Seal without Corrosion: Pernicious of Graphite (Part B)

In continuation of the series on Expanded Graphite and its role in Sealing components, please also refer to Part A.

Expanded graphite is truly impressive for its overall sealing properties and high-temperature resistance, making it a preferred choice in various industrial applications. It has very low water absorption hence makes it desirable for Steam applications. However, like many materials, it is not immune to the effects of corrosion. This article explores the corrosion issues associated with expanded graphite and offers effective strategies for its mitigation.

Understanding Corrosion in Expanded Graphite

Corrosion is the gradual destruction of materials due to chemical reactions in their environment. For expanded graphite, corrosion can occur in several forms, primarily influenced by its exposure to water, aggressive chemicals, humidity, and temperature variations.

The main concerns include:

Chemical Corrosion: Expanded graphite is generally resistant to many chemicals, but certain aggressive substances, such as strong acids and bases, can degrade its structure over time. This degradation can lead to reduced sealing efficiency and premature failure.

Oxidation: At elevated temperatures, expanded graphite may undergo oxidation, especially in the presence of oxygen. Theoretically, Expanded Graphite starts oxidation at 450 Deg C and has a limit of 550 Deg C. However Graphite of <98% Purity will start oxidizing even earlier at 400 Deg C . Even if the Purity is 99%, the speed or rate of oxidation also plays an important role on how long it can retain its structure. This process can weaken the material, compromising its mechanical integrity and performance as a seal.

Moisture Absorption: While expanded graphite has low permeability, it can absorb moisture, which may lead to hydrolysis reactions with certain chemicals. This can create an environment conducive to corrosion, particularly in applications involving aqueous environments.

Strategies to Reduce Corrosion

To combat this corrosion issues associated with expanded graphite, we can divide the strategies into two categories. Type A which can be mitigated by the Material Manufacturer and Type B which can be mitigated at customer or consumer level.

For the sake of this article, we are not focusing on Type B. Most of the ideas for Type B are similar in nature to standard protection measures for Corrosion Protection. Since as a Sealing Material, the Expanded Graphite is in contact with the Media, there is more control at a Material Manufacturer level rather than at consumer level.

Clean Room Controlled Environment: Minimizing exposure to aggressive chemicals and humidity can greatly reduce the risk of corrosion. Certain components are required to be cleaned in a Controlled dust free environment that ensures there is no contamination during production process. These are specific required in highly corrosive environments or Oxygen environments.

Material Selection

The grade and quality selected based on the chemical parameters is far important when it comes to the performance of the Expanded Graphite in the long term. Choosing the right grade of expanded graphite for specific applications is crucial for maintaining flange integrity. Some grades are engineered with additives that improve corrosion resistance and antioxidation inhibitors.. These additives are further divided into passive and active. Selecting the correct grades can enhance durability especially in harsh environments.

The technical parameters of the Expanded Graphite material gives great insights into material composition and its behaviour at application level. We will try to decipher each parameter and identify the ones critical for Graphite selection.

• Carbon % & Ash%: The purity of Carbon is one of the most important parameter. Lowest purity possible is 95% and highest purity possible is 99.9%. However the standard rule is Minimum 98% for Industrial Applications. 99.9% for Nuclear. Anything between 98% to 95% is generally regarded as low grade Graphite. It is important to note, that since Graphite is a mined component, the quality of the Mine directly co-relates to the purity of the final Graphite.

The Ash content is connected to the carbon % and generally higher the Ash content, more is the impurities in the overall Graphite grade.

• Sulphur, Chlorine, Fluorine, Halogens (ppm): As per earlier article, Expanded Graphite goes through a chemical expansion process. This expansion is done through multiple means. The most economical is using Sulphur based chemicals. These chemicals then are absorbed by the Graphite Flakes and are released during compression and application. Since Sulphur promotes corrosion any leachable Sulphur more than 200ppm is undesirable in a good sealing material. Additionally, higher the Sulphur content will co-relate directly with lower purity of Carbon %.

Similarly, Chlorine and Fluorine based chemical expansion methods must ensure they stay below their threshold of 40ppm and 20ppm respectively.

• Corrosion Inhibitors: Incorporating corrosion inhibitors into the system can be effective to ensure a longer lasting Graphite especially at temperatures above 450 Deg C.

Passive Inhibitors: Focus on creating a protective barrier to prevent corrosion; effective in less aggressive environments and provide long-term protection without altering material properties.

Active Inhibitors: Engage in chemical reactions that mitigate corrosion; highly effective in aggressive environments but may alter material properties and require more frequent applications.

Since Passive Inhibitors offer more long term sealing and do not cater to any specific chemical reactions or requirement of media compatibility of the inhibitor, certain technical specs such as IOGP and MESC recommend passive inhibitors.

• Weight Loss %: As mentioned earlier, the speed or rate at which oxidation effect has on the finished component is also dependent upon the quality of the raw material selected. At 670 Deg C, the Graphite readily oxidises and the weight loss is recorded to understand how fast will the Graphite Ring lose its structure. This directly co-relates to the effect of the Graphite material and the longevity of the overall Seal in high temperature applications.

Conclusion

While expanded graphite is a formidable sealing material, it is not without its challenges, particularly concerning corrosion and purity. By understanding the types of corrosion that can affect expanded graphite and implementing effective mitigation strategies, industries can ensure the longevity and reliability of their sealing applications. Usage of poor quality Graphite will invariably introduce high sulphur and pitting corrosion into the flange, compromising overall joint integrity over the long term.

With proactive measures in place, and the right quality of expanded graphite we can continue to Seal in demanding environments and safeguarding the integrity of critical systems.