Erosion-Corrosion

Key Takeaways

• The synergistic effect of erosion-corrosion causes damage rates 2-10 times higher than the sum of the individual erosion and corrosion rates measured separately, which is why erosion-corrosion failures occur rapidly and unpredictably in systems where each mechanism individually might be considered manageable: a carbon steel pipe elbow in a stream with 50 ppm suspended sand might experience 0.1 mm per year from erosion alone, 0.3 mm per year from CO2 corrosion alone, but 1.5-3 mm per year from the combined erosion-corrosion mechanism because the sand continuously removes the corrosion film that would otherwise limit the corrosion rate; this synergistic acceleration explains why erosion-corrosion failures can occur in systems that were believed to be protected by either a corrosion inhibitor program (which requires the inhibitor film to remain on the surface, which erosion prevents) or a material specification (which was selected based on individual corrosion resistance without accounting for the erosion component that degrades that resistance); materials and inhibitor dosages that are adequate for pure corrosion or pure erosion service must be re-evaluated for combined erosion-corrosion service using testing and modeling that captures the interaction between the two mechanisms.
• Corrosion inhibitor films are particularly vulnerable to erosion-corrosion because the effectiveness of chemical corrosion inhibitors depends on their ability to adsorb onto the metal surface and form a protective molecular film (typically an organic amine or imidazoline compound that orients its polar head toward the metal surface and its non-polar tail away from the environment, creating a hydrophobic barrier), and high-velocity particle impingement physically dislodges the inhibitor molecules from the surface faster than continuous dosing can replace them; the critical particle flux and velocity above which inhibitor film integrity is compromised depends on the specific inhibitor chemistry, the particle size, hardness, and concentration, and the carrier fluid velocity and turbulence; operators managing sand-producing wells that require corrosion inhibitor programs often need to increase inhibitor dosage significantly above the rate that would be effective in a sand-free stream, and may need to combine chemical inhibition with alloy upgrades (corrosion-resistant alloys that do not rely on a separate inhibitor film for protection) or with sand management measures that reduce the particle flux reaching critical locations.
• Material selection for erosion-corrosion service requires alloys that are both corrosion-resistant and erosion-resistant, because the best corrosion-resistant materials are often soft alloys with poor erosion resistance and vice versa: martensitic stainless steels (13Cr and super 13Cr) are widely used in oil and gas tubing for their corrosion resistance to CO2 and mild H2S, but their hardness (Rockwell C 22-28) provides moderate erosion resistance that is adequate for low to moderate sand concentrations; duplex and super-duplex stainless steels provide higher strength and hardness along with excellent corrosion resistance to chloride-containing produced water, but they are susceptible to H2S-induced cracking if H2S partial pressure exceeds NACE MR0175 limits; cobalt-chromium alloys (Stellite) and tungsten carbide hard-facing are used for extremely severe erosion-corrosion service in choke valves and pump components; the material selection must be validated against the specific combination of fluid chemistry, temperature, pressure, and particle parameters of the service, not against any single parameter in isolation.
• Flow-induced corrosion, turbulent flow corrosion, and impingement attack are all related terms describing specific manifestations of the erosion-corrosion mechanism in different flow geometries: flow-induced corrosion occurs in straight pipe sections where the turbulent velocity is high enough to prevent passivation film formation through fluid shear forces even without solid particles (relevant in pure liquid service at velocities above approximately 1-3 m/s for uninhibited carbon steel in CO2-saturated water); turbulent flow corrosion is the same phenomenon described in the context of multiphase gas-liquid flow, where the intermittent impact of liquid slugs against the pipe wall creates severe local shear stresses at the slug front; impingement attack describes the localized material loss at the point of direct fluid or particle impact, as observed at the outside of pipe bends, at baffle plates in heat exchangers, and at the face of pump impellers; all of these flow geometry-driven degradation modes are accelerated when solid particles are present in the flow stream, transitioning from pure hydrodynamic corrosion to the fully synergistic erosion-corrosion regime.
• Real-time erosion monitoring using ultrasonic thickness measurement (UT), electrical resistance probes, or fiber-optic strain sensors provides the condition-based maintenance data needed to manage erosion-corrosion damage in sand-producing wells and in high-velocity processing equipment before failures occur: permanently installed UT transducers at high-risk locations (pipe bends, choke body, wellhead connections) measure the metal wall thickness continuously and transmit the data to the control system, triggering inspection or maintenance actions when wall thinning exceeds defined thresholds; electrical resistance (ER) probes expose a thin metal wire or strip to the production stream and measure the electrical resistance as the element thins by erosion-corrosion, providing a continuously updated metal loss rate that the operator can correlate with sand concentration, flow rate, and chemical treatment parameters; these monitoring technologies have become standard in deepwater and HPHT wells where the consequences of undetected erosion-corrosion failures (topside loss of containment, subsea pipeline failure) are catastrophic and where the cost of monitoring is small compared to the cost of failure.