basic principle behind anodizing, types, applications and benefits

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KNOWLEDGE SHARING SESSION
TOPIC : ANODIZING
Presented by-
SATHISHKUMAR S
(06-02-2019)

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 Anodizing is an electrochemical process that converts the
metal surface into a decorative, durable, corrosion-resistant,
anodic oxide finish.
 Aluminum is ideally suited to anodizing, although other
nonferrous metals, such as magnesium and titanium, also can
be anodized.
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 Anodizing is accomplished by immersing the aluminium into
an acid electrolyte bath and passing an electric current
through the medium.
 A cathode is mounted to the inside of the anodizing 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 anodized.
 Anodizing is, therefore, a matter of highly controlled oxidation
—the enhancement of a naturally occurring phenomenon.
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Principle

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Principle
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Anodizing Overview
 The metal part to be treated (generally aluminum) is first submerged in an
electrolytic solution bath along with a cathode. When a current is passed
through the acid solution hydrogen is released from the cathode and oxygen
forms on the surface of an anode. This results in a metal oxide film growing on
the surface of the part being treated.
 Depending on the end-use application and the anodizing process used, the
oxide layer can be expanded greatly. The layer that can be grown on an
aluminum part can be more than 100 times as thick as an oxide layer that
would naturally exist on an aluminum part that is only exposed to oxygen.
 Common sense dictates that because the metal part being treated forms the
anode in this electrolytic circuit the process is referred to as 'anodizing'.
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 In the 1920's, aluminium turned the world of metals upside down with its
benefits of light weight, strength, fabrication flexibility, and durability.
 The first and most important of these was the development of the anodizing
process, which converts the aluminium surface into an extremely hard,
durable, corrosion resistant, long-lasting aluminium oxide finish.
 While various metals, including titanium, hafnium, zinc, and magnesium, can
be protected by applying an anodized layer, by far the process is most
commonly applied to aluminum and aluminum alloys.
A Historical Perspective
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Types of Anodizing
 Type – I : Chromic Acid Anodizing.
 Type – II : Sulphuric Acid Anodizing.
 Type – III : Hard Acid Anodizing.
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Soft and Hard Anodizing
 Chromic anodizing utilizes a chromic acid electrolyte and yields the thinnest
coatings, only 0.5 to 2.5 microns thick. Chromic anodizing has the least effect
on fatigue strength and is less corrosive.
 Sulphuric anodizing is the most common method of anodizing, utilizing
sulphuric acid to yield coatings up to 25 microns thick. Sulphuric anodizing
provides corrosion resistance and is very durable.
 Hard anodizing utilizes a higher concentration sulphuric acid electrolyte at a
lower temperature resulting in a tough outer skin(greater than 25 microns)
with excellent abrasion resistance, corrosion resistance, colour fade resistance,
dielectric strength and surface hardness
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Anodizing Process
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Anodizing Process
 When aluminum is anodized conventionally, direct electrical current (DC) is
passed through a bath of sulphuric acid -- the electrolyte -- while the aluminum
being treated serves as the anode. This produces a clear film of aluminum
oxide on the aluminium's surface.
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Anodizing Process
 Preclean – If necessary, the parts are cleaned to remove tenacious soils or markings.
For example, some ink markings can only be removed by solvent wiping. The
complexing compound frequently used is sodium polyphosphate.
 Mask - Masking is needed to cover areas that don’t require an anodic coating. For
example, masking might be needed if one side of the part is anodized and the other
side conversion coated. Tape and liquid maskants are commonly used products and
specification listed.
 Degrease - The parts are degreased if oil or grease is present. Either aqueous or
vapour degreasing can be used.
 Alkaline Clean - Parts are then alkaline cleaned to remove fingerprints, perspiration,
and other soils picked up during handling.
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Anodizing Process
 Deoxidize – Acids are used to remove oxides (of varying thickness and composition)
from the surfaces of the parts. An aluminum oxide layer that is thin and of uniform
thickness forms naturally on the part surfaces after withdrawal from the deoxidizing
tank. An oxide layer forms when the deoxidized metal is exposed to air or water,
which contains oxygen. The new natural oxide etches evenly during anodizing,
promoting the formation of an anodic coating that has uniform thickness and
appearance.
 Etching - is most often achieved by the use of a warm, 10 % (2.5 mol L-1) sodium
hydroxide solution. It roughness the surface to provide smooth finish. And it also
removes aluminium of about (0 to 65 microns) per side.
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Anodizing Process
 Anodizing – When the current is flowing in the cell the following sequence of events
is believed to occur. Sulphuric acid begins to decompose, the hydrogen ions moving
to the cathode where they are reduced to hydrogen gas:
2H+ + 2e- → H2(g)
 Simultaneously, negatively charged anions, i.e. hydroxide, sulphate and maybe oxide
ions move to the anode. The electrical charge in the circuit causes positively charged
aluminium ions (Al3+) to be generated in the anode and in turn move toward the
cathode. At the anode surface they react with the oxide/hydroxide ions to form
aluminium oxide
Equations of the anode reactions
Al → Al3+ + 3e-
2Al3+ + 3O2- → Al2O3
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 The acid in the electrolyte tries to dissolve
this oxide and produces a porous oxide film
on the aluminium surface. Coating thickness
up to 25 micron is recommended for
external use
 Once the required thickness of anodic film is
obtained, the aluminium is removed from
the electrolyte and rinsed thoroughly to
remove the acids from the pores in the film.
The anodic film produced from sulphuric-
based electrolytes is now ready for
colouring
 Seal/Dry/Paint – After anodizing, the parts
may be dyed, sealed, or simply dried.
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Anodizing Process
 Seal/Dry/Paint – After anodizing, the parts may be dyed, sealed, or simply dried.
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 Protects satellites from the harsh environment of space.
 Used in one of the world's tallest buildings --- the Sears Tower in Chicago,
Illinois.
 Provides attractive, minimum-maintenance, highly durable exteriors, roofs,
curtain walls, ceilings, floors, escalators, lobbies and staircases in skyscrapers
and commercial buildings throughout the world.
 Revolutionized the construction of computer hardware, exhibit displays for
trade shows, scientific instruments, and a constantly expanding array of home
appliances, consumer products, and building materials.
 Considered environmentally safe, producing few, if any, harmful effects on
land, air, or water.
Benefits of Anodized Aluminium
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 The unique anodized finish is the only one in the metals industry that
satisfies each of the factors that must be considered when selecting a
high performance aluminum finish:
 Durability. Most anodized products have an extremely long life span
and offer significant economic advantages through maintenance and
operating savings. Anodizing is a reacted finish that is integrated with
the underlying aluminum for total bonding and unmatched adhesion.
 Colour Stability. Exterior anodic coatings provide good stability to
ultraviolet rays, do not chip or peel, and are easily repeatable.
Benefits of Anodized Aluminium
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 Ease of Maintenance. Scars and wear from fabrication, handling,
installation, frequent surface dirt cleaning and usage are virtually non-
existent. Rinsing or mild soap and water cleaning usually will restore
an anodized surface to its original appearance. Mild abrasive cleaners
can be used for more difficult deposits.
 Aesthetics. Anodizing offers a large increasing number of gloss and
colour alternatives and minimizes or eliminates colour variations.
Unlike other finishes, anodizing allows the aluminum to maintain its
metallic appearance.
Benefits of Anodized Aluminium
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 Cost. A lower initial finishing cost combines with lower maintenance
costs for greater long-term value.
 Health and Safety. Anodizing is a safe process that is not harmful to
human health. An anodized finish is chemically stable, will not
decompose; is non-toxic; and is heat-resistant to the melting point of
aluminum (1,221 degrees F.)
 Since the anodizing process is a reinforcement of a naturally occurring
oxide process, it is non-hazardous and produces no harmful or
dangerous by-products.
Benefits of Anodized Aluminium
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 NASA Space Station trusses and handrails
Designer/Company: Boeing
Medium: Aluminum extrusions with anodized finishes
Trusses: During truss component assembly in space, the on-orbit thermal
environment's effects are challenging. According to Boeing's Manager of
Outboard Truss Elements at Boeing Canoga Park who worked on the project,
direct sunlight and shadowing cause wide swings in temperature; the anodized
surface finish of the extruded aluminium trusses provides superior optical
properties and, because anodized aluminium doesn't get as hot, allows for
better overall thermal control.
Anodized Aluminium Applications
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 Common applications for anodized finishes:
• Structures and architectural categories of all types
• Appliances
• Commercial and residential building products
• Food preparation equipment
• Furniture
• Sporting goods and boats
• Motor vehicle components
• Jewellery and artwork
Anodized Aluminium Applications
Anodized ceiling curtain wall
Anodized Gold Doors
Anodized parts for aesthetic purpose
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 Examples of end products using anodized aluminium:
• Building exteriors, such as storefronts, curtain walls and roofing systems.
• Appliances such as refrigerators, dryers, coffee brewers, ranges,
televisions, microwave equipment.
• Vents, awnings, dust covers, light fixtures, storm doors, window frames,
mailboxes, bathroom accessories, patio covers, and wall switch plates for
buildings.
• Display cases, pans, coolers, and grills for the food industry.
• Tables, beds, files and storage chests for homes and offices.
• Golf carts, boats, and camping/fishing equipment for the leisure industry.
• Hundreds of components for motor vehicles of all kinds such as trim parts,
wheel covers, control panels, and name plates.
• Exterior panels for aerospace vehicles, clocks and electronic products, fire
extinguishers, photo equipment, solar panels, telephones, picture frames,
and bathroom accessories.
• Interior decoration and trim.
Anodized Aluminium Applications
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