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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 where a material's natural tendency to corrode or dissolve is significantly reduced due to the formation of a thin, adherent oxide film on the surface, acting as a protective barrier. Two common examples of passive alloys are stainless steel and aluminum alloys. Stainless steel contains a minimum of 10.5% chromium, forming a chromium oxide film, while aluminum alloys form an aluminum oxide layer, both offering corrosion-resistant properties.

Step by step solution

01

Definition of Passivity

Passivity is a phenomenon where a material's natural tendency to corrode or dissolve is significantly reduced due to the formation of a thin, adherent oxide film that forms on the material's surface. This passive film acts as a barrier that protects the material from further corrosion, thus increasing its durability and longevity.
02

Example 1: Stainless Steel

Stainless steel is a common passive alloy that is widely used in various applications due to its corrosion-resistant properties. This family of alloys contains a minimum of 10.5% chromium, which reacts with the surrounding environment to form a thin oxide layer (mainly chromium oxide) on the surface. The chromium oxide film is highly stable, self-repairing, and strongly adherent to the surface, which effectively prevents further reactions with corrosive agents. This passivation makes stainless steel a popular choice for products that require long-lasting corrosion resistance, such as kitchen appliances, cutlery, and medical instruments.
03

Example 2: Aluminum Alloy

Aluminum alloys are another example of materials that can undergo passivation. Natural or induced passive films on aluminum surfaces are typically composed of aluminum oxide, which has excellent corrosion resistance properties. When exposed to the environment, the aluminum surface reacts rapidly with oxygen, forming a thin aluminum oxide layer that is highly inert and chemically stable, protecting the underlying material from further deterioration. Due to its lightweight and corrosion-resistant properties, aluminum alloys are commonly used in various industries, including automotive, aerospace, and construction.

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Most popular questions from this chapter

For the following pairs of alloys that are coupled in seawater, predict the possibility of corrosion; if corrosion is probable, note which metal/alloy will corrode. (a) Aluminum and cast iron (b) Inconel and nickel (c) Cadmium and zinc (d) Brass and titanium (e) Low-carbon steel and copper

(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 \(\mu \mathrm{A} / \mathrm{cm}^{2}\left(10^{-6} \mathrm{~A} / \mathrm{cm}^{2}\right)\).

Briefly describe the two techniques that are used for galvanic protection.

(a) From the galvanic series (Table \(17.2\) ), cite three metals/alloys that may be used to galvanically protect cast iron. (b) As Concept Check \(17.4\) (b) notes, galvanic corrosion is prevented by making an electrical contact between the two metals in the couple and a third metal that is anodic to the other two. Using the galvanic series, name one metal that could be used to protect a nickel-steel galvanic couple.

Why does chromium in stainless steels make them more corrosion resistant than plain carbon steels in many environments?
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