3.2 Galvanic Corrosion, Concentration Cell Corrosion & Dealloying

Galvanic Corrosion — API RP 571 Section 3.31

1. Fundamental Electrochemical Principles

Galvanic Corrosion (also known as dissimilar metal corrosion) is an accelerated form of electrochemical attack that occurs when three mandatory conditions are simultaneously satisfied:

1. Two electrochemically dissimilar metals or electrically conductive non-metals (such as graphite) possess distinct corrosion potentials ($E_{\text{corr}}$).
2. The dissimilar materials are in direct, low-resistance physical and electrical contact.
3. Both materials are immersed in a shared, continuous conductive electrolyte (such as cooling water, brine, or condensed moisture).

When coupled, an electron flow is established driven by the potential difference. The more electronegative, chemically active metal becomes the anode and suffers accelerated oxidation (metal dissolution): $M_{\text{anode}} \rightarrow M^{z+} + z e^-$ The more electropositive, noble material becomes the cathode, where electron consumption occurs via reduction reactions (such as oxygen reduction or hydrogen evolution). The cathodic member experiences negligible corrosion and is effectively cathodically protected by the sacrificial dissolution of the anode: $O_2 + 2H_2O + 4e^- \rightarrow 4OH^- \quad \text{(Aerobic Environments)}$ $2H^+ + 2e^- \rightarrow H_2 \quad \text{(Acidic Environments)}$

2. The Galvanic Series in Seawater / Industrial Waters

The relative nobility of industrial alloys is defined by the Galvanic Series. In flowing seawater and aqueous electrolytes, metals rank from most active (anodic) to most noble (cathodic):

[ACTIVE / ANODIC - Most Readily Corroded]
  ▲  Magnesium and Magnesium Alloys
  │  Zinc (Used as sacrificial galvanizing / anodes)
  │  Aluminum Alloys (e.g., 6061, 5083)
  │  Carbon Steel and Cast Iron
  │  Low-Alloy Steels (Cr-Mo alloys)
  │  Active Stainless Steels (304, 316 with disrupted passive films)
  │  Lead-Tin Solders
  │  Admiralty Brass, Yellow Brass, Naval Brass
  │  Copper and Silicon Bronze
  │  Cupronickels (90/10, 70/30)
  │  Inconel 600 (active state)
  │  Passive Stainless Steels (304, 316, 2205 Duplex with intact Cr2O3)
  │  Alloy 825, Alloy 625, Alloy C-276
  │  Titanium (Grades 2, 7, 12)
  │  Graphite (Carbon packing, graphite gaskets - severely cathodic!)
  ▼  Platinum and Gold
[NOBLE / CATHODIC - Protected Member]

Critical Field Note: Graphite is highly electrically conductive and exceptionally noble. Using graphite-filled gaskets or graphite valve stem packing in direct contact with carbon steel or stainless steel components in aqueous services frequently results in severe, unexpected galvanic grooving of the metallic sealing faces.

3. The Critical Area Ratio Effect

The physical rate of galvanic penetration is governed mathematically by the Area Ratio of the cathode to the anode. By charge conservation, the total anodic oxidation current must equal the total cathodic reduction current ($I_a = I_c$). Because current equals current density multiplied by surface area ($I = i \times A$): $i_a \times A_a = i_c \times A_c \implies i_a = i_c \left( \frac{A_c}{A_a} \right)$

Where:

• $i_a$ = Corrosion current density of the anode (proportional to metal penetration rate in mpy)
• $i_c$ = Reduction current density on the cathode
• $A_c / A_a$ = Ratio of cathodic surface area to anodic surface area

UNFAVORABLE RATIO (Catastrophic Penetration):          FAVORABLE RATIO (Negligible Additional Attack):
┌────────────────────────────────────────┐          ┌────────────────────────────────────────┐
│           CATHODE (Large Area)         │          │            ANODE (Large Area)          │
│    Type 316L Stainless Steel Shell     │          │           Carbon Steel Pipe Wall       │
└───────────────────┬────────────────────┘          └───────────────────┬────────────────────┘
                    │ Massive cathodic current                          │ Diffuse cathodic current
                    ▼ concentrated on tiny anode                        ▼ spread over vast anode
          ┌───────────────────┐                               ┌───────────────────┐
          │ ANODE (Small Area)│                               │CATHODE(Small Area)│
          │Carbon Steel Nipple│                               │316L SS Thermowell │
          └───────────────────┘                               └───────────────────┘

• Unfavorable Area Ratio (Large Cathode / Small Anode): When a large cathode is coupled to a small anode ($A_c \gg A_a$), the immense total cathodic reduction current is forced through a tiny anodic footprint. This produces astronomical anodic current densities ($i_a$), resulting in rapid, localized gouging and perforation (e.g., a carbon steel drain plug installed in a bronze pump casing, or carbon steel rivets joining copper sheets).
• Favorable Area Ratio (Small Cathode / Large Anode): When a small cathode is coupled to a vast anode ($A_c \ll A_a$), the galvanic current is diluted across a large surface area. The resulting increase in anodic corrosion rate is negligible (e.g., an austenitic stainless steel thermowell installed in a large carbon steel pipeline).

4. Common Refinery Galvanic Couples

• Carbon steel heat exchanger shells, channels, or tubesheets coupled to stainless steel, cupronickel, or titanium tubing.
• Bronze or brass valve trim installed inside cast iron or carbon steel valve bodies.
• 300-series stainless steel internal trays or packing retainers welded directly to carbon steel distillation column shells without clad isolation.
• Carbon steel structural saddles welded to stainless steel pressure vessels without dielectric isolation pads.

Concentration Cell (Crevice) Corrosion — API RP 571 Section 3.19

1. Mechanism of Differential Aeration Cells

Concentration Cell Corrosion (predominantly manifesting as Crevice Corrosion) is localized degradation resulting from chemical concentration gradients within the electrolyte across contiguous regions of the same metal surface. The most pervasive form in industrial plants is the Oxygen Concentration Cell (Differential Aeration Cell).

Crevice corrosion initiates within tight mechanical clearances shielded from the bulk fluid. Initially, uniform corrosion occurs both inside and outside the crevice. However, dissolved oxygen within the confined crevice volume is rapidly consumed by cathodic reduction ($O_2 + 2H_2O + 4e^- \rightarrow 4OH^-$) and cannot be replenished due to restricted convective mass transfer. Outside the crevice, dissolved oxygen remains abundant.

Bulk Electrolyte (High Dissolved O2, pH ~ 7.0)
====================================   <-- External Cathode: O2 + 2H2O + 4e- → 4OH-
[ Metal Plate ]
------------------------------------
[ Crevice Gap: 0.025 to 0.1 mm ]     <-- Restricted Diffusion Zone
  - Oxygen completely depleted (Deoxygenated Anode: Fe → Fe2+ + 2e-)
  - Excess Fe2+ cations attract migrating Cl- anions
  - Hydrolysis reaction: FeCl2 + 2H2O → Fe(OH)2 + 2HCl
  - Intracrevice pH plunges to 1.5 - 2.0 (Extreme Pitting & Dissolution)
------------------------------------
[ Mating Flange / Deposit ]

Once the crevice is fully deoxygenated, oxygen reduction ceases inside the gap. The metal inside becomes exclusively anodic, while the exterior exposed surface acts as a vast cathode. To preserve electroneutrality against accumulating metal cations ($Fe^{2+}$ or $Cr^{3+}$), negatively charged chloride ions ($Cl^-$) migrate into the crevice. The metal chlorides undergo hydrolysis, generating free hydrochloric acid ($HCl$): $FeCl_2 + 2H_2O \rightarrow Fe(OH)_2\downarrow + 2H^+ + 2Cl^-$ $CrCl_3 + 3H_2O \rightarrow Cr(OH)_3\downarrow + 3H^+ + 3Cl^-$ This hydrolysis plunges the intracrevice pH to 1.5 to 2.5, destroying the protective oxide film and establishing a self-sustaining (autocatalytic) localized pitting cell.

2. Critical Crevice Dimensions and Geometry

• Critical Clearance Dimension: Concentration cell corrosion initiates in narrow geometric gaps typically between 0.025 mm and 0.1 mm (1 to 4 mils) wide.
  • Gaps wider than 0.1 mm (4 mils) allow sufficient convective circulation to replenish dissolved oxygen, preventing stable differential aeration cells from establishing.
  • Gaps narrower than 0.025 mm (1 mil) frequently prevent electrolyte entry altogether.
• Vulnerable Geometries: Flange gasket seating surfaces, threaded pipe connections, socket weld crevices, lap joints, rolled tube-to-tubesheet joints, and zones underneath accumulated sludge, sand, bio-slime, or delaminated coatings.

Dealloying (Selective Leaching) — API RP 571 Section 3.24

1. Mechanism and Forms of Dealloying

Dealloying, historically termed selective leaching, is a specialized corrosion mechanism in which one specific electrochemically active constituent element of an alloy is selectively dissolved and removed from the solid solution lattice. This leaves behind a porous, mechanically weakened, brittle sponge-like framework composed of the remaining noble constituent(s).

1. Dezincification of Brasses:

   • Metallurgy: Occurs in copper-zinc alloys containing greater than 15% zinc (e.g., Muntz metal 60Cu-40Zn, Yellow Brass 67Cu-33Zn, and Uninhibited Admiralty Brass 70Cu-29Zn-1Sn). Zinc is selectively dissolved, leaving behind porous copper.
   • Plug-Type Dezincification: Highly localized, deep penetrating cylindrical plugs of porous copper. Prevalent in hot, stagnant, neutral to alkaline cooling waters or brines; causes sudden, catastrophic through-wall perforation.
   • Layer-Type Dezincification: Uniform leaching across extensive surface areas. More frequent in acidic, low-pH waters or soft waters with high chloride content.

2. Dealuminification:

   • Selective dissolution of aluminum from aluminum bronze alloys (containing >8% Al) exposed to acidic marine environments, hot sour waters, or high-chloride brines, leaving a brittle, porous copper-rich matrix.

3. Denickelification:

   • Selective removal of nickel from 70/30 or 90/10 cupronickel alloys exposed to hot, high-velocity, ammoniacal streams or high-sulfide brines.

4. Graphitic Corrosion (API RP 571 Section 3.33):

   • A classic selective leaching process occurring in gray cast iron, where the metallic iron matrix selectively dissolves, leaving behind an intact, brittle network of graphite flakes filled with corrosion products.

2. Visual Hallmarks: The "Dimensional Deception"

A defining characteristic of dealloying is dimensional deception: dealloyed components typically retain their original macroscopic shape, dimensions, and sharp machine contours (such as pipe threads or valve stems) with zero obvious external wall loss. However, their structural integrity is completely destroyed:

• Color Change: Yellow brass changes to a dull, spongy reddish-copper color; dealuminified bronze shifts to dark coppery brown.
• Acoustic / Mechanical Test: When struck lightly with a ball-peen inspection hammer, dealloyed components emit a dull, hollow "thud" rather than a crisp, metallic ringing resonance. The porous layer can frequently be gouged or carved easily with a hardened steel inspection blade.
• Mechanical Brittleness: Parts suffer sudden brittle failure under minimal mechanical loading or normal operating pressure.

3. Prevention, Mitigation & Inspection Techniques

Engineering Prevention Best Practices:

• Galvanic Mitigation:
  • Dielectric Isolation: Install complete dielectric flange isolation kits (G-10/G-11 insulating gaskets, full-length bolt sleeves, and non-conductive washers).
  • The Golden Coating Rule: NEVER coat only the anodic member! If an anode is coated, unavoidable coating holidays, pinholes, or scratches concentrate the full cathodic current onto the microscopic bare anode footprint, causing instantaneous pinhole perforation. The correct practice is to coat the cathode (starving the cathodic reduction reaction) or coat both metals completely.
  • Sacrificial Cathodic Protection: Install sacrificial anodes (zinc, aluminum, magnesium) directly in water boxes to protect less noble tubesheets.
• Concentration Cell Mitigation:
  • Eliminate crevices through engineering design: replace threaded fittings with full-penetration butt-welds; convert lap joints and socket welds to continuous seal welds; use non-wicking, non-porous elastomeric flange gaskets.
• Dealloying Mitigation (Inhibited Metallurgy):
  • For brass components in cooling water, specify inhibited brass alloys containing 0.02% to 0.06% arsenic (As), antimony (Sb), or phosphorus (P) (e.g., Inhibited Admiralty Brass UNS C44300, Naval Brass UNS C46500). The addition of arsenic forms a stable monolayer that halts the redeposition of copper.
  • Upgrade to dealloying-resistant alloys: 70/30 Cu-Ni (UNS C71500), 2205 Duplex Stainless Steel, or Titanium.

NDE Inspection Methods:

• Visual Testing (VT): Color inspection of disassembled heat exchanger tubesheets, flange gasket faces, and valve internals; checking for reddish copper coloration or galvanic grooving.
• Eddy Current Testing (ECT): The primary NDE method for non-ferrous heat exchanger tubing; detects subsurface dealloyed porosity and wall degradation.
• Ultrasonic Testing (UT / PAUT): Scanning dissimilar weld boundaries and flange necks to map galvanic step thinning.
• Metallographic Examination: Field metallographic replication (FMR) or laboratory cross-sectional micro-hardness testing to confirm dealloying depth.