Anodising is an electrochemical process used to produce durable and decorative finishes on components made of aluminium or aluminium-based alloys.
Aluminium, in an untreated form produces a protective oxide coating on exposure to the atmosphere. This oxide is inherently protective, but being very thin provides little resistance to long term corrosion. The coating can, however, be thickened by anodising.
Sulphuric acid anodising which is extensively used for general industrial and architectural applications, produces a coating (typically silver or pale grey) which can be left natural or dyed to produce a wide range of colours.
Sulphuric acid anodising produces coatings of moderate thickness 1.8 μm to 25 μm (0.00007″ to 0.001″) and are known as Type II in North America, as named by MIL-A-8625, while coatings thicker than 25 μm (0.001″) are known as Type III, hardcoat, hard anodising, or engineered anodising. Very thin coatings similar to those produced by chromic anodising are known as Type II B. Thick coatings require more process control, and are produced in a refrigerated tank near the freezing point of water with higher voltages than the thinner coatings.
Standards for thin (Soft/Standard) sulphuric anodising are given by BS3989, BS 1615, BS EN 2284, MIL-A-8625 Type II, Def Stan 03-25, AMS 2471 (undyed), and AMS 2472 (dyed), BS EN ISO 12373/1 (decorative), BS EN 3987 (Architectural) .
Hard anodising covers a number of processes which are based on the use of specialist acid solutions and high voltage and current density electrical conditions, to produce particularly hard anodic oxide coating with excellent wear and corrosion-resistant properties. These coatings have particular advantages on aluminium used for specialist engineering components where the properties of lightweight plus a hard surface are of particular benefit.
Engineering finish (similar to decorative sulphuric anodising, but with greater film depth and closer structure). Can emulate the surface hardness of stainless steel.
Hard Anodising in engineering application is normally used in its as-produced colour. This colour is usually a grey, but the colour is dependent on alloy and processing conditions. Because of this, the only appropriate dyed finish is black, but the process may be customised to produce an attractive range of pale to dark grey finishes for cosmetic use e.g. facia panels.
Hard anodic coatings have specific advantages on aluminium used for engineering components, where the properties of light weight and ease of machining plus a hard surface are of particular benefit.
When specifying hard anodising careful attention should be given to the choice of alloy, not only for the colour of the film, but for the quality of coating. Whilst free cutting alloys may be hard anodised, they are not ideal, as the copper content (which does not anodise) gives rise to an open softer film.
Chemically Brightening is a process of metal finishing that can create a bright, shiny surface on aluminum parts. Unlike other plating techniques, Chemically Brightening does not leave a deposit on the surface of the part. After the Chemically Brightening process the part can be anodized for increased corrosion, wear, and cosmetic enhancement.
Chemical Etching the surface to obtain a uniform surface finish. This is normally done in a strong alkaline solution such as caustic soda with special chemical additives to give that silver matt finish
Semi bright (also known as 156 brightener) is a mixture of chemically brightening and chemically etching leaving you with a pearl finish.
The most common anodising processes, for example sulphuric acid on aluminium, produce a porous surface which can accept dyes easily. The number of dye colors is almost endless; however, the colors produced tend to vary according to the base alloy. Though some may prefer lighter colors, in practice they may be difficult to produce on certain alloys such as high-silicon casting grades and 2000-series aluminium-copper alloys. Another concern is the “lightfastness” of organic dyestuffs—some colors (reds and blues) are particularly prone to fading. Black dyes and gold produced by inorganic means (ferric ammonium oxalate) are more lightfast. Dyed anodising is usually sealed to reduce or eliminate dye bleed out.
Splash effects are created by dying the unsealed porous surface in lighter colors and then splashing darker color dyes onto the surface. Aqueous and solvent based dye mixtures may also be alternately applied since the colored dyes will resist each other and leave spotted effects.
You show us the samples and with the knowledge or our experienced engineers we will mix the colour using organic dyes.
Produces gold to orange conversion coatings and is ideal for coating all types of aluminium and its alloys, including high silicon pressure dye castings. It is recommended for use on aluminium wherever maximum corrosion resistance is required. Coatings are flexible and will withstand bending, and inserts of steel, brass or copper are not affected by the process. Alocrom 1200 is approved to DEF-STAN 03-18 and to MIL-C-05541, but must not be used to treat foodstuff containers as it contains Hexavalent chromium.
Alocrom 1200 is a versatile treatment for industrial and electrical components, vehicle parts including body panels, domestic appliances, and aircraft components. It is approved to DEF STAN 03-18 for use in aircraft (Certificate 031801), including specific approval for repairing damaged anodic coatings. It is useful where maximum corrosion resistance is required.
The Alocrom process can be applied to aluminium and aluminium alloys including high silicon pressure die-castings. It is not is affected by steel, brass or copper inserts and can be used for treating zinc and aluminium in the same bath. It should not be used for decorative effects on unpainted alloys exposed to exterior weathering, since some colour change may occur.
Alocrom 1000 Chromate conversion offers a cost effective non-electrolytic process for coating, and protecting aluminium. Its wear properties are inferior to sulphuric anodising, and it is easily scratched, but provides an excellent surface for painting, bonding and where electrical conductivity is needed. The end colour is silver. Note: Alocrom 1000 is not suitable for foodstuffs as it contains hexavalent chromium.
The oldest anodising process uses chromic acid. It is widely known as the Bengough-Stuart process. In North America it is known as Type I because it is so designated by the MIL-A-8625 standard, but it is also covered by AMS 2470 and MIL-A-8625 Type IB. In the UK it is normally specified as Def Stan 03/24 and used in areas that are prone to come into contact with propellants etc. There are also Boeing and Airbus standards. Chromic acid produces thinner, 0.5 μm to 18 μm (0.00002″ to 0.0007″) more opaque films that are softer, ductile, and to a degree self-healing. They are harder to dye and may be applied as a pretreatment before painting. The method of film formation is different from using sulphuric acid in that the voltage is ramped up through the process cycle.
Why it is important to paint aircraft parts within a set time of chromic acid anodising?
Chromic acid anodising is at times used a bond preparation rather than corrosion prevention in the aircraft industry. (Particularly in Europe). The formation of the oxide coating if exposed to the atmosphere particularly a warm and wet atmosphere can lead to hydrolysis of the oxide structure. This is detrimental to the bond strength and durability. The presence of water in the oxide layer can lead to the formation of an oxy-hydroxide which is more massive than the oxide leading to a build-up of stresses at a crack tip and can promote crack propagation along the adhesive, metal interface. Ultimately leading to bond failure. Adhesion mode of failure.
If the anodise coating has primer applied to the surface it prevents this degradation of the oxide structure and extends the bond strength and in particular its resistance to a warm wet environment. The finger like structure of the oxide also ensures any crack tip is maintained in the adhesive layer rather than migrating to the metal surface. This structure also acts to disperse any forces at a crack tip effectively stop drilling any crack. Cohesive failure mode.
Various different grades ranging from 0-800 grit available.
The process used in electroplating is called electrodeposition. It is analogous to a galvanic cell acting in reverse. The part to be plated is the cathode of the circuit. In one technique, the anode is made of the metal to be plated on the part. Both components are immersed in a solution called an electrolyte containing one or more dissolved metal salts as well as other ions that permit the flow of electricity. A power supply supplies a direct current to the anode, oxidizing the metal atoms that comprise it and allowing them to dissolve in the solution. At the cathode, the dissolved metal ions in the electrolyte solution are reduced at the interface between the solution and the cathode, such that they “plate out” onto the cathode. The rate at which the anode is dissolved is equal to the rate at which the cathode is plated, vis-a-vis the current flowing through the circuit. In this manner, the ions in the electrolyte bath are continuously replenished by the anode.
Below are various finishes that can be obtained within the electroplating process:
Electroless nickel plating, also known as enickel and NiP, offers many advantages: uniform layer thickness over most complicated surfaces, direct plating of ferrous metals (steel), superior wear and corrosion resistance to electroplated nickel or chrome. Much of the chrome plating done in aerospace industry can be replaced with electroless nickel plating, again environmental costs, costs of hexavalent chromium waste disposal and notorious tendency of uneven current distribution favor electroless nickel plating.
Electroless nickel plating is self-catalyzing process, the resultant nickel layer is NiP compound, with 7–11% phosphorus content. Properties of the resultant layer hardness and wear resistance are greatly altered with bath composition and deposition temperature, which should be regulated with 1 °C precision, typically at 91 °C.
During bath circulation, any particles in it will become also nickel plated, this effect is used to advantage in processes which deposit plating with particles like silicon carbide (SiC) or polytetrafluoroethylene (PTFE). While superior compared to many other plating processes, it is expensive because the process is complex. Moreover, the process is lengthy even for thin layers. When only corrosion resistance or surface treatment is of concern, very strict bath composition and temperature control is not required and the process is used for plating many tons in one bath at once.
Electroless nickel plating layers are known to provide extreme surface adhesion when plated properly. Electroless nickel plating is non-magnetic and amorphous. Electroless nickel plating layers are not easily solderable, nor do they seize with other metals or another electroless nickel plated work piece under pressure. This effect benefits electroless nickel plated screws made out of malleable materials like titanium. Electrical resistance is higher compared to pure metal plating.
The Iridite NCP, Non Chrome Passivate has been developed in response to the ELV, RoHS and WEEE directives. The Iridite NCP does not contain Lead, Cadmium, Chromium (hexavalent or trivalent), Mercury or PBB / PBDE compounds. Iridite NCP is an environmentally friendly chemical process that produces a protective conversion coating on aluminium and its alloys. This coating exhibits bare, unpainted, corrosion resistance that is equal to hexavalent chromates on many aluminium alloys. The coating can be used as a final finish and can also serve as a base for paints, high performance topcoats, powder paints, lacquers, or as a base for rubber bonding. When the application is for a paint base only, then Iridite EXD (110563) may be considered.
The Iridite NCP process is composed of Iridite NCP Start (110561) and Iridite NCP Replenish (110544). Iridite NCP Start is formulated to build a new bath that does not require any break-in to produce a conversion coating that will provide excellent NSS corrosion resistance. Iridite NCP Replenish is the maintenance additive that has been formulated to maintain the bath in the correct operating range.
The working range of Iridite NCP is very flexible. Suitable adjustments of the concentration can accommodate wide variations in immersion time, the corrosion protection desired and the alloy to be treated.
Cadmium plating is widely used in some applications such as aerospace fasteners and it remains in military and aviation specs.
Cadmium plating (or “cad plating”) offers a long list of technical advantages such as excellent corrosion resistance even at relatively low thickness and in salt atmospheres, softness and malleability, freedom from sticky and/or bulky corrosion products, galvanic compatibility with aluminum, freedom from stick-slip thus allowing reliable torquing of plated threads, can be dyed to many colors and clear, has good lubricity and solderability, and works well either as a final finish or as a paint base.
If environmental concerns matter, in most aspects cadmium plating can be directly replaced with gold plating as it shares most of the material properties, but gold is more expensive and cannot serve as a paint base.
The preferred alloy composition for zinc-nickel plating is 12 – 15% nickel, with the remainder being zinc. This alloy gives exceptional sacrificial corrosion resistance and can be readily passivated. To achieve this alloy, zinc-nickel is usually plated from an alkaline electrolyte. For some applications, including the plating of brake castings, acid zinc-nickel can be used. – See more at: http://industrial.macdermid.com/cms/anti-corrosion/zinc-zinc-alloy-systems/zinc-nickel-plating/index.shtml#sthash.DQMXex13.dpuf
Advantages of zinc-nickel plating processes
Enviralloy Ni 12-15 G2 Zinc-nickel…the Choice for “Bright” Barrel Deposits
Zinc-nickel is the choice for high performance coatings in automotive applications. Enviralloy Ni 12-15 G2 is the process of choice for zinc-nickel applicators who demand bright and consistent deposits from their alkaline barrel zinc-nickel plating solutions.
Building on the success of our Enviralloy systems, Enviralloy Ni 12-15 G2 delivers the same consistency and ease of use with increased deposit brightness. This is achieved even at the lower end of the nickel alloy range, offering users further potential in use cost savings. The attractive plated finish has a nickel content between 12-15% and operates across a wide range of current densities and operating parameters.
The Enviralloy Ni 12-15 G2 additives are designed to maintain high efficiency over time without the use of anode boxes or regeneration technology. Existing Enviralloy Ni 12-15 can also be converted to the new system. The deposit
Zinc coatings prevent oxidation of the protected metal by forming a barrier and by acting as a sacrificial anode if this barrier is damaged. Zinc oxide is a fine white dust that (in contrast to iron oxide) does not cause a breakdown of the substrate’s surface integrity as it is formed. Indeed the zinc oxide, if undisturbed, can act as a barrier to further oxidation, in a way similar to the protection afforded to aluminium and stainless steels by their oxide layers. The majority of hardware parts are zinc plated, rather than cadmium plated.
We are one of the very few companies in the UK that operates the electro-brightening (Brytal) process. Using super pure grades of aluminium, electro-brightening is able to produce very high levels of specular and total reflectivity, which can be retained indefinitely after anodising. This process is particularly suited to high quality reflector spinning’s that require maximum levels of light output and longevity.
For parts used in less demanding applications we are able to offer chemically brightened or chemically etched finishes. If necessary, these can be dyed in just about any colour so as to be able to cover for any commercial requirement.