Weight loss (ASTM G1), penetration rate, and electrochemical (ASTM G102) methods — results in MPY, MM/Y, and MDD, with a severity classification against typical industry bands.
Per ASTM G1: corrosion rate (mpy) = (3.45×10⁶ × weight loss) ÷ (density × area × time in hours). Weight loss method assumes uniform corrosion.
CORROSION RATE
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MPY
MM/Y—
MDD—
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Depth is converted directly to mils per year, then adjusted for temperature using the common approximation that corrosion rate roughly doubles every 10°C — a rule of thumb, not a precise model for every reaction.
PENETRATION RATE
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MPY
MM/Y—
Temp. factor—
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Per ASTM G102: corrosion rate (mpy) = (0.1288 × corrosion current × equivalent weight) ÷ density. Weight loss over the measurement period is derived separately using Faraday's law.
CORROSION RATE
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MPY
MM/Y—
Predicted loss—
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Quick reference
Common units
MPY — mils per year (thousandths of an inch/year), most common in the US
MM/Y — millimeters per year, the metric equivalent
MDD — milligrams per square decimeter per day, common in lab weight-loss tests
What influences corrosion rate
Material composition and alloying elements
Temperature, humidity, and pollutant levels
pH — both highly acidic and highly alkaline environments accelerate corrosion
Chloride concentration and oxygen availability
Uniform vs. localized corrosion
Uniform corrosion is even across the surface, predictable, easier to design for
Localized (pitting, crevice, galvanic) occurs at specific sites and can cause failure even when the overall rate looks low
Pitting factor = max pit depth ÷ average metal loss; >3 indicates severe localized attack
Prevention
Select corrosion-resistant materials for the specific environment
Protective coatings, cathodic protection, or environmental control
Avoid crevices and galvanic couples; ensure proper drainage
Regular inspection and monitoring against trend data