Rust Decay Calculator

Estimate average corrosion rate, remaining thickness, and time to half thickness from field measurements.

Corrosion Progression Estimate

This screening calculation assumes measured thickness loss continues at a constant average rate.

Initial Material Thickness (mm)

Current Rust Depth (mm)

Exposure Time (years)

About Rust and Corrosion Estimates

Rust is the visible product of iron or steel reacting with oxygen and moisture. As corrosion progresses, sound metal is converted into oxides that occupy more volume but provide less structural strength. Measuring the resulting depth or thickness loss over a known exposure period provides a simple average corrosion rate. This calculator divides current rust depth by exposure time and uses that linear rate to project future loss. The remaining-thickness result subtracts measured rust depth from the original material thickness. The service projection reports how much additional time would pass before half of the original thickness remains if the same rate continues. Half thickness is only a convenient comparison point, not a universal failure limit. Safe minimum thickness depends on load, geometry, pressure, fatigue, defects, design codes, and the consequences of failure. Real corrosion rarely remains perfectly linear. A protective oxide layer can slow attack, while damaged coatings, trapped water, salt, acids, heat, galvanic contact, or changing oxygen levels can accelerate it. Pitting can penetrate deeply even when average loss is small, and rust scale can make direct depth measurements misleading. Measurements should therefore be taken on cleaned surfaces with suitable ultrasonic, mechanical, or laboratory methods and repeated at consistent locations. Use this result for preliminary screening, maintenance planning, and comparison between inspection periods. It can help identify whether a measured loss is broadly slow or rapid and show the significance of continued exposure. It is not a structural assessment or a substitute for applicable corrosion standards. Inspect critical equipment through a qualified engineer or corrosion professional, especially for pressure vessels, bridges, lifting equipment, vehicles, pipelines, and occupied structures. If the corrosion rate changes over time, calculate separate interval rates and use the most relevant conservative value rather than assuming the lifetime average will continue. Keep inspection records that identify the exact measurement location, instrument, surface preparation, and date. Consistent records make trend calculations more meaningful and help distinguish genuine material loss from measurement variation.

Rust Decay Examples

Measurements	Average Result	Interpretation
10 mm initial, 2 mm loss, 4 years	0.5 mm/year; 8 mm remains	Half thickness in 6 more years
12 mm initial, 1.5 mm loss, 5 years	0.3 mm/year; 10.5 mm remains	Half thickness in 15 more years
8 mm initial, 0.8 mm loss, 10 years	0.08 mm/year; 7.2 mm remains	Half thickness in 40 more years

How to Estimate Rust Decay

Find the original specified or reliably measured material thickness.
Measure current metal loss after removing loose rust scale as appropriate.
Enter the exposure time represented by the measured loss.
Select Calculate Rust Decay and compare the screening result with inspection requirements.

Frequently Asked Questions

Is corrosion loss normally linear?

Not always, because environment, coating condition, and oxide layers change over time. Linear projection is a simple screening assumption and should be checked against repeated measurements.

Is rust depth the same as rust scale thickness?

No, expanded rust scale can be much thicker than the sound metal that was consumed. Use actual metal thickness loss rather than the loose oxide thickness whenever possible.

What is a safe remaining thickness?

There is no single safe percentage for every component. Required thickness depends on design loads, material, geometry, defects, codes, and safety factors.

How should I measure corroded metal?

Ultrasonic thickness testing is commonly used when access and material conditions permit. Critical measurements should be made or reviewed by a qualified inspector using calibrated equipment.

Does this calculator account for pitting?

It treats the entered depth as representative and does not model pit shape or distribution. Local pitting may control failure even when average wall thickness appears acceptable.