Phosphating is one of the most important and widespread metal surface treatments in precision mechanics. It achieves significant corrosion resistance, improves material durability and facilitates paint adhesion.
The core of the entire process is the application of a crystalline layer, which acts as a protective coating and gives the part the above-mentioned properties.
Metal Phosphating: What it is
During the metal phosphating process, a thin layer consisting of the superposition of small phosphate crystals is deposited on the surface of the treated workpiece. It acts as a barrier against the corrosive effect of oxygen and water, preventing oxidation reactions and increasing the life of the material.
One of the most interesting effects of phosphating is the formation of micro-roughnesses, which are essential for better anchoring of coatings. These roughnesses can have a needle-like or prismatic crystalline structure, and their purpose is to provide a surface that allows the colour to adhere firmly to the part.
It is important to emphasise that this is a highly specialised process that requires careful consideration of the specific requirements of the part and the environmental conditions in which it will be used.
How to Perform Phosphating
Before taking a closer look at how phosphating is performed, let us clarify an important point: to be carried out, the process requires specially designed equipment and qualified personnel.
The treatment takes place inside tanks that require periodic care and cleaning: the workers must take care of the removal of the phosphate sludge that is created during acceleration, as well as the maintenance of the equipment.
The treatment phases are as follows:
It must be borne in mind that the process is rather lengthy and that certain stages, such as pre-degreasing and washing, are repeated. These two, together with degreasing, are the preparatory steps to the actual treatment.
Activation is aimed at increasing the number of crystallisation centres, the latter being directly proportional to the quantity of crystals and inversely proportional to their weight and size. This is how an evenly distributed microcrystalline layer is obtained.
For phosphating baths, phosphate solutions are used to coat the metal part. Accelerating products are recommended to speed up the treatment. Shortening the time means making the process more efficient and economical, so their use is strongly recommended. Oxidising elements of a mineral nature are generally used, capable of oxidising near the anode and depolarising near the cathode.
With passivation, all crystals other than those desired are removed, which could cause cracks in the paint and subsequent chipping. This is followed by a rinsing phase, in which the part is treated with distilled water to prevent the formation of limescale, which is responsible for compromising the performance of the crystalline layer.
It is evident how complex the realisation of the cycle is, as much for the equipment required as for the numerous steps in which it is articulated. Each step must be performed with great precision and care to ensure a high quality end result.
Zinc and Manganese Phosphating
There are two types of phosphating, which can be distinguished according to the coating that is created: in one case based on zinc and iron phosphates; in the other on manganese phosphate. However, both fall under the same DIN standard, DIN-50942.
Zinc phosphating is commonly used to improve paint anchorage on the surface of the metal component. After creating the layer of zinc and iron phosphates, the paint is applied to the part, which has now turned grey. In this way, the colour adheres perfectly and, at the same time, greater corrosion resistance of the final product is guaranteed.
Manganese phosphating, on the other hand, is performed when the sliding of the part is preferred. Such a treatment is particularly suitable to protect against corrosion and oxidation, while at the same time allowing friction stress relief. This is the case, for example, with gears, hydraulic components and gear wheels. At the end of machining, the colour obtained on the part ranges from dark grey to black, but the result depends on the nature of the material.
The choice of phosphating type is subject to the specific requirements of the part and the objectives to be achieved. The precision machine shop must therefore carefully evaluate which of the two alternatives to use in order to achieve the best result in terms of strength, durability and performance of the end product.