---
title: "Galvanic and stress corrosion"
subject: "Material Science"
chapter: 8
key: "semester_3/material_science:galvanic-and-stress-corrosion"
source: https://www.bctengineer.com/q/semester_3/material_science/galvanic-and-stress-corrosion
site: BCT Engineering
---

# Galvanic and stress corrosion

Material Science · Chapter 8: Environmental Effects

- Asked in: year not given, 6 marks
- Video: [Materials Science: Corrosion, Part 1](https://www.youtube.com/watch?v=yuVQ_NOpqGc) — Welt der Werkstoffe
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/material_science/galvanic-and-stress-corrosion
- Whole chapter: https://www.bctengineer.com/md/semester_3/material_science/chapter-8

## Question

Explain galvanic corrosion and stress corrosion cracking. State the conditions under which each occurs and give one example of each.

## Short answer

Galvanic corrosion: two dissimilar metals in electrical contact in an electrolyte, anode corrodes (e.g., steel bolts on copper). Stress corrosion cracking: brittle cracking under tensile stress and a specific corrosive environment (e.g., chloride SCC of stainless steel). Condtions: for galvanic, potential difference, electrolyte, small anode area; for SCC, sustained tensile stress, susceptible alloy, specific medium.

## Answer

**Corrosion** is the deterioration of a metal by chemical or electrochemical reaction with its environment.

#### Galvanic corrosion
When two **dissimilar metals** are in electrical contact in the presence of an electrolyte (water, moisture), a galvanic cell forms. The more active metal (**anode**) corrodes and the more noble metal (**cathode**) is protected.

- Anode: $M \rightarrow M^{n+} + n e^-$
- Cathode (in neutral water): $O_2 + 2H_2O + 4e^- \rightarrow 4OH^-$

The metals' positions in the galvanic series decide which is the anode (e.g., Mg, Zn and Al are anodic to steel; steel is anodic to copper and stainless steel).

Corrosion is more severe with a larger potential difference, a small anode area compared with the cathode area, and a highly conductive electrolyte.

Example: steel bolts used on a copper plate, or an aluminium sheet riveted with steel rivets in sea water.

Prevention: select metals close together in the galvanic series, insulate them from each other, keep the anode area large, apply coatings, or use sacrificial anodes.

#### Stress corrosion cracking (SCC)
SCC is brittle cracking of a normally ductile metal under the combined action of **tensile stress** (applied or residual) and a **specific corrosive environment**.

| Condition | Explanation |
|---|---|
| Tensile stress | Residual stress from welding or cold work is enough |
| Specific environment | e.g., chlorides for austenitic stainless steel; ammonia for brass (season cracking); caustic for carbon steel (caustic embrittlement) |
| Susceptible alloy | Pure metals are generally immune |

At the crack tip, the stress breaks the protective film and the exposed metal dissolves, so the crack advances (intergranularly or transgranularly) with little overall metal loss.

Example: season cracking of brass cartridge cases, and chloride SCC of 304 stainless steel pipes.

Prevention: stress-relief annealing, shot peening, changing the alloy, controlling the environment.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What is required for galvanic corrosion?
   A. A single pure metal in dry air
   B. A tensile stress and a specific environment
   C. Two dissimilar metals in electrical contact with an electrolyte
   D. A coating of zinc
   Answer: C. A cell forms between dissimilar metals in contact through an electrolyte; the more active metal is the anode and corrodes. (Key point: Galvanic cell conditions)

2. Which area ratio makes galvanic corrosion most severe?
   A. Large anode, small cathode
   B. Equal areas
   C. Small anode, large cathode
   D. No anode
   Answer: C. A small anode with a large cathode concentrates the corrosion current on a small area. (Key point: Area effect)

3. Which combination is needed for stress corrosion cracking?
   A. Compressive stress and any environment
   B. Tensile stress, a specific corrosive environment and a susceptible alloy
   C. Two dissimilar metals and an electrolyte
   D. High temperature only
   Answer: B. SCC needs tensile stress (applied or residual), a specific environment for that alloy, and a susceptible alloy; pure metals are generally immune. (Key point: SCC conditions)

4. Which is an example of SCC?
   A. Steel bolts on a copper plate in sea water
   B. Zinc protecting steel
   C. Uniform rusting of a steel sheet
   D. Season cracking of brass in ammonia
   Answer: D. Brass in ammonia-containing environments cracks under stress; the steel-on-copper case is galvanic corrosion. (Key point: SCC examples)

## Related in other subjects

- [Conductor tension under varying conditions](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Atension-varying-conditions-equation) — Transmission and Distribution System Design, chapter 4
- [Conditions for axisymmetric stress](https://www.bctengineer.com/question.md?key=semester_8%2Fcomputational_techniques_in_civil_engineering%3Aaxisym-stress-conditions) — Computational Techniques in Civil Engineering, chapter 3
- [Breakdown mechanisms in solids under working conditions](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Asolid-breakdown-actual-conditions) — High Voltage Engineering, chapter 6
- [Ni/Cd galvanic cell: electrodes, emf](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Ani-cd-cell) — Engineering Chemistry, chapter 1