Rustproofing

Rustproofing is the prevention or delay of rusting of iron and steel objects, or the permanent protection against corrosion. Typically the protection is achieved by a process of surface finishing or treatment. Depending on mechanical wear or environmental conditions, the degradation may not be stopped completely, unless the process is periodically repeated. The term is particularly used in the automobile industry.

Application

Rustproofing may be applied at the time of manufacture, or later during use of the product.

Factory

In the factory, car bodies are protected with specially chemical formulations, typically phosphate conversion coatings. Some firms galvanize part or all of their car bodies 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 rubberized underseal can become brittle on older cars and is particularly liable to this.

Aftermarket

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 can be applied by aerosol can, brush, low pressure pump up spray (which has a reputation for mixed results), and Shutz compressor fed spray gun which gives best results but is the most messy - both the sprayers come with extension pipes for enclosed body sections. Shutz spray guns are much simpler and cheaper than top coat spray guns. An alternative for sills/rocker panels, is to block drain holes and simply fill them up with wax and then drain most of it out (able to be stored and reused), leaving a complete coating inside. Anti-rust wax like phosphoric acid based rust killers/neutralisers 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. Structural rust should be cut back to good metal and new metal welded in. Wax may 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. Application is easier in hot weather rather than cold because even when pre-heated, the products are viscous and don't flow and penetrate well on cold metal.

Aftermarket "underseals" can also be applied. They are particularly useful in high-impact areas like wheel arches. There are two types - drying and non-drying. The hardening and drying products are also known as 'Shutz' and 'Anti Stone Chip' with similar potential problems to the original factory underseals. These are available in black, white, grey and red colours and can be over painted. These are best used for the area below the bumpers on cars that have painted metal body work in that location, rather than modern plastic deep bumpers. The bitumen based products that do not dry and harden so they cannot become brittle, like the confusingly named 'Underbody Seal with added Waxoyl' made by Hammerite which can be supplied in a Shutz type cartridge labelled 'Shutz' for use with a Shutz compressor fed gun. Mercedes bodyshops use a similar product supplied by Mercedes-Benz.^[1] The non drying types contain anti-rust chemicals similar to those in anti-rust waxes. 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 top side exterior application and unsafe in areas where it can be slipped on. They also cannot be painted.

Some of these products have performed better than others in long term magazine tests, and there are large price differences, so reading up to date product review information before buying could prove useful.

Car bodies

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 Corrosion

The rate at which vehicles corrode is dependent upon:

• The local climate and use of ice-melting chemicals (salt) upon the roads.
• The metal that it is made from. The quality of the original mild steel affects the rate of rusting. Some cars have been made from aluminium which is more corrosion resistant than steel, but it can be problematic for boats. DeLorean cars had stainless steel bodies.
• The particular process of rustproofing used. E.g. Galvanising.
• 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 and types of metal used, for example, some dissimilar metals can accelerate the rusting of steel bodywork through electrolytic corrosion.
• Atmospheric pollution which can cause paint damage. E.g. Acid rain
• 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.

See also

Wikimedia Commons has media related to Rust.