Corrosion Rate
Definition
The rate of material loss due to corrosion or erosion, typically expressed in millimeters per year (mm/yr) or mils per year (mpy). Short-term corrosion rate is calculated between the two most recent thickness measurements; long-term corrosion rate is calculated from the original (or earliest) thickness to the most recent measurement. The more conservative (higher) rate is typically used for remaining life calculations.
Guide
Context
Corrosion rate is the key parameter driving remaining life calculations and inspection interval determination. In Reliatic, both short-term and long-term corrosion rates are calculated for each CML, with the maximum used for conservative remaining life estimation. Sudden changes in corrosion rate trigger alerts and investigation.
Formula
CR = (T_previous - T_current) / Time Interval; Long-term CR = (T_original - T_current) / Total Time
Examples
- Short-term CR = (11.2 - 10.8) / 1.0 year = 0.40 mm/yr; Long-term CR = (12.7 - 10.8) / 10.0 years = 0.19 mm/yr. The short-term rate suggests accelerating corrosion.
- A corrosion rate increase from 0.1 to 0.5 mm/yr triggers an alert, investigation reveals that a process change increased operating temperature into the high-sulfidation regime.
Applicable Standards
- API 510
- API 570
- API 581
Related Terms
A measured wall thickness value obtained at a CML using non-destructive examination techniques, typically ultrasonic thickness gauging. Thickness readings are the primary quantitative data for calculating corrosion rates and remaining life of static equipment. Each reading is recorded with the CML identifier, measurement date, measured value, measurement technique, and the technician.
Remaining LifeThe calculated time until the wall thickness at a CML or the minimum measured thickness on a component reaches the retirement thickness, based on the current measured thickness and the applicable corrosion rate. Remaining life is the inspection-data-driven equivalent of Remaining Useful Life (RUL), specifically calculated from thickness data for static equipment.
Condition Monitoring Location(CML)A designated, permanently marked point on equipment where thickness measurements or other condition assessments are taken at regular intervals. CMLs (also called TMLs — Thickness Measurement Locations) are positioned at locations where degradation is expected to be most severe or most representative — areas of high velocity, turbulence, impingement, deadlegs, injection points, and heat-affected zones. Each CML has a unique identifier and documented coordinates.
Damage MechanismA specific process or phenomenon that causes degradation of material properties, leading to eventual failure of equipment. Damage mechanisms include corrosion (uniform, localized, galvanic), erosion, fatigue, creep, hydrogen damage, stress corrosion cracking, embrittlement, and other metallurgical or environmental attack modes. API 571 catalogues over 60 damage mechanisms relevant to refining and petrochemical equipment.
Integrity Operating Window(IOW)Defined limits for process variables (temperature, pressure, flow, composition, pH, chloride content, etc.) that, when maintained, ensure the equipment operates within the conditions assumed by the integrity management program. IOWs are more specific than design limits — they define the operating envelope within which damage mechanisms progress at the predicted rate. Exceedances may accelerate degradation beyond what the inspection plan accounts for.
Where this appears in Reliatic
This concept is used in the Inspections module.