Process description: Conversion coating(chromium)
Back to Conversion coating (chromium) in surface industry
- Chromium conversion coatings
Chromium conversion coatings are used to enhance corrosion protection on various metal surfaces, including electroplated zinc and cadmium, zinc die castings, tin, aluminium, magnesium and magnesium alloys, copper, brass and bronze, nickel, silver and stainless steel. Without such protection, zinc electroplated steel surfaces have a strong tendency to white (zinc oxide) corrosion. Often referred to as “chromating” because the process originally used only hexavalent chromium as the chromate ion (Cr2O42-), it is used in nearly all areas of the steel processing industry and is an essential post treatment step in zinc plating. The wide spread use of the original yellow chromating was increased by the development of further layer systems of blue and black chromating, which have decorative effects in addition to corrosion protection.
Phospho-chromating exists with both hexavalent chromium (Cr(VI)) and trivalent chromium (CR(III)) versions and is used in the treatment of aluminium prior to painting.
The layer thickness of the chromating coats lie between 0.1 and 2 μm. They are well suited as adhesion mediators for subsequent treatment with paint layers or synthetic material coatings. Lubricants can be included in order to decrease the friction values of treated pieces.
The corrosion protection of the chromium coatings can be increased by further sealing layers.
The typical composition of solutions are chromic acid, dichromate, chloride, fluorides, sulphates, borates, nitrates and acetates, which are used in different combinations and concentrations to produce different colours and layer characteristics.
Chromium (VI) conversion coatings
Conventional processes are all based on the use of acid oxidising solutions, the oxidant being hexavalent chromium (Cr(VI)). The protection mechanism is based on the dissolution of the Cr(VI) present at the surface of the film. The presence of chromate works locally to inhibit any corrosive action on the exposed metal surface.
Chromium (VI) conversion coatings on electroplated zinc layers
Coatings are applied by chemical reaction in aqueous solutions. Numerous proprietary conversions coating processes are available, producing decorative and protective films with colours ranging from clear through iridescent yellow to brass, brown, olive drab and black. Broadly, the deeper the colour is, the better is the corrosion resistance.
Chromium (VI) conversion coatings on copper, brass and bronze
Chromate solutions to treat copper and copper alloys are prepared with proprietary materials. The treatment not only passivates the surface but also provides effective chemical surface polishing. Consequently, chromate treatments on copper and copper alloys are used both as a final finish and as a whole or partial substitute for mechanical buffing prior to nickel or chromium plating. The passivity obtained is effective in reducing corrosion and sulphide tarnishing.
Chromium (VI) conversion coatings on aluminium
Chromate or phospho-chromate conversion films, ranging from clear yellow for chromate to green for phospho-chromate can be produced on aluminium. The film colour depends on the immersion time, pH, solution concentration and, to some extent, on the composition of the alloy to be treated. The main use is as a pretreatment prior to painting or powder coating, although it is used for components in aerospace, electronics and other applications.
The ability of the pretreatment to provide corrosion resistance without a corresponding loss of conductivity is particularly useful for electronic applications.
Solutions containing low Cr(VI) are available (Personal communication, ESTAL).
Chromium (VI) conversion coatings on magnesium and its alloys
Because of their corrosion characteristics, chromate treatment is still the only common method to treat magnesium and magnesium alloys. This treatment is often applied at the raw material source, in order to ensure good storage characteristics. Chromate treatment is alos used to prepare magnesium and magnesium alloys for subsequent plating, particularly for nickel autocatalytic plating.
There are two typical process methods in use:
- Pickling is mainly used to protect parts during storage and shipment.
- Dichromate treatment provides maximum corrosion protection and improved adhesion properties for paint.
Trivalent chromium (Cr(III)) conversion coatings on aluminium and electroplated zinc
Trivalent chromium (Cr(III)) conversion coating processes were first developed about 20 years ago as a more environmentally acceptable alternative to hexavalent chromium (Cr(VI)) processes, mainly on electroplated zinc. These conversion coating systems and treatment baths do not contain hexavalent chromium. Trivalent chromium-based blue coating processes can produce the appearance traditionally associated with hexavalent processes over acid, cyanide or alkaline non-cyanide zinc plating systems.
Trivalent chromium conversion coating processes usually produce clear or blue conversion coatings only. Recently, trivalent chromium passivates producing a higher film thickness and greater corrosion resistance have been developed that perform well in comparison with iridescent hexavalent chromium passivation.
At least one process is available with coatings that are Cr(VU)-free. They have a unique light green, yellow iridescent colour and provides up to 120 hours to first white corrosion when barrel processing and up to 240 hours when rack processing. These numbers vary depending on the process and if a topcoat or sealer has been added. This passivation can be used on all zinc plating (acid, alkali non-cyanide and cyanide) as well as zinc alloys (Zn-iron, Zn-cobalt and Zn-nickel). It is claimed that treated parts can be baked at 200 °C for four hours and still retain up to 90% of their corrosion protection abilities.
Black Cr(VI)-free passivation have also been developed which, with sealers, give the comparable corrosion test results.
However, the thickest conversion coatings, olive drab, that give greater corrosion protection for zinc can only be achieved using hexavalent chromium conversion coating processes.
Compared to hexavalent chromium conversion coating process, the trivalent chromium conversion coating:
- can provide equal or more corrosion-resistant for the same colour finish
- does not require a running in period for freshly prepared passivation solutions
- the colour and performance of the processes are more uniform throughout the solution life
- solution is normally at least twice that of conventional hexavalent chromium and limited by impurities rather than consumption of the active chromium compounds, resulting in less solution disposal
- cannot produce the range of colours ad corrosion resistance given by hexavalent coatings
- requires more process control
- may require a top coating or sealer to produce similar corrosion performance to darker Cr(VI) passivations
Topcoatings for chromate conversion coatings
Both hexavalent and trivalent chromate conversion films are porous and adsorbent in nature and their thickness is very limited. Their protective action can be enhanced by applying a subsequent top coating film, either organic (e.g. methacrylate, see electroplating and lacquering) or inorganic (e.g. metasilicate) and/or mixed inorganic-organic. These Topcoatings have additional functions: a self-healing effect, providing protection against local mechanical scratches due to improper handling during processing; a substantial reduction in the quantity of Cr(III) leaching out from the treated surface, and a reduction in friction parameters. Top coating protection is mainly due to the physical barrier of the coating itself.
Source: BAT Surface Treatment of Metals and Plastic, Aug. 2006.
