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Chapter 17: Problem 20

Briefly describe the phenomenon of passivity. Name two common types of alloy that passivate.

Short Answer

Expert verified
Answer: Passivity is a phenomenon in which a material (usually a metal or alloy) exhibits high resistance to corrosion and oxidation due to the formation of a thin, inert, and adherent surface oxide film that acts as a protective barrier. Two common types of alloys that exhibit passivation are stainless steel and aluminum alloys.

Step by step solution

01

Definition of Passivity

Passivity is a phenomenon in which a material (typically a metal or alloy) exhibits an unusually high resistance to corrosion and oxidation in a given environment. This occurs because the formation of a thin, inert, and adherent surface oxide film provides a protective barrier that shields the underlying material from further reaction with corrosive agents, such as water and oxygen.
02

Explanation of Passivity

In certain conditions, when a metal or its alloy comes into contact with a corrosive environment, the initial oxidation forms a thin, compact, and stable oxide layer. This layer acts as a barrier that prevents further oxidation or corrosion of the material. The formation and maintenance of this passivating oxide layer are dynamic processes, involving the continuous dissolution of the oxide into the environment and its reformation. The rate of these processes determines the stability and effectiveness of the passivating layer.
03

First Common Alloy: Stainless Steel

Stainless steel is one of the most common types of alloy that exhibits passivation. It is an iron-based alloy containing at least 10.5% chromium, which forms a passive chromium oxide layer on the surface when exposed to oxygen. This property makes stainless steel highly resistant to the corrosive effects of water, air, and certain chemicals when compared to regular steel.
04

Second Common Alloy: Aluminum Alloys

Aluminum alloys are another common type of alloy that exhibits passivation. Pure aluminum forms a natural, thin aluminum oxide layer when exposed to oxygen, making it naturally resistant to corrosion. In aluminum alloys, the addition of other elements, such as magnesium or silicon, can further enhance the corrosion resistance by promoting the passivation phenomenon. These alloys are widely used in industries such as aerospace, automotive, and construction due to their lightweight, high strength, and corrosion-resistant properties.

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Most popular questions from this chapter

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

A piece of corroded metal alloy plate was found in a submerged ocean vessel. It was estimated that the original area of the plate was \(800 \mathrm{cm}^{2}\) and that approximately \(7.6 \mathrm{kg}\) had corroded away during the submersion. Assuming a corrosion penetration rate of \(4 \mathrm{mm} / \mathrm{yr}\) for this alloy in seawater, estimate the time of submersion in years. The density of the alloy is \(4.5 \mathrm{g} / \mathrm{cm}^{3}\)

(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 CPR in mpy and i in \(\mu A / \mathrm{cm}^{2}\) \(\left(10^{-6} \mathrm{A} / \mathrm{cm}^{2}\right)\)

(a) Cite the major differences between activation and concentration polarizations. (b) Under what conditions is activation polarization rate controlling? (c) Under what conditions is concentration polarization rate controlling?

According to Table \(17.3,\) the oxide coating that forms on silver should be nonprotective, and yet Ag does not oxidize appreciably at room temperature and in air. How do you explain this apparent discrepancy?
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