The motor industry has tried all manner of coatings to protect vehicles against rust, with varying degrees of success. The relatively thin steel sections of car bodies mean that the ravages of water, grit and salt can soon wreak havoc on them.
Vehicles which don't have a separate chassis but rely on a monocoque body for their structural integrity can be destined for an early grave if not adequately rustproofed. The use of low technology surface coatings in the early years of monocoque production meant that most such cars were lucky to survive more than 10 years.
The introduction of the MoT test highlighted the problem, particularly when corrosion damage had been done around weight-bearing areas such as sills and suspension mountings. Not surprisingly, many companies sprang up during the 1960s and '70s offering a thorough rustproofing treatment, but the benefit of such products was sometimes questionable. Treating a new car might increase protection, but applying rustproofer to a used vehicle often only succeeded in masking existing corrosion and locking in already present moisture.
It was well known that non-ferrous metals were not prone to corrosion in the same way as steel. Zinc is just one additive which was included in primers, and applied to components and bodies to alleviate the problem with varying degrees of success. But the corrosion-inhibiting effects of any surface coating are severely limited if moisture or air is trapped between the paint and the steel. Even the most effective primers are compromised by atmospheric conditions during application and the condition of the steel to which they are applied.
Galvanised Land Rovers
Rover was well aware of the rust problem when the first prototype Land Rovers were assembled. It knew that these vehicles would spend much of their life in all sorts of corrosive environments, from sea water to animal dung. This problem was first addressed by hot dip galvanising the chassis and various body components. The abandonment of this policy and painting the chassis may well affect the number of survivors of these models.
The value of galvanising is abundantly demonstrated where previously galvanized 90 and 110 body cappings were replaced by those with a painted finish. It is unusual to see one of these circa-1986 vehicles without either the tell-tale holes or even completely missing sections.
So what exactly is galvanising, and why is zinc used? Zinc is the seventeenth most common element in the earth's crust, a natural element that is found in all plants and animals. It plays a crucial role in the health of skin, teeth, bones, muscles and even brain function. Zinc even occurs in the air we breathe. The recommended daily allowance (RDA) is 15mg for a male adult. Some groups, such as the elderly and pregnant women, need up to 19 mg a day.
The zinc galvanising process was first used over 150 years ago to protect corrugated roofing sheets. Zinc is an ideal element to use, as it can be recycled without any loss of its physical or chemical properties, so it's environmentally sustainable too. Unlike other protective coatings, a galvanised zinc coating is metallurgically bonded to the steel.
The zinc galvanising process was first used over 150 years ago to protect corrugated roofing sheets
The key to successful galvanising is a chemically clean surface. It is essential that the surface is free from dirt, oil and mill scale before galvanising. To achieve this, the article to be dipped is first degreased in an acidic or alkaline degreasing agent before being rinsed with clean water. The next stage entails dipping in hydrochloric acid. However, welding slag, paint and heavy grease are not removed by these steps. These contaminants must be removed before an item is sent for galvanising, usually by grit blasting.
State of flux
The hydrochloric acid is rinsed off before the next stage, dipping in a flux solution, which is usually 30 percent zinc ammonium chloride heated to about 65-80 degrees C. Some galvanising plants use a flux blanket on top of the galvanising bath. This method removes the last traces of oxides as an item is dipped.
The molten zinc, usually about 450 degrees C, forms a series of zinc-iron alloy layers by metallurgical reaction as the item is submerged in the bath. The main thickness of the coating is applied during this rapid process, which often creates considerable agitation on the surface of the molten zinc. The optimum time period for immersion is four to five minutes. No significant increase in thickness is gained by extending the immersion time.
An additional layer is taken on top of this layer as the item is withdrawn from the tank. It is this layer that gives galvanised items their familiar shiny finish. The process is then complete and the item is quenched or left. There are three additional factors that govern the thickness of the zinc layer which is taken up during dipping.
Centrifuged or spun galvanising is used for coating threaded components or small parts. The parts are placed in a wire basket before dipping, and the basket is then spun at high speed to throw off the excess zinc.
Roughening the surface of in item by grit blasting can increase the zinc layer thickness by up to 50 percent. This process is often used on items destined for hostile environments such as closeness to sea water or chemical effluents.
Steel with a high silicon content, which is added as a de-oxidant during the steel-making process, will take on a thicker zinc layer. This is because the silicon changes the composition of the zinc-iron layers so that they continue to grow while immersed in the zinc bath. The layer continues to grow for the duration of immersion, unlike what happens with conventional steel. Virtually any size item can be galvanised, from bolts and nuts to large steel sections. Sections that are too long or too high for a bath can be double dipped by turning them around before re—dipping.
The initial shiny finish of zinc coating is replaced in time by a dull grey patina as the surface reacts with oxygen, water and carbon dioxide in the atmosphere. The most important contaminant of zinc is sulphur dioxide (502) which is present in the atmosphere. Concentrations of 502 vary across Britain, and the lifespan of a zinc-galvanised coating is affected accordingly. Quoted lifespan figures vary from 170 years in some areas, down to "only" 34 years in areas with higher concentrations of 502.
In general, galvanising is highly resistant to most types of environment, including hot water, sea water, most chemicals except strong acids or alkalis, high temperatures or even burial underground. In some applications a sacrificial anode (a component intended to corrode instead of the structural item) will be attached to the galvanised material for added protection.
Comparisons are often made between zinc galvanising and zinc plating but the typical thickness of zinc plating is only 10 microns, compared with 85 microns for galvanised items when dipped to BS EN ISO 1461.
Sometimes reference is made to the self-healing properties of galvanising. The truth behind this fact lies in the preference of zinc to corrode instead of steel. If the surface incurs damage, the galvanic cells produced from this corrosion activity are deposited onto the steel, sealing it from the atmosphere and therefore stopping corrosion. The steel is also protected as it is cathodic (negatively charged) in relation to the zinc coating.
Plating metals which are more electro-positive than the steel underneath, such as copper, chromium or nickel, results in pitting which is visible on the surface if it becomes damaged.
With a painted surface, any break in the coating would have to be immediately repaired to prevent further corrosion which, left untreated, would creep under the painted surface.
For Land Rover owners, the cost of restoring previously galvanised items can be expensive for only one vehicle, so it is clear that painting would be much cheaper than galvanising. But on an industrial scale, the once-only nature of galvanising can result in huge long-term savings over most paint systems, which will have to be re-applied sooner or later.
Galvanisers usually work on a minimum charge and charge by weight after that. The minimum charge can often be anything from £50 to £100. If you wanted to galvanise a relatively small quantity of items you could band together with other members of a club and negotiate with a local galvaniser, or you could ask an engineering firm which uses galvanisers if you could include your bits in its next consignment. But beware — small items are easily lost or damaged either before, during or after dipping (particularly if they are not collected promptly after dipping), so think carefully before sending any rare or valuable parts.
Thin sections can distort because of the relatively high temperatures used. Flat sheets can often have a wavy appearance after dipping because of temperature distortion. Galvanisers may be reluctant to dip items such as classic car chassis, although the thicker section steel used in Land Rover chassis means that this isn't a problem.
It is worth spending some time ensuring that the surface is perfectly clean before you send the items away for treatment. You'll need to make sure that there are venting holes in box or closed hollow sections; these are required to prevent explosion during the dipping process. Galvanisers are pretty keen on this for obvious reasons. If you don't make the necessary holes, the galvaniser will charge you for doing so.
Galvanised surfaces can be quite easily painted if required. It can be a good idea to allow the item to weather for about six months to provide a key for the paint. It is also advisable to use a phosphate or chromate etch primer beforehand.
Many people see a galvanised surface as more than a utilitarian coating. The current vogue for minimalism has even led architects to choose it solely for its appearance. If you decide to galvanise, you will be safe in the knowledge that you have chosen the most effective method of protecting steel components.
Somehow the concept and appearance of galvanising seems to suit a Land Rover very well. It is a shame that Land Rover has abandoned a process that is synonymous with longevity.