Manual Fatigue and Corrosion in Metals

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Some types of corrosion, such as pitting or crevice corrosion, are easily identifiable by visual inspection.

What Is Metal Corrosion and Why Does It Occur?

Others, like cavitation or exfoliation, may need additional examination to identify. But there are three types of corrosion, each a form of environmental cracking, that can only be detected with an optical microscope or an electron microscope: stress corrosion cracking SCC , corrosion fatigue, and hydrogen embrittlement. Read on to discover environmental cracking and the damage these three types of it can each cause to carbon steel and other metals.


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Environmental cracking refers to the brittle fracture of a normally ductile metal, caused by the effects of corrosion in the environment. The cracks form and spread at right angles to the direction of the tensile stresses. The material fractures under lower stress levels than would normally cause fractures if the material was in a non-corrosive environment.

Corrosion Fatigue

Grain boundaries are not positioned in a regular crystal array. An anodic electrode potential results from increased strain energy. Consequently, corrosion can arise selectively along grain boundaries. Stress corrosion cracking SCC is the result of tensile strength in a corrosive environment. SCC typically occurs in specific combinations of alloy and environmental stress.

Failure may happen without warning, and with minimal material lost. SCC fractures appear brittle to the naked eye.

Introduction

Corrosion fatigue occurs when corrosion is combined with repeated cyclic loading at lower stress levels to cause metal fractures. Corrosion fatigue ruptures the protective passive film, causing quicker corrosion. Because of its exposure to a corrosive environment, the metal failure can take place within a shorter time or fewer cycles than in a non-corrosive environment. Hydrogen embrittlement is a type of corrosion that causes metals to become brittle and fracture due to the ingress and diffusion of hydrogen into the material.

The higher the strength of the steel, the more susceptible it is to hydrogen embrittlement. This makes aluminum alloys, titanium alloys, and electrolytic tough pitch copper the main contenders for this kind of corrosion.

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There are two recognized types of hydrogen embrittlement: internal hydrogen embrittlement and environmental hydrogen embrittlement. Corrosion Fatigue Crack Initiation: Influence of environment on fatigue crack initiation of a material is illustrated in S-N curves Stress vs Log of number of stress cycles to failure , which compare the smooth-specimen stresslife obtained from inert and aggressive environments.

Corrosion Fatigue Crack Propagation: Although the phenomena for corrosion fatigue are diverse, several factors are known to influence crack growth rate:. Effect of variables such as metallurgy, temperature, load history and waveform, stress state, and environment composition are unique to specific materials and environments. Corrosion Fatigue.

Corrosion Fatigue Crack Propagation: Although the phenomena for corrosion fatigue are diverse, several factors are known to influence crack growth rate: Stress intensity range - In embrittling environments, crack growth rate generally increases with increasing stress intensity D K ; Load frequency - Cyclic load frequency is the most important variable that influences corrosion fatigue for most materials.

The rate of brittle cracking above that produced in vacuum generally decreases with increasing frequency.

Fatigue

Frequencies exist above which corrosion fatigue is eliminated. Stress ratio - Rates of corrosion fatigue crack propagation are typically increased by increase in the stress ratio.