millimeter per year
SpeedCorrosion rateexact by definition
The millimetre per year is about 3.17 × 10⁻¹¹ metres per second, using this site's Julian year of exactly 365.25 days. It is the unit of geologically slow motion: tectonic plates separate at 20 to 150 mm/yr, sea level is rising at roughly 3.5 mm/yr, and general corrosion rates on steel are tabulated in it.
Watch out: Corrosion literature more often uses mils per year (mpy), thousandths of an inch, where 1 mpy is 0.0254 mm/yr. A rate quoted as "5" is a very different problem in the two units.
| 1 mm/yr (Speed) | 3.1688088e-11 m/s |
| 1 mm/yr (Corrosion rate) | 1 mm/yr |
The millimetre per year is the international unit of corrosion rate, exactly 0.001 m/yr by decimal prefix. Its convenient property is that a rate in mm/yr multiplied by a design life in years gives the corrosion allowance directly in millimetres of wall thickness. General guidance in process work calls below 0.1 mm/yr excellent, 0.1 to 0.5 acceptable for most service, and above 1 mm/yr a material selection problem.