Impregnated Graphite Corrosion Resistance Testing and Selection Guide

Impregnation treatment is a key process for improving carbon graphite density and corrosion resistance. By impregnating metals, resins, or other materials into carbon graphite pores, chemical stability in specific media can be significantly improved. As a core technology of Huahao Sealing Co., Ltd. (霍邱县华豪密封件有限公司), impregnated graphite products are widely used in chemical, metallurgical, pharmaceutical, and desalination applications with strongly corrosive conditions. This article systematically presents corrosion resistance data of different impregnated graphite materials, standard test methods, and selection guidance based on operating conditions.

1. Classification of Impregnated Graphite

1.1 Metal Impregnation

• Antimony-impregnated: 800-900°C, porosity reduced below 2%, density 2.3-2.5 g/cm³
• Copper-impregnated: 1100-1200°C, density 2.4-2.6 g/cm³
• Babbitt-impregnated: 300-400°C, density 2.2-2.4 g/cm³
• Silver-impregnated: 960°C, density 2.5-2.7 g/cm³ (high cost)

1.2 Resin Impregnation

• Phenolic resin-impregnated: acid-resistant, not alkali-resistant, temperature limit 180°C
• Furan resin-impregnated: acid and alkali resistant, temperature limit 200°C
• Epoxy resin-impregnated: alkali-resistant, not acid-resistant, temperature limit 150°C

1.3 Special Impregnation

• PTFE-impregnated: resists virtually all media, temperature limit 200°C
• Glassy carbon-impregnated: temperature limit >500°C, mainly for high-temperature oxidizing conditions

2. Standard Corrosion Test Methods

2.1 ASTM C580 Standard

American Society for Testing and Materials standard for impervious graphite equipment corrosion evaluation:

• Specimen: 50×50×10 mm, precision ground surface
• Immersion: atmospheric pressure, media at 50°C, 80°C, or boiling point, for 30 days
• Evaluation indicators:
• Weight loss rate (g/m²·h)
• Compressive strength retention (%)
• Dimensional change (%)
• Rating:
• Excellent: <0.1 mm/year (weight loss <0.05 g/m²·h)
• Good: 0.1-1.0 mm/year
• Usable: 1.0-3.0 mm/year
• Unsuitable: >3.0 mm/year

2.2 GB/T 13465 Standard

Chinese national standard for impervious graphite material corrosion testing:

• Similar specimen and immersion conditions to ASTM
• Additional impact strength retention indicator
• Four-level rating by corrosion rate (mm/year)

2.3 Field Simulation Testing

For critical applications, field simulation testing is recommended:

• Use actual media, simulate temperature, pressure, and flow
• Test duration 90-180 days
• Monitor corrosion rate, strength change, sealing performance

3. Corrosion Data in Typical Media

3.1 Sulfuric Acid

| Impregnation Type | 70% H₂SO₄ (80°C) | 98% H₂SO₄ (50°C) | 98% H₂SO₄ (Boiling) |

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

| Antimony | 0.05-0.10 mm/yr | 0.10-0.20 mm/yr | 0.50-1.00 mm/yr |

| Copper | 0.10-0.20 mm/yr | Unsuitable | Unsuitable |

| Phenolic | 0.05-0.10 mm/yr | 0.10-0.30 mm/yr | 0.30-0.50 mm/yr |

| Furan | 0.05-0.10 mm/yr | 0.10-0.20 mm/yr | 0.20-0.50 mm/yr |

| PTFE | <0.02 mm/yr | <0.05 mm/yr | 0.05-0.10 mm/yr |

3.2 Hydrochloric Acid

| Impregnation Type | 30% HCl (50°C) | 36% HCl (Boiling) |

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

| Antimony | 0.10-0.30 mm/yr | 0.50-1.00 mm/yr |

| Phenolic | 0.05-0.10 mm/yr | 0.10-0.30 mm/yr |

| Furan | <0.05 mm/yr | 0.05-0.20 mm/yr |

| PTFE | <0.02 mm/yr | <0.05 mm/yr |

3.3 Hydrofluoric Acid

| Impregnation Type | 20% HF (Ambient) | 40% HF (Ambient) |

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

| Antimony | Unsuitable | Unsuitable |

| Phenolic | 0.10-0.30 mm/yr | Unsuitable |

| Furan | 0.05-0.10 mm/yr | 0.05-0.10 mm/yr |

| PTFE | <0.02 mm/yr | <0.05 mm/yr |

3.4 Alkali Solutions

| Impregnation Type | 20% NaOH (80°C) | 40% NaOH (Boiling) |

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

| Antimony | 0.05-0.10 mm/yr | 0.10-0.30 mm/yr |

| Copper | 0.02-0.05 mm/yr | 0.05-0.10 mm/yr |

| Phenolic | Unsuitable | Unsuitable |

| Epoxy | 0.05-0.10 mm/yr | 0.10-0.20 mm/yr |

| PTFE | <0.02 mm/yr | <0.05 mm/yr |

4. Selection Principles and Recommendations

4.1 Decision Process

1.Determine media composition, concentration, temperature, pressure, and flow

2.Look up corrosion data for each impregnated graphite type

3.Select types with annual corrosion rate <0.1 mm/year (excellent grade)

4.Verify operating temperature does not exceed upper limit

5.Balance cost, processability, and availability

4.2 Typical Application Selections

• Concentrated sulfuric acid, phosphoric acid: antimony or furan-impregnated, up to 200°C
• Hydrochloric acid, hydrofluoric acid: furan or PTFE-impregnated, latter preferred
• Alkali solutions (NaOH, KOH): copper or epoxy-impregnated
• Aqua regia, strong oxidizing mixed acids: only PTFE-impregnated
• Neutral salt solutions: antimony or resin-impregnated
• High-temperature (>200°C) corrosive conditions: glassy carbon-impregnated

4.3 Cost vs Performance

• Antimony: medium cost, good versatility, first recommendation
• Resin-impregnated: lower cost, moderate application range
• PTFE: higher cost, best corrosion resistance
• Silver: highest cost, only for special applications
• Glassy carbon: high cost, only for high-temperature oxidizing conditions

5. Application Examples

5.1 Phosphoric Acid Concentration

Conditions: 75% phosphoric acid, 150°C, 5% solids

Selection: antimony-impregnated seal ring, after 3 years face wear 0.2 mm, still serviceable

5.2 Ion-Exchange Membrane Caustic Soda

Conditions: 32% NaOH, 90°C, atmospheric

Selection: copper-impregnated seal ring, no significant corrosion after 5 years

5.3 Hydrofluoric Acid Reactor

Conditions: 40% HF, 60°C

Selection: PTFE-impregnated seal ring, corrosion rate <0.05 mm/year after 2 years

5.4 Multi-Effect Seawater Desalination

Conditions: seawater concentrated 3x, 110°C, 12% salinity

Selection: furan resin-impregnated, no significant corrosion after 4 years

6. Precautions

6.1 Impregnation Quality Inspection

When purchasing impregnated graphite products, inspect:

• Bulk density: meets grade requirement (e.g., antimony-impregnated ≥2.3 g/cm³)
• Porosity: <2% (impervious graphite)
• Compressive strength: ≥150 MPa
• Hydraulic test: 1.5x working pressure for 30 minutes without leakage

6.2 Condition Change Effects

When media concentration or temperature exceeds design values, corrosion rate may multiply. Reassess applicability when conditions change, and replace impregnation type if needed.

6.3 Regular Inspection and Replacement

For critical equipment:

• Quarterly seal ring thickness inspection
• Replace when cumulative corrosion reaches 20% of original thickness
• Annual media composition analysis to confirm no changes

Conclusion

Proper selection of impregnated graphite corrosion resistance directly affects equipment operational safety and service life. Through scientific test methods and data comparison, engineers can make correct selection decisions. Huahao Sealing Co., Ltd. provides a full range of impregnated graphite products — metal, resin, and PTFE impregnation — with customized selection recommendations based on specific operating conditions. Contact our technical team for detailed technical consultation.