Carbon dioxide (CO2) corrosion, commonly known as sweet corrosion, is one of the leading causes of premature failure in gear pumps that handle water-bearing fluids in oil and gas production, CO2 injection, and carbon capture systems. This technical article analyzes the electrochemical mechanisms of CO2 attack on gear pump components, the environmental factors that accelerate degradation, and the standardized laboratory methods used to evaluate material performance. Understanding how dissolved CO2 corrodes gears, shafts, and housings enables engineers to select corrosion-resistant materials, extend maintenance intervals, and prevent costly unplanned downtime.
Technical Overview
Gear pumps are positive-displacement machines widely used to transfer crude oil, produced water, liquefied or supercritical carbon dioxide, carbonated beverages, and various chemical media. When the pumped fluid contains free water and dissolved carbon dioxide, an aggressive corrosive environment forms inside the pump. CO2 dissolves in the aqueous phase to produce carbonic acid, which depresses the pH and drives rapid electrochemical dissolution of ferrous components. In oilfield fluid-handling equipment, CO2 corrosion is one of the most frequently diagnosed degradation mechanisms, manifesting as wall thinning, pitting, seizure of close-clearance parts, seal leakage, and progressive loss of volumetric efficiency. Because gear pumps operate with running clearances that may be smaller than 0.1 mm, even modest corrosion damage can quickly compromise performance, which is why systematic corrosion analysis is essential for pumps in wet CO2 service.
Electrochemical Mechanism of CO2 Corrosion
The corrosivity of wet CO2 arises from the formation and dissociation of carbonic acid. The governing reactions are summarized below:
- Dissolution and hydration: CO2 (g) dissolves into the water phase and hydrates to form carbonic acid, CO2 + H2O reversible to H2CO3.
- Dissociation: H2CO3 releases protons in two steps, H2CO3 reversible to H+ + HCO3−, and HCO3− reversible to H+ + CO3 2−, lowering the local pH.
- Cathodic reaction: 2H2CO3 + 2e− yields H2 + 2HCO3−. Direct reduction of undissociated carbonic acid provides an additional cathodic pathway, which is why CO2-saturated water corrodes steel far faster than a strong acid solution of the same pH.
- Anodic reaction: Fe oxidizes to Fe2+ + 2e−, releasing iron ions into the electrolyte.
- Scale formation: Fe2+ combines with carbonate ions to precipitate iron carbonate, FeCO3 (siderite), which may form a partially protective scale under favorable conditions.
Key Environmental Factors Influencing Corrosion Rate
- CO2 partial pressure: According to the widely used de Waard–Milliams model, the corrosion rate of carbon steel is approximately proportional to the CO2 partial pressure raised to the power of 0.67 under scale-free conditions.
- Temperature: Corrosion kinetics increase with temperature up to roughly 60–100 °C, where the iron carbonate scale remains porous and non-protective; this range is widely regarded as the worst-case regime. At higher temperatures, dense adherent siderite scale can substantially reduce the corrosion rate, while high flow velocities can strip the scale away.
- pH value: Higher bicarbonate content and residual alkalinity raise the pH and promote protective scaling; low pH accelerates active dissolution.
- Flow velocity and turbulence: High shear at gear tips, discharge ports, and close clearances removes corrosion product films, producing flow-accelerated corrosion and erosion–corrosion synergy.
- Chloride concentration: Chlorides stainless steel passivation films and stabilize pitting, and at elevated temperature they raise the risk of chloride stress corrosion cracking in austenitic alloys.
- Co presence of H2S: Even small H2S contamination converts the environment from sweet to sour, changing the failure mechanism toward sulfide stress cracking, which must then be evaluated under ISO 15156 material requirements.
- Oxygen ingress: Trace oxygen dramatically accelerates attack and must be excluded from test environments and from sealed pump systems.
Why Gear Pumps Are Particularly Vulnerable
- Tight running clearances between gear tips, side plates, and housing mean that a corrosion loss of only a few tens of micrometers measurably reduces discharge pressure and volumetric efficiency.
- High contact stress and sliding velocity on tooth flanks continually rupture passive films, producing tribocorrosion that is far more aggressive than static immersion.
- Dissimilar metal couples, such as a hardened steel shaft running in a stainless or bronze housing, create galvanic cells with the less noble component acting as the anode.
- Crevices at seal seats, keyways, press fits, and bearing bores promote localized crevice corrosion and differential aeration cells.
- Pumping flashing CO2-rich or high-vapor-pressure fluids can induce cavitation at the pump inlet, and collapsing cavities mechanically remove protective corrosion products, combining cavitation erosion with chemical attack.
- Entrained solids such as sand turn the corrosion mechanism into full three-body erosion–corrosion at the discharge side plate and gear tip regions.
Testing Samples
Gears and Drive Shafts
Gear pump gears are commonly manufactured from carburizing steels such as 20CrMnTi, through-hardening steels such as 42CrMo or AISI 4140, and corrosion-resistant grades such as 17-4PH, 431, or 316L for chemical service. Shafts are typically 4140, 17-4PH, or duplex stainless steel. Typical corrosion findings include pitting along tooth pitch lines that acts as fatigue crack initiation sites, uniform thinning of tooth tips, journal corrosion under bearings, and micro-galling accelerated by acidic media.
Housing, Casing, and Side Plates
Housings and covers are frequently produced from gray cast iron, ductile cast iron such as QT450-10, cast steel, or 316/CF8M stainless steel, while wear plates may be duplex stainless, bronze, or carbon-graphite. Common degradation includes mesa-type localized attack at the discharge passage, graphitic corrosion of gray iron in chloride-bearing water, washout erosion downstream of the discharge port, and scale buildup that restricts flow.
Bearings, Bushings, and Seal Components
Needle or journal bearings, sleeve bushings, mechanical seal faces, and elastomeric O-rings are examined for raceway corrosion, pitting-induced spalling, face blistering, and swelling or embrittlement caused by CO2-rich media. These small components are often the first failure point in wet CO2 service.
Material Coupons and Weldments
For material selection programs, flat coupons, cylindrical electrodes, and welded joint specimens representing candidate alloys are prepared with surface finishes that match production parts, allowing direct ranking of alloys under simulated service chemistry.
Corrosion Products and Process Fluid Samples
Deposits, scales, sludge, and samples of the pumped fluid are collected to determine chloride content, pH, dissolved CO2, total dissolved solids, suspended solids, and iron concentration, all of which are required to reconstruct the corrosive environment in the laboratory.
Testing Items
A complete corrosion analysis of a gear pump evaluates the component condition, the aggressiveness of the medium, and the suitability of candidate replacement materials. Typical test items include:
- Uniform corrosion rate determined by gravimetric weight loss, expressed in mm/a or mils per year.
- Localized corrosion assessment: maximum and average pit depth, pit density, pit morphology, and mesa attack rating.
- Scale characterization: chemical composition, crystalline phase, thickness, porosity, and adhesion of iron carbonate or mixed deposits.
- Microstructural examination: decarburization, integrity of carburized case layers, grain boundary attack, intergranular corrosion, inclusion-assisted pitting, and weld zone condition.
- Mechanical property degradation: hardness change across sections, microhardness profiles for effective case depth, tensile properties, impact toughness, and gear tooth bending fatigue assessment.
- Hydrogen-related damage evaluation, including hydrogen content determination and blister inspection, particularly when H2S is present alongside CO2.
- Dimensional verification of critical clearances, tooth thickness, bore diameters, and shaft journal roundness against drawing specifications.
- Fluid chemistry: pH, chloride and sulfate content, bicarbonate alkalinity, dissolved and free CO2, total iron and ferrous ion concentration, and water content of the CO2 stream.
- Coating and plating integrity, including porosity of electroless nickel deposits and adhesion of sprayed cermet coatings.
Testing Methods
Immersion and Autoclave Testing
Baseline corrosion rates are measured by full immersion testing in accordance with ASTM G31, using deoxygenated brine formulations matching the field water chemistry, saturated with CO2 at the service temperature and partial pressure. For downhole injection, CO2 flood, and CCUS conditions, high-pressure/high-temperature autoclave testing following ASTM G111 reproduces the actual dissolved CO2 concentration, and rotating cage or flow loop attachments introduce the hydrodynamic shear that gear pump internals experience. Test durations typically range from 72 hours for screening to 30 days or longer for scale protectiveness studies.
Electrochemical Testing
Electrochemical techniques provide rapid quantitative data in CO2-saturated electrolytes: potentiodynamic anodic polarization per ASTM G5 establishes active–passive behavior, linear polarization resistance per ASTM G59 gives instantaneous corrosion rates, cyclic potentiodynamic polarization per ASTM G61 identifies pitting initiation and repassivation potentials of stainless alloys, and electrochemical impedance spectroscopy evaluates film and scale resistance over time. Galvanic current measurements per ASTM G71 quantify the accelerated attack of the less noble member in mixed-material pump assemblies.
Localized Corrosion Evaluation
After exposure, pitting is examined, measured, and rated in accordance with ASTM G46, using surface profiling and metallographic cross sections to determine maximum pit depth and its relationship to component wall thickness and running clearance. External atmospheric corrosion resistance of housings and painted or plated parts is verified by neutral salt spray testing per ASTM B117, ISO 9227, or GB/T 10125.
Metallographic and Fractographic Analysis
Failed components are sectioned and prepared per ASTM E3 and etched per ASTM E407 for microstructural examination. Grain size is rated per ASTM E112, and fracture surfaces are examined by scanning electron microscopy to distinguish corrosion fatigue striations, intergranular hydrogen-assisted cracking, and ductile overload, which is essential for root-cause determination.
Scale and Deposit Characterization
Corrosion scales and deposits are analyzed by scanning electron microscopy with energy-dispersive X-ray spectroscopy and by X-ray diffraction to confirm siderite, iron oxides, calcium carbonate, or corrosion-influenced deposits, and to assess scale protectiveness through thickness mapping and adhesion testing on cross sections.
Mechanical and Hardness Testing
Rockwell hardness testing per ASTM E18 and micro-Vickers testing per ASTM E384 verify heat treatment condition, detect softening from decarburization or hydrogen exposure, and measure effective case depth of carburized gears, while tensile and impact testing quantify any general embrittlement of the base material.
Fluid and Environmental Analysis
Water chemistry is determined by standard titration, ion chromatography, and inductively coupled plasma optical emission spectrometry, with pH measured per ASTM D1293 and substitute seawater prepared per ASTM D1141 where marine conditions apply. These data feed industry-accepted CO2 corrosion prediction models to cross-check laboratory rates and field inspection findings.
Corrosion Prevention and Mitigation
Material Selection
- For mild CO2 service with low chlorides and inhibited brines, carbon steel or ductile iron with a corrosion allowance and filming amine inhibitor may be acceptable when the predicted rate is below approximately 0.1 mm/a.
- For moderate service, martensitic and precipitation-hardening stainless steels such as 17-4PH offer a combination of corrosion resistance and the high hardness required for gear teeth and shafts.
- For high chloride or high CO2 partial pressure service, duplex stainless steels such as 2205 and 2507 provide superior pitting resistance plus high strength, and nickel alloys are reserved for the most severe environments.
- Electroless nickel-plated cast iron housings and high-velocity oxy-fuel cermet-coated gears extend life in borderline environments, provided coating porosity and edge coverage are controlled.
Design and Operating Practices
- Keep CO2 streams dry: corrosion is negligible below the water solubility limit, and CO2 transport specifications such as those referenced in ISO 27913 for carbon capture pipelines restrict water content to parts-per-million levels precisely to prevent corrosive free-water formation.
- Provide adequate suction pressure margin to suppress cavitation, and install strainers to exclude abrasive solids that drive erosion–corrosion.
- Eliminate crevices and stagnant dead zones, insulate dissimilar metals where practical, and specify drainable casing designs.
- Monitor condition continuously using electrical resistance probes, iron counts in the produced fluid, and periodic ultrasonic thickness measurement of the housing.
Applications
CO2 corrosion analysis of gear pumps is critical across numerous industries and processes:
- CO2 enhanced oil recovery injection skids and water-alternating-gas flood plants handling wet CO2 and CO2-saturated brine.
- Carbon capture, utilization, and storage systems, including pre-injection boost pumps, dense-phase CO2 transfer, and injection pump stations.
- Supercritical CO2 pumping for extraction, precision cleaning, and power cycle working fluid circulation.
- Beverage, brewery, and carbonated soft drink plants where pumps handle chilled carbonated water and syrup.
- Oilfield produced water transfer, oily water treatment, and multiphase wellhead transfer with dissolved acid gas.
- Transcritical CO2 refrigeration systems using R744 as the working fluid.
- CO2 fire suppression storage and distribution systems where condensation can create localized corrosive conditions.
- Refinery amine units, sour water service, and acid gas handling where CO2 coexists with H2S and requires combined sweet–sour assessment.
FAQ
Why is CO2 corrosion called sweet corrosion?
The term sweet refers to the absence of hydrogen sulfide, distinguishing it from sour corrosion. It does not imply that the attack is mild; wet CO2 environments can produce corrosion rates on carbon steel exceeding several millimeters per year under high partial pressure and turbulent flow.
Can completely dry CO2 corrode a gear pump?
No significant electrochemical corrosion occurs without free water. As long as the water content stays below its solubility limit in the CO2 phase, carbon steel components remain essentially unaffected. Corrosion analysis therefore always begins with determining whether free water can condense anywhere in the operating envelope, including during startup, shutdown, and ambient-temperature standby.
At what temperature is CO2 corrosion most severe?
The most damaging range is generally 60 to 100 °C, where the iron carbonate scale that forms is porous and poorly adherent while reaction kinetics are already fast. Below about 40 °C the kinetics are slower, and above about 110 °C in quiescent brines a dense protective siderite layer often develops, although high flow velocity can destabilize it.
What corrosion rate is considered acceptable for gear pump components?
For inhibited carbon steel systems, a long-term rate below roughly 0.1 mm/a is commonly targeted as the design limit. Rates above 0.25 to 0.5 mm/a, or any localized pitting approaching half the wall thickness or the running clearance, normally justify upgrading to a corrosion-resistant alloy or applying protective coatings.
How do chlorides interact with CO2 in stainless steel gear pumps?
CO2 lowers the pH and increases the driving force for localized breakdown of the passive film, while chlorides attack that film directly and stabilize growing pits. The combination is markedly more aggressive than either species alone, and at elevated temperatures it also raises the risk of chloride stress corrosion cracking in standard austenitic grades, favoring duplex alloys with higher pitting resistance equivalent numbers.
How often should gear pumps in wet CO2 service be inspected?
A risk-based interval is recommended, but common practice for moderate service includes clearance and seal checks every 6 to 12 months, monthly fluid iron counts as a corrosion trend indicator, and annual ultrasonic thickness measurement of the housing. Pumps in severe uninhibited CO2 service, or those showing rising iron counts, should be examined more frequently by accredited third-party laboratories or in-house metallurgical teams using the standardized methods described above.
