OverviewAnodizing is the process of increasing aluminum’s strengthand corrosion resistance by forming a layer of oxide on the surface.Because aluminum is a widely-used metal, anodizing finds usein many industries, such as the aerospace and automotive industries due to thelight weight of aluminum. Construction uses aluminum in places like roofs andtrim work, while furniture, food preparation, and even art make use of it toproduce objects such as tables, appliances, display cases, and jewelry.ProcessLarge-scale anodizing began in 1923, when seaplane partswere coated to increase corrosion resistance in a process patented by Bengoughand Stuart.
This process was upgraded in 1927 by Gower and O’Brien to usesulfuric rather than chromic acid, and this technique has become the standardone in use today. In the 1950s, the Japanese discovered a version of theprocess that used oxalic acid, which had the main benefit of producing a hardcoating faster. The chromic acid anodizing process is used to give good corrosionresistance with a relatively thin layer. This coating is softer than thecoating produced by other processes and is easily damaged. There are twosubtypes of this; one ramps the voltage up to 40 volts, while the other uses amaximum voltage of 20 to 22 volts. The sulfuric acid process has twosubvarieties as well. The first one, type II, is used for decorative orprotective purposes, and the second is used in the aerospace industry.Due to concerns over hexavalent chromium’s toxcitiy, chromicacid is being phased out of the electrolytic portion of anodizing operations.
The replacement is boric sulfuric acid, which produces a coating with superiorpaint adhesive ability to a coating produced by the chromic acid process. Thisprocess is voltage controlled and ramps up to 15 volts.Anodizing is an electric process. The aluminum acts as ananode in an electrolytic bath. Direct current, usually between 15 and 20 voltsis run through the bath, producing hydrogen at the cathode and oxygen at theanode, the aluminum part, converting the surface into an oxide. The acid andoxygen actions combine to form nanopores in the oxide surface, allowing theoxidation process to penetrate farther into the metal and thicken the coating.These pores are later sealed, sometimes with a dye, to prevent futurecorrosion. The coating thickness varies based on use, from 0.
5 micrometers to150 micrometers. Sometimes the anodizing is done combined with chromateconversion. The anodizing increases the wear resistance of the piece, while thechromate conversion gives it electrical conductivity.This process takes place in a long, multi-stage tank. Theparts to be anodized are placed on a rack hung that runs along a track. Theyare washed and rinsed in various baths to remove potential contaminants.
Next,they are dipped in the anodizing bath and left there for some time while thecurrent is applied. At this stage, it is critical to control factors such aspH, electrolyte strength, current, and temperature for an even coat. Afterthat, they are rinsed, sealed, and dried before inspection. BenefitsAn anodized coating has benefits over paints and powdercoats.
It is thinner and more durable, lasting longer than other options. Itcosts less and can be recycled, unlike paint and powder coating. ConsiderationsWhen searching for an anodizer, consider what you will beusing the part for. Chromate anodizing coatings are self-healing to an extentbut lack the durability of the sulfuric acid coatings. It works best when thereare few alloying elements in the aluminum. Sulfuric acid anodizing offers thebest possibility for dying the final product.
If a thick coating is needed,such as for aerospace applications, the Type III process is what is used. Findan anodizer who is willing to examine your parts and listen to what your needsare so that the most useful procedure occurs. There is a partial list above foryou to examine. Some anodizing work has been outsourced, affecting time takento receive parts.Anodizing TypesChromic Acid (Procedure I) – A chromic acid electrolyte isused to conduct the charge; being phased out due to health and environmentalconcernsSulfuric Acid (Procedures II and III) – Produces coatingsthat are harder than those made using chromic acidBoric-Sulfuric Acid – Replacement procedure for chromic acidanodizingOxalic acid – Sulfuric acid – An alternative process forproducing hardcoats, used more in Japan and GermanyAnodizing termsAbrasion – The process of using friction to grind andwear away at a surface. Activation – Changing a metal surface into a chemically-activestate. Alloy – A substance with metallic properties composed of two or morechemical elements.
At least one element must be metal. Alkaline Etch – Cleaning step used to give parts a mattefinish and remove any anodic coating in preparation for anodizingAnodizing Sheet – A sheet that has suitablemetallurgical characteristics and surface quality for the development ofprotective and decorative films by anodic oxidation processes. Bath – The chemical environment in which the anodizing takes place. Brazing – Joining metals together by flowing a layer of molten fillermetal between them. Chemical Film – The application of chromate conversion coating onaluminum. Chromate conversion – Passivation technique that retains ametal’s conductivityCorrosion – The gradual chemical or electrochemicalwearing of a surface.
Also, this is the deterioration due to environmentalforces. Cryolite – A white mineral used in making aluminum Deburring – The removal of burrs, sharp edges and fins by mechanical,chemical or electrochemical means. Edging – Dressing metal edges by rolling, filling and drawing.Electrolyte – Solution that carries an electrical chargeHardener – An alloy of aluminum and at least one othermetal that is used to make additions to molten aluminum.
Impurities – Undesirable elements in aluminum compounds. Inhibitor – A substance used to reduce the rate of a chemical orelectrochemical reaction, commonly corrosion or pickling. Melting Point – The minimum temperature at which a metal will become aliquid. Passivation – Changing a chemically active metal into a passivestate.
Plating – The process of laying a thin coat of metal on another. Pores – Microscopic openings in an anodized surface, leading to amicroscopic tube. Although porous, the anodized surface is very dense andhard. Rack – The electrically-conductive device used to hold the parts to beanodized as they are lowered into the bath. Racks are made in different sizes,types and shapes to hold the large variety of parts and products encountered inanodizing. Refined Aluminum – Aluminum that is in a very pure state.
Smelt – Fusing or melting ore in order to extract or refine the metal itcontains. Striation – Longitudinal lines that are not uniform, caused by unevencoating. Ultimate Strength – The maximum stress a material can withstand.