Slug Flow and Erosion-Corrosion: Why Average Corrosion Rates Can Miss Localized Damage

What Is Slug Flow and Why It Matters for Integrity

Slug flow is a multiphase flow regime where liquid slugs and gas pockets alternate along the pipe. As slugs travel through elbows, tees, reducers, and other geometric features, the resulting impact forces, velocity changes, and liquid impingement can significantly increase the rate of corrosion and erosion at those specific locations.

The key integrity challenge is that slug flow creates localized damage that may not be representative of the rest of the pipe. A general corrosion rate calculated from a few mid-span thickness readings may appear acceptable while a downstream elbow or tee is experiencing rapid wall loss.

Locations of Concentrated Erosion-Corrosion Damage

• Elbows and bends — especially those downstream of sources of flow turbulence
• Tees and branch connections — where slug impacts and turbulence concentrate
• Reducers and diameter changes — where velocity changes create localized turbulence
• Injection points — where chemical injection, water, or process additions create turbulence and chemistry mixing
• Dead-leg interfaces — where stagnant fluid chemistry and flow disturbance interact
• Low points in horizontal systems — where water accumulation and slug formation are most likely

Why Average Corrosion Rates Miss Localized Damage

Average corrosion rates from representative CMLs assume that corrosion is reasonably uniform along the circuit. When erosion-corrosion is the controlling mechanism, this assumption fails. The damaged location may be a short section of elbow experiencing rapid local wall loss while the surrounding pipe shows minimal general thinning. An RBI or inspection program based on average circuit corrosion rate will underestimate the real risk at the feature of concern.

Improving Inspection Strategy for Slug Flow Systems

For systems with known or suspected slug flow, erosion-corrosion, or high-velocity turbulent flow, the inspection strategy should: specifically identify and target elbows, tees, reducers, and injection points as priority CML locations; use UT corrosion mapping or PAUT to characterize local wall loss profiles at suspect features; avoid relying on mid-span CMLs alone to calculate corrosion rates for the circuit; consider more frequent inspection at high-consequence turbulent features; and update CML locations if new damage is found at previously uninspected features.

Connection to RBI and Damage Mechanism Review

Erosion-corrosion and slug flow should be identified explicitly in the damage mechanism review for affected circuits. The RBI assessment should reflect the localized nature of the mechanism by assigning separate corrosion rate assumptions and inspection strategies to the high-turbulence features within the circuit, rather than applying a single circuit-average rate.