Rust Decay Calculator — Steel Corrosion & Remaining Thickness
Estimate how much steel a given environment corrodes each year, how much thickness is left after an exposure period, and how long before the metal rusts through — based on ISO 9224 atmospheric corrosivity categories.
mm
Exposure environment
years
Estimated metal remaining after corrosion over the exposure period
- 1
Corrosion depth
0.07 × 20 = 1.4Rate (mm/year) × exposure period (years) - 2
Remaining thickness
6 − 1.4 = 4.60
How does this calculator work?
Corrosion depth = rate × years; remaining thickness = original − depth. Unprotected carbon steel loses ~0.07 mm/year in an urban atmosphere (ISO C3), so a 6 mm plate thins by 1.4 mm over 20 years and would rust through in about 86 years. Rates are editable ISO 9224 estimates (C1–C5). Always combine with measured data and engineering judgement for structural decisions.
Formula
How this is calculated
Iron and steel exposed to moisture and oxygen oxidise to form iron oxide (rust) at a rate that depends strongly on the chemical environment. ISO 9224 / ISO 9223 categorise atmospheric corrosivity into five classes: C1 (indoor, dry) to C5-M (marine splash zone). Typical linear uniform corrosion rates for unprotected carbon steel range from under 0.01 mm/year in a dry indoor space to 0.3–0.5 mm/year in a marine splash zone. These are editable estimates based on widely published corrosion data; actual rates vary with alloy grade, surface condition, temperature and humidity.
The calculator applies a simple linear model: corrosion depth = rate × years. This is conservative and widely used in structural-integrity assessments. In reality, early rust layers can sometimes slow further oxidation (forming a partial barrier), but surface moisture, salt and acid rain can accelerate it — so the linear model is a practical baseline. The "years to perforation" figure assumes constant rate throughout, which is appropriate for atmospheric corrosion of plain carbon steel but may overestimate service life in immersed or buried conditions.
For structural decisions, combine this estimate with an on-site thickness measurement (ultrasonic testing), apply an appropriate safety factor, and consult a qualified corrosion engineer. Protective coatings, galvanising or cathodic protection can reduce rates by 90–99% and are not modelled here.
Frequently asked questions
Unprotected carbon steel corrodes at roughly 0.001 mm/year indoors (C1), 0.025 mm/year in a rural atmosphere (C2), 0.07 mm/year urban (C3), 0.15 mm/year industrial/coastal (C4), and 0.4 mm/year in a marine splash zone (C5). These are averages from ISO 9224; actual rates vary significantly with humidity, salt and air pollutants.
For atmospheric corrosion, some studies show a slight self-limiting effect as a rust layer develops, better modelled as a power law (depth ∝ t^0.5 to t^0.7). However, for conservative engineering estimates and plain carbon steel, a linear rate is widely used. In wet or marine immersion conditions, there is generally no significant self-limiting effect.
Common protection methods include paint or powder-coating systems, hot-dip galvanising (zinc layer), epoxy coatings, cathodic protection (sacrificial zinc/magnesium anodes or impressed current), and selecting a more corrosion-resistant alloy such as stainless steel or weathering steel (Corten). Each method dramatically slows the corrosion rate — often by 90–99%.
Also known as
TG we-Calculate Editorial Team. (2026). Rust Decay Calculator — Steel Corrosion & Remaining Thickness [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/rust-decay-calculator
TG we-Calculate Editorial Team. "Rust Decay Calculator — Steel Corrosion & Remaining Thickness." TG we-Calculate. 2026. https://we-calculate.com/calculator/rust-decay-calculator.
TG we-Calculate Editorial Team, "Rust Decay Calculator — Steel Corrosion & Remaining Thickness," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/rust-decay-calculator
@misc{wecalculate_rust_decay_calculator, title = {Rust Decay Calculator — Steel Corrosion & Remaining Thickness}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/rust-decay-calculator}}, year = {2026}, note = {TG we-Calculate} }
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