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Chapter 17: Problem 24

Briefly explain why, for a small anode-tocathode area ratio, the corrosion rate will be higher than for a large ratio.

Short Answer

Expert verified
Answer: The anode-to-cathode area ratio has a significant impact on the corrosion rate. A smaller ratio results in higher corrosion rates due to increased current density and concentrated electron flow, whereas a larger ratio leads to lower corrosion rates with less intense electron flow per unit area.

Step by step solution

01

Understand Anode and Cathode in Corrosion

An anode is an area of a metal surface where corrosion takes place due to the loss of electrons (oxidation). Cathodes, on the other hand, are areas on a metal surface that protect it from corrosion because they attract and consume electrons (reduction). The anode-to-cathode area ratio refers to the relative sizes of an anodic or corroding area (anode) to the nearby protective area (cathode).
02

Examine the Behavior of a Small Anode-to-Cathode Area Ratio

In an electrochemical cell, the anode loses electrons. As the anode-cathode area ratio decreases with a smaller anode area relative to the cathode, the concentration of the electron flow and the corrosion rate will increase. A small ratio has a greater current density (i.e., more electrons flowing per unit area), leading to a more intense corrosion process.
03

Examine the Behavior of a Large Anode-to-Cathode Area Ratio

A large anode-to-cathode area ratio means that the area available for the anode to lose electrons compared to the cathode area increases. Consequently, the current density or electron flow per unit area will be relatively less, leading to a slower corrosion process.
04

Compare Small and Large Ratios

When analyzing the behavior of a small anode-to-cathode area ratio and a large one, we can note that with a small ratio, there is a higher rate of corrosion due to the increased current density or concentrated electron flow. Conversely, a large ratio leads to a lower corrosion rate as electron flow per unit area is less intense.
05

Conclusion

In summary, the corrosion rate will be higher for a small anode-to-cathode area ratio compared to a large one. This is due to the increased current density, which accelerates the corrosion process in a smaller anode area relative to the cathode area.

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Most popular questions from this chapter

For each form of corrosion, other than uniform, do the following:(a) Describe why, where, and the conditions under which the corrosion occurs. (a) Describe why, where, and the conditions under which the corrosion occurs. (b) Cite three measures that may be taken to prevent or control it.

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) Compute the voltage at \(25^{\circ} \mathrm{C}\) of an electrochemical cell consisting of pure lead immersed in a \(5 \times 10^{-2} M\) solution of \(\mathrm{Pb}^{2+}\) ions, and pure tin in a \(0.25 M\) solution of \(\mathrm{Sn}^{2+}\) ions. (b) Write the spontaneous electrochemical reaction.

(a) Write the possible oxidation and reduction half-reactions that occur when magnesium is immersed in each of the following solutions: (i) HCl, (ii) an HCl solution containing dissolved oxygen, (iii) an HCl solution containing dissolved oxygen and, in addition, \(\mathrm{Fe}^{2+}\) ions. (b) In which of these solutions would you expect the magnesium to oxidize most rapidly? Why?

Using the results of Problem \(17.13,\) compute the corrosion penetration rate, in mpy, for the corrosion of iron in \(\mathrm{HCl}\) (to form \(\mathrm{Fe}^{2}\) ions if the corrosion current density is \(8 \times 10^{-5} \mathrm{A} / \mathrm{cm}^{2}\)
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