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Chapter 17: Problem 23

Briefly explain why cold-worked metals are more susceptible to corrosion than noncoldworked metals.

Short Answer

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Answer: Cold-worked metals are more susceptible to corrosion than non-cold worked metals due to factors such as structural changes, residual stress, increased reactivity, and micro-galvanic corrosion. The deformation process in cold working creates structural imperfections, lattice strains, and increased dislocation density, which provide favorable sites for corrosion attacks. Residual stresses induced during cold working can lead to stress corrosion cracking. Additionally, higher reactivity and the formation of micro-galvanic cells in cold-worked metals further contribute to their increased corrosion susceptibility.

Step by step solution

01

Introduction to Cold-Worked Metals

Cold working is a metal forming process where the metal is deformed below its recrystallization temperature. This process results in many structural and mechanical changes in the metal that, in turn, affects its corrosion resistance. Let's see the reasons behind this increased susceptibility to corrosion in cold-worked metals.
02

Structural Changes in Cold-Worked Metals

During the cold working process, the metal grains are elongated and deformed, causing a large number of dislocations and lattice imperfections in the metal structure. These imperfections and anisotropy in grain structure create favorable sites for corrosion attacks. Additionally, the cold working may also lead to the formation of inclusions or defects, which act as localized points for corrosion to initiate.
03

Residual Stress in Cold-Worked Metals

Cold working induces residual stresses into the metal due to the deformation process. These stresses can enhance the rate of corrosion as they create zones of high stress, which can lead to stress corrosion cracking (SCC). The presence of residual stress increases the susceptibility of cold-worked metals to various forms of localized corrosion.
04

Higher Reactivity in Cold-Worked Metals

Cold working results in a higher dislocation density and strained lattice in the metal, which increases the chemical reactivity of the cold-worked metals as compared to the non-cold worked ones. Increased chemical reactivity contributes to the higher rate of various corrosion mechanisms like galvanic, pitting, and crevice corrosion in cold-worked metals.
05

Galvanic Corrosion in Cold-Worked Metals

The structural changes and defects formed during cold working create micro-galvanic cells within the metal. These micro-galvanic cells result in accelerated corrosion as more anodic areas (defects, inclusions) provide electrons to the nearby more noble regions, thus creating a galvanic corrosion mechanism on a micro-scale. In conclusion, cold-worked metals are more susceptible to corrosion than non-cold worked metals due to several factors like structural changes, residual stress, increased reactivity, and micro-galvanic corrosion.

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