Quartz and Alumina Tube Compatibility in K₂CO₃ and KOH

This case study presents how we helped a customer resolve long-standing material compatibility concerns when operating quartz tubes in nitrogen atmospheres containing alkaline compounds.
By evaluating corrosion behavior, temperature limits, and flange design, a practical quartz–alumina tube solution was recommended and successfully applied.

Customer Background

A leading petroleum company in the Middle East, within a frontier development research institute, focuses on biomass conversion experiments under high-temperature alkaline catalytic reaction environments.

Customer Inquiry:
We need high-purity fused quartz tubes with an outer diameter of 35±0.5 mm, an inner diameter of 30±0.5 mm, a length of 200±1 mm, and matching flanges. However, I still have some follow-up questions to ask:

Can these flanges be used at high temperatures? In our application, the temperature will not exceed 900°C, and the system will run continuously for 6–8 hours.
We will use some salts such as potassium carbonate (K₂CO₃) and potassium hydroxide (KOH) in the crucible, then mix them with samples, place them into the quartz tube, and process them in a dry or humid nitrogen atmosphere. Under these application conditions, are the quartz tubes and flanges suitable?
After the quartz tube and flange are assembled together, what is the total length?

Will K₂CO₃ and KOH Corrode Quartz Tubes and Stainless Steel Flanges? (Part 1 of 6)

K₂CO₃ and KOH are solids at room temperature, but under high temperatures or in humid environments, they may exist in vapor or solution form. Below is an analysis of their corrosiveness to quartz tubes and stainless steel flanges:

1. Corrosion Resistance of Quartz Tubes (SiO₂)

Potassium Hydroxide (KOH):
KOH has significant corrosiveness to quartz tubes, especially under high-temperature or molten conditions. KOH reacts with SiO₂ to form soluble potassium silicate (K₂SiO₃). The reaction formula is:

Therefore, KOH vapor or melt will severely corrode quartz tubes.

Potassium Carbonate (K₂CO₃):
At high temperatures (such as molten state), K₂CO₃ will also react with SiO₂ to generate potassium silicate and carbon dioxide.

However, its corrosiveness is weaker than KOH, requiring higher temperatures or longer contact times to become significant.

Conclusion:
KOH is extremely corrosive to quartz tubes, while K₂CO₃ also poses corrosion risks at high temperatures.

2. Corrosion Resistance of Stainless Steel Flanges

Potassium Hydroxide (KOH):
Stainless steel (such as 304 or 316) has some corrosion resistance to KOH solutions at room temperature, but high-temperature and high-concentration KOH can cause stress corrosion cracking (particularly in austenitic stainless steel). KOH vapor may also accelerate corrosion.

Potassium Carbonate (K₂CO₃):
Stainless steel has relatively good corrosion resistance to K₂CO₃ solutions, but molten states at high temperatures may lead to pitting or intergranular corrosion, especially when chloride impurities are present.

Conclusion:

In room-temperature humid environments: Stainless steel can withstand K₂CO₃ and low-concentration KOH.
In high-temperature/molten states: Stainless steel may be corroded by KOH or K₂CO₃; nickel-based alloys (e.g., Inconel) or special coatings are required.

Recommendations

Quartz Tubes: Avoid contact with KOH or high-temperature K₂CO₃. Instead, use alumina tubes (Al₂O₃) or nickel tubes.
Stainless Steel Flanges: In high-temperature alkaline environments, Hastelloy or nickel-plated treatment is recommended.

If more specific solutions are needed, please provide detailed operating conditions (temperature, concentration, pressure, etc.).

Attachment: Technical characteristics of alumina tubes.

Will Dry or Moist Nitrogen Corrode Quartz Tubes? (Part 2 of 6)

Whether nitrogen (N₂) is dry or moist, it will not corrode quartz tubes (SiO₂) directly. Quartz tubes remain highly stable in such environments, making them an ideal material for containers or pipelines.
However, moist environments (humid nitrogen) can significantly accelerate corrosion caused by other factors, and there is one very important exception. A detailed analysis is provided below.

1. Dry Nitrogen (Dry N₂)

Corrosiveness: None at all.

Reason: Dry nitrogen is an inert gas with very stable chemical properties. It does not react with the main component of quartz tubes, silicon dioxide (SiO₂), at room temperature or even at high temperatures. Quartz tubes have a dense glassy structure that nitrogen molecules cannot penetrate or corrode.

Application: Dry nitrogen is an ideal atmosphere for protecting quartz tubes and their internal samples (such as semiconductor materials, metal heat treatment, etc.), preventing oxidation and ensuring a stable environment.

2. Moist Nitrogen (Humid N₂)

Nitrogen itself: Still non-corrosive. Water vapor (H₂O) also does not strongly react with SiO₂.

Key Risks: Humid nitrogen can indirectly cause problems for quartz tubes in two main ways:

a) Accelerating Corrosion from Alkali Metal Contaminants (Main Risk)

Mechanism: If even trace amounts of alkaline substances are present (e.g., sodium ions Na⁺ from fingerprints, dust in the air, or KOH from other parts of the system), water vapor can dissolve these substances, forming an alkaline liquid film on the quartz tube surface.

Consequence: Even extremely dilute alkaline solutions can severely corrode quartz tubes.

(For KOH, a similar reaction occurs, producing soluble potassium silicate.)

Conclusion: Moist nitrogen acts as a carrier and catalyst for corrosive agents (alkaline contaminants), dramatically increasing the corrosion rate compared to a dry environment.

b) Accelerating Quartz Devitrification at High Temperatures

Mechanism: Quartz glass is a metastable amorphous material. At high temperatures (>1000 °C), water vapor disrupts the Si-O-Si network structure of quartz, lowers its viscosity, and accelerates its transformation into a more stable crystalline form (cristobalite).

Consequence: The quartz tube surface loses its smooth glassy state and becomes a white, opaque crystalline structure. This process is called “devitrification.” Devitrified areas have drastically reduced mechanical strength, becoming brittle and highly prone to cracking under thermal shock.

Conclusion: High temperature combined with water vapor is the main cause of quartz tube devitrification. Dry nitrogen can greatly delay this process.

A Critical Exception: High-Temperature Hydrogen-Oxygen Reaction

If your system contains not only moist nitrogen but also hydrogen (H₂), and operates at high temperatures (>1000 °C), special caution is required:

At high temperatures, H₂O decomposes: $\ce{H2O <=> H+ + OH-}$

These reactive species (particularly OH⁻) react with SiO₂ to produce volatile silicon hydroxide, Si(OH)₄.

Consequence: This process gradually thins the quartz tube wall, roughens the surface, and eventually leads to vapor-phase corrosion, wall thinning, perforation, or rupture. This type of corrosion is especially severe under high temperature, high pressure, and high water vapor content conditions.

Technical Characteristics of Alumina Tubes (Part 3 of 6)

Alumina tubes (main component Al₂O₃ ≥99%) are a type of high-performance ceramic material, widely used in high-temperature, corrosive environments, and precision industrial applications. Their main properties are as follows:

1. High-Temperature Resistance

Long-term working temperature: 1600℃
Maximum short-term tolerance temperature: 1800℃
Thermal shock resistance: withstands rapid cooling and heating (1550℃ water quenching 6 times without cracking)
Thermal expansion coefficient: 8.2×10⁻⁶/℃ (20–1000℃), with thermal stability superior to metals

2. Mechanical Strength

Compressive strength: ≥80 MPa (some products up to 350 MPa or even 12000 MPa)
Flexural strength: >350 MPa
Hardness: Mohs hardness 9 (second only to diamond and silicon carbide), with wear resistance more than 20 times that of carbon steel

3. Chemical Corrosion Resistance

Acid and alkali resistance: excellent resistance to most acids, bases, and molten salts (such as KOH, K₂CO₃) (though high-temperature molten alkalis may still cause corrosion)
Oxidation resistance: suitable for vacuum, inert, and reducing atmospheres

4. Physical Properties

Density: 3.80–3.95 g/cm³, porosity <1%, with high compactness
Thermal conductivity: 25 W/m·K, with excellent insulation performance (resistivity at 20℃ >10¹⁴ Ω·cm)
Dielectric constant: 2 (1 MHz), suitable for high-frequency insulation applications

5. Application Fields

High-temperature furnace tubes: vacuum tube furnaces, atmosphere furnaces (such as molybdenum wire furnaces, sintering furnaces)
Industrial measurement: thermocouple protection tubes, continuous steel temperature measurement
Wear-resistant pipelines: transportation of abrasive media in metallurgy and power industries
Special environments: chemical, military, and electronic industries requiring corrosion resistance

6. Specifications and Customization

Size range: inner diameter 0.5 mm to 150 mm, length up to 2500 mm
Custom options: can be processed into single-hole, multi-hole, threaded tubes, and other special structures

Conclusion:
Due to its ultra-high temperature resistance, mechanical strength, and chemical inertness, alumina tubes are an ideal substitute for quartz tubes (not resistant to alkali) and stainless steel (prone to oxidation at high temperatures), showing exceptional performance especially in extreme environments below 1800℃.

How to Seal Both Ends of a Alumina Tube (Part 4 of 6)

When alumina tubes (Al₂O₃) are used in high-temperature or vacuum environments, the sealing materials at both ends must have high-temperature resistance, corrosion resistance, and good airtightness.
Below are the common sealing materials and corresponding solutions:

1. Flange Sealing System (Most Common)

Material: 304/316 stainless steel flange (high-temperature and corrosion resistant)

Sealing gaskets:

Double-layer silicone O-ring: Suitable for medium and low temperatures (≤250 °C), with airtightness up to 4.04×10⁻³ Pa.
Graphite wound gasket: Withstands high temperatures (≤650 °C), suitable for high-pressure or vacuum environments.
Alumina-filled PTFE gasket: Corrosion-resistant and wear-resistant, suitable for acidic or alkaline conditions.
Water-cooled flange: Cooling water can be circulated under high-temperature conditions to prevent O-ring aging.

2. High-Temperature Adhesive Sealing (For Temporary or Small-Scale Experiments)

High-temperature adhesive: Such as aluminum phosphate adhesive (resistant up to 650 °C), though it tends to crack during long-term use.
Ceramic adhesive: Withstands temperatures above 1000 °C, but is difficult to remove once cured.

3. Mechanical Sealing Components

Metal bellows seal: Used for ultra-high-vacuum applications (e.g., molecular pump interfaces); made of stainless steel or nickel-based alloys.
Spring-loaded sealing cap: Compresses the end face of the alumina tube using spring pressure, usually in combination with a metal gasket.

4. Special Condition Adaptations

Strongly corrosive environments: Use Hastelloy flanges or nickel-plated components.
Ultra-high-temperature (>1600 °C): Employ alumina-metal composite flanges with an inner alumina ring to reduce thermal stress.

Notes

Thermal expansion matching: The thermal expansion coefficient of alumina (8×10⁻⁶ / °C) should be close to that of the sealing material to avoid cracking caused by thermal cycling.
Installation and maintenance: Regularly clean the flange contact surfaces to prevent dust or oil contamination that could affect sealing performance.

Applicability of Alumina Tubes in Humid Nitrogen Environments with KOH (Part 5 of 6)

The applicability of alumina tubes (Al₂O₃) under humid nitrogen (with water) combined with KOH depends on several factors—mainly temperature, KOH concentration, and the form of water present.

1. Room Temperature or Low-Temperature (≤100°C) Humid Nitrogen + KOH Solution

Applicability: Short-term use is acceptable, but long-term use requires caution.

Alumina exhibits good corrosion resistance to low-temperature KOH solutions (concentration <30%), as a passivation layer can form on the Al₂O₃ surface.
However, with continuous contact with liquid water, KOH will slowly dissolve Al₂O₃ (though the reaction rate is low). Over time, this may lead to wall thinning or surface roughening.

Sealing risk: Water vapor may infiltrate flange sealing gaskets (such as silicone O-rings), causing swelling or aging.

Recommendations:

Use PTFE seals or fluoroelastomer O-rings, which resist alkali and moisture.
Regularly inspect the inner wall of the alumina tube for fogging or corrosion marks.

2. High-Temperature (>100°C) Humid Nitrogen + KOH Vapor / Molten Alkali

High-temperature steam (>200°C):
Water vapor accelerates the corrosion of alumina by KOH, forming potassium aluminate (KAlO₂) and aluminum hydroxide gel.

The higher the temperature, the faster the reaction rate—for example, at 500°C, the corrosion rate may reach 0.1 mm/year.

Molten KOH (>400°C):
Alumina tubes are rapidly attacked, especially in the presence of water vapor, which promotes alkali ion migration.

Conclusion:
When the temperature exceeds 200°C, alumina tubes are not recommended for use in humid nitrogen + KOH environments.

Alternative Materials:

Nickel-based alloy tubes (e.g., Inconel 600): Highly resistant to high-temperature alkali corrosion but more costly.
Molybdenum or tantalum tubes: Suitable for anhydrous high-temperature alkali conditions (note that tantalum oxidizes easily in humid oxygen).
Quartz tubes (for anhydrous conditions only): Though not resistant to KOH, they can be used short-term at low temperature in humid nitrogen, provided that direct liquid water contact is avoided.

3. Design Recommendations for Risk Reduction

If the use of an alumina tube is unavoidable, the following measures can reduce risk:

Protective inner coating: Apply yttria (Y₂O₃) or zirconia (ZrO₂) coating on the inner wall for enhanced alkali resistance.
Drying pre-treatment: Before introducing humid nitrogen, purge the system with dry nitrogen to remove residual liquid water.
Thermal isolation: Place KOH in the central high-temperature section of the alumina tube, while keeping both flanges at low temperature (water-cooled) to prevent vapor condensation.

Summary

Condition	Alumina Tube Applicability	Risk Factors	Recommended Alternative
Room temperature humid N₂ + dilute KOH	Short-term use acceptable	Slow corrosion, seal aging	PTFE seal + periodic inspection
High-temperature humid N₂ + KOH vapor	Not suitable	Rapid corrosion, wall perforation	Nickel-based alloy tube
Molten KOH + water vapor	Strictly prohibited	Damage within hours	Molybdenum/Tantalum tube (anhydrous)

Solution: Customized Alumina Tubes and Alkali-Resistant Flange System (Part 6 of 6)

July 2, 2025 – Customer Inquiry

We are pleased to learn that you can supply alumina tubes.
If alumina tubes can be used together with KOH in a nitrogen atmosphere containing moisture (humid nitrogen), we would prefer to select alumina as the tube material.

Regarding the flange, would you recommend using a ceramic flange or a nickel-based alloy flange under our operating conditions?

In addition, please note that the total assembled length (tube + flange) must not exceed 1.3 meters.

We look forward to your reply.

July 3, 2025 – Our Reply

Figure 1 shows a standard flange combined with an alumina tube of 50 mm outer diameter and 5 mm wall thickness.
The alumina tube is made of alkali-resistant alumina material and can be used in corrosive gas environments such as potassium carbonate (K₂CO₃) and potassium hydroxide (KOH). Its maximum temperature resistance is 1700 °C, with a long-term operating temperature of 1600 °C.

However, the silicone sealing ring used in standard flanges is only rated up to 200 °C.
If higher temperature resistance is required, the standard flange can be replaced with a water-cooled flange (Figure 2), which can withstand temperatures up to 300 °C.
If this is still insufficient, increasing the length of the alumina tube can be considered to reduce the temperature at both tube ends.

Regarding flange corrosion resistance, the flange can be internally lined with PTFE (Teflon), as shown in Figure 3, to improve corrosion resistance.

Given the special nature of your experimental environment, we recommend preparing both quartz tubes (low-cost options suitable for one-time experiments) and alumina tubes simultaneously, which can help reduce development time.

In addition, except for standard flanges and alumina tubes with 50 mm outer diameter and 5 mm wall thickness, all other special products require customization.

July 3, 2025 – Customer Requirements

We would like to order both types of tubes (quartz tubes and alumina tubes).
Our specific requirements are as follows:

Tube structure: straight tubular structure, open at both ends
Operating temperature: stable operation up to 1200 °C
Tube length: 1.2 m (excluding flanges)
Outer diameter: 3.4 cm
Inner diameter: 3.0 cm
Wall thickness: 0.2 cm
Operating condition: atmospheric pressure
Quantity: 10 tubes in total (5 alumina tubes and 5 quartz tubes)
Stability requirement: stable when water vapor is introduced into the tube

Flange requirements:

Quantity: 10 pieces
Internal lining: PTFE (Teflon)

We are still concerned about the thermal stability of the flanges and are considering whether extending the length of the quartz or alumina tubes could help reduce the temperature at the flange area.
The question is: what additional length would be appropriate?

Given that our target operating temperature is 850 °C, and the total assembled length (including flanges) should not exceed approximately 1.5 m, what would you recommend?

If a total length of 1.4–1.5 m can resolve the thermal stability issue, please provide a quotation and the estimated delivery time.

July 4, 2025 – Our Reply

A tube length of 1.2 m is appropriate, and we do not recommend increasing it further.

For alumina tubes, a wall thickness of 2 mm is not feasible.
The minimum wall thickness we can provide for alumina tubes is 5 mm.

Quartz tubes can be manufactured according to your requested dimensions.
However, please note that the outer diameters of quartz tubes and alumina tubes would differ, which would make it impossible to use a shared flange.

To ensure that one flange type can be compatible with both tube materials, we recommend adjusting the dimensions as follows:

Alumina Tubes

Outer diameter: 40 mm
Wall thickness: 5 mm
Inner diameter: 30 mm
Length: 1200 mm
Tolerance: ±1 mm
Quantity: 5 pcs
Price: USD $*** / pc
Lead time: 12 days

Quartz Tubes

Outer diameter: 40 mm
Wall thickness: 3 mm
Inner diameter: 34 mm
Length: 1200 mm
Quantity: 5 pcs
Price: USD $**** / pc
Lead time: 12 days

Flange Assembly (for 40 mm OD tubes)

Internal lining: PTFE (Teflon)
Equipped with water-cooling structure
Cooling ring and quick-release flange are independent components
Quantity: 5 sets (10 individual flanges)
Price: USD $**** / set
Lead time: 20 days

Regarding the flange design:

We recommend adding a water-cooling system.
The additional cost of the cooling ring is very limited, but it significantly improves the temperature resistance of the sealing gasket.

The water-cooling system is designed as an independent and normally open structure.
When not in use, it can remain idle.
Its structure is completely isolated from the internal gas environment, so it does not affect the purge gas and does not introduce any leakage risk at the cooling ports.

To ensure compatibility between quartz tubes and alumina tubes, the final recommended configuration is:

Alumina tubes: OD 40 mm, WT 5 mm, ID 30 mm, length 1200 mm, tolerance ±1 mm, quantity 5 pcs
Quartz tubes: OD 40 mm, WT 3 mm, ID 34 mm, length 1200 mm, quantity 5 pcs
Flange assemblies: 5 sets for 40 mm tubes, PTFE-lined, with external water-cooling system

August 21, 2025 – Customer Feedback

We have received the products and have successfully and continuously conducted the long-awaited experiments.
Thank you very much for providing such a professional and practical solution, which perfectly resolved a problem that had troubled us for several months.

Your company’s professional capability has truly impressed us and has provided significant momentum for our research.
We sincerely thank Casper and your technical team for their strong support.
We greatly appreciate it and look forward to our next cooperation.

Discuss Your Application Requirements

We also manufacture high-purity alumina tubes for high-temperature and corrosive environments.

If your application involves nitrogen atmospheres, alkali compounds, or thermal stability concerns, our engineers can help evaluate suitable solutions.