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All About Anodising - SMLease Mechanical Design

Anodising is an electrolytic process for producing thick oxide coatings, usually on aluminium and its alloys. Magnesium and Titanium also can be anodised. It is used to give improved surface resistance to wear and corrosion, or as a decorative layer.

Machining, drilling or welding of the component should be done before anodising.


Aluminium Anodising Specifications : MIL-A-8625 

Military Specification MIL-A-8625 is the most common specification for anodizing in use.

  • Type II Sulfuric Acid Anodising (common or “standard” anodising)
  • Type III Sulphuric Acid Anodising (hard or “hard coat” anodising)
  • Class 1 – Undyed (clear)
  • Class 2- Dyed
  • MIL-A-8625 would be called out on a drawing as follows: “MIL-A-8625, Type II, Class 2, black” 


Types of Anodising

Sulphuric Anodising

Sulphuric Anodising is an electrochemical process that converts the aluminium surface into a decorative, durable, corrosion-resistant, anodic oxide finish

Characteristics

The anodic oxide structure originates from the aluminium substrate and is composed of aluminium oxide. Aluminium oxide is fully integrated with Aluminium, so it can not chip or peel. It is a highly porous structure that allows for the secondary process such as colouring and sealing

Anodising is done by immersing the aluminium into an acid electrolyte solution and passing a current through the aluminium. A cathode is mounted to the inside of the anodising tank; the aluminium acts as an anode, so that oxygen ions are released from the electrolyte to combine with the aluminium atoms at the surface of the part being anodised. 

Finishes That can be achieved with Anodising

  • Satin Silver, Black, Bronze & Gold
  • Bright Silver, Gold, Black
  • Colours in Bright, Bright Etch and Satin finishes.

Clear Anodising

Clear anodising varies from a very light gray (for very thin coatings 2.54 to 7.5 microns) to a dark olive color for thick Type III coatings (50 microns). Very thin clear anodizing (sometimes called “cosmetic” clear) coatings mimic the look of bare aluminium (though the coating is actually slightly grey), hence the term “clear” anodising. But on thick hard-coated parts, the coloring is anything but clear. 

 


Hard Anodising

Hard Anodising is anodic coatings with film thickness, corrosion and abrasion resistance as their primary characteristics.

The nominal thickness of coatings applied is 25-50 microns but a controlled thickness can be obtained from just a few microns up to 100 microns depending on the alloy used


Hardness & Wear Resistance

The wear characteristics of hard anodised aluminium are most favourable at low loads and compare most favourably with hard chromium and high speed steel, in most cases better.

The micro hardness of the coating varies significantly depending on the alloy chosen.

Generally the high strength alloys i.e. 2000 and 7000 alloys produce a micro hardness figure ranging from 250 to 350 micro Vickers and 6000 alloy range 400-500 micro Vickers.

It has been found that a hard anodised coating measuring 450mv will have a wear resistance of tool steel of 950mv and even an anodic coating of high copper alloy such as 2014 measuring 250-280mv will often offer as much wear resistant as tool steels.

Corrosion Resistance

Hard anodic coatings have better abrasion resistance than conventional coatings of the same thickness. Corrosion resistance is further improved by sealing but with some loss in the abrasion resistance (about 20%).

Colour

Depending on alloy chosen and film thickness the colour varies from light grey to dark grey.

Adhesion

Due to the fact that the hard anodic film is an integral part of the aluminium, you will find that the adhesion properties are superior to that of any other coating applied to aluminium

Fatigue Strength

Hard anodising will reduce fatigue strength considerably - up to 47%

Heat Resistance

Hard anodic coatings can withstand short exposures up to 2000 °C. Long exposure to high temperature shows no adverse effect on performance, other than surface crazing



Anodise vs Alodine (Chem Film)

Alodine is a trade name for chemical conversion coating of aluminium (also called “chem film” or “chemical conversion coating”). While both are conversion coatings, chem film is much thinner than anodising and is also created by immersion but without use of electrical current. Unlike anodising, chem films (which are commonly seen in gold or ROHS clear versions) provide a conductive coating, and are sometimes used in conjunction with masking in anodized parts. The coating provides corrosion protection and also is an excellent base primer for paints. The common military specification for chemical conversion coating is MIL-DTL-5541.



Threads, Holes and Masking

Even with thinner, Type II anodizing, coating build-up can cause problems with tolerancing, for example, with very fine threaded holes, precision pin holes, and fine sliding fits. The most common solution to this problem is to mask the features. This is done using soft plastic plugs, for round holes, or plastic tape or painted-on liquid plastics for flat areas. As noted above, masking is often not required for Type II anodizing. However, it should be noted on print call outs when specifying thicker Type III coatings and when small thread or tight tolerance features exist



Cost factors in Anodising

Factors that affect part cost:

  • Hard vs. regular anodizing: Since hard Type III anodising is performed at a much lower temperature than regular Type II anodising, and at higher current and voltage, it requires much higher energy use, and is usually considerably more expensive.
  • Masking: Masking usually involves a significant amount of hand work, which adds to cost. Masking materials often cannot be reused and add to costs as well.
  • Racking: Difficult-to-rack parts can add to cost. If required racking features need to be added
  • Small features which trap chemistry: Small holes, especially blind holes, as well as small, deep pockets or other enclosed features force extensive rinsing of parts and raise labour costs.


Defects in Metal and their Effects on Anodising

Because anodizing is a conversion coating, and relies on the aluminium substrate being converted to aluminium oxide, any defects in the aluminium will have marked effects on the quality of the anodising. aluminum finish must be consistent, clean and fully exposed for the anodic layer to form properly. Common problems occur with welds, extrusions, cold or hot-worked areas, or poorly machined surfaces. It is common for problems in the material or mechanical finish to have the appearance of a problem with the anodizing. Often material defects will look like chemical spotting or streaking, when the issue is material based. For the anodic layer to be perfect, the aluminum substrate has to be perfect as well.  



Aluminium Alloys and their Effects on Anodising

Colour Aluminium alloys can also have an effect on anodising. Alloying elements such as copper and Zinc do not anodise, so high-copper or zinc alloys such as 7075 or 2024 will give noticeably different colors than lower copper alloys such as 6061, both with dyed parts and with clear anodising. Generally, 6061, the 3000 and 5000 series alloys will anodize reasonably consistently with each other. 7075 will show a yellow tint in clear anodizing, due to the high Zinc content, and black will not be as deep. 1000 series, while rarely specified, actually anodizes the best of all, being the closest to pure aluminium.



Marking and Engraving Anodising

Anodising is an excellent background finish for either mechanical or laser engraving.

Using carbide tooling, e.g. a small 90o chamfer mill, letters, numbers or other symbols can engraved directly through the anodic layer with a CNC mill or router. The aluminium substrate gives an excellent contrast to the characters.

Laser Engraving also produces excellent results. Rather than burn through the anodic layer, a laser engraver will bleach the dye in the layer, generally giving an excellent contrast to the characters. An advantage of laser engraving on colored anodizing, is that no additional chemicals need be added to the surface to accentuate the marking, which is not the case with bare aluminum, stainless steel, or most other metals.

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