Rust-proofing is the name process whereby the rate at which objects made of iron and/or steel begin to rust is reduced (in the long term it cannot be stopped, unless the 'rustproofing is periodically renewed), so that the places in which they are rusting can be spotted in time before a catastrophic failure and repaired. The term is particularly used for the automobile industry.

There are two ways in which rustproofing is applied:

  • In the factory, car bodies are soaked in chemicals (phosphates), often electrically charged so that layers of protection are added. Some firms galvanize their car bodies. This is done before the primer coat of paint is added. If a car is body-on-frame, then the frame and its attachment methods 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 a rubberised or PVC based coating is sprayed on. These products will be breached eventually, and can lead to unseen corrosion that spreads unseen under the underseal. 1960s and 70s rubberised underseal can become brittle on older cars and is particularly liable to this.
  • There are aftermarket kits to gain access to the enclosed sections, for example sills/rocker panels (see monocoque), in the body of a car so as to spray it with, (usually wax based) phosphoric acid chemicals by the owner. Hand pumped kits are generally ineffective. A special compressor powered gun is needed. Alternatively, sealing drain holes and filling the enclosed sections is very effective, but very messy. Loose or thick rust must be removed before anti-rust wax is used. Wax does not penetrate spot welded seams or thick rust well. A penetrating anti-rust product like WD40 followed by anti-rust wax is more effective. The best type of aftermarket 'underseal' is bitumen based, never dries/hardens, so cannot become brittle and contains similar anti-rust chemicals to the anti-rust waxes. It is particularly useful in 'high impact' areas like wheel arches.

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.

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, designed on newly available computers and the integrity of the body-shell for strength. A light car was a fast and/or economical car. Unfortunately the design of corrosion prevention had not kept pace with this new technology.

Cars that rust out quickly get a bad reputation on the market, some of the most notorious European car makes were Austin, Vauxhall and Fiat. Japanese cars until the 1980s also had a poor reputation for rusting. In Japan the usual life of a car is only five years. This is due to the 'Shaken' system that was designed by the Japanese government to keep demand high for new cars in Japan, with progressively harder and more expensive road worthiness tests. On their home market poor corrosion protection was largely irrelevant, but not when they exported. Today this problem still applies, but only to exported used Japanese 'grey market' vehicles, first registered in Japan.

The rate at which vehicles rust is dependent upon:

  • The local climate / 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, dis-similar metals like aluminium can accelerate the rusting of steel bodywork through electrolytic corrosion.

Click here for the article on Corrosion.

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