Wet conversion or deposition processing are carried out by immerging the parts to be treated in a bath which contains the coating material. When the process is only driven by chemical reactions, it is called an electroless process. If the process requires an electrical current to induce a redox reaction it is called an electrochemical process.
Wet process treatments often need the immersion in different baths with very specified treatment durations. To limitate the number of manual operations, automated process lines are used to prepare and treat the parts. Also, rinsing steps are really important and must not be overlooked. They allow to avoid the pollutions form one bath to the next.
Metallic coating can be divided into two categories ; noble and sacrificial coatings.
Noble coatings are used because they have a far better corrosion resistance than the substrate. To be really efficient those coatings must be totally impermeable to the corrosion medium, thus they must not be deteriorated in service.
As an example, for iron-based substrates, chromium, nickel, copper or lead can be used as noble protective coating.
Sacrificial coatings are called this way because they dissolve in the corrosion medium before the substrate. As a consequence, the coating thickness most be sufficient to avoid its complete dissolution.
Unlike noble coating, sacrifical ones do not need to be completely impermeable. For example, iron-based substrate can be protected by zinc or cadmium coatings.
Before carrying out any treatments, parts must be prepared in order to assure the best possible adhesion of the coating. Fabrication and transformation processes of the substrates can leave some pollutions on the surfaces (greases, machining oils…). Futhermore, heat treatments often induce the formation of oxide layers.
Transportation and storage of the parts can also lead to the pollution of the surfaces. Different type of surface prepartions are available :
- Mechanical preparation such as sand-blasting, shot-peening, mass finishing or polishing to eliminate oxides, scratches, burrs and unwanted coatings.
- Electrochemical or electroless surface preparation such as degreasing, stripping, brightening to get rid of greases, oxidation layers, stains or unwanted coatings.
Prohibitions on some solvent, namely carbon tetrachloride, 1,1,1-tricholoroethane, bromoethane due to their carcinogenic risks have led to the replacement of organic solvent by alkaline solvents for degreasing.
It is well known that wet processes have some drawbacks such as environmental hazards or health and safety risks, nevertheless some coatings can’t be done by other surface treatment methods, especially for parts with a complex geometry.
Electrochemical processes allow low thickness metallic coating on metals surface with peculiar properties (corrosion resistance, decorative, hardness and electrical conductivity).
This process is based on redox reactions. Parts to be coated are immerged in a bath containing dissolved metallic salt or soluble anodes. An electrical current is applied to force the chemical reaction. In this process, the part to be coated plays the role of the cathode and is placed between anodes. Those anodes can be soluble (copper or nickel) or not soluble (lead, platinum coated titanium).
Main electrochemical processes are :
- Zinc electroplating
This process is used to protect iron or low alloyed steel parts from corossion. Depending on the nature and pH of the bath, nature and thickness of oxidation products and thus corrosion resistance of the coating will be different.
- Copper electroplating
Cyanide-based or pyrophosphate-based copper plating are used to deposit copper underlayer on zinc alloys, aluminium alloys and steels. Generally this treatment is followed by a nickel plating, a tin plating or any other type of displacement plating. The copper underlayer reduces the surface irregularities, increase the corrosion resistance by filling up the prosities and also limits the embrittlement by hydrogen formation.It is noticeable that cyanide-based baths are being gradually replaced by alkaline, cyanide free baths for envirommental hazards and health and safety reasons.
- Nickel electroplating
Nickel coatings are used for decorative and corrosition resistance applications. They’re made of several coatings which slow down the corrosion of the substrate and keep the surface shiny as long as possible. Their applications can go from jewellery, where they can be associated with gold coating, to microprocessors or copper thin sheets. They protect substrates against corrosion and harden the surfaces. Nickel allergenic properties restrict its usage for jewellery application where skin contact is involved.
- Chromium electroplating
This process has two main applications : decorative plating and hard chromium plating. Generally coated on top of a nickel underlayer, it confers a shiny aspect to the coated surface.Hard chromium coating is used to improve the corrosion resistance, the wear resistance and decrease the friction coefficient.Normative evolutions in term of environmental hazards and health and safety have lead to the replacement of chromium VI-based bathes due to their carcinogenic properties.
- Noble metals
Silver, gold, palladium, rhodium and platinum are the most commonly used noble metals for coatings.Osmium and ruthenium are also used but more rarely. These coatings can’t be corroded, have good electrical conductivity and their colors are really appreciated for decorative applications.
- Conversion coatingsThose treatments transform the surface of the parts without input of matter. Titanium or aluminium anodizing are two examples of conversion coatings. Anodizing of aluminium leads to the formation of a hard oxide layer on the surface which increases the corrosion resistance. This procces can be followed by a chemical or elctrochemical colorizing and a clogging which further improves the corrosion resistance. The thickness of the conversion layer can go from few hundreds of nanometers to few hundreds of micrometers.
Thoses processes are based on chemical reactions between the surface to be coated and the bath.
Metal coatings on meallic surface are called chemical metallizing. The coating is created by the reaction between those two species or by the input of a metal coupled with the substrate. The ionization of the metal is done by metallic contact or displacement.
The metallic catalysts can be iron, nickel, palladium or cobalt but also, more rarely, copper, silver or gold. Depending on the process carried out, coating thickness can be different but generally around the tenth of a micrometer. Electroless processes have coating rate lower than electrochemical plating processes. For each subtrates type (copper alloys, aluminium alloys, etc…), composition bath and electrons sources are different.
Main electroless processes are :
- Zinc plating on aluminium
Zinc plating can be done twice in a row to improve the coating quality.
- Tin plating on copper and copper alloys
The chromating process can be done on zinc plating, aluminium, silver, cadmium, magnesium and copper. Chromatation coatings improve the corrosion resistance, can have some decorative applications by colorizing the surface. Also this process has the adhesive propertie of an organic coating.
- Nickel plating
This treatment is generally carried out on cast irons, steels, aluminium alloys, copper, silver, gold, titanium, polymers, glasses or ceramics. It can be done with the addition of hypophosphite which brings phosphourous atoms in the coating in order to increase the hardness. The addition to the bath of chemical components which contain boron atoms is also possible, to create a nickel-boron coating. Nickel plating brings a good corrosion resistance and a good wear resistance.
Nickel plating can also be carried out before the enamelling process to improve the adhesion.
- Phosphate conversion coating
Phosphate conversion coating can be carried out on steels, cast irons, zinc alloys, zinc or cadmium plated steels and aluminium steels. It can be used in numerous manufactoring sectors to lower the friction coefficient in order to avoid jamming or to reduce noises due to friction.
- Silver plating and gold plating
These processes are generally used for jewellery or for alloys which only require a low coating thickness.
- Applied patina
Well-known since antiquity, applied patina consists in colorizing metallic surfaces by chemicals. It is done by provoking a controlled superficial oxidation.