Corrosion Rate Uncertainty in RBI: Moving Beyond Single-Point Corrosion Rates

Why Corrosion Rate Is Often Uncertain

Major sources of corrosion rate uncertainty include:

• Limited number of thickness readings
• Measurement error and repeatability issues with UT instruments
• Poor repeatability of CML locations between inspections
• Surface condition, coating, or access limitations
• Localized corrosion or pitting missed by sparse CMLs
• Flow turbulence at injection points, elbows, tees, and low points
• Process chemistry changes — pH, H2S, CO2, chlorides, oxygen ingress, inhibitor performance
• Intermittent service, shutdown/start-up cycles, and thermal cycling
• Mixing different damage mechanisms in one circuit
• Historical data quality issues from different contractors or procedures

The Problem with Single-Point Corrosion Rates

Using one average corrosion rate can hide the range of possible wall loss behaviours. Common examples include: mid-span CMLs showing stable thinning while an elbow downstream of an injection point corrodes rapidly; average thickness readings missing pitting at low points; CUI occurring at supports or terminations while inspected areas look acceptable; and corrosion rate changing after process chemistry or inhibitor performance changes.

How to Address Corrosion Rate Uncertainty

Several approaches can help address corrosion rate uncertainty in RBI and integrity programs:

• Distinguish between circuit-average corrosion rates and location-specific rates at known high-risk areas
• Identify and separately characterize areas of known localized damage such as injection points, dead legs, elbows, and supports
• Consider corrosion rate as a range rather than a single number, especially where data scatter is high
• Use short-interval inspection to establish real corrosion rates at high-uncertainty locations before committing to longer intervals
• Trigger corrosion rate review when process conditions change, inhibitor performance changes, or inspection findings deviate from expectation
• Ensure CMLs are placed at locations where damage is credible, not only where access is convenient

Link to Remaining Life Estimation

Remaining life calculations use corrosion rate directly. If the rate is uncertain, the remaining life estimate is also uncertain. A program that treats a range of possible corrosion rates will produce a more realistic assessment of when reassessment or action may be required. This is more useful for planning than a single remaining life number that appears precise but is based on data of uncertain quality.

Evergreening Corrosion Rate Assumptions

Corrosion rate assumptions should not be static inputs. They should be updated as new inspection data, process changes, inhibitor monitoring results, or operator experience accumulates. An RBI program that is never updated may become increasingly unreliable as operating conditions drift from the original assumptions.