mils per year penetration, Oil & Gas Glossary

Mils per year penetration, abbreviated mpy, is a unit of corrosion rate expressing how fast metal is being lost from a surface, defined as the thickness of material removed in thousandths of an inch over one year, since a mil is one one-thousandth of an inch (0.001 in, or 25.4 micrometres). An mpy figure is almost always derived from a corrosion coupon, a pre-weighed metal specimen of known surface area and alloy that is installed in a pipeline, vessel, or flowline, exposed to the process fluid for a measured interval, then retrieved, cleaned of corrosion product, and reweighed so the mass lost can be converted to an average penetration rate. The governing standard in the oil and gas industry is NACE SP0775 (formerly RP0775), which specifies coupon preparation, installation, exposure, and the calculation, and the working formula is mpy = (weight loss in grams times 22,300) divided by the product of coupon area in square inches, metal density in grams per cubic centimetre, and exposure time in days. The 22,300 constant folds together the unit conversions between grams, inches, and years. Because the calculation returns a single averaged number, it reports general or uniform corrosion well but understates localized attack such as pitting, which can perforate a wall at a far higher local rate than the coupon's area-averaged mpy suggests, so operators pair coupon mpy with pit-depth measurement and other monitoring. NACE SP0775 also provides a classification that turns a bare number into an actionable judgement: for carbon steel in oilfield service, 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 classed on a separate and stricter scale because a deep pit is more dangerous than uniform thinning. In the Western Canadian Sedimentary Basin, mpy is a daily operational metric because so much of the basin's production is sour or carbonated: hydrogen sulphide and carbon dioxide dissolved in produced water drive aggressive internal corrosion of carbon steel gathering lines and downhole tubing, and the AER regulates the integrity of this infrastructure under Directive 056 and pipeline rules that flow from CSA Z662. Operators on sour Leduc, Nisku, and deep Montney gas run coupon and probe programs, pulling coupons every 90 to 180 days, tracking mpy trends, and using the numbers to set chemical corrosion-inhibitor dose rates, to schedule inline inspection, and to decide when a carbon-steel line must be upgraded to corrosion-resistant alloy or protected under NACE MR0175/ISO 15156 sour-service metallurgy. A rising mpy trend is an early warning that inhibitor coverage has failed or that water cut has climbed, and catching it on a coupon is far cheaper than discovering it as a leak.

Key Takeaways

One mil is 0.001 inch: Mils per year measures penetration rate as thousandths of an inch of metal lost per year, so 10 mpy means a wall thins by 0.010 in annually. The unit dominates North American oilfield practice, while metric programs often report the equivalent millimetres per year, where 1 mpy equals 0.0254 mm/y, and both describe the same average wall loss.
Coupon weight loss is the source: An mpy value comes from a pre-weighed corrosion coupon of known area and alloy exposed for a set time, then cleaned and reweighed. The NACE SP0775 formula, mpy = (grams lost times 22,300) over (area times density times days), converts that mass loss to an averaged penetration rate that anchors an integrity program.
NACE SP0775 rate classes: For carbon steel, below 1 mpy is low, 1 to 5 mpy moderate, 5 to 10 mpy high, and over 10 mpy severe, with pitting judged on a separate stricter scale. These bands let an integrity engineer translate a raw coupon number into a decision about inhibitor dosing, inspection frequency, or material upgrade.
Averages hide pitting: Because a coupon reports corrosion averaged over its whole area, it understates localized pitting that can perforate a wall long before the general mpy looks alarming. WCSB sour-service programs therefore combine coupon mpy with pit-depth gauging, ER probes, and inline inspection to catch localized attack.
Central to WCSB sour service: H2S and CO2 in produced water make internal corrosion the dominant integrity threat on WCSB gathering systems, regulated under AER Directive 056 and CSA Z662. Coupons pulled every 90 to 180 days give the mpy trend that sets inhibitor dose and flags when carbon steel must yield to NACE MR0175/ISO 15156 alloys.

From Coupon to Inhibitor Dose

The practical value of mpy is that it closes the loop on chemical corrosion control. An operator on a sour Montney gathering line installs weight-loss coupons at high-risk locations such as low spots where water pools, pulls them at 90 days, and computes a rate. If the coupon returns 3 mpy the inhibitor program is holding the line in the moderate band, but a jump to 12 mpy signals inhibitor breakthrough or a rise in water cut, prompting an increase in continuous-inject dose rate, a batch treatment, or a pigging campaign. The mpy trend, not any single reading, drives the decision, and a stable low number is the evidence an integrity file needs to defend the run life of a carbon-steel line to the AER.

Material Selection and Sour-Service Limits

When coupon mpy stays stubbornly high despite full inhibition, the answer shifts from chemistry to metallurgy. Persistent rates above 10 mpy on a wet sour line signal that carbon steel is uneconomic over the design life, and the operator evaluates internally clad or solid corrosion-resistant alloy, or a non-metallic liner, against the cost of frequent replacement. In sour WCSB service the metallurgy must also satisfy NACE MR0175/ISO 15156 to resist sulphide stress cracking, a failure mode independent of the general mpy rate, so material selection weighs both uniform corrosion measured in mpy and the cracking resistance the standard mandates for the H2S partial pressure present.

Fast Facts

The 22,300 constant embedded in the NACE weight-loss formula is not arbitrary; it is the numerical product of the conversions that reconcile grams of metal lost, the density of the alloy, a coupon area in square inches, and days of exposure into a final answer in thousandths of an inch per year. Change any base unit and the constant changes: the same physical corrosion expressed in millimetres per year uses a different multiplier entirely, roughly 87,600 times the mass-loss term for metric output, which is why an integrity engineer must confirm the constant matches the unit system before trusting a spreadsheet's mpy result.

Mils per year is the output of a weight-loss coupon program and is the standard yardstick for the overall rate of corrosion attacking oilfield steel. In the WCSB the dominant drivers of that rate are CO2 corrosion, the sweet carbonic-acid attack that thins wet gas lines, and the aggressive chemistry of sour gas, whose H2S content both accelerates weight loss and introduces the separate cracking risk that governs sour-service material selection.

Real-World WCSB Scenario: Rising mpy on a Sour Gathering Line

An operator running a sour Leduc gas gathering system near Drayton Valley, Alberta, pulls quarterly weight-loss coupons under a NACE SP0775 program. The spring coupon returns 4 mpy, comfortably moderate, but the summer coupon jumps to 14 mpy after water cut rises on an aging pool, pushing the line into the severe band and flagging that the continuous corrosion inhibitor is no longer keeping pace with the wetter, more sour stream.

The integrity team raises the inhibitor dose, adds a weekly batch treatment, and runs a cleaning pig to clear accumulated water and solids, and the next coupon falls back to 5 mpy. Acting on the mpy trend costs a few thousand CAD in extra chemical and pigging, far below the hundreds of thousands a sour-gas leak, cleanup, and AER incident file would have imposed had the line perforated.