapping waves promote the initiation of corrosion fatigue cracks, resulting in a sharp reduction in corrosion fatigue life. (2) Material factors: Based on air fatigue strength, both water and a 3% NaCl aqueous solution significantly reduced the fatigue strength of typical structural materials, although the degree of reduction varied depending on the material. (3) Environmental factors: The corrosivity of the medium has a significant impact on corrosion fatigue. Generally, when the corrosivity of the medium is high, the corrosion fatigue strength is low; however, if the corrosivity is increased further, the cracking rate actually slows down. Preventive measures: 1. Select materials appropriately. Generally speaking, materials with high pitting resistance also have higher corrosion fatigue strength, whereas materials prone to stress corrosion cracking have lower corrosion fatigue strength. The higher the strength of a steel grade, the greater its susceptibility to corrosion fatigue, and the lower its corrosion fatigue threshold value – this is similar to the situation with stress corrosion. Choosing a steel grade with lower strength can completely avoid this issue. 2. Reduce stress. Improve the design to reduce stress. Avoid sharp notches to reduce stress concentration; use heat treatment to eliminate residual internal stresses; apply surface treatments such as shot blasting to create compressive stresses on the material surface, thereby inducing residual compressive stresses in the surface layer. 3. Reduce stress fluctuations. For static equipment such as petrochemical vessels and storage tanks, it is necessary to maintain stability in the production process as much as possible in order to reduce pressure fluctuations; for dynamic equipment such as compressors and pumps, attention should be paid to minimizing vibration. 4. Reduce corrosion. Gaps should be avoided in the design. In addition, common measures include coating, corrosion inhibitors, and electrochemical protection