Anaerobic sealant is a liquid adhesive that cures into a solid only when trapped between tight-fitting metal parts, cut off from air. It stays liquid in the bottle indefinitely but hardens once you assemble the joint, filling microscopic gaps to create a seal that resists vibration, pressure, and chemical exposure. You’ll find it used on threaded fasteners, pipe fittings, flanged surfaces, and press-fit bearings across automotive, plumbing, and industrial applications.
How Anaerobic Sealant Cures
The curing process depends on two conditions happening at once: the absence of oxygen and contact with metal. When you apply the liquid sealant to a metal surface and close the joint, the tight fit squeezes out air. Metal ions on the surface then trigger a chain reaction that converts the liquid monomers into a hard, cross-linked plastic. This is a free-radical polymerization process, meaning the metal surface essentially kicks off a chemical chain reaction that spreads through the entire film of sealant.
Without metal contact, curing is extremely slow, almost impractical. Without excluding air, it won’t cure at all. This is actually a major advantage: you get unlimited working time to position parts, and the sealant only locks in once you’ve completed the assembly. Most anaerobic sealants cure at room temperature with no mixing, heating, or UV light required.
Which Metals Work Best
Not all metals trigger curing at the same speed. Copper and brass are the most reactive and cure anaerobic sealants fastest. Steel, nickel, and aluminum fall in the middle, curing at a reliable rate that works well for most applications. These are all considered “active” metals.
Passive metals slow things down. Stainless steel, chromed surfaces, anodized aluminum, and other passivated metals have surface coatings or oxide layers that interfere with the curing reaction. On these surfaces, you’ll typically need a primer or activator, a fast-evaporating liquid you brush or spray onto the parts before applying the sealant. Primers deposit a thin layer of reactive chemistry that substitutes for the metal ions the sealant needs. Porous surfaces sometimes need two coats of activator to get a reliable cure.
Gap Filling and Fit Requirements
Anaerobic sealants are designed for closely mated surfaces, not for bridging wide gaps. The practical upper limit is about 0.30 mm (roughly 0.012 inches). Beyond that, even high-viscosity formulations struggle to cure fully because the sealant film is too thick for the metal-surface reaction to reach all the way through. This is why surface flatness matters on flanged joints, and why anaerobic threadlockers work best on properly sized bolt-and-nut combinations with minimal thread clearance.
Strength Grades and Color Coding
Anaerobic threadlockers use a color-coding system that tells you the bond strength at a glance:
- Purple (low strength): About 55 inch-pounds of breakaway torque. Meant for small fasteners under 1/4 inch that you’ll need to adjust or remove regularly with standard hand tools.
- Blue (medium strength): About 115 inch-pounds of breakaway torque. The most common grade for general fasteners. Prevents loosening from vibration but still allows disassembly with normal wrenches.
- Red (permanent strength): About 180 inch-pounds of breakaway torque. Intended for fasteners you don’t plan to remove. Disassembly typically requires heat.
- Green (penetrating grade): About 80 inch-pounds of breakaway torque. A thin, wicking formula applied to fasteners that are already assembled. It seeps into threads by capillary action.
These colors are largely standardized across manufacturers, though you should always check the product label for the specific strength rating.
How It Compares to Silicone Gasket Maker
Silicone (RTV) gasket makers are the other common option for sealing flanged joints, but anaerobic sealants outperform them in several situations. Anaerobic sealants resist oils, fuels, and industrial chemicals better than silicone, making them the standard choice for engine oil pans, transmission cases, and fuel system components. They also hold up better under high pressure and vibration because the cured material bonds rigidly to the metal rather than remaining flexible like silicone.
Silicone has its own advantages: it handles larger gaps, works on non-metal surfaces, and stays flexible for joints that move or expand. But if you’re sealing metal-to-metal surfaces with tight clearances and exposure to petroleum products, anaerobic sealant is the stronger option.
Surface Preparation
Clean surfaces are essential. Any oil, grease, or old sealant residue will interfere with the bond. Wipe both mating surfaces with a solvent like acetone or isopropyl alcohol and let them dry completely before applying the sealant. If you’re resealing a joint, remove all traces of the old gasket or sealant with a wire brush, wire wool, or fine abrasive paper before cleaning.
For passive metals like stainless steel, apply a primer to both surfaces and let it dry (usually 1 to 3 minutes) before adding the sealant. Skipping this step on passive metals can result in incomplete curing or a weak bond that fails under load.
Removing Cured Anaerobic Sealant
Low and medium-strength products come apart with standard hand tools. Permanent (red) grades are a different story. Applying direct heat with a torch or placing the assembly in an oven weakens the bond significantly, but you need to work quickly: once the parts cool, the original strength returns. Heat the joint, then immediately attempt disassembly while everything is still hot.
For retaining compounds used on press-fit bearings and shafts, heat or chemical solvents are often the only option. Soaking assembled parts in acetone or methylene chloride overnight can soften the bond enough to separate them the next day. Anaerobic gasket makers on flanged joints are easier to deal with. The cured material is strong in compression but weak under peel stress, so wedging a flat-bladed screwdriver between the flanges and tapping it with a hammer is usually enough to pop the seal.
After disassembly, cured anaerobic sealant typically appears as a white, powdery residue. Scrub it off with a wire brush or abrasive pad, then wipe with acetone before reassembling.
Certifications for Specialty Applications
Certain anaerobic pipe sealants carry NSF/ANSI 61 certification, meaning they’re approved for contact with drinking water. These are used on metal pipe threads in plumbing systems where a leak-proof, permanent seal is needed. If you’re working on potable water lines, look for products specifically labeled with this certification rather than using a general-purpose anaerobic sealant.