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Rustproofing is a condition of preservation or protection, by a process or treatment whereby the rate at which objects made of iron and/or steel begin to rust is reduced. The degradation in the long term can not be stopped completely, unless the rustproofing is periodically renewed. The term is particularly used for the automobile industry.
There are two ways in which rustproofing can be applied:
In the factory
In the factory, car bodies have chemicals, typically phosphates, applied often using electrically charged systems so that layers of protection are added to the metal body. Some firms galvanize part or all of their car bodies. This is done before the primer coat of paint is applied. If a car is body-on-frame, then the frame (chassis) and its attachments must also be rustproofed. Paint is the final part of the rustproofing barrier between the body shell (apart from on the underside) and the atmosphere. On the underside an underseal, a rubberised or PVC-based coating is sprayed on. These products will be breached eventually and can lead to unseen corrosion that spreads underneath the underseal. 1960s and '70s rubberised underseal can become brittle on older cars and is particularly liable to this.
Aftermarket kits are available to apply rustproofing compounds both to external surfaces and inside enclosed sections, for example sills/rocker panels (see monocoque), through either existing or specially drilled holes. The compounds are usually wax-based and applied by pressure spray, but phosphoric acid can also be painted on already rusted areas. Loose or thick rust must be removed before anti-rust wax like Waxoyl or a similar product is used. Wax does not penetrate spot welded seams or thick rust well. A penetrating anti-rust product like WD-40 followed by anti-rust wax can be more effective. Aftermarket "underseals" can also be applied. These are often bitumen-based products that do not dry and harden, so they cannot become brittle. They contain anti-rust chemicals similar to those in anti-rust waxes. They are particularly useful in high-impact areas like wheel arches. Petroleum based rust-inhibitors provide several benefits, including the ability to creep over metal, covering missed areas. Additionally, a petroleum, solvent-free rust inhibitor remains on the metal surface, sealing it from rust accelerating water and oxygen. Other benefits of petroleum based rust protection include the self-healing properties that come naturally to oils, which helps undercoatings to resist abrasion caused by road sand and other debris. The disadvantage of using a petroleum based coating is the film left over on surfaces, rendering these products too messy for exterior application and unsafe in areas where it can be slipped on.
The chemicals which are sold in bottles to paint on rust, and which react with the rust to destroy it, turning it blue/black, are not called rustproofing but "rust killers" and need to be used as part of a paint system. The active agent in this process is Phosphoric acid.
Car body corrosion was a particular problem from the 1950s to the 1980s when cars moved to monocoque or uni-body construction from a separate chassis frame made from thick steel. This relied on the shaped body panels and the integrity of the body shell for strength. Unfortunately the design of corrosion prevention had not kept pace with this new technology.
Rate of rusting
The rate at which vehicles rust is dependent upon:
- The local climate and use of ice-melting chemicals (salt) upon the roads.
- The particular process of rustproofing used.
- The design of "rust traps" (nooks and crannies that collect road dirt and water).
- The plastic/under-seal protection on the car underside.
- The thickness and composition of the metal, for example, some dissimilar metals can accelerate the rusting of steel bodywork through electrolytic corrosion.
- Exposure to salt water in the case of boats, which strips off the protective paint and causes rust much quicker than ordinary rain water would.
Rustproof iron alloys
Stainless steel, also known as "inox steel" does not stain, corrode, or rust as easily as ordinary steel. Pierre Berthier, a Frenchman, was the first to notice the rust-resistant properties of mixing chromium with alloys in 1821, which led to new metal treating and metallurgy processes and eventually the creation of usable stainless steel.
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