3 Early Warning Signals of Carbon Graphite Seal Failure in Concentrated Sulfuric Acid Service and Troubleshooting

1. Unique Corrosion Mechanism of Concentrated Sulfuric Acid

98% concentrated sulfuric acid (H2SO4 mass fraction 95%-99%) attacks carbon graphite seals through a mechanism entirely different from dilute acid. Dilute sulfuric acid acts via electrochemical corrosion, whereas concentrated acid, with its extremely low water content, exerts a strong dehydrating effect. The reaction splits into two temperature regimes:

1.1 Passivation Regime (Room Temp – 80°C)

Concentrated sulfuric acid reacts with the carbon graphite surface and metal mating ring surfaces to form insoluble passivation films: the carbon graphite surface oxide layer reaches approximately 3-8μm thickness, and the SiC mating ring surface produces a SiO2 passivation film ~1-2μm thick. In this regime the corrosion rate is below 0.02mm/year and seal life can exceed 18 months.

1.2 Activation Regime (Above 120°C)

Above 120°C, thermal disturbance disrupts and dissolves the passivation film, switching the corrosion mechanism to a vigorous oxidation reaction:

• C + 2H2SO4 (conc.) → CO2↑ + 2SO2↑ + 2H2O
• The corrosion rate leaps from 0.02mm/year in the passivation regime to 0.8mm/year in the activation regime — a 40-fold amplification

Measured data from a copper smelter: after pump outlet temperature increased from 95°C to 135°C, M120K seal ring service life collapsed from 16 weeks to 2 weeks, with maximum seal face wear depth reaching 0.9mm (design wear margin is only 1.2mm).

2. Three Typical Warning Signals

2.1 Signal One: Abnormal Seal Face Discoloration

Under normal conditions, phenolic or furan-resin-impregnated carbon graphite seal faces exhibit a uniform silver-gray color with Ra ≤ 0.4μm. The following color changes demand immediate attention:

• Dark Green Deposit Film: Localized or full-face dark green deposits confirmed by EDS as Fe2(SO4)3·nH2O crystals. This signal indicates corrosion product deposition from mating stainless steel rings or pump cavities, proving the seal cavity temperature has exceeded the passivation film dissolution temperature. Incidence 62%, the most frequent precursor.
• Brown Spots: Localized oxidative decomposition of the resin-impregnated layer, exposing the carbon graphite matrix which is then oxidized by sulfuric acid into activated carbon precursors that adsorb colored substances. Appears 3-5 days later than dark green deposits but with higher precision — 88% probability of leakage within 7-10 days after onset.
• Gray-White Etch Pits: Seal face shows pits of 0.3-1.5mm diameter and 0.05-0.2mm depth. This is direct evidence of impregnated resin being selectively dissolved by concentrated sulfuric acid. Seal leakage typically follows within 48 hours.

Field inspection method: After shutdown, clean the seal face with anhydrous ethanol, blow-dry with cold air, photograph under 100× stereo microscope and compare against standard reference images. Huahao Sealing provides a free seal face color comparison atlas.

2.2 Signal Two: Friction Torque Rise ≥25% Week-over-Week

Normal mechanical seal friction torque consists of liquid film shear force (60%-75%) and face asperity contact force (25%-40%). Concentrated sulfuric acid viscosity at 25°C is approximately 24.5 cP — 24 times that of water — so the baseline torque is already high.

Two mechanisms for torque spikes:

1.Semi-dry Friction on Seal Faces: Excessive seal face temperature causes liquid film evaporation, dropping film thickness from 0.5-1.5μm to below 0.1μm. Direct asperity contact ratio increases, pushing the torque coefficient from 0.08 to 0.15-0.20.

2.Abrasive Effect of Wear Debris: Carbon graphite wear debris hardness is HB150-250, which is not easily flushed away in high-viscosity sulfuric acid, creating three-body abrasive wear that further degrades mating ring surface roughness — a vicious cycle.

Client data from a phosphoric acid plant: baseline torque 14.8N·m, rose to 18.9N·m (+27.7%) after 7 days of continuous monitoring, then to 22.5N·m (+52.0%) on day 10, followed by sudden seal leakage on day 12. Recommend a 20% week-over-week torque rise as yellow alert threshold and 25% as red alert.

2.3 Signal Three: Tank Media Viscosity Rises ≥15% within 24h

Easily overlooked but actually the most reliable early warning indicator. When concentrated sulfuric acid leaks into the reaction or circulation system, its dehydrating action causes organic media molecules to dehydrate and condense into oligomers, rapidly increasing system viscosity.

Monitoring and determination method:

• Install an online viscometer at the circulation pump outlet with sampling frequency ≥1/hour
• Use the 7-day rolling average viscosity as baseline
• ≥10% viscosity rise within 24h triggers yellow alert, ≥15% triggers red alert
• Simultaneously test circulation tank samples for SO4²- concentration (ion chromatography); a rise from background <50ppm to >200ppm confirms leakage

This signal provides a 2-5 day advance window compared to direct leakage monitoring, and is well-suited for DCS automatic alarm systems.

3. Standardized Troubleshooting Flow

Upon detecting warning signals, follow this prioritized troubleshooting sequence:

Step 1: Rapid Seal Face Temperature Measurement (<5 min)

• Tool: Portable IR thermometer (accuracy ±2°C), or pre-installed PT100 sensor
• Measurement point: Seal cavity exterior, 5mm from the stationary ring O-ring (to avoid flange heat conduction interference)
• Thresholds: Temperature ≤105°C = normal; 105-120°C = yellow alert, continue monitoring; >120°C = red alert, shut down immediately
• Common false-positive source: Pump case radiant heat, ambient temperature >40°C — apply background temperature correction

Step 2: Leakage pH and Appearance Test (post-shutdown, <10 min)

• Method: Collect liquid from the seal drain port onto a clean glass slide, measure with wide-range pH paper (0-14)
• Interpretation: pH 2-4 with clear, transparent liquid = normal dilute acid flush; pH<1 with black, turbid liquid = abnormal condition
• Further confirmation: Titrate with BaCl2 solution; white BaSO4 precipitate confirms sulfuric acid leakage

Step 3: Post-Disassembly Face Roughness Inspection (<30 min)

• Tool: Portable surface roughness meter (Ra range 0.01-10μm), measure 3 evenly distributed points on the face and average
• Carbon graphite soft ring Ra upper limit: ≤0.8μm; SiC hard ring Ra upper limit: ≤0.2μm
• Over-limit handling: If hard ring Ra>0.2μm, it must be returned to Huahao Sealing for re-grinding, otherwise soft ring wear rate will double

4. Root Remediation Solutions

4.1 Material Upgrade Plan

Comparison of mainstream materials for concentrated H2SO4 service:

| Material Grade | Impregnation | Temp Limit | Corrosion Rate (98% H2SO4) | Recommended Life |

|---------------|-------------|-----------|---------------------------|-----------------|

| M120K | Phenolic Resin | ≤100°C | 0.025 mm/year | 8-12 weeks |

| M180K | Furan Resin | ≤140°C | 0.006 mm/year | 28-36 weeks |

| M254K | PTFE | ≤120°C | 0.008 mm/year | 24-32 weeks |

| M106K | Antimony Metal | ≤180°C | 0.015 mm/year | 20-28 weeks |

Huahao Sealing recommends M180K furan-resin-impregnated carbon graphite as first choice: furan resin cross-link density is 2.3 times higher than phenolic resin, and at 120°C in concentrated sulfuric acid the impregnation layer retention rate is 89% (phenolic only 41%). Already batch-replaced for 12 TiO2 pigment, phosphate fertilizer and copper smelter clients, with a statistical average service life increase from 11 weeks to 32 weeks — a 191% improvement.

4.2 Operating Parameter Optimization

• Flush plan: Adopt Plan 23 circulation flush with flush flow ≥3 L/min at 0.1-0.2 MPa above media pressure
• Temperature control: Seal cavity jacket cooling water flow ≥5 m³/h, ensuring temperature rise ≤30°C
• Startup procedure: Open flush valve 3 minutes before pump start to avoid instantaneous dry-start temperature spikes

Huahao Sealing technical services team provides complimentary concentrated sulfuric acid service seal audits, delivering complete reports including material selection recommendations, flush plan optimization, and life prediction.