Rust Decay Calculator

Estimate a metal's corrosion (rust) rate from mass-loss test data using the standard ASTM G1 formula, then see the rate in mm/year and mils/year plus an estimated time to perforation.

Quick Facts

Formula
ASTM G1 mass-loss method: CR = (K × W) / (A × T × D)
K = 8.76×10⁴ gives the corrosion rate in millimeters per year.
Severity scale
Excellent < 0.02, Good 0.02–0.1, Fair 0.1–0.5, Poor 0.5–1.0, Unacceptable > 1.0 mm/yr
Common corrosion-engineering classification for structural metals.

Your Results

Calculated
Corrosion rate
-
Millimeters per year (mm/yr)
Corrosion rate
-
Mils per year (mpy)
Time to perforation
-
Years to corrode through starting thickness
Severity
-
Classification for this rate

Ready

Enter mass-loss test data, then press Calculate.

Understanding the Rust Decay Calculator

This tool estimates how fast a metal is corroding using the mass-loss method, the standard laboratory and field technique described in ASTM G1 ("Standard Practice for Preparing, Cleaning, and Evaluating Corrosion Test Specimens"). A sample of known area is exposed to a corrosive environment for a measured time, then cleaned of rust and re-weighed. The mass it lost translates directly into a corrosion rate.

The formula

The mass-loss corrosion rate is:

CR = (K × W) / (A × T × D)

  • W — mass loss in grams (initial mass minus final mass, measured after removing rust from the sample)
  • A — exposed surface area in cm²
  • T — exposure time in hours
  • D — metal density in g/cm³ (7.85 g/cm³ for mild steel)
  • K — a units constant; this calculator uses K = 8.76 × 10⁴ so CR comes out in millimeters per year, then converts to mils per year (1 mm = 39.3701 mils)

Once the annual corrosion rate is known, dividing a starting material thickness by that rate gives a simple straight-line estimate of how many years remain before the metal corrodes all the way through — a useful planning number for pipe walls, tanks, brackets, or any thin structural section.

Reading the severity classification

Corrosion engineers commonly sort mass-loss rates into five bands: Excellent (below 0.02 mm/yr), Good (0.02–0.1 mm/yr), Fair (0.1–0.5 mm/yr), Poor (0.5–1.0 mm/yr), and Unacceptable (above 1.0 mm/yr) for general structural service. Coated, stainless, or alloyed metals typically land in the Excellent–Good range; bare carbon steel exposed outdoors without protection often falls in Fair to Poor.

Assumptions and limits

This is a straight-line (linear) projection: it assumes the corrosion rate measured during the test period stays constant into the future. Real corrosion is often faster early on and can slow as protective oxide layers form, or speed up under pitting, galvanic contact with a dissimilar metal, chlorides, or cyclic wet-dry conditions. Treat the time-to-perforation figure as an order-of-magnitude planning estimate, not a guaranteed failure date, and re-test periodically for anything safety-critical.

Frequently Asked Questions

What formula does this rust decay calculator use?
It uses the ASTM G1 mass-loss method: corrosion rate CR = (K × W) / (A × T × D), where W is the mass lost during exposure (initial mass minus final mass after removing corrosion products), A is the exposed surface area, T is the exposure time, D is the metal's density, and K is a units constant (8.76×10⁴ for millimeters per year). The result is also converted to mils per year and combined with a starting thickness to estimate years until the material corrodes through.
What counts as a normal corrosion rate for steel?
Corrosion engineers commonly classify mass-loss corrosion rates as Excellent below 0.02 mm/year, Good from 0.02 to 0.1 mm/year, Fair from 0.1 to 0.5 mm/year, Poor from 0.5 to 1.0 mm/year, and Unacceptable above 1.0 mm/year for typical structural applications. Bare mild steel exposed outdoors without a protective coating often falls in the Fair to Poor range.
Why remove rust before weighing the final mass?
The ASTM G1 method measures metal actually consumed by corrosion, not the rust product that builds up on the surface. Iron oxide weighs more than the metallic iron it replaced, so weighing a still-rusted sample would understate the true mass loss and corrosion rate.

Related calculators