Convert a weight-loss coupon test into a penetration rate. Enter the mass lost, the metal's density, the exposed area, and the exposure time, and the calculator returns the uniform corrosion rate in mils per year, millimetres per year, and micrometres per year — with a NACE rating that tells you whether the material is suitable for service.
The weight-loss coupon test is the most direct way to measure how fast a metal corrodes in a given environment: expose a sample of known area for a known time, clean it, and weigh how much metal was lost. This calculator converts that mass loss into a penetration rate — how quickly the surface recedes — expressed in mils per year, millimetres per year, and micrometres per year, and assigns a NACE relative-resistance rating so you can judge whether the alloy is fit for service.
CR = (K · W) / (ρ · A · t)
W is the mass loss in grams, ρ the metal density in g/cm³, A the exposed surface area in cm², and t the exposure time in hours. The constant K carries the unit conversion: K = 3.45 × 10⁶ gives the rate in mils per year (mpy, thousandths of an inch per year), and K = 8.76 × 10⁴ gives it in millimetres per year. Micrometres per year is simply the mm/yr value multiplied by 1000. The rate represents an average uniform penetration over the whole exposed surface.
The penetration rate is mass loss divided by density (a volume) divided by area (a depth) divided by time. The numerical constant K folds together the conversions between grams, cubic centimetres, the chosen length unit, and the conversion from hours to years. Using the matching K is essential: 3.45 × 10⁶ for mils/year and 8.76 × 10⁴ for mm/year. Picking the wrong constant is a common source of order-of-magnitude errors, so the calculator computes all units consistently from the same inputs.
A coupon of known composition and surface finish is weighed, exposed in the service environment (or a simulated one) for a set period, then retrieved, cleaned of corrosion product without removing sound metal, dried, and reweighed. The difference is the mass loss W. Standardized procedures (such as ASTM G1 for preparing and evaluating specimens, and ASTM G31 for immersion testing) specify cleaning, minimum exposure durations, and how to handle multiple coupons so the measured rate is repeatable and representative.
This calculation assumes uniform corrosion — the metal thins evenly across the whole surface. That assumption is valid for general attack but can be dangerously optimistic when corrosion is localized. Pitting, crevice corrosion, and stress-corrosion cracking concentrate the damage in small areas, so even a low average rate can hide deep pits that perforate a wall. Always inspect coupons for localized attack and supplement weight-loss data with pit-depth measurements when localized mechanisms are possible.
Mpy is mils per year — the number of thousandths of an inch (1 mil = 0.001 inch = 25.4 µm) that the metal surface recedes per year due to uniform corrosion. It is the traditional U.S. unit for corrosion rate; the metric equivalent is millimetres or micrometres per year.
One mil per year equals about 0.0254 mm/yr, so 1 mm/yr ≈ 39.4 mpy. This calculator avoids manual conversion by applying the appropriate K constant to the same inputs, reporting mpy, mm/yr, and µm/yr simultaneously so the three values are always consistent.
Per the common NACE relative-resistance scale, below 1 mpy is outstanding, 1–5 mpy excellent, 5–20 mpy good, 20–50 mpy fair, 50–200 mpy poor, and above 200 mpy generally unacceptable. The right threshold depends on design life and wall thickness — a thick vessel may tolerate a higher rate than thin tubing.
Because the equation gives an average uniform rate. If attack is localized — pitting, crevice corrosion, or cracking — a small area can be penetrating far faster than the average suggests. A coupon showing a low weight-loss rate can still develop a through-wall pit, so localized corrosion must be assessed separately by inspection and pit-depth measurement.
Long enough for the rate to stabilize and the mass loss to be measurable above weighing error. Short tests can overstate the rate because the protective oxide or passive film has not fully formed. Standards often recommend exposures of days to weeks, and running several coupons for different durations helps confirm a steady-state rate.