The point of this process is to add a specific superficial property - which may be decorative, protective against corrosion, or more generally, physical or mechanical properties that are different from those of the substrate (in terms of hardness, friction, conductivity, adhesion of organic products, barrier layers, etc.).
The filler metal - initially in ionic form in a bath - is subjected to an electrochemical reduction reaction, transforming it to a metal state. This reaction is provoked on the surface of the parts to be coated through the addition of electrons from an external circuit.
The operation takes place within an electrolytic cell comprising the following elements:
- A tank containing the electrolytic bath
- The electrodes immersed in the bath
- The negatively polarised cathode comprising the workpiece to be coated, and the seat of the reduction reaction resulting in the plating. This electrode can also be the seat of other reduction reactions, including the electrolysis of the water with the release of hydrogen – which can be a source of embrittling for certain substrates.
- The anode, seat of one or more oxidation reaction(s). This can be soluble or indissoluble. Where it is soluble, it comprises metal for plating and is subject to the opposite reaction of that which is produced at the cathode. Where it is insoluble, the composition of the bath varies continuously throughout the course of the electrolysis.
- The electrical circuit is made up of conductors supplying the electrodes connected to a generator of current.
The electrolytic cell shown is suitable for medium and large sized workpieces. Small workpieces are processed in bulk, in an appliance known as a "barrel". Other devices are used for specific applications (out-of-tank plating). Certain installations designed for specific applications look more like machine-tools than electrolytic tanks.
In order to obtain superior corrosion resistance, a conversion treatment is required after coating. This treatment, known as passivation, is carried out in trivalent chrome-based baths (Cr3).
Passivation creates a protective barrier of zinc coating (see "Protection Principles")
The surfaces to be treated require careful preparation, and the conversion treatments must be carried out after electrolysis. The treatment options are, therefore, complex procedures of which electrolysis represents only one stage. Example of the electrolytic galvanizing range:
Flake-type coatings were designed to offer increased resistance to corrosion in comparison with conventional electro-zinc coatings, as well as for their other advantages in the manufacture of small components, and fasteners in particular.
This type of coating is widely used in the automotive industry. Its composition and characteristics also enable friction coefficients to be controlled, where a repeatable control of mechanical torque is necessary (in the case of automated mounting for mass production).
This type of coating is principally composed of zinc flakes, combined with aluminium flakes in certain cases.
The use of zinc flakes rather than zinc dust or powder in the formulation of this material is primordial, because the presence of flakes allows us to obtain an extremely dense coating. Their structuring parallel to the substrate surface during drying and baking considerably improves the protection offered by the coating. The flakes can be connected by an organic or inorganic matrix, depending on the specific materials used.
These coatings are highly conductive, offering steel sacrificial cathodic protection. They are often also given a top coat to strengthen resistance to corrosion as well as barrier effect - in which case the coating ultimately achieved can cease to be conductive.
In terms of performance, zinc flake coating offers the following advantages:
- Excellent resistance to atmospheric corrosion
- Reduced ‘white rust’ effect (products of zinc corrosion) or other products of corrosion
- Resistance to salt spray superior to that of many other coatings resulting from conventional processes, such as: electro-galvanizing or sherardizing.
- Protection against numerous “non-aggressive” chemical products and solvents, such as brake fluids and fuel, etc.
- No embrittling through hydrogen because the application is non-electrolytic.
- Generally highly conductive.
- Electroplate protection of the zinc-enriched coating allows satisfactory reduction of bimetallic corrosion on contact with steel, aluminium, zinc and cadmium in most situations.
- Allows complexly-shaped workpieces, with hollows and holes, to be coated with optimal material.
- Reduced thickness makes it possible to obtain corrosion resistance equivalent to that of conventional, more thickly-layered coatings.
The zinc flake coating process (barrel) - the most common method- can be broken down into 4 phases:
1) Preparation phase, which seeks to remove all dirt, lubricants and oxidation from the workpieces to be treated: this phase can itself be broken down into an alkaline degreasing + drying stage, followed by either mechanical (shotblast) or chemical pickling (phosphate);
2) Dipping-centrifugation phase: the fasteners placed in cylindrical baskets are dipped in a liquid zinc flake bath preparation. The baskets are then subjected to centrifugation, which results in an even coating over the entire workpiece surface. The baskets are then tilted and tossed to empty the hollow bodies, before being centrifugated again;
3) Drying phase: the workpieces are tipped out onto a mat in a drying cabinet;
4) Baking phase: immediately after drying, the workpieces travel through the baking oven.
Phases 2 to 4 are then repeated, as many times as there are layers of plating. Two layers of plating for a Grade A coating; three layers of plating for a Grade B coating. The maximum size of these parts, for the barrel method, is up to 150 mm x 20 mm in diameter, for a weight of around 0.5 kg.
For larger dimensions and weights, the barrel process is no longer appropriate and the jig coating process is used. The parts are positioned on jigs. The jigs are dipped in a liquid zinc flake bath. Then, the parts thus coated undergo a draining-centrifugal process designed to eliminate all excessive liquid. The maximum acceptable size of parts is up to 1100 x 500 mm for a weight of 30 kg.
There is a third type of plating method, known as spraying. The parts (brake discs, etc.) are placed on jigs before being treated by spraying. This treatment gives them increased protection against corrosion, and a finish that is more aesthetically pleasing.
The various flaked treatments offered by LA CLUSIENNE-CLUFIX are listed in the "Coatings offered" page.
Control of the friction coefficients of zinc flake coatings is guaranteed:
- either solely through the intrinsic qualities of the layers deposited - in which case we speak of zinc flake with integrated lubricant
- or by the addition of an extra protective film or topcoat to the layers of zinc flake to guarantee friction coefficient control.
The friction coefficients of the zinc flake zincs offered by LA CLUSIENNE-CLUFIX are listed in the "Coatings offered" page.
The workpieces commonly zinc flake coated are as follows:
- Threaded fasteners, in particular, those of strength grades 10.9 and 12.9
- Stamped pieces, springs, fasteners for the automotive industry, household appliances and construction industry.
- High-strength steel (≥ 1000 N/mm2 in particular) and cemented workpieces, for surface protection without any risk of embrittling through hydrogen.
- Sintered steel, cast steel and cast iron parts
- Complexly-shaped workpieces, with holes and hollows
- Connectors with multiple screw threads, e.g. locking parts and hose connectors