mil per year
Corrosion rateexact by definition
The mil per year, abbreviated mpy, is the North American corrosion-engineering unit: thousandths of an inch of metal thickness lost per year. One mil is exactly 0.0254 mm, so 1 mpy is exactly 2.54e-5 m/yr and the factor is exact by definition. Corrosion coupons in cooling systems are reported almost universally in mpy.
Watch out: mpy and mm/yr are both in daily use and differ by a factor of 39.37, so 5 mpy is 0.127 mm/yr and not 5 mm/yr. Be careful too with the abbreviation: mpy sometimes appears in older literature as mdd, milligrams per square decimetre per day, which is a mass-loss rate and needs the metal's density to convert. And a low general rate does not rule out pitting.
| 1 mpy | 0.0254 mm/yr |
The unit survives because it puts real corrosion rates in the range of single digits. A mild steel coupon at 2 mpy loses two thousandths of an inch a year, a number an engineer can compare to a wall thickness without reaching for a calculator.
About the mil per year
Mils per year is the corrosion coupon unit, and the coupon is still the standard test in cooling water treatment. A pre-weighed strip of mild steel or copper is mounted in a bypass rack, left in the flowing system for 60 to 90 days, then cleaned and reweighed. The mass lost, the exposed area, the metal's density and the exposure time give the penetration rate by the ASTM G1 formula:
\(\text{mpy} = \dfrac{534 \, W}{D \, A \, T}\), with \(W\) the mass loss in milligrams, \(D\) the density in g/cm³, \(A\) the area in square inches and \(T\) the time in hours. The metric form is \(\text{mm/yr} = 87.6 \, W /(D A T)\) with area in square centimetres. Both convert a weight loss into an equivalent uniform thinning, which is a fiction, but a useful one.
The numbers people work to in cooling water: mild steel below 1 mpy is excellent, below 3 mpy is generally acceptable, 3 to 5 is marginal and above 5 calls for a change in the treatment program. Copper and its alloys are held to a far tighter standard, typically below 0.3 mpy, both because the tubes are thin and because dissolved copper plating out onto steel sets up galvanic attack elsewhere in the system. Those thresholds are program guidance rather than a code requirement, but they are what a coupon report is measured against.
Two things the number does not tell you. First, it is an average over the whole coupon surface, so localised attack is invisible: a coupon can report 0.8 mpy and show a pit halfway through its thickness. Always look at the coupon, not just the report. Second, the conversion to the international unit is 39.37, since a mil is a thousandth of an inch: 5 mpy is 0.127 mm/yr. Process industry tables written in mm/yr and water treatment reports written in mpy describe the same phenomenon on scales that differ by a factor of forty.