Three standard methods used in asset integrity and corrosion monitoring work: mass loss
(per ASTM G1), thickness loss from inspection data, and remaining life estimation against
a minimum allowable thickness. For general, uniform corrosion screening, not a substitute
for facility inspection codes such as API 570 or API 580.
Mass Loss Method
Based on ASTM G1: CR = (K × W) / (A × T × D), where W is mass loss, A is exposed surface area, T is exposure time, and D is metal density.
Corrosion Rate
—
Note: assumes uniform corrosion over the full exposure period on a
representative coupon or component. Does not capture localized attack such as pitting,
crevice corrosion, or microbially influenced corrosion. K constants per ASTM G1-03
(mpy: 3.45×10⁶, mm/yr: 8.76×10⁴, µm/yr: 8.76×10⁷, ipy: 3.45×10³).
Thickness Loss Method
Derives corrosion rate directly from two thickness measurements (e.g. UT readings) taken at different times. CR = (initial thickness − final thickness) / elapsed time.
Corrosion Rate
—
Note: reflects average corrosion rate between the two inspection points
only. Where possible, use trend data from three or more inspection intervals rather than
a single two-point reading, since localized thinning or measurement error in a single UT
reading can distort a two-point rate.
Remaining Life Estimator
Estimates remaining service life from current measured thickness, minimum allowable (retirement) thickness, and a known or assumed corrosion rate.
Estimated Remaining Life
—
Note: minimum allowable thickness should come from a code-based
calculation (e.g. ASME B31.3, API 510/570) for the specific component, not assumed.
Recommended next inspection interval shown here follows the common risk-based
inspection rule of thumb of half the remaining life, capped per applicable code or
company policy; always confirm against your facility's RBI programme and inspection
code requirements.
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