3.1 Microbiologically Influenced Corrosion (MIC) & Brine Corrosion

Microbiologically Influenced Corrosion (MIC) — API RP 571 Section 3.45

1. Mechanism Description and Microbial Ecology

Microbiologically Influenced Corrosion (MIC) refers to the accelerated deterioration of metals resulting from the metabolic activities of living microorganisms. Microorganisms do not directly digest or consume structural metals in the bulk sense; rather, they colonize metal surfaces, establishing sessile communities embedded within self-secreted matrices of extracellular polymeric substances (EPS), commonly known as biofilms.

Biofilms alter the immediate electrochemical microenvironment at the metal-solution interface. Within these dense biological layers, pH, dissolved oxygen concentration, inorganic ion concentrations, and redox potentials diverge dramatically from bulk solution parameters. The biofilm creates severe localized concentration gradients and harbors highly corrosive biogenic byproducts (such as hydrogen sulfide and organic acids) directly against the bare metal substrate, driving intense localized pitting and crevice corrosion.

Bulk Fluid (Bulk Oxygen, Neutral pH, Low Sulfide)
   │
   ▼  Biofilm Boundary Layer (EPS Matrix / Tubercles)
   │  - Aerobic slime formers consume dissolved O2
   │  - Acid-Producing Bacteria generate volatile fatty acids (pH drops to 3.5)
   │  - Anaerobic core: Sulfate-Reducing Bacteria (SRB) thrive
   ▼
Metal Surface (Anodic dissolution: Fe → Fe2+ + 2e-; Fe2+ + S2- → FeS)

2. Primary Microbial Classifications

Refinery damage investigations focus on four primary bacterial and fungal classifications:

1. Sulfate-Reducing Bacteria (SRB):

   • Metabolism: Obligate anaerobes (most notably Desulfovibrio and Desulfotomaculum) that thrive in strictly oxygen-depleted niches, such as beneath dense aerobic slime blankets, thick iron scale, or sediment beds.
   • Electrochemistry: SRB utilize molecular hydrogen ($H_2$) or organic carbon as electron donors to reduce inorganic sulfates ($SO_4^{2-}$) to corrosive biogenic sulfide ions ($S^{2-}$ or dissolved $H_2S$): $SO_4^{2-} + 8H^+ + 8e^- \xrightarrow{\text{SRB}} S^{2-} + 4H_2O$ $Fe^{2+} + S^{2-} \rightarrow FeS \quad \text{(Iron Sulfide)}$
   • Diagnostic Field Identifier: Black, soft, gelatinous or sooty iron sulfide ($FeS$) sludge deposits. When tested in the field with a droplet of 15% dilute hydrochloric acid (HCl), the deposit effervesces rapidly and releases the pungent, distinctive rotten-egg odor of hydrogen sulfide gas ($H_2S$).

2. Acid-Producing Bacteria (APB):

   • Metabolism: Facultative or obligate anaerobic heterotrophic bacteria (e.g., Clostridium, Bacteroides) that ferment hydrocarbons, glycol, or organic debris into low-molecular-weight organic acids, predominantly acetic acid ($CH_3COOH$), formic acid, and lactic acid.
   • Impact: Lowers the localized interfacial pH to 3.0–4.0 beneath the biofilm, directly dissolving protective passive films and accelerating hydrogen reduction reactions.

3. Iron-Oxidizing Bacteria (IOB):

   • Metabolism: Aerobic bacteria (such as Gallionella ferruginea, Sphaerotilus, Leptothrix, and Crenothrix) that oxidize soluble ferrous ions ($Fe^{2+}$) into insoluble ferric hydroxide ($Fe(OH)_3$): $4Fe^{2+} + O_2 + 10H_2O \xrightarrow{\text{IOB}} 4Fe(OH)_3\downarrow + 8H^+$
   • Physical Manifestation: Build bulky, filamentous, voluminous reddish-orange or brown mounds known as tubercles. These porous tubercles restrict oxygen diffusion to the metal directly beneath them, establishing aggressive differential aeration cells where the deoxygenated metal beneath the tubercle acts as an active anode while surrounding oxygenated surfaces act as the cathode.

4. Slime-Forming Bacteria and Fungi:

   • Microorganisms (Pseudomonas, Aerobacter, and various mold fungi) that produce copious quantities of insoluble polysaccharide slime. This EPS matrix mechanically glues silt, sand, and corrosion debris to the pipe wall, creating anaerobic micro-habitats that protect underlying SRB colonies from exposure to circulating biocides.

3. Susceptible Materials and Metallurgy

MIC attacks virtually all common industrial alloys, though damage morphology varies:

• Carbon Steel and Low-Alloy Steels: Highly susceptible, experiencing rapid hemispherical pitting and deep gouging beneath tubercles and sludge.
• Austenitic Stainless Steels (300 Series: 304, 304L, 316, 316L): Extremely vulnerable in untreated waters. MIC manifests as minute, pinhole-sized surface entries that conceal extensive subsurface cavernous voids, hollow tunnels, and under-deposit tunneling pits, frequently concentrating along weld seams, fusion boundaries, and heat-affected zones (HAZs).
• Copper-Base Alloys (Admiralty Brass, 90/10 and 70/30 Cu-Ni): Susceptible to sulfide attack generated by SRB. Biogenic sulfides form non-protective copper sulfide films that induce severe localized pitting.
• Aluminum and Aluminum Alloys: Experience severe pitting attack under fungal and bacterial colonization in water-contaminated fuel storage environments.
• Duplex Stainless Steels and Nickel Alloys (Alloy 625, C-276): Possess superior resistance, but can suffer secondary crevice attack if bio-deposits create aggressive hyper-saline, acidic crevices.

4. Critical Operating Variables

• Fluid Velocity: Flow velocity is the single most critical mechanical variable controlling biofilm establishment:
  • Stagnant or low velocity (<3 ft/s or <0.9 m/s): Permits planktonic bacteria to anchor securely to pipe walls and synthesize protective EPS matrices without hydrodynamic shearing.
  • Continuous velocity (>5 to 8 ft/s or >1.5 to 2.4 m/s): Imparts sufficient shear stress to sweep away planktonic cells, deterring stable biofilm formation in cooling water circuits.
• Hydrotest Water Management: Leaving untreated river water, well water, or unchlorinated municipal water in piping or vessels after hydrostatic testing is a frequent source of unexpected MIC. RP 571 recommends that systems be drained and dried promptly after hydrotest (or that the water be treated), and many owners set short internal deadlines for draining and drying.
• Temperature Range: RP 571 notes that microorganisms can survive and grow over a very wide range, from about 0 °F to 235 °F (-17 °C to 113 °C). Many common species are most active near ambient temperatures, but do not assume that warm water systems are immune.
• pH Range: RP 571 notes survival over a pH range of about 0 to 12, with or without oxygen and light. Low-flow or stagnant conditions, deposits, and nutrient sources (hydrocarbons, organics, sulfates) matter more than any single pH value.

Brine Corrosion — API RP 571 Section 3.10

1. Mechanism and Environmental Chemistry

Brine Corrosion is the accelerated general and localized degradation of metallic components exposed to concentrated aqueous solutions of dissolved inorganic salts (predominantly sodium chloride [NaCl], calcium chloride [CaCl2], and magnesium chloride [MgCl2]). Brines are widespread throughout petroleum refining, occurring in:

• Crude unit desalter aqueous effluent streams and water draw-offs
• Water separation boots in crude and intermediate product storage tanks
• Sour water stripper bottoms and desalting brine wash systems
• Cooling tower recirculating brines and seawater cooling circuits

Brine corrosion operates as an intense electrochemical process. Dissolved salts dissociate into mobile cations ($Na^+$, $Ca^{2+}$, $Mg^{2+}$) and anions ($Cl^-$), dramatically increasing the electrical conductivity of the electrolyte. High conductivity lowers the internal ohmic resistance of electrochemical circuits, allowing micro-galvanic corrosion cells and localized pitting anodes to operate across greater geometric separations at vastly accelerated penetration rates.

2. Critical Factors Governing Brine Corrosion

• Dissolved Oxygen Concentration: Dissolved oxygen ($O_2$) is the primary cathodic depolarizer in neutral to mildly alkaline brines: $O_2 + 2H_2O + 4e^- \rightarrow 4OH^-$ In deaerated brines (<10 ppb dissolved $O_2$), carbon steel exhibits negligible corrosion rates (<1 to 2 mpy). However, in oxygen-saturated brines, corrosion rates regularly exceed 20 to 50 mpy.
• Temperature Effects (Open vs. Closed Systems):
  • Open Atmospheric Systems: In aerated open vessels (e.g., cooling basins), corrosion rates initially increase with temperature due to faster ionic diffusion. However, at temperatures between 160 °F and 180 °F (71 °C to 82 °C), the solubility of dissolved oxygen plummets, causing the corrosion rate to plateau and drop sharply as boiling conditions strip oxygen from the water.
  • Closed Pressurized Systems: In pressurized refinery systems (e.g., crude desalter effluent piping, heat exchanger bundles), dissolved gases cannot escape. Consequently, the corrosion rate continues to increase exponentially with rising temperature, exceeding hundreds of mils per year if oxygen or acid gases ingress.
• pH Influences: Acidic brines (pH < 6.0) induce severe, rapid general dissolution accompanied by hydrogen gas evolution. Neutral to slightly alkaline brines (pH 7.0 to 8.5) promote localized pitting. At elevated pH (>10.0), protective iron carbonate or magnetite scales may passivate carbon steel, provided high fluid turbulence does not scour the films.
• Chloride-Induced Pitting and Crevice Corrosion in Stainless Steels: Austenitic stainless steels (Type 304/304L, 316/316L) depend upon a microscopic, passive chromium oxide film ($Cr_2O_3$). High chloride concentrations penetrate and destabilize this passive barrier, initiating severe localized pitting and crevice corrosion under gaskets, O-rings, and scale deposits.

3. Comparative Overview: MIC vs. Brine Corrosion

4. Prevention, Mitigation & Inspection Methodologies

Prevention and Mitigation Strategies:

1. MIC Control:
   • Biocide Regimens: Implement alternating dosing schedules of oxidizing biocides (chlorine dioxide, sodium hypochlorite, ozone) to burn down biomass, coupled with non-oxidizing biocides (glutaraldehyde, THPS [tetrakis hydroxymethyl phosphonium sulfate], isothiazolinones) to penetrate and eradicate sessile colonies without selecting for resistant strains.
   • Hydrotest Water Rigor: Hydrotest equipment strictly with clean, demineralized water or municipal water treated with biocides and oxygen scavengers. Drain completely and promptly after testing, then dry the system (for example, with dry air) so no standing water remains.
   • Mechanical Cleaning: Perform periodic mechanical pigging of piping headers and chemical flushing of deadlegs to strip slime matrices.
2. Brine Mitigation:
   • Chemical Treatment: Inject chemical oxygen scavengers (sodium bisulfite, carbohydrazide) into closed brine circuits to maintain dissolved $O_2$ <10 to 20 ppb. Inject film-forming corrosion inhibitors (filming amines).
   • Materials Selection: Upgrade carbon steel in high-salinity hot brines to 2205 or 2507 duplex stainless steels (Pitting Resistance Equivalent Number, $\text{PREN} \ge 35\text{--}42$) or nickel-base alloys (Alloy 825, Alloy 625, Alloy C-276). Seawater heat exchanger tubing requires titanium (Grade 2 or Grade 12).

NDE and Inspection Techniques:

• Visual Testing (VT): High-resolution video borescopes and crawler cameras to inspect internal surfaces for bio-mounds, tubercles, and under-deposit pustules.
• Pit Depth Gauges & Profilometry: Calibrated dial pit depth gauges to quantify maximum pit penetration beneath removed tubercles.
• Ultrasonic Thickness Testing (UT / PAUT): High-density grid ultrasonic scans and phased array imaging to detect localized wall thinning and subsurface voids.
• Profile Radiography (PRT): Exceptional technique for identifying localized pitting and deposit build-up in small-bore piping, bypass lines, and deadlegs without stripping thermal insulation.
• Biological Assays: Field testing using Adenosine Triphosphate (ATP) luminometry for rapid quantification of total active biomass; serial dilution culture bottles (Most Probable Number - MPN) to enumerate viable SRB, APB, and IOB; quantitative Polymerase Chain Reaction (qPCR) for exact metagenomic DNA mapping.