Problem 122 - Chemistry An Atoms First Approach

Question

Which of the following statements concerning corrosion is(are) true? For the false statements, correct them. a. Corrosion is an example of an electrolytic process. b. Corrosion of steel involves the reduction of iron coupled with the oxidation of oxygen. c. Steel rusts more easily in the dry (arid) Southwest states than in the humid Midwest states. d. Salting roads in the winter has the added benefit of hindering the corrosion of steel. e. The key to cathodic protection is to connect via a wire a metal more easily oxidized than iron to the steel surface to be protected.

Step-by-Step Solution

Answer

False statements and their corrections: a. Corrosion is an example of an electrolytic process. Correction: Corrosion is an example of a galvanic process. c. Steel rusts more easily in the dry (arid) Southwest states than in the humid Midwest states. Correction: Steel rusts more easily in the humid Midwest states than in the dry Southwest states. d. Salting roads in the winter has the added benefit of hindering the corrosion of steel. Correction: Salting roads in the winter accelerates the corrosion of steel. True statements: b. Corrosion of steel involves the reduction of iron coupled with the oxidation of oxygen. e. The key to cathodic protection is to connect via a wire a metal more easily oxidized than iron to the steel surface to be protected.

Step 1: Statement a

Corrosion is an example of an electrolytic process. False. Corrosion is an example of a galvanic process, not an electrolytic one. In an electrolytic process, an electric current is applied to drive a nonspontaneous redox reaction, while in a galvanic process, the redox reaction happens spontaneously.

Step 2: Statement b

Corrosion of steel involves the reduction of iron coupled with the oxidation of oxygen. True. The corrosion of steel mainly consists of two half-reactions: the reduction of oxygen (O2) to form hydroxide ions (OH-) and the oxidation of iron (Fe) to form iron(II) ions (Fe²⁺). The hydroxide ions and iron(II) ions react with each other to form a hydrated complex of iron(III) oxide, also known as rust.

Step 3: Statement c

Steel rusts more easily in the dry (arid) Southwest states than in the humid Midwest states. False. Steel rusts more easily in humid environments than in dry environments. The presence of moisture and oxygen is necessary for the corrosion process to occur. In the humid Midwest states, there is a higher amount of moisture in the air, which makes the corrosion process faster.

Step 4: Statement d

Salting roads in the winter has the added benefit of hindering the corrosion of steel. False. Salting roads in the winter actually accelerates the corrosion of steel. The salt lowers the freezing point of water, allowing the presence of more liquid water. Additionally, dissolved salt increases the conductivity of the water, providing a more conducive environment for the electrochemical corrosion process to occur.

Step 5: Statement e

The key to cathodic protection is to connect via a wire a metal more easily oxidized than iron to the steel surface to be protected. True. Cathodic protection is a method used to prevent corrosion by making the metal surface serve as the cathode in a galvanic cell. By connecting a more easily oxidized metal (known as the sacrificial anode) to the steel surface via a wire, the sacrificial anode gets oxidized instead of the iron, thus protecting the steel from corrosion.

Key Concepts

Galvanic Process

Corrosion of steel is often misunderstood as an electrolytic process, when in fact, it is an example of a galvanic process. Unlike electrolytic processes, a galvanic process does not require an external power source to drive the reaction. Instead, it occurs naturally when two different metals, or areas on a piece of metal that have dissimilar potentials, come into contact in the presence of an electrolyte, such as water containing dissolved oxygen and ions.

A simple example of a galvanic cell involves a piece of steel connected to a piece of a more noble metal like copper. In this cell, steel acts as the anode, corroding over time, while the copper acts as the cathode and remains protected. The electrolyte facilitates the movement of ions allowing the circuit to complete, while the electron flow from steel to copper is what drives the reaction forward. Understanding this spontaneous electrochemical process is crucial in creating prevention strategies for the corrosion of steel.

Cathodic Protection

Cathodic protection is a highly effective method to prevent steel corrosion. It involves transforming the steel into the cathode of an electrochemical cell. By attaching a more reactive metal, known as the sacrificial anode, to the steel, the anode will preferentially corrode instead of the steel. The steel is thus 'protected' as it now becomes the cathode.

Sacrificial Anodes

Common sacrificial anodes are made of zinc, magnesium, or aluminum, all of which are more anodic than iron. They will corrode, or 'sacrifice' themselves, to protect the steel. This technique is widely used in protecting pipelines, ship hulls, and water heaters. Proper monitoring and replacement of the sacrificial anodes are necessary to ensure ongoing protection, as they will eventually corrode completely.

Electrochemical Corrosion

Electrochemical corrosion is the gradual deterioration of metal as a result of chemical reactions between it and its environment. It is a natural occurrence where metal atoms lose electrons and form positive ions, often leading to rust in the case of iron or steel.

Rusting, the most common form of electrochemical corrosion, is accelerated by the presence of electrolytes such as saltwater, which increases the conductivity and enables the redox reaction to occur more readily. Factors such as pH, temperature, and environmental conditions like humidity also influence the rate of this corrosion.

Role of Environments

The higher humidity in areas like the Midwest of the United States creates a more conducive environment for rust to form than the dry Southwest. Similarly, salting roads in winter does not help to prevent rust; it actually lowers the freezing point of water, allows liquid brine to form, and significantly accelerates the electrochemical reaction that leads to steel corrosion.