Circuitization Beats Guesswork: Building Credible Corrosion Circuits for Piping Integrity

What Is a Corrosion Circuit?

A corrosion circuit is a section of piping that is reasonably expected to experience similar corrosion behaviour — similar damage mechanism, rate, and inspection requirements — based on material, process fluid, operating conditions, and system geometry. Circuits are used to group CMLs, calculate corrosion rates, and establish inspection intervals.

Why Circuitization Matters

Circuits that are too broad may combine areas with very different damage mechanisms or corrosion rates. The resulting average corrosion rate may be meaningless and CML placement may miss the actual damage location. Circuits that are too granular may create administrative burden without engineering benefit. The goal is meaningful groupings that reflect the actual integrity behaviour of the piping.

Key Criteria for Circuit Definition

• Dominant damage mechanism — general corrosion, localized corrosion, CUI, erosion-corrosion, injection-point corrosion, SCC, or fatigue
• Flow regime — single-phase, multiphase, slug flow, turbulent elbows, low points, dead legs
• Operating conditions — temperature, pressure, pH, water content, velocity
• Material of construction — carbon steel, alloy steel, stainless steel
• Consequence of failure — high consequence circuits merit finer segmentation
• Inspection history — circuits with known damage at specific locations may be segmented around those features

CML Placement Within Circuits

CMLs should be placed at locations within the circuit that are most likely to experience the dominant damage mechanism, not simply at convenient access points. For general corrosion, representative points may be adequate. For localized corrosion, pitting, erosion-corrosion, or injection-point corrosion, CMLs must be placed where damage concentration is credible — which may be very different from a convenient mid-span access point.

Circuitization and Inspection Effectiveness

The inspection effectiveness assigned to a circuit in RBI depends on whether the inspection method and CML locations can actually detect the active damage mechanism. A circuit with CMLs misplaced relative to the expected damage location may have low inspection effectiveness regardless of how recently or frequently it was inspected.

Maintaining and Updating Circuits

Circuitization is not a one-time exercise. Circuits should be reviewed when damage is found in unexpected locations, when process conditions change, when operating envelope changes, or when existing corrosion rate data show unexpected scatter or trends. The circuit definition should evolve as inspection data improves.