Corrosions Terms

Overview

Engineering reference data for Corrosions Terms in material science and properties.

Key Formulas

Stress

$\sigma = \frac{F}{A}$

Force per unit area.

Strain

$\varepsilon = \frac{\Delta L}{L_0}$

Change in length per original length.

Hooke's Law

$\sigma = E \varepsilon$

Stress proportional to strain in elastic region.

Thermal Expansion

$\Delta L = \alpha L_0 \Delta T$

Length change due to temperature.

Variables

Symbol	Description	Unit
$\sigma$	Stress	Pa
$\varepsilon$	Strain	—
$E$	Young's modulus	Pa
$\alpha$	Thermal expansion coefficient	1/°C
$\Delta T$	Temperature change	°C

Corrosion Types

Corrosion can be classified by its mechanism, environment, or the form it takes. Understanding the type is critical for selecting the correct protection strategy and interpreting rate data.

• Uniform/General Corrosion: Occurs evenly across the entire exposed surface. It is relatively easy to predict and manage. The corrosion rate ($CR$) is the primary metric.
• Galvanic Corrosion: Occurs when two dissimilar metals are in electrical contact in a corrosive electrolyte. The more anodic (active) metal corrodes preferentially.
• Pitting Corrosion: A localized form that creates small pits or cavities. It is dangerous because it can lead to rapid penetration failure while the overall material loss is minimal. Often represented by maximum pit depth ($P_d$).
• Crevice Corrosion: A localized form that occurs within shielded areas (e.g., under gaskets, washers, or deposits) where stagnant solution chemistry differs from the bulk.
• Intergranular Corrosion: Occurs at the grain boundaries of an alloy, often due to sensitization (e.g., in stainless steels from heat treatment).
• Stress Corrosion Cracking (SCC): The combined action of a tensile stress and a specific corrosive environment leads to cracking. Neither stress nor the environment alone would cause failure.
• Erosion-Corrosion: Results from the combined effect of mechanical abrasion (erosion) and electrochemical corrosion. It is common in piping systems with high fluid velocities, turbulence, or suspended solids.

Measurement Units

The Corrosion Rate ($CR$) is often expressed in multiple units. The formulas in the existing Key Formulas section facilitate conversion between these fundamental units.

Unit	Symbol	Typical Application
Millimeters per year	mpy	Common international metric unit.
Mils per year	mpy	Common US customary unit. 1 mil = 0.001 inch.
Microinches per year	µin/yr	Often used for very low corrosion rates or in electronics.
Grams per square meter per hour	g/m²/hr	Common in laboratory weight-loss tests.

Note: To convert from a mass-loss-based corrosion rate (e.g., g/m²/hr) to a penetration-based rate (e.g., mpy), you need the density of the corroding material.

Practical Considerations

When applying corrosion rate data, consider the following:

1. Environment Aggressiveness: The $CR$ measured in one environment (e.g., distilled water) is not directly applicable to another (e.g., seawater). The specific ions, pH, temperature, and dissolved gases dramatically alter corrosion kinetics.
2. Flow Velocity: Quiescent conditions are not directly comparable to flowing or turbulent conditions. Erosion-corrosion effects are velocity-dependent.
3. Time Dependency: Corrosion rates are not constant. Initial rates may differ significantly from long-term averages due to the formation of protective films (patina, scale, passive oxide layers). Extrapolating short-term data to long-term life predictions requires caution.
4. Material Condition: The metallurgical state (annealed, cold-worked, sensitized), surface finish, and prior history all influence corrosion resistance. Data from pristine lab samples may not reflect field performance of welded, fabricated components.

References

• Original Source