A Closer Look at Common Stress Corrosion Cracking Problems

Introduction

Stress corrosion cracking (SCC) poses a significant threat across various industries, often leading to unexpected and potentially catastrophic failures. While the fundamental requirements for SCC involve:

A susceptible material,
A specific environment, and
Sufficient tensile stress,

certain combinations of materials and environments are particularly vulnerable to this insidious form of corrosion.

This article explores some of these “particular problem systems” in detail, based on the insights from the “Guides to Good Practice in Corrosion Control.”

Topics covered: Stress Corrosion Cracking Problems

Quiz on Stress Corrosion Cracking Problems

SCC Quiz

1. What are the three fundamental requirements for stress corrosion cracking (SCC) to occur?

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2. In what kind of environment does season cracking of brass commonly occur?

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3. Which type of cracking morphology is typically observed in austenitic stainless steels due to SCC in chloride environments?

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4. What strength level makes high strength steels especially vulnerable to hydrogen embrittlement?

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5. What is the primary type of SCC observed in high-strength aluminium alloys?

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Season Cracking of Brass in Ammonia Environments

One of the earliest identified SCC issues involved brass failures in ammonia-containing environments.

Historical Context: Noticed in brass cartridge cases used by the British Army in India. The ammonia came from organic matter decomposition.
Name Origin: The term “season cracking” was coined either because failures were most common in the rainy season, or due to the visual similarity with cracks in seasoned wood.
Failure Type: This SCC typically results in intergranular cracking.

Despite the rise of plastics in brass applications, this form of SCC is still a concern.

Austenitic Stainless Steels in Chloride-Containing Environments

Austenitic stainless steels are vulnerable to SCC in hot chloride solutions.

Trigger Conditions:
- High chloride concentrations.
- Heated surfaces (due to boiling or evaporation).
- Localized corrosion (e.g., pitting or crevice corrosion).
Even tap water can cause SCC under certain conditions.
Temperature Sensitivity: SCC generally occurs above 70 °C, though acidic environments can lower this threshold.
Stress Sources: Often residual stress from welding or fabrication.
Crack Morphology: Typically transgranular, but may become intergranular if sensitization occurs.

Carbon and Low Alloy Steels in Passivating Environments

These steels are prone to SCC in environments that form protective passivating films, such as:

Strong caustic solutions
Phosphates, nitrates, carbonates
Hot water

Unlike hydrogen embrittlement, SCC in these systems usually doesn’t occur with general corrosion.

Historic Examples:

Caustic cracking in 19th-century steam boilers due to caustic concentration at rivet leaks → led to boiler explosions.
Modern pipelines have cracked in carbonate solutions formed beneath coatings due to cathodic protection → cracks often run longitudinally along pipes.

Fracture Mode: Mostly intergranular (I).

Hydrogen Embrittlement in High Strength Steels: Stress Corrosion Cracking Problems

All steels can be affected by hydrogen, but high strength steels are especially vulnerable to hydrogen embrittlement (HE).

Key Factors:

Strength Threshold:
- <600 MPa: HE is generally unlikely.
- 1000 MPa: HE becomes a significant concern.
Hydrogen Sources:
- Welding, pickling, electroplating
- Hydrogen-containing gases
- In-service corrosion
Hydrogen Behavior:
- Migrates to high triaxial tensile stress zones (e.g., ahead of cracks or notches).
- Facilitates fracture via cleavage or intense plastic deformation.
Fracture Type: Can be intergranular, transgranular, or mixed (M).
Prevention Tip: Baking components at ~200 °C post-hydrogen exposure can reduce embrittlement risk.

SCC in High Strength Aluminium Alloys: Stress Corrosion Cracking Problems

Even though aluminium alloys have a face-centred cubic (fcc) structure (slower hydrogen diffusion than steels), high strength variants are also vulnerable to SCC.

Important Points:

Cracking Type: Mostly intergranular (I).
Strength Dependency: Higher strength → greater SCC risk.
Microstructure & Heat Treatment: Strongly influence SCC resistance.
Environments:
- Humid air
- Salt solutions

Some high-strength aluminium alloys show excellent SCC resistance and are widely used in aircraft.

Conclusion: Engineering Awareness is the First Line of Defence

Understanding these specific systems and the conditions under which SCC is likely to occur is essential for:

Engineers and material scientists
Designing robust systems
Selecting appropriate materials
Implementing effective corrosion control measures

By recognizing the interplay between material susceptibility, aggressive environments, and residual/applied stress, proactive steps can be taken to prevent costly and dangerous failures.