mpy, Oil & Gas Glossary | Oil Authority

MPY is the abbreviation for mils per year, a unit of measurement that expresses how fast metal is being lost to corrosion. One mil is one thousandth of an inch, so a corrosion rate of 5 mpy means the exposed steel surface is thinning at five thousandths of an inch, about 0.127 mm, every year. The number is the single most quoted figure in oilfield corrosion monitoring because it converts an abstract chemical process into a concrete rate of wall loss that an engineer can compare against the remaining wall thickness of a pipe or vessel and turn into a remaining-life estimate. MPY is most commonly derived from a corrosion coupon, a small pre-weighed metal specimen of known area installed in the flow stream through an access fitting. After a measured exposure period, usually 30 to 90 days, the coupon is retrieved, cleaned of corrosion product, and reweighed. The weight lost, combined with the coupon area, the alloy density and the exposure time, gives an average corrosion rate through the standard relationship mpy equals 22,300 times the weight loss in grams, divided by the coupon area in square inches times the density in grams per cubic centimetre times the exposure time in days. This methodology, and the interpretation thresholds that go with it, are laid out in NACE Standard SP0775, "Preparation, Installation, Analysis, and Interpretation of Corrosion Coupons in Oilfield Operations," the reference every Western Canadian Sedimentary Basin operator uses. NACE SP0775 also gives the industry its rating bands: for carbon steel a general corrosion rate below about 1 mpy is considered low, 1 to 5 mpy moderate, 5 to 10 mpy high, and above 10 mpy severe, with pitting rates rated on a separate and stricter scale because localized attack punches through a wall far faster than uniform loss. In WCSB service those numbers drive real decisions. A sour gas gathering line in the Montney or a produced water disposal line moving brine full of chlorides and dissolved carbon dioxide can corrode at rates that demand continuous chemical inhibition, and the coupon-derived mpy is how the operator proves the inhibitor program is working. The metric equivalent, millimetres per year, is used alongside mpy on Canadian data sheets, where 1 mpy equals 0.0254 mm/y, so a rate reported as 4 mpy is the same as about 0.10 mm/y. Because mpy from a coupon is a time-averaged number, it is normally read together with faster-responding tools such as electrical resistance probes, linear polarization resistance meters and inline inspection data, but it remains the auditable, physical benchmark against which every other corrosion measurement on the system is checked.

Key Takeaways

One mil is one thousandth of an inch: A rate of 5 mpy means 0.005 inch, or 0.127 mm, of wall thickness is lost per year. Multiplying the rate by the years in service and comparing against the pipe's corrosion allowance gives a direct remaining-life estimate, which is the whole reason corrosion is reported as a rate rather than a total loss.
Coupon weight-loss is the standard source: A pre-weighed steel coupon of known area is exposed 30 to 90 days, cleaned and reweighed. The mpy formula, 22,300 times grams lost divided by area in square inches times density times days, follows ASTM G1 and G31 practice and is codified for oilfield use in NACE SP0775, giving an auditable, physical corrosion rate.
Rating bands drive action: Per NACE SP0775 for carbon steel, under 1 mpy is low, 1 to 5 mpy moderate, 5 to 10 mpy high, and over 10 mpy severe general corrosion. Pitting is judged on a stricter scale because localized penetration perforates a wall far faster than uniform metal loss and is the more common failure mode on WCSB sour and water lines.
Metric equivalent runs in parallel: Canadian data sheets carry both units, with 1 mpy equal to 0.0254 mm/y. A produced-water line reported at 4 mpy is corroding at roughly 0.10 mm/y, and integrity engineers convert freely between the two because inline inspection tools and wall-thickness surveys are reported in millimetres.
Time-averaged, so paired with fast probes: A coupon gives one averaged number over its exposure window and cannot catch a short upset. Operators read mpy alongside electrical resistance and linear polarization resistance probes for near-real-time trends, using the coupon as the calibrated benchmark that confirms what the electronic probes report.

How a Coupon Becomes an MPY Number

The workflow is deliberately simple so it can be audited. A technician installs a flush or ladder coupon of known weight and surface area on an access fitting in the pipeline or vessel, records the install date, and leaves it in the live stream. After the planned exposure, commonly 90 days on a WCSB gathering system, the coupon is pulled under pressure through the same fitting, chemically cleaned to remove scale and corrosion product without attacking base metal, dried and reweighed on a laboratory balance. The weight loss, area, alloy density and elapsed days feed the mpy equation. Any visible pitting is measured separately with a depth gauge and reported as a pitting rate, because a low general rate can still hide a deep pit that threatens the wall.

MPY in a Montney Sour Gas Inhibition Program

On a sour gas gathering system carrying carbon dioxide, hydrogen sulphide and produced water, uninhibited carbon steel can corrode well into the double digits in mpy. The operator injects a filming amine corrosion inhibitor and sets a target, often below 3 to 5 mpy, that the chemical program must hold. Coupons pulled each quarter are the report card: if the mpy climbs, dosing is increased, the injection point is checked for coverage, and a biocide may be added if microbially influenced corrosion is suspected. The coupon mpy, cross-checked against electrical resistance probe trends and periodic inline inspection, is what an AER integrity audit expects to see documented over the life of the line.

Fast Facts

The constant 22,300 that turns coupon weight loss into mils per year is not arbitrary; it bundles together the unit conversions between grams, cubic centimetres, square inches, inches and years into a single factor, so a technician only needs weight, area, density and time. Steel density near 7.86 g per cubic centimetre is baked into most field spreadsheets, and because the answer scales directly with exposure time, a coupon left in too briefly during an upset can wildly overstate the long-run rate, one reason NACE SP0775 recommends minimum exposure periods of about 30 to 90 days.

MPY is the reporting unit for corrosion, the electrochemical loss of metal it exists to quantify, so the two terms are read together on every integrity data sheet. It measures the same wall attack that produces the iron sulphide and rust found in pipeline gunk, meaning a rising mpy and a sulphide-heavy pig return tell the same story from two angles. On sour service, the acceptable mpy target is set with reference to the H2S and chloride severity of the stream, connecting the number back to the fluid chemistry that a corrosion inhibitor program is dosed to control.

Real-World WCSB Scenario: A Produced Water Disposal Line

An operator running a produced water disposal line near Lloydminster injects brine loaded with chlorides and dissolved carbon dioxide into a deep disposal zone. Quarterly coupons come back showing a general corrosion rate of 8 mpy, roughly 0.20 mm/y, well into the high band under NACE SP0775, with shallow pitting visible on the coupon face. At that rate the 6.4 mm wall of the carbon steel line would exhaust its corrosion allowance in only a few years. The chemical spend before the change is about CAD 40,000 a year in inhibitor.

The operator raises the filming amine dose, moves the injection quill to improve coverage, and adds a batch biocide to knock back sulphate-reducing bacteria. The next two coupons return 3 mpy and then 2 mpy. The extra CAD 25,000 a year in chemical is trivial against the cost of a brine release, cleanup and a Directive 077 reportable failure, and the documented drop in mpy is what closes out the integrity concern.