Understanding how a nut or bolt is "frozen" is the first step to getting it loose. Some fasteners freeze in place due to oxidation between the threads—plain ol’ rust. Others are literally welded by galvanic corrosion, a chemical reaction between unlike metals. Disassembling rusty fasteners requires a different strategy than disassembling fasteners that are frozen by galvanic corrosion.
Badly rusted fasteners are best addressed with lubricants and patience. If possible, liberally soak the fastener with a penetrating lubricant for at least 24 hours before attempting to disassemble.
At the microscopic level, oxidation in fasteners leaves the threads pitted and scarred. These irregularities mechan-ically "lock" the threads into place. The dry, red, powdery residue we call rust fills those irregularities and acts like sand to increase friction between the threads.
Penetrating oils chemically dissolve rust while lubricating damaged threads. The trick is to get the penetrant deep inside the rusted fastener, which takes time and extremely low surface tension to encourage capillary action.
Capillary action is the force that pulls water slightly higher inside a drinking straw inserted into a glass. Given time, the low surface tension of a good penetrating lubricant will allow it to work its way deep between the threads of a frozen fastener and ease its disassembly.
Tom Hrabak, an engineer with Blaster Chemical Companies, demonstrates capillary action by placing a rusty 1" bolt head down in a pan of PB Blaster Penetrating Oil. In 24 hours, capillary action allows the lubricant to "crawl" up the length of the bolt.
Once a penetrating lubricant has had time to soak its way between the threads of a frozen fastener and chemically dissolve the rust, it still takes brute force to initially break the bond. If brute force fails, heat is the next option.
Apply heat from a propane or acetylene torch to the female portion of the fastener. If a nut is frozen to a bolt, heat the nut. If a bolt is frozen in a casting, heat the casting. The strategy is to expand the confining threads away from the bolt. Many fasteners yield to "hot-to-the-touch" heat. Stubborn fasteners require heating to "dull red" before applying a breaker bar, cheater pipe or air-powered wrench while the fastener is glowing hot.
Extreme cases might require alternating applications of penetrating lubricant and heat. Heat expands the metal and allows the lubricant to penetrate even deeper into the threads. The cooling action of the liquid encourages separation. Use caution when applying penetrating lubricants to hot surfaces—most penetrating lubricants are flammable.
Once a frozen fastener yields, continue to be patient as it is removed. Rust dust can re-wedge fasteners. Twist the fastener out a couple turns, apply lubricant, then twist it back in for a turn or two to lubricate the threads.
Break the bond. Penetrating lubricants and patience will break loose most frozen fasteners. The frustrating exceptions are fasteners that are frozen by galvanic corrosion. This, as noted above, is the result of a chemical reaction between unlike metals over time. If you’ve ever tried to break loose a steel spark plug from an aluminum engine cylinder head, you’ve dealt with galvanic corrosion.
The only solution is to mechanically break the bond with a wrench, breaker bar or air-powered wrench. If the galvanic corrosion is severe, it might be impossible to crack the fastener loose without damaging or stripping threads. A penetrating lubricant will help removal once the fastener is loose, but it will not dissolve galvanic corrosion.
Prevention is the best strategy for dealing with galvanic corrosion. Apply a heat-resistant anti-seize compound when bolts are installed into nonsimilar metals. Or, as some car dealerships do, "exercise" (remove and reinstall with anti-seize compound) spark plugs threaded into aluminum cylinder heads.
Cool Tool of the Month
Tap-and-die sets are designed to cut new threads; using a tap or die to "clean" threads on a bolt that has rolled threads theoretically weakens those threads. Rethreading kits are designed to clean corrosion and clear damaged threads without decreasing their strength. Rethreading kits are available for standard nut and bolt sizes and up to wheel spindle size. $100 to $300.