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Chapter 17: Problem 23

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

Short Answer

Expert verified
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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Most popular questions from this chapter

An electrochemical cell is composed of pure copper and pure lead electrodes immersed in solutions of their respective divalent ions. For a \(0.6 M\) concentration of \(\mathrm{Cu}^{2+}\), the lead electrode is oxidized, yielding a cell potential of \(0.507 \mathrm{~V} .\) Calculate the concentration of \(\mathrm{Pb}^{2+}\) ions if the temperature is \(25^{\circ} \mathrm{C}\).

The corrosion rate is to be determined for some divalent metal M in a solution containing hydrogen ions. The following corrosion data are known about the metal and solution: \begin{tabular}{rr} \hline \multicolumn{1}{c}{ For Metal \(M\)} & For Hydrogen \\ \hline\(V_{\left(M M^{2}+\right)}=-0.47 \mathrm{~V}\) & \(V_{\left(\mathrm{H}^{+} / H_{2}\right)}=0 \mathrm{~V}\) \\ \(i_{0}=5 \times 10^{-10} \mathrm{~A} / \mathrm{cm}^{2}\) & \(i_{0}=2 \times 0^{-9} \mathrm{~A} / \mathrm{cm}^{2}\) \\ \(\beta=+0.15\) & \(\beta=-0.12\) \\ \hline \end{tabular} (a) Assuming that activation polarization controls both oxidation and reduction reactions, determine the rate of corrosion of metal \(\mathrm{M}\left(\mathrm{in} \mathrm{mol} / \mathrm{cm}^{2} \cdot \mathrm{s}\right)\) (b) Compute the corrosion potential for this reaction.

(a) Demonstrate that the CPR is related to the corrosion current density \(i\left(\mathrm{~A} / \mathrm{cm}^{2}\right)\), through the expression $$ \mathrm{CPR}=\frac{K A i}{n \rho} $$ where \(K\) is a constant, \(A\) is the atomic weight of the metal experiencing corrosion, \(n\) is the number of electrons associated with the ionization of each metal atom, and \(\rho\) is the density of the metal. (b) Calculate the value of the constant \(K\) for the \(\mathrm{CPR}\) in \(\mathrm{mpy}\) and \(i\) in \(\mu \mathrm{A} / \mathrm{cm}^{2}\left(10^{-6}\right.\) \(\left.\mathrm{A} / \mathrm{cm}^{2}\right)\)

A piece of corroded steel plate was found in a submerged ocean vessel. It was estimated that the original area of the plate was 10 in. \(^{2}\) and that approximately \(2.6 \mathrm{~kg}\) had corroded away during the submersion. Assuming a corrosion penetration rate of \(200 \mathrm{mpy}\) for this alloy in seawater, estimate the time of submersion in years. The density of steel is \(7.9 \mathrm{~g} / \mathrm{cm}^{3}\)

An electrochemical cell is constructed such that on one side a pure nickel electrode is in contact with a solution containing \(\mathrm{Ni}^{2+}\) ions at a concentration of \(3 \times 10^{-3} M\). The other cell half consists of a pure Fe electrode that is immersed in a solution of \(\mathrm{Fe}^{2+}\) ions having a concentration of \(0.1 M\). At what temperature will the potential between the two electrodes be \(+0.140 \mathrm{~V} ?\)
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