Question

The following quotation is taken from an article dealing with corrosion of electronic materials: "Sulfur dioxide, its acidic oxidation products, and moisture are well established as the principal causes of outdoor corrosion of many metals." Using \(\mathrm{Ni}\) as an example, explain why the factors cited affect the rate of corrosion. Write chemical equations to illustrate your points. (Note: \(\mathrm{NiO}(s)\) is soluble in acidic solution.)

Short answer

In the corrosion of metals like nickel (Ni), sulfur dioxide (SO₂) reacts with oxygen (O₂) and moisture (H₂O) to form corrosive sulfurous acid (H₂SO₃) and corresponding salts. This process promotes the oxidation of nickel, forming soluble metal sulfates that lead to corrosion: \[ 2\, \mathrm{Ni} + 2\, \mathrm{SO}_2 + O_2 + 2\, H_2O \to 2\, \mathrm{NiSO}_4 + 2\, H_2O \] Acidic oxidation products, such as sulfuric acid (H₂SO₄), react with metal oxides like nickel oxide (NiO) to form soluble salts. The presence of sulfuric acid dissolves NiO, promoting corrosion: \[ \mathrm{NiO} + \mathrm{H}_2\mathrm{SO}_4 \to \mathrm{NiSO}_4 + \mathrm{H}_2\mathrm{O} \] Moisture is essential for corrosion to occur, enabling the electrochemical process and helping dissolve metal oxides. In summary, sulfur dioxide, acidic oxidation products, and moisture contribute to the corrosion of metals by promoting the formation of soluble salts and dissolving metal oxides, as illustrated by the chemical equations.

Step-by-step solution

1. Overview of the corrosion process

Corrosion is an electrochemical process where a metal loses its metallic properties (such as conductivity) as a result of its reaction with its environment, leading to the formation of a more stable compound such as an oxide or a sulfate. Two main reactions occur in corrosion: the oxidation reaction (anodic reaction) and the reduction reaction (cathodic reaction).

2. Sulfur dioxide's role in corrosion

Sulfur dioxide (SO₂) affects the corrosion of metals by reacting with oxygen (O₂) and moisture (H₂O) in the air to produce sulfurous acid (H₂SO₃) and corresponding salts, which are corrosive agents. In the case of nickel, SO₂ reacts with oxygen and moisture to promote the oxidation of nickel, forming soluble metal sulfates that lead to corrosion. The overall chemical process is: \[ 2\, \mathrm{Ni} + 2\, \mathrm{SO}_2 + O_2 + 2\, H_2O \to 2\, \mathrm{NiSO}_4 + 2\, H_2O \]

3. The acidic oxidation products' role in corrosion

Sulfurous acid (H₂SO₃) and other acidic oxidation products, such as sulfuric acid (H₂SO₄), are produced as a result of the reaction between sulfur dioxide (SO₂), oxygen (O₂), and moisture (H₂O). These acidic substances can react with metal oxides, such as nickel oxide (NiO), to form soluble metal salts. In the case of nickel, the presence of sulfuric acid dissolves nickel oxide (NiO), thereby promoting corrosion. The chemical equation illustrating this process is: \[ \mathrm{NiO} + \mathrm{H}_2\mathrm{SO}_4 \to \mathrm{NiSO}_4 + \mathrm{H}_2\mathrm{O} \]

4. Moisture's role in corrosion

Moisture is essential for the corrosion process to occur. Water is necessary for the metal to undergo an electrochemical process. In the case of nickel, the presence of water enables the reaction between nickel, oxygen, and sulfur dioxide, which leads to the formation of soluble salts and continued corrosion. Furthermore, water helps acidic oxidation products to dissolve metal oxides, as shown in the second step. In conclusion, sulfur dioxide, its acidic oxidation products, and moisture play significant roles in the corrosion of metals. In the case of nickel, these factors lead to the formation of soluble salts, promoting the corrosion process. The provided chemical equations illustrate the involvement and impact of these substances in the corrosion mechanism.

Key concepts

Electrochemical process

Corrosion of metals involves an electrochemical process. This means that the metal undergoes both chemical and electrical changes simultaneously. In simple terms, you can think of it as a battery. In corrosion, a metal like nickel (\( \mathrm{Ni} \)) reacts with elements in its environment, losing its properties and gaining a more stable form, such as an oxide or a sulfate.

• Anodic reaction: The metal loses electrons (oxidation) and becomes positively charged.
• Cathodic reaction: The metal reacts with and reduces another substance, often oxygen from the air, gaining electrons in the process.

These reactions involve the transfer of electrons, making electrochemical principles a key part of understanding corrosion. The loss of electrons at the anodic site results in the formation of metal ions, subsequently reacting to form corrosion products like oxides and sulfates.

Sulfur dioxide

Sulfur dioxide (\( \mathrm{SO}_2 \)) is a gas that plays a significant role in the corrosion process. In the atmosphere, it reacts with oxygen (\( \mathrm{O}_2 \)) and moisture (\( \mathrm{H}_2\mathrm{O} \)) to form sulfurous acid (\( \mathrm{H}_2\mathrm{SO}_3 \)) and, potentially, sulfuric acid (\( \mathrm{H}_2\mathrm{SO}_4 \)), both of which are very corrosive.

• Sulfur dioxide can accelerate the oxidation of metals like nickel, converting them to soluble sulfates that contribute to corrosion.
• These reactions also produce acidity, lowering the pH of any moisture in contact with metals, further promoting the corrosion rate.

In summary, sulfur dioxide not only contributes to the formation of acidic components but also facilitates the dissolution of metal oxides, leading to increased corrosion.

Nickel oxide dissolution

Nickel oxide (\( \mathrm{NiO} \)), unlike some other metal oxides, is soluble in acidic environments. When exposed to acidic oxidation products like sulfuric acid (\( \mathrm{H}_2\mathrm{SO}_4 \)), the nickel oxide reacts to form soluble nickel sulfate (\( \mathrm{NiSO}_4 \)):\[\mathrm{NiO} + \mathrm{H}_2\mathrm{SO}_4 \to \mathrm{NiSO}_4 + \mathrm{H}_2\mathrm{O}\]

• In this reaction, nickel oxide dissolves, creating nickel sulfate, a compound that is not protective like nickel oxide.
• This dissolution encourages further exposure of fresh nickel metal to the corrosive environment, which perpetuates the corrosion cycle.

Understanding this dissolution process is crucial since it breaks down the protective layers that might otherwise hinder ongoing metal oxidation.

Moisture in corrosion

Moisture is a catalyst in the corrosion of metals. It enables the essential electrochemical reactions that drive corrosion. Water not only provides a medium for ions to move, enhancing the electrochemical processes, but it also participates directly in reactions.

• Moisture allows sulfur dioxide and oxygen to form sulfuric acid, intensifying corrosion.
• Water further helps dissolve metal oxides, like nickel oxide, by providing solvent pathways, thereby promoting even more metal dissolution.

Thus, the presence of moisture is indispensable in breaking down metal, providing the necessary environment for corrosion reactions to proceed and form corrosive products.