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What Is a Corrosion Rate? | Piping Engineering Glossary
What Is a Corrosion Rate?
In piping engineering and process engineering, a corrosion rate is the speed at which a material loses thickness or mass due to corrosion, usually expressed in millimeters per year (mm/y) or mils per year (mpy), where one mil is a thousandth of an inch. It is one of the most important numbers in mechanical integrity, because it determines how long equipment can safely remain in service.
How Corrosion Rate Is Measured
The most common method compares wall thickness measurement values taken at the same location at different times. Dividing the thickness lost by the time between readings gives the rate. Inspection codes distinguish a long-term rate, based on the earliest reading, from a short-term rate, based on the two most recent readings, and use the more representative or conservative value.
Corrosion rates are also measured with weight-loss corrosion coupon specimens, electrical resistance probes, and other online corrosion monitoring tools. These methods detect changes faster than periodic thickness surveys and help link corrosion to process conditions.
Applications
Remaining Life Calculations
A remaining life assessment divides the thickness still available above the minimum required thickness by the corrosion rate. The result sets the next inspection date and any planned replacement.
Material Selection and Design
Expected rates guide material choice and the corrosion allowance added to wall thickness. Designers compare predicted rates with an allowable corrosion rate for the intended service life.
Process Changes
Changes in feedstock, temperature, or velocity can change corrosion rates quickly. Monitoring helps detect problems such as erosion corrosion soon after a process change.
Benefits
Predictable Integrity Planning
Known rates let owners schedule inspections, repairs, and replacements before failures occur. This reduces unplanned shutdowns and emergency repairs.
Early Warning
An increasing rate is often the first sign of a process upset or a new damage mechanism. Investigating early can prevent leaks.
Better Economic Decisions
Accurate rates help decide whether to upgrade materials, add inhibitors, or simply plan replacement. Spending can then match the actual risk.
Limitations
Averages Hide Localized Attack
A rate calculated from spot readings may not reflect pitting or other localized corrosion. General corrosion is much easier to track than localized damage.
Measurement Error
Small thickness changes over short periods can be lost in measurement uncertainty. Short-term rates can therefore be misleading.
Rates Change Over Time
Corrosion is rarely constant. Changes in operation can make historical rates poor predictors of future damage.
FAQ
How do I convert mpy to mm/y?
Multiply mils per year by 0.0254. For example, a rate of 10 mpy equals 0.254 mm per year.
What counts as a low corrosion rate?
There is no universal threshold, but many guidelines treat rates below about 0.1 mm per year (4 mpy) as low for carbon steel, while rates above roughly 0.5 mm per year (20 mpy) usually need attention.
What is the difference between long-term and short-term corrosion rates?
The long-term rate uses the earliest available thickness reading, while the short-term rate uses the two most recent readings. Comparing them shows whether corrosion is speeding up or slowing down.
How does corrosion rate affect inspection intervals?
Higher rates mean shorter remaining life and more frequent inspection. Codes often limit the interval to a fraction of remaining life, such as half, up to a maximum number of years.
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