Metal Surface Coatings: What They Are and What They're For

What Are Surface Treatments For?

The main utility of surface treatments is to protect the metal from the slow deterioration caused by wear and atmospheric agents.

Indeed, carbon steels and alloy steels have excellent mechanical characteristics but a strong tendency to react with atmospheric elements. Without appropriate protection, any object made of this material is destined to undergo chemical aggression and to become covered with a layer of rust in a short time.

This phenomenon leads to the total corrosion of the product and the loss of its physical properties: depending on the type of agent causing the deterioration of the manufactured good, different types of corrosion occur, which in turn require specific treatments to combat it.

Thus, the main reasons for the invention of metal surface coatings are:

• To prevent their deterioration;
• To restore their functionality;
• To restore their aesthetics.

There are different types of metal workings, each with different characteristics and peculiarities.

To choose the correct procedure, it is necessary to always keep in mind the type of product to be treated and the chemical and physical agents it will be exposed to during its existence: only by considering these factors will it be possible to make the right choice and ensure the best protection.

What Are the Main Anti-corrosion Treatments?

With technological progress, various types of coatings have been developed capable of protecting metals from the oxidation phenomenon.

In the case of fasteners, the issue is of utmost importance, as it is necessary to respect the coupling tolerances and allow the tightening tools to function correctly: the surface treatment must protect while also being thin and allowing the correct use of the fastening elements.

The existing metal coatings are generally indicated by the metal they are formed from (copper, zinc, nickel, etc.) and can be obtained through 3 different methods:

• Mechanical (spray coatings or immersion);
• Chemical (chemical reduction or electrodeposition);
• Physical (immersion in molten metal or metal diffusion).

An important distinction concerning metal treatment is the difference between galvanic and non-galvanic coatings.

Galvanic and Non-galvanic Treatments: Differences

The surface coatings that can be applied to metals are mainly distinguished into two categories: galvanic treatments and non-galvanic treatments.

The main characteristic that distinguishes one from the other is the electrolytic bath, an intrinsic peculiarity of the galvanic type treatment: it is a process that involves the polarization of an anode and a cathode immersed in a solution compatible with the coating to be obtained.

Galvanic processes are regulated by the UNI EN 4042 standard and are the most widespread coatings among those used in the world of fasteners, thanks to their ability to guarantee protection and give aesthetic quality to the products.

Within this category, we find the following processes:

• Electrolytic zinc plating
  
  
• Zinc-nickel treatment
  
  
• Copper plating
  
  
• Nickel plating
  
  
• Tin plating
  
  
• Silver plating
  
  
• Hot-dip galvanizing - UNI EN ISO 10684
  
  
• Mechanical galvanizing in powders – UNI EN ISO 12683
  
  
  
  
  
  

List of the main galvanic treatments and their characteristics

Non-galvanic treatments, on the other hand, include all those surface coating operations that do not involve the electrolytic bath.

They are regulated by the UNI EN ISO 13811 standard and include:

• Zinc plating by sherardization
• Application in zinc flakes UNI EN ISO 10683
• Application of organic coatings.

There is a further category of processing that includes treatments carried out with chemical substances: in this case, we are talking about non-metallic coatings.

This type of treatment includes:

• Oiling
• Blackening
• Phosphating UNI EN 12476

Before subjecting materials to any of these surface coatings, it is recommended to provide thorough surface preparation: from simple washing with solvents to more complex procedures such as passivation and pickling, it is necessary to choose the best process depending on the material, its coating, and its consistency.

Zinc Plating

Zinc plating is the most well-known and recognized metal coating.

It involves the application of a layer of zinc on the surface of the manufactured good, giving it durable and resistant protection.

The zinc plating process is carried out through three different techniques:

• Hot-dip galvanizing
• Cold galvanizing
• Electrolytic galvanizing

To choose the best alternative to use, it is necessary to consider the dimensions, weight, and composition of the part to be treated.

Mechanical Galvanizing

Mechanical galvanizing is a particular type of galvanizing obtained through a compaction process of zinc. Its main advantage is offering an anti-corrosive coating free from hydrogen embrittlement problems.

This treatment involves inserting the object to be treated into a tumbling barrel (a rotating cylinder heated to very high temperatures) containing fine zinc powders along with appropriate additives and metal spheres. By subjecting the manufactured good to rotation and heat, the hammering of the spheres crushes the zinc powder onto the surface, making it adhere to the material.

It is a treatment suitable for parts with complex geometries.

Phosphating

Phosphating is a chemical process that involves the application of a protective layer of metallic phosphates on the surface. It is a coating suitable for manufactured goods found in aggressive environments and exposed to adverse conditions or mechanical components subjected to abrasive loads.

It is also used as a base for the application of subsequent coatings such as paints and lubricants, improving their adhesion and effectiveness.

The treatment can be carried out using different chemical formulations depending on the type of metal and the desired properties of the phosphate layer. The three main types of phosphating are:

• Manganese phosphating;
• Zinc phosphating;
• Iron phosphating.

Each coating has particular advantages and characteristics, and the choice must be made based on the final result to be achieved.

What Is Meant by Blackening?

Blackening is a chemical process that involves the application of a dark, glossy coating on metals. It is a treatment that, in addition to conferring corrosion resistance, serves to improve the aesthetics of the treated objects.

There are two variants: chemical-electrolytic blackening and thermal blackening.

Chemical-electrolytic blackening is a long process composed of various sequential steps:

• Degreasing by immersion
• Water washing
• Pickling
• Water washing by immersion
• Chemical-electrolytic blackening
• Water washing by immersion
• Final washing
• Hot air blowing

Thermal blackening is instead a processing used especially in the production cycle of high-strength screws and involves the immersion of the product in a special oil bath. The passage in the oil and then the treatment at high temperatures give the surface a matte patina.

Oiling of Metals

Oiling consists of the immersion application of a liquid oily substance on metals to protect them from oxidation and to perfect their aesthetic appearance.

The operation is applicable to all finishes and greatly increases the corrosion resistance of the manufactured goods.

It is important to distinguish oiling from lubrication, which consists of the application of a fluid product containing substances capable of reducing friction.

How Does Copper Plating Work and What Is It For?

Copper plating is a galvanic treatment that involves the application of a thin layer of copper on an object. It is usually performed on materials such as brass, aluminum, iron, and stainless steel and makes possible subsequent treatments with other metals without prejudicing their adhesion.

Copper plating is carried out by immersing the manufactured good in electrolytic baths where copper metal becomes the anode and the material to be coated the cathode.

The baths for copper plating can be of two types:

• Alkaline – the electrolyte is constituted by complex cyanide;
• Acidic – the electrolyte is formed by sulfuric acid and copper sulfate.

These two processes have very different characteristics and peculiarities:

• Acidic copper plating produces very shiny deposits, ensures a stable solution over time, and offers high leveling power;
• Alkaline copper plating has a high penetrating power, produces a semi-glossy deposit, and can be applied directly to the surfaces of a large number of materials.

As mentioned, galvanic copper plating is often used as a preparatory treatment for subsequent coatings. Among these, the most common is nickel plating.

Nickel Plating: Which Metals Can Be Nickel-Plated?

The nickel plating treatment can be of two different types: chemical or electrolytic.

Chemical nickel plating is the surface coating that is most suited to be applied to precision mechanical parts. It is indeed a process capable of guaranteeing:

• Uniform thickness
• High hardness
• Wear resistance
• Excellent corrosion resistance.

Chemical nickel plating involves immersing the manufactured good in a nickel solution, which deposits on the surface creating a coating with uniform thicknesses and capable of penetrating even the most internal parts and complex geometries.
It is applicable to all iron alloys, aluminum, copper, and titanium.

Electrolytic nickel plating is a process that involves the use of the galvanic bath to deposit a layer of nickel on the surface of the metal. Its main characteristics are:

• Precise control of the thickness of nickel to be deposited;
  
  
• High adhesion of the nickel coating to the underlying metal;
  
  
• Variety of alloys and additives that can be used in the electrolytic bath, which allow obtaining different results both from an aesthetic point of view and in terms of resistance.

The two processes have very different characteristics, and the choice between the two methods will depend on the specific needs and the desired result.

Thickness Difference Between Electrolytic Nickel Plating and Chemical Nickel Plating

What Is Tin Plating For?

Tin plating consists of the uniform deposition of a layer of tin on the surface of a material. The main purpose, as with all treatments, is protective.

The coating is applied directly to the manufactured good or can be preceded by degreasing, pickling, and activation operations.

There are three tin plating systems:

• Thermal tin plating, which is used for large sheets and semifinished products;
  
  
• Galvanic tin plating, characterized by the ability to control the thickness of the coatings of various objects;
  
  
• Chemical tin plating.

The characteristics of this surface treatment are multiple:

• Excellent electrical conductivity;
• Excellent oxidation resistance on copper alloys;
• Stability against chemical agents;
• Anti-galling on stainless steel.

The tin plating process finds application especially in the food industry thanks to its capacity for protection against atmospheric agents, waters, and various substances.

Silvering of Metals: How It Works

Silvering of metals is a process used both to confer hardness and anti-corrosive capacity to an object and to give it great brilliance. It indeed creates a solid, protective, and shiny film.

Being a galvanic treatment, it involves immersion in silver electrolytic baths. Depending on the concentration of this metal, it is possible to make a distinction between:

• Common baths – envisage a variable quantity of silver in the solution, so they are used for the most varied needs, including small parts;
  
  
• Concentrated baths – allow obtaining a rapid and heavy silvering, but the thickness of the protective film is greater.

The galvanic silvering process allows obtaining an excellent result thanks to the conductive and protective capacities of silver. Moreover, it is possible to perform a selective treatment only on some particular areas of interest of the manufactured good.

To block the sulfurization process, typical of silver, it is often necessary to perform a further step through passivation.

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