What Is Anaerobic Sealant?

  • Post last modified:February 4, 2026

What Is Anaerobic Sealant? A Comprehensive Guide to Industrial Bonding and Sealing

In the world of industrial manufacturing, automotive repair, and precision engineering, the integrity of a joint can mean the difference between a high-performing machine and a catastrophic failure. While there are many types of adhesives available, one specific category stands out for its unique curing properties and specialized applications: anaerobic sealants. But what exactly is an anaerobic sealant, and why is it preferred over traditional mechanical locking devices or other adhesives?

This comprehensive guide explores the chemistry, applications, benefits, and technical nuances of anaerobic sealants, providing you with the knowledge needed to select the right product for your specific industrial needs.

Understanding the Basics: What Defines an Anaerobic Sealant?

The term “anaerobic” comes from the Greek words meaning “living without air.” In the context of sealants and adhesives, this describes the unique curing mechanism of the material. Anaerobic sealants are one-part, liquid or semi-liquid resins that remain stable in a liquid state as long as they are exposed to oxygen. However, when they are confined between two closely fitting metal surfaces—effectively cutting off the oxygen supply—they undergo a chemical reaction and polymerize into a tough, cross-linked plastic.

This “curing in the absence of air” is only half of the equation. For the reaction to occur efficiently at room temperature, the presence of metal ions (typically iron or copper) is required to act as a catalyst. This dual-requirement—exclusion of oxygen and contact with metal—makes anaerobic sealants incredibly reliable for specific mechanical assemblies.

The Chemistry Behind the Cure

At a molecular level, anaerobic sealants are primarily composed of acrylic monomers (specifically dimethacrylates). The formulation also includes hydroperoxides to initiate the reaction, accelerators to speed up the process, and stabilizers that prevent the product from hardening while inside the bottle. Because oxygen acts as a stabilizer, anaerobic bottles are never filled to the top; the “headspace” of air is vital to keep the product liquid during storage.

When the sealant is applied to a bolt and threaded into a nut, the oxygen is squeezed out. The metal surfaces provide the necessary electrons to trigger the breakdown of the hydroperoxides, initiating a free-radical polymerization. The result is a solid, thermoset plastic that fills all the microscopic voids between the metal threads or surfaces.

The Four Main Types of Anaerobic Sealants

Not all anaerobic products are created equal. Depending on the viscosity, strength, and intended use, they are generally categorized into four primary groups:

1. Threadlockers

Threadlockers are perhaps the most well-known type of anaerobic sealant. They are designed to prevent the loosening of threaded fasteners caused by vibration, shock, or thermal expansion. Before the invention of threadlockers, engineers relied on mechanical devices like spring washers, nylon-insert nuts, and split pins. While effective, these mechanical methods do not seal the threads, leaving them vulnerable to corrosion.

  • Low Strength (Purple): Used for small screws and parts that require frequent adjustment or disassembly with hand tools.
  • Medium Strength (Blue): The most versatile grade, capable of being disassembled with standard tools but strong enough to resist heavy vibration.
  • High Strength (Red): Designed for permanent assemblies. Disassembly usually requires localized heating (above 450°F) and heavy-duty tools.
  • Wicking Grade (Green): A very low-viscosity liquid that can be applied to pre-assembled fasteners via capillary action.

2. Thread Sealants

While threadlockers focus on holding parts together, thread sealants are formulated to prevent the leakage of gases and liquids through pipe joints. Unlike PTFE tape (Teflon tape) or pipe dope, anaerobic thread sealants do not shred or creep. They provide an instant low-pressure seal and, once cured, can withstand the burst pressure of most piping systems. They also provide lubrication during assembly to prevent galling on stainless steel or aluminum threads.

3. Retaining Compounds

Retaining compounds are used to secure non-threaded, cylindrical assemblies, such as bearings onto shafts or pins into housings. They allow for “slip fits” instead of “press fits,” which reduces the stress on the metal components and eliminates the need for expensive high-tolerance machining. By filling the 100% of the gaps between the parts, retaining compounds provide much higher load-bearing capabilities than mechanical methods alone.

4. Anaerobic Gasketing (Flange Sealants)

In flange sealing, anaerobic sealants act as “formed-in-place” gaskets. Unlike traditional paper, cork, or rubber gaskets, which can compress, shrink, or squeeze out over time, anaerobic gaskets create a rigid, high-strength seal between two metal flanges. They are excellent for preventing leaks in gearboxes, engine casings, and pump housings. Because they do not shrink, they maintain the structural integrity of the joint and eliminate the need for retorquing bolts.

Key Advantages of Using Anaerobic Sealants

Why choose an anaerobic sealant over a silicone RTV or a mechanical fastener? The benefits are numerous and impact both the assembly process and the long-term durability of the product.

  • Vibration Resistance: Because the cured plastic fills the entire space between threads, it prevents any side-to-side movement, which is the primary cause of bolt loosening.
  • Corrosion Protection: By sealing the joint completely, anaerobic sealants prevent moisture, chemicals, and oxygen from entering the threads, effectively “locking out” rust and galvanic corrosion.
  • Single-Component Simplicity: There is no mixing required, reducing the margin for error and simplifying the production line.
  • Controlled Strength: Engineers can choose the exact shear strength required for the application, ensuring that parts can be disassembled for maintenance when necessary.
  • Chemical Resistance: Once cured, anaerobic plastics are highly resistant to most industrial fluids, including fuels, lubricants, hydraulic fluids, and many solvents.
  • Cost Efficiency: A single bottle of sealant can replace thousands of specialized washers and mechanical locking devices, reducing inventory costs.

Factors Affecting the Cure: Active vs. Inactive Metals

One of the most critical aspects of working with anaerobic sealants is understanding the substrate. Since the cure is catalyzed by metal ions, different metals react at different speeds.

Active Metals

These metals have a high concentration of ions on the surface and facilitate a fast cure. They include:

  • Brass
  • Copper
  • Bronze
  • Iron
  • Soft Steel

Inactive Metals

These metals have few ions or a heavy oxide layer that inhibits the chemical reaction. While the sealant will eventually cure, it may take significantly longer or require the use of a chemical activator (primer). Inactive metals include:

  • Stainless Steel
  • Aluminum
  • Galvanized Steel
  • Zinc
  • Titanium
  • Plated parts

If you are working with inactive metals or in very cold environments where the reaction is sluggish, it is highly recommended to use an anaerobic primer. This ensures a consistent cure and maximum strength. If you are unsure which product is best for your specific metal substrate, [Contact Our Team](https://www.incurelab.com/contact) for expert guidance.

Anaerobic Sealants vs. Other Adhesives

It is helpful to compare anaerobic sealants to other common adhesives to understand where they fit in the industrial toolkit.

Anaerobic vs. Cyanoacrylate (Super Glue)

Cyanoacrylates cure through moisture on the surface of the substrate. They are incredibly fast and great for bonding plastics and rubbers. However, they have poor heat resistance and are brittle. Anaerobic sealants are much better suited for metal-to-metal mechanical joints where vibration and temperature fluctuations are present.

Anaerobic vs. RTV Silicone

RTV (Room Temperature Vulcanizing) silicones cure by reacting with atmospheric moisture. They are excellent for filling large gaps and remain flexible. However, they have low shear strength and can “creep” under pressure. Anaerobic sealants are rigid and high-strength, making them superior for structural metal joints and precision flanges where a thin bond line is required.

Best Practices for Application

To achieve the best results with anaerobic sealants, proper application technique is essential. Follow these steps for a reliable seal:

1. Surface Preparation

The surfaces must be clean and dry. Use a degreaser to remove any oils, cutting fluids, or old sealant residue. Even though some modern anaerobic sealants are “oil-tolerant,” a clean surface will always provide the highest bond strength.

2. Choosing the Right Viscosity

Viscosity is the “thickness” of the liquid. For fine threads or tight-fitting parts, use a low-viscosity (thin) liquid. For large diameter pipes or worn parts with larger gaps, use a high-viscosity (paste-like) sealant to ensure it doesn’t run out of the joint before curing.

3. Application Method

For through-holes, apply the sealant to the bolt at the nut engagement area. For blind holes (holes that do not go all the way through), apply the sealant to the bottom of the female threads. This ensures that as the bolt is turned, the air is forced out and the sealant is forced up, covering the entire engagement area.

4. Curing Time

While anaerobic sealants typically achieve “fixture strength” (enough to hold the part in place) within 10 to 20 minutes, a full cure usually takes 24 hours. Do not put the assembly under full load or pressure until the cure cycle is complete.

Removal and Disassembly

A common misconception is that “high strength” means “impossible to remove.” While high-strength anaerobic sealants are designed to stay put, they can be disassembled if necessary.

  • Low and Medium Strength: These can be removed using standard hand tools. Once the “breakaway torque” is overcome, the cured plastic turns back into a powder-like substance that can be wiped away.
  • High Strength: If a bolt is stuck, apply localized heat using a heat gun or blowtorch to reach approximately 250°C (482°F). The heat softens the thermoset plastic, allowing the fastener to be unscrewed while hot.

Common Applications Across Industries

Because of their reliability, anaerobic sealants are used in almost every sector of modern industry:

  • Automotive: Sealing engine blocks, water pumps, and transmission housings; locking suspension bolts and brake caliper bolts.
  • Aerospace: Securing fasteners subject to extreme vibration and temperature cycles.
  • Manufacturing: Mounting bearings in conveyor systems and securing hydraulic fittings on heavy machinery.
  • Power Generation: Sealing large flanges in turbines and locking critical fasteners in wind power assemblies.
  • Plumbing and HVAC: Sealing high-pressure gas lines and water pipes where leaks are not an option.

The Future of Anaerobic Technology

As environmental regulations become stricter, the chemistry of anaerobic sealants is evolving. Modern formulations are increasingly “user-friendly,” with reduced skin irritation risks and lower VOC (Volatile Organic Compound) profiles. Furthermore, “surface-insensitive” formulas are being developed to provide faster cure times on inactive metals like stainless steel without the need for primers, streamlining the manufacturing process even further.

Conclusion: Why Anaerobic Sealants Are Essential

What is anaerobic sealant? It is more than just “glue for metal.” It is a sophisticated engineering solution that addresses the fundamental weaknesses of mechanical fasteners. By filling 100% of the space between metal surfaces, these sealants eliminate movement, prevent corrosion, and ensure the long-term reliability of mechanical systems.

Whether you are preventing a single screw from backing out of a consumer electronic device or ensuring the integrity of a high-pressure industrial pipeline, anaerobic sealants provide the security and performance that modern engineering demands. By understanding the different grades, the importance of metal substrates, and the proper application techniques, you can significantly improve the quality and lifespan of your assemblies.

Choosing the right sealant requires a balance of material science and practical application knowledge. If you are looking for high-performance bonding solutions or need technical assistance for your next project, the experts at Incure are here to help.

Visit [www.incurelab.com](https://www.incurelab.com) for more information.