Carbon Graphite Chemical Compatibility Guide: From Strong Acids to Strong Alkalis

In chemical, pharmaceutical, and metallurgical industries, the variety of media — from strongly oxidizing acids to high-temperature alkalis, from chlorinated solvents to molten salts — means the chemical compatibility of a seal material often determines whether equipment can run reliably long-term. Carbon graphite, with its strong covalent carbon bonds and layered crystal structure, resists most media. However, the impregnant that fills the internal pores often becomes the material's weakest link. Based on years of experience in chemical pumps and reactor seals, Huahao Sealing Co., Ltd. has compiled this compatibility guide for engineers.

1. Inherent Corrosion Resistance of Unimpregnated Graphite

The intrinsic chemistry of baked carbon graphite is defined by carbon itself. At room temperature, carbon does not react with most acids, alkalis, or salts; it only slowly oxidizes under strongly oxidizing conditions. Below 100°C, unimpregnated graphite tolerates 98% sulfuric acid, 37% hydrochloric acid, 65% nitric acid (short term), phosphoric acid, hydrofluoric acid (non-oxidizing), and sodium hydroxide and sodium carbonate at any concentration. However, its high porosity (10%~15%) means unimpregnated graphite is mainly used for static components like linings and heat exchangers, not as a sealing friction pair.

2. Corrosion Resistance of Resin-Impregnated Graphite

2.1 Phenolic-Impregnated Graphite (M106H)

Cured phenolic resin forms a highly crosslinked network with good resistance to non-oxidizing acids, salt solutions, and dilute alkalis. It can serve long-term in:

• Hydrochloric acid: any concentration, up to 100°C
• Sulfuric acid: up to 70%, up to 90°C
• Phosphoric acid: any concentration, up to 120°C
• Hydrofluoric acid: up to 40%, up to 60°C
• Sodium hydroxide: up to 30%, up to 80°C

Phenolic-impregnated graphite performs poorly in strongly oxidizing media. Nitric acid above 50%, sulfuric acid above 70%, chromic acid, and hypochlorous acid cause oxidative degradation of the resin, reopening the pores.

2.2 Furan-Impregnated Graphite (M106K)

Furan resin outperforms phenolic in both acid and alkali resistance and is ideal for alternating acid-alkali service. At 50% sodium hydroxide and 100°C, furan-impregnated graphite runs stably long-term. Our IJ centrifugal pump bushings supplied to a chlor-alkali plant, made of furan-impregnated graphite, have run 18 months without failure.

2.3 Epoxy-Impregnated Graphite

Epoxy offers good resistance to alkalis and many organic solvents, but temperature limit is lower (≤ 150°C), suitable for room-temperature media containing organic solvents.

3. Corrosion Resistance of Metal-Impregnated Graphite

3.1 Antimony-Impregnated Graphite (M254G)

Antimony is relatively stable in alkalis and tolerates mid-to-low concentration sulfuric acid. It excels in:

• Sodium hydroxide: up to 50%, up to 200°C
• Sulfuric acid: up to 60%, up to 120°C
• Steam: up to 450°C
• Thermal oil: up to 400°C

Antimony-impregnated graphite must be used cautiously in chloride-containing media. Chloride ions break the passive film on antimony, causing pitting. For seawater cooling pumps and brine wastewater service, resin-impregnated grades or silicon carbide mating parts are recommended.

3.2 Copper-Impregnated Graphite

Copper-impregnated graphite has excellent thermal conductivity but is not resistant to ammonia, ammonium salts, or strong acids. It is mainly used for heat-dissipating seals in high-temperature, low-corrosion service.

4. Organic Solvent Service

Carbon graphite itself is stable against most organic solvents, including benzene, toluene, xylene, ethanol, acetone, chloroform, and dichloromethane. The impregnant's tolerance, however, must be checked carefully: phenolic and furan resins may swell in polar solvents; epoxy degrades in ketones. For pumps handling organic solvents, we recommend unimpregnated high-purity graphite or PTFE-impregnated graphite.

5. Quick Compatibility Reference

| Medium | Conc. | Temp. | Phenolic | Furan | Antimony |

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

| HCl | any | boil | OK | OK | No |

| H2SO4 | 70% | 90°C | OK | OK | Caution |

| HNO3 | 30% | RT | No | No | No |

| NaOH | 50% | 100°C | Caution | OK | OK |

| Seawater | — | RT | OK | OK | Caution |

| Benzene | 100% | RT | Caution | OK | OK |

| Steam | — | 400°C | No | No | OK |

6. Common Selection Pitfalls

1.Treating carbon graphite as universally corrosion-proof, ignoring impregnant effects and causing premature failure in oxidizing media.

2.Ignoring temperature effects: media tolerable at room temperature may accelerate corrosion at high temperature.

3.Overlooking galvanic corrosion of mating parts: carbon graphite acts as a cathode and may accelerate corrosion of mating metal components.

Conclusion

The chemical compatibility of carbon graphite is the combined result of impregnant and carbon body performance. We recommend that engineers define medium concentration, temperature, flow velocity, and mating material, then use this compatibility table for initial screening. For complex service or novel media, please share operating parameters with our engineering team for a joint evaluation to ensure a reliable, cost-effective sealing solution.