Corrosion Characterization and Measurement Technique

Corrosion Characterization and Measurement Techniques: Corrosion Rate – Definitions and Units ● Corrosion Current Density ● Cross-sectional Loss ● Thickness Loss ● Mass Loss ● Unit Conversion ● Instantaneous Corrosion Rate ▬ Series: Corrosion Characterization and Measurement Techniques
Different definitions of corrosion rate and the units commonly used to express it
Corrosion rate (corrosion velocity): The extent of metal loss due to corrosion per unit of time; used to measure the speed of corrosion. Expressed in two main ways: mass loss/gain methods, depth methods (annual corrosion depth), and current density (electrochemical).

1. 1. Mass-based corrosion rate (mass loss method; the most classic approach)
The mass of metal lost to corrosion per unit area and per unit of time.
Can be interconverted with depth-based corrosion rates and current density.

1.2 Depth-based Corrosion Rate (annual corrosion depth; most common in engineering applications; standard unit for ER [Electrical Resistance] and impedance-based corrosion rate measurements)
This metric converts mass loss into the thickness of metal lost to corrosion annually; it is the most intuitive measure and is used for engineering-based corrosion severity assessments.
Units: mm/a (millimeters per year; internationally standard); μm/a (micrometers per year; for low corrosion rates); mpy (mils per year; 1 mpy = 0.0254 mm/a; commonly used in the European and American petrochemical industries).

Common Corrosion Severity Levels (for carbon steel; reference values ​​in mm/a):
•    <0.05 mm/a: Excellent (low corrosion)
•    0.05–0.1 mm/a: Good
•    0.1–0.5 mm/a: Acceptable/Fair
•    >0.5 mm/a: Severe corrosion

Also used is "Corrosion Depth" (CD), representing the corrosion depth over a 30-day period; typically measured in Angstroms (Å). (This unit is often used for atmospheric corrosion monitoring in server rooms, power distribution cabinets, museums, electronics factories, etc.)

ISO 9223 Atmospheric Corrosivity (reference for server rooms):
C1 (Lowest corrosivity; ideal server room environment): Copper corrosion rate ≤ 0.1 μm/a = 1000 Å/a.

1.3 Current Density (used in electrochemistry and by atmospheric water-film corrosion sensors employing this technology)
Current density is typically used in scientific research.
According to Faraday's Law, corrosion current directly reflects the rate of corrosion:
Unit: μA/cm² (microamperes per square centimeter). The amount of charge passed corresponds to the weight of metal lost. Carbon steel plates are frequently used for atmospheric water-film corrosion testing, allowing for the conversion between weight loss and corrosion depth values.