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Acid pickling and etching

The use of strong acids is commonplace in industry in the manufacture of many components, and they have a number of purposes that might be useful in the context of race engines and transmissions.

Although not widely used for this purpose, there are applications where precision pickling produces tight-tolerance final dimensions on components, owing partly to the simultaneous effects of improvement of surface finish (roughness reduction) and removal of burrs from production that would require ‘artisan’ – that is, manual and non-repeatable – intervention. Depending on the acid and the material being treated, pickling can produce a surface with high quality and low roughness. As the acid preferentially attacks and removes material with a high ratio of surface area to volume, it quickly removes undesired features such as burrs as well as peaks in the surface finish of the material.

However, while pickling or etching to a precise final dimension is not widely used, the general principle of using strong acids to remove material as a way of reducing the thickness of sheet materials can be used both on raw materials and finished components, especially fabricated parts. The selective pickling of thin fabricated assemblies can be accomplished by masking critical areas (typically welds) followed by immersion in a bath of acid so that the remaining unmasked areas are ‘eroded’ to the desired thickness.

The amount of time required depends on the alloy, the particular acid used and its strength and temperature. It is therefore necessary to carry out regular tests of the rate of material removal before use. As the cumulative effect of pickling/etching on the acid is to weaken it and reduce the rate of material removal, if these tests are neglected then you may find produce components that are oversize and/or overweight. The advantage of using acid pickling/etching is that materials that are too thin to weld reliably can be specified, so it is possible to fabricate components such as oil tanks and water tanks, for example, that are much lighter by etching down to the desired thickness after welding.

Pickling is also used to remove surface contamination from some components. For example, stainless steel or titanium fabrications can be pickled to remove traces of steel from their surface, which can cause problems in later processing or might provide an initiation site for a corrosion problem. In the same way, some castings are acid-etched to remove the surface layer, which can include some foreign material (casting sand or investment casting mould materials) that might be a fatigue crack initiation site.

This technique is likely to be used on titanium gearbox main-case castings, as found in Formula One for example. Ferrari pioneered the use of titanium for gearbox main cases, although this was as a fabricated assembly rather than a casting. Minardi used cast-titanium main cases from 2000 onwards, and the technique was quickly adopted by other teams.

The rate of material removal varies widely with material type and alloy, so care needs to be taken and sufficient data gathered before using the process. It should also be noted that, because material is preferentially removed where the ratio of surface area to volume is high, sharp external corners are likely to be removed. This may or may not be the desired effect.

Written by Wayne Ward

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