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Mechanical or chemical metal pickling


Mechanical metal pickling removes the hardest part of oxides, reducing the use of acids

18 January 2024
The pickling of the pipes is a surface cleaning treatment aimed at removing oxide layers through the use of acids or with mechanical methods. Generally, this operation is necessary because, during the high temperature processing phases, the components tend to oxidize superficially due to the presence, in fact, of the temperature and an atmosphere rich in oxygen, as can be the air itself. In fact, if this layer were not removed in some way, it would not be possible to lay another layer on the surface which may be necessary to increase the resistance to corrosion.

Mechanical pickling of pipes

Mechanical pickling is an operation that is carried out using methods such as shot peening and sandblasting.

- Shot peening: it is a superficial operation that is carried out by applying a powerful jet of spherical pellets. The effect, in addition to inducing a residual tension state of compression, is to eliminate possible oxide layers and create a suitable surface for the deposition of a surface layer required by the project.
- Sandblasting: the component surface, which in this case is the pipe, is treated and scraped by applying a jet of sand. Once again, when the oxide layer is removed, this operation guarantees the possibility of anchoring a further surface layer.

Among the materials used for these operations are silica sands, corundum, glass microspheres and other suitable materials. It can also be used as a preliminary phase of acid pickling.

Chemical pickling of pipes

The chemical pickling of metals, unlike the mechanical pickling which involves the use of an external body to remove the oxide layer, is based on the chemical reaction that occurs between the surface oxide and an acid compound. This reaction starts a dissolution process that leads to the formation of a liquid solution between the dissolved oxide and the acid. The metal therefore acts as a base which, reacting with the acid leads to the formation of a salt and a gas (mainly hydrogen).
There are three main factors to consider for this type of pickling:
- Chemical Composition: as always, the composition of the metal is critical to the process. In particular, it is noted that the more the steel has high amounts of alloy elements (high-alloy steel), the more it will be necessary to use substances that perform a more thorough pickling and for a longer time;
- Temperature: the control of this parameter is essential to carry out an optimized and effective pickling, without producing aesthetic or functional alterations of the component;
- Type of welding: depending on the type of welding that can be applied to a pipe that requires a pickling operation, different times and conditions may be required. For example, a MIG welding produces a greater quantity of oxides than a TIG welding and, therefore, requires a certainly longer treatment time.

Advantages and disadvantages

The two methods are both effective even if they have different strengths and weaknesses from each other. On the one hand, mechanical pickling allows to reduce the use of acids and chemical compounds with all the complexities that derive from the treatment before and after use. However, it presents the problem of the correct choice of materials and parameters to prevent them from being incorporated into the metal, creating defects.
On the other hand, acid pickling is more effective than mechanical pickling for the purpose of the result to be obtained, despite the fact that the cost is higher.
Furthermore, mechanical pickling, as mentioned, can act as a preliminary operation of the chemical one and, therefore, the two methods can also work synergistically.

In general, it can be said that depending on the situation, one or the other method may be preferred: the fundamental aspect for obtaining a quality product is the operator's ability to manage the treatment appropriately.

Metallic materials subjected to pickling treatment

Whether it is mechanical pickling or chemical pickling, the importance of this treatment clearly emerges for obtaining qualitatively excellent components that can last over time. In fact, pickling is a widely used surface treatment at an industrial level and which concerns various types of metals:
- carbon steel: being a material which, due to its chemical composition, is subject to mechanical and thermochemical phenomena, such as wear and corrosion, it is often subjected to pickling treatment both after the forming and welding phases, and in particular conditions of storage and treatment with certain types of instruments;
- stainless steel: while resisting better than carbon steel to the phenomena listed above thanks to the layer that forms on the surface, it can suffer surface damage caused by the intrinsic fragility of this layer and pickling is the best solution to restore the lost characteristics;
- aluminum: surface oxidation due to the various working phases can generate problems from a mechanical and aesthetic point of view, making pickling necessary;
- titanium: this treatment reduces the premature aging of the material;
- copper: as with titanium, the pickling treatment eliminates aging but above all solves problems of an aesthetic nature (the classic "copper green" color) resulting from the formation of oxide.

The characteristics of the pickling process

Carrying out a correct and state-of-the-art metal pickling process is not easy but requires the presence of adequately trained personnel. In fact, an approximate pickling can lead to minimal advantages or even to a worsening of conditions leading to a waste of substances, both energetic and economical.
Among the most important considerations to make, as previously mentioned, there is the choice on the type of pickling which influences both the properties obtained and the methodologies of carrying out. The parameters, which are also influenced by the first choice, are fundamental to obtain an optimal removal of the oxide layer and to favor subsequent depositions. Among these is certainly the unwinding temperature as, especially in chemical pickling, there is a direct proportionality relationship between the aggressiveness of the treatment and the increase in temperature. Furthermore, temperature control allows to reduce deterioration and uncontrolled reactions that can develop due to incorrect treatment.
The liquids and agents that are used during the treatment are harmful to the environment and require special disposal following the inherent environmental regulations, avoiding uncontrolled spills with damage to persons and buildings that are located in the vicinity of these systems: maintenance therefore represents an essential requirement both for pickling and for the environment.

An in-depth look at mechanical pickling: shot peening

Speaking of mechanical pickling, the process of shot peening of materials was introduced. As specified, this technique consists of shooting numerous metal microspheres with compressed air through nozzles, which have the precise task of creating craters on the surface of the piece in question.
What are the main characteristics to take into consideration?
The shot peening process must be carried out by making the best use of the characteristics and limiting the defects that can be created to a minimum. In particular, the following must be taken into consideration:
- the width of the shot peening beam that converges on the surface;
- the speed of impact with the surface;
- the speed of movement of the flow;
- the exposure time to the beam of the surface of interest.


What are the benefits that can be achieved?

In addition to the advantages expressed in the specific paragraph relating to pickling, there are some benefits that can be obtained which are not negligible:
1. improvement of fatigue resistance: in fact, shot peening creates a state of compressive tension which increases the critical fatigue stress and, therefore, extends the useful life;
2. improvement of the surface hardness of the component, also reducing wear problems;
3. material is not removed but only the surface is modified, creating craters.
These aspects make us lean towards the use of this technique in the industrial and hydraulic fields.

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