An anaerobic sealer is a liquid adhesive that cures into a solid polymer only when trapped between tight-fitting surfaces and cut off from air. The name comes from “anaerobic,” meaning “without oxygen.” These sealers stay liquid in the bottle indefinitely because oxygen keeps them from hardening, but once squeezed between two metal parts where air can’t reach, they undergo a chemical reaction that locks the joint tight and creates a leak-proof seal. They’re used across automotive, plumbing, industrial, and military applications wherever metal parts need to stay fastened or fluid-tight.
How Anaerobic Sealers Cure
Anaerobic sealers are acrylic-based, single-component liquids. Inside the formula, a chemical initiator (a type of peroxide) sits dormant as long as oxygen is present. When you apply the sealer between two metal surfaces and press them together, two things happen simultaneously: oxygen gets squeezed out, and metal ions from the surface trigger the initiator. That initiator kicks off a chain reaction called free-radical polymerization, which transforms the liquid monomers into a hard, cross-linked plastic.
Metals like copper and iron are especially effective at driving this reaction. Copper ions actually lower the energy needed to start the curing process, which is why anaerobic sealers cure readily at room temperature on copper or steel parts. On less reactive metals like stainless steel, zinc, or aluminum, the cure still happens but takes longer. For non-metal surfaces or sluggish metals, you can apply a surface conditioner (essentially a chemical primer) that supplies the missing metal ions and speeds things up, though this can slightly reduce the final bond strength.
Cure Time: Fixture vs. Full Strength
Anaerobic sealers reach initial “fixture strength” within minutes at room temperature. At this stage, the parts are held in place but haven’t reached maximum holding power. Full chemical cure takes about 24 hours under normal conditions. If you need to speed that up, applying moderate heat (around 120°C for 30 minutes) can achieve a complete cure much faster. This two-stage timeline matters in practice: you can assemble and lightly handle parts quickly, but you should avoid full operating loads until the 24-hour mark.
Four Main Types of Anaerobic Sealers
Anaerobic sealers fall into four functional categories, each designed for a different joint geometry.
- Threadlockers bond and seal threaded metal fasteners like bolts and screws, preventing loosening from vibration or thermal cycling while also blocking corrosion on the threads.
- Thread sealants (pipe sealants) seal threaded pipe connections in plumbing, HVAC, fire sprinkler systems, and hydraulic lines, replacing or outperforming traditional thread tape.
- Retaining compounds bond cylindrical assemblies like bearings pressed into housings or shafts fitted into gears. They fill microscopic voids to create 100% surface-to-surface contact, compared to roughly 20% contact from a mechanical press fit alone.
- Form-in-place gaskets (FIP) are high-viscosity anaerobic sealers applied directly onto a flange surface, where they cure into a custom-shaped gasket. This eliminates the need to stock pre-cut gaskets in multiple sizes and improves load distribution across the flange.
Threadlocker Strength Grades and Colors
Threadlockers are the most commonly encountered anaerobic sealers, and they follow an industry-standard color system that signals their holding strength. The colors correspond to how much torque it takes to break the fastener free after the sealer has fully cured.
- Purple (low strength): Around 53 inch-pounds of breakaway torque. Designed for small fasteners or parts you’ll need to adjust frequently.
- Blue (medium strength): Breakaway torque typically ranges from 74 to 230 inch-pounds depending on the specific formula. This is the most popular grade for general maintenance, suitable for fasteners you may need to remove later with standard hand tools.
- Red (high strength to permanent): Breakaway torque from 161 to 290 inch-pounds. Intended for critical fasteners that should not come loose under any normal circumstances. Disassembly requires localized heat.
- Green (wicking grade): A thinner formula designed to wick into already-assembled fasteners by capillary action. Available in medium and low strengths.
Choosing the right color depends on whether you’ll ever need to take the joint apart. Blue is the safe default for most repair and maintenance work. Red is for permanent or safety-critical assemblies.
Removing Cured Anaerobic Sealer
Low and medium-strength formulas can usually be broken free with standard wrenches. High-strength (red) formulas are a different story. The only reliable way to disassemble a joint sealed with high-strength anaerobic sealer is to apply localized heat above 250°C (about 480°F) directly to the joint. At that temperature, the cured polymer breaks down enough to release the fastener. A propane torch or heat gun on its highest setting can reach this threshold. Once disassembled, any remaining cured material can be scraped or wire-brushed off the threads.
Temperature Limits
Standard anaerobic sealers perform well up to about 150°C (300°F) in continuous service. Interestingly, bond strength can actually peak around 100°C because the elevated temperature pushes any incompletely cured adhesive to finish hardening. Beyond that range, standard formulas begin to weaken. Specialized high-temperature versions extend the operating range to around 230°C (450°F), and experimental formulations with ceramic-type additives have shown stability at temperatures as high as 400°C, though those are not yet common in consumer products.
Anaerobic Sealers vs. RTV Silicone
If you’re sealing a flange or gasket surface, you’ll often see both anaerobic gasket makers and RTV (room-temperature vulcanizing) silicone recommended. They serve similar purposes but behave quite differently.
Anaerobic gasket makers cure rigid and work best on metal-to-metal joints with tight, consistent gaps. They resist vibration and high pressure well, which makes them the standard choice for transmission pans, differential covers, and similar precision-machined flanges. RTV silicone, on the other hand, cures into a flexible rubber. It handles wider and more irregular gaps, tolerates greater thermal expansion, and works on non-metal surfaces without a primer. If your flange surfaces are warped, pitted, or made of mixed materials, RTV is generally the better option. For clean, flat metal flanges with minimal gap, anaerobic sealers provide a stronger, more pressure-resistant seal.
Surface Prep and Application Tips
Anaerobic sealers need clean, oil-free metal surfaces to cure properly. Wipe both mating surfaces with a solvent like acetone or isopropyl alcohol before application. Apply the sealer to one surface in a thin, even film, then assemble the parts. The sealer will fill gaps in the micro-texture of the metal, but these products are not designed for large gaps. Most standard formulas work best with clearances under 0.25 mm (0.010 inches). Flanged joints can tolerate slightly larger gaps with high-viscosity FIP formulas.
On passive metals like stainless steel, aluminum, or plated surfaces, cure times can stretch significantly. Applying an anaerobic primer or surface activator to one of the mating surfaces before assembly solves this problem by supplying the reactive metal ions the formula needs. Just keep in mind that primers can reduce the ultimate bond strength by a small margin, so they’re a tradeoff between cure speed and maximum performance.