Galvanic Corrosion

Problem: Galvanic corrosion on an accelerated timeline observed in a fully electrically isolated fluid handling system composed of carbon steel and stainless steel components.  Some of the corrosion patterns observed, were the circular corrosion around the stainless bolt up components in the gunbarrell and the semicircle patterns on the manway, as well as the rapid material loss on the elbow on the transition.   The inlet and outlet of the system are electrically isolated, and the entire system is not grounded, effectively forming a sealed electrochemical “wet cell”.

Internal electrical continuity between dissimilar metals combined with a highly conductive, high-temperature brine drives internal galvanic reactions, leading to rapid, localized corrosion—particularly at welds and transition points.

System Description

• Fluid: Produced water from oil and gas gathering
• Chloride: 154,000 ppm
• Bromide: 452 ppm
• Sulfate: 975 ppm
• Temperature: ~150°F (65°C)
• Materials: Carbon steel, stainless steel (304/316), ERW piping, forged components and welded connections
• Configuration:
• Electrically isolated at both inlet and outlet
• Not grounded
• Contains internal electrical continuity between dissimilar metal components
• Result: A closed-loop galvanic environment (wet cell)

Mechanism of Corrosion in Wet Cell Conditions

1. Wet Cell Formation

Even when a system is isolated from ground, internal electrical continuity between dissimilar metals, immersed in a conductive fluid, creates an electrochemical cell. In this case:

• Anode: Carbon steel (especially at welds)
• Cathode: Stainless steel (more noble)
• Electrolyte: High-TDS brine with Cl⁻, Br⁻, SO₄²⁻
• Electrical Path: Internal metallic continuity
• No ground: But grounding is not necessary for internal corrosion

This mimics the behavior of a wet battery, where spontaneous current flows within the system due to electrode potential differences.

References:

• Fontana, Corrosion Engineering, Ch. 3
• NACE SP0177 – Electrical Isolation and AC Effects
• Davis, Corrosion: Understanding the Basics, ASM, 2000

2. Effect of High Anion Flux in Isolated System

• High mobility of anions (Cl⁻, Br⁻, SO₄²⁻) leads to:
• High electrolyte conductivity (resistivity typically < 1,000 µS/cm in this brine)
• Sustained ionic current between anodic and cathodic regions
• Chloride ions penetrate passive oxide layers, destabilizing stainless steel and increasing cathodic activity
• Bromide ions are more polarizable than chloride and may accelerate pit initiation
• Sulfate ions promote localized under-deposit corrosion and may contribute to microbial activity
• The constant flow of these ions, coupled with temperature elevation, maintains an environment of continuous electrochemical activity, even without outside grounding

References:

• ASTM G102 – Electrochemical Potentials and Corrosion Rates
• Tverberg, “Localized Corrosion in High Chloride Systems,” Materials Performance, 2003
• Little & Lee, Microbiologically Influenced Corrosion, 2007

3. Localized Corrosion at Welds

• Welds create electrochemical heterogeneity due to:
• Dissimilar filler materials
• Residual stress
• Heat-affected zone (HAZ) grain boundary effects
• Welds in carbon steel act as local anodes when adjacent to stainless components
• Result: Rapid thinning, pitting, and early failure—especially under flow conditions

References:

• API 582 – Welding Guidelines
• AWS D1.1 – Commentary on corrosion at weld interfaces
• ASM Handbook, Vol. 13B – Section on HAZ corrosion

Estimated Corrosion Behavior

• Carbon steel in brine @150°F, exposed to stainless steel:
• Galvanic corrosion rates of 5–12 mm/year (200–500 mpy)
• Weld zones can fail in <12 months if not coated or isolated
• Pitting and crevice corrosion observed at low-flow regions and joints

References:

• NORSOK M-001 – Materials Selection Corrosion Tables
• API RP 14E – Design Corrosion Allowance in Offshore Piping
• NACE SP0169 – Corrosion Control in Piping Systems

Recommendations for mitigating:

Use of Sacrificial Anodes in Electrically Isolated Systems

In a system where internal galvanic corrosion is occurring due to dissimilar metals in a highly conductive electrolyte (i.e., produced water with 154,000 ppm Cl⁻), sacrificial anode systems offer a proven and passive means of mitigation, even when the system is not grounded to earth.

How Sacrificial Anodes Work in an Isolated “Wet Cell”

• In the absence of a path to ground, the internal galvanic circuit is self-contained, with carbon steel acting as the anode and stainless steel acting as the cathode.
• By introducing a more active (less noble) metal, such as magnesium, aluminum, or zinc, you deliberately shift the anodic reaction away from vulnerable carbon steel welds and base metal.
• The anode corrodes preferentially, protecting adjacent steel via galvanic action.
• Placement inside vessels or piping (commonly threaded or welded to access ports) provides localized protection to the internal wetted surface.

Anode Material Selection for High-Salinity Produced Water

Recommended:
Use Al-Zn-In alloy anodes rated for elevated temperature and chloride-rich environments, in accordance with NACE SP0387 and DNV-RP-B401 guidelines.

Design and Placement Considerations

• Internal Anodes in Tanks: Weld-on or bolt-on brackets for anodes placed in the flow path near welds and transition zones.
• Pipe Spool Insertion: For in-line systems, a spool section with an anode boss allows replacement and monitoring.
• Flow Rate Impact: Ensure placement in areas of moderate flow—not stagnation or turbulence—to avoid shielding or erosion.
• Electrical Continuity: Anode must be electrically bonded to the component it is intended to protect, but not to the stainless steel section.
• Monitoring: Install with inspection ports or test leads to verify open-circuit potential and consumption over time.

Benefits

• Passive, continuous protection with no external power required.
• Reduces corrosion rates at carbon steel interfaces to <1 mm/year, even under severe brine conditions.
• Critical backup protection where coatings are damaged or dissimilar welds exist.

Key Standards and References

• NACE SP0387 – Metallurgical Selection of Anodes for Cathodic Protection of Steel Water Tanks
• DNV-RP-B401 – Cathodic Protection Design
• ASTM G1 – Anode Mass Loss and Corrosion Rate Measurement
• API RP 651 – Cathodic Protection of Aboveground Storage Tanks