Precision machine shops work every day to produce safe and professional products. In order to guarantee high quality standards, they turn to specialised professionals and carefully consider the choice of components. Indeed, the same part can have different characteristics and properties, depending on the treatment it has undergone.
In this article, let's discover together what it is, what it is used for and how it works, the burnishing of metal, the material par excellence in the precision mechanics sector.
What is burnishing?
Burnishing, also known as metallodising, is a chemical treatment that darkens the surface colour of a metal. There are two variants, hot and cold, which are carried out in different ways.
It is a very old practice, but the rudimentary processes of the past have been progressively perfected until today. The first chemical burnishing process ever described in Europe dates back to the second half of the 18th century, reported in the German magazine Hannover Magazin and focused on firearms.
At the time, this particular treatment was of interest to the military and was aimed at removing reflective capabilities from metal surfaces. When hit by sunlight, they could not only blind the weapon's owner, but also signal his position.
What is the purpose of burnishing?
Nowadays, the burnishing of metal is used to make its appearance more pleasing, with the certainty of not significantly altering its structural characteristics, such as shape, size and thickness. The object subjected to the process takes on a darker colouring, tending towards shiny black-blue.
Added to the aesthetic improvements are advantages of a practical nature: metallisation makes a metal component much more resistant and durable over time, protecting it from corrosion (both from substances and direct weathering). It also guarantees a rust-proofing action and prevents oxidation.
Although it is always true that a burnished part is more resistant than an untreated one, it is also true that this result can only be appreciated if protective oiling is carried out.
Hot burnishing is performed by immersing the metal piece in an aqueous solution brought to a temperature of between 100°C and 140°C. Various chemicals, such as copper sulphate, lead acetate and sodium hyposulphite, are found in it.
On contact with the metal surface, they generate a chemical reaction that leads to the formation of a black oxide layer, known as magnetite. The latter has a thickness of about 1 micron and therefore does not result in a significant change.
The intensity of the colour increases with immersion time: the hue varies from blue to black, passing through intermediate tones such as purple and grey. Once the desired tone is reached, the object is taken out, rinsed (first with hot water, then with cold water) and left to cool. In this way, the surface coating is stabilised and made permanent.
This is the ideal process for iron, steel and cast iron.
Cold-burnishing is known for its economical and low-pollution character, but also for its ease of execution. In this case, no temperature change is exploited, but only the reacting power of chemicals.
More specifically, the metal undergoes a chemical conversion process that leads to a black oxidation. The thickness undergoes a negligible variation of 1.3 to 3 microns, so the physical characteristics of the metal object remain virtually unchanged.
The result is a uniform tint, which is also suitable for threaded objects and grooved surfaces. The materials on which it can be performed are iron, steel, brass, bronze, copper and zinc.
Phosphating or burnishing?
Phosphating and burnishing are both treatments involving the surface of metals. The execution of the two practices is very similar, but they lend themselves to different contexts.
In particular, phosphating adds a phosphate layer consisting of the superimposition of small crystals. This coating has a strong absorbing power and can subsequently be coated with varnish or treated with protective oils.
In one case, colour adhesion will be preferred, in the other, the flow of the part. Two distinct purposes are achieved through the use of two chemical substances: zinc and manganese respectively.
The choice between phosphating and burnishing depends on what is required of the metal component. Burnishing is an excellent compromise between aesthetics and resistance; whereas phosphating is indicated if the focus is primarily on preventing corrosion.