What is Anodizing?
A coating of aluminum oxide is grown from the aluminum by passing an electrical current through an acid electrolyte bath in which the aluminum is immersed. The coating thickness and surface characteristics are tightly controlled to meet end product specifications.
It is an electrochemical process that thickens and toughens the naturally occurring protective oxide. The resulting finish, depending on the process, is the second hardest substance known to man, second only to the diamond. The anodic coating is part of the metal, but has a porous structure which allows secondary infusions, (i.e. organic and inorganic coloring, lubricity aids, etc.)
What is the purpose of Anodizing?
The purpose of anodizing is to form a layer of aluminum oxide that will protect the aluminum beneath it. The aluminum oxide layer has much higher corrosion and abrasion resistance than aluminum. There are some types of anodizing that produce a porous oxide layer that can be colored with organic dyes or metallic pigments giving the aluminum a decorative and protective finish.
Anodizing will protect the aluminum parts by making the surface much harder than natural aluminum. Aluminum oxide is grown out of the surface during anodizing and then becomes aluminum hydrate that is extremely hard. The porous nature of the anodized layer allows the product to be dyed any color that is required. Type II anodizing (room temperature) gives an anodized layer of .0002″ to .001″ (half which is grown into the surface and half out of the surface). Parts anodized will become slightly larger by about .0005″.
Type III (hard coat) anodizing is done at much colder temperatures and at higher current densities and can reach thick nesses of .002″. Type III anodized surfaces can typically only be dyed black or dark green due to the denser pore size.
“Hard-anodization is an electro-chemical process that hardens the surface of the aluminum. (Hard-anodized aluminum is 30% harder than stainless steel). During hard-anodization, aluminum is submerged in an acid bath and then subjected to electrical charges. The result is a chemical reaction where the surface of the aluminum combines with oxygen to become aluminum oxide. This reaction is also known as oxidation, a process which occurs spontaneously in nature. Hard-anodization is actually controlled, accelerated oxidation.
They use this process on aluminum used on satellites to protect them from the harsh environment of space! Hard-anodizing leaves a virtually nonporous surface. Works great on our “non-stick” pots, pans, BBQ grilles, etc, not on something we are trying to paint. Those of us who cook with a fork, know how durable the skillets are. The strange thing is, the dishwasher, of all things, will ruin a “teflon coated non stick pan”. (before you ask, I do not know how they add the Teflon in the hard-anodizing process, you need to ask the white coats that one. I cut, weld and paint, but not in a laboratory.)
can make the surface of aluminum 30-35 points from being as hard as a diamond. You can’t cut it with a file. It is about 65-70 Rockwell hard. That is one of the reasons it is used in so many critical applications,”
It is less expensive than stainless steel. It machines twice as fast and doesn’t weigh as much. This saves on shipping costs. “And it is more durable than hard chromium plating because the coating is integral to the part, not just on the part
Anodizing is a process that converts aluminum to its oxide. The oxide is thicker than the aluminum that is consumed, which means the dimension of the anodized part changes. The amount of change will depend on the anodizing process conditions (temperature, current density, etc.) and alloy. Under nominal Type II anodizing conditions, the rule of thumb is 2/3 in 1/3 out; for example, a coating that is 0.6 mil thick will have consumed 0.4 mil of aluminum. Under hard coat (Type III) anodizing conditions the ratio changes to ½ in ½ out. Keep in mind that when calculating the shrinkage of a hole, you must double the amounts given because a hole has two sides, for example, the hole diameter reduction for a 0.6 mil Type II coating would be (1/3 of 0.6 mil) x 2 = 0.4 mil. Another example to consider is the hole reduction of a 1.5 mil Type III coating (1/2 of 1.5mil) x 2 = 1.5 mil.
|Figure 1. Hard-anodizing buildup and penetration|
Hard coat anodizing should not be confused with sulfuric anodizing, which places a very thin coating of aluminum oxide on the surface of the aluminum. Hard anodizing requires an electrolysis process that produces a dense layer of aluminum oxide both on and in the aluminum surface. The thickness of this hard-anodized coating ranges from 1-3 mils or more. (Thicker coatings may burn, crack or have a powdery appearance.) The coating thickness is a function of current density, time in solution, temperature, the composition of the alloy and the solution itself, which is supplied by SIC Technologies.
Properties of Hard Anodic Oxide Coatings
Hardness. Wear characteristics compare favorably with hard tool steel under low loads. Microhardness tests on hard-anodized aluminum typically give values of 500-530VPN. This number refers to the weight required for a diamond indenter to produce an indentation in the coating.
Microhardness is nearly independent of coating thickness, up to 1.5 mils. An aluminum coating with a hardness value of 450VPN exhibits the same wear characteristics as tool steel that has twice the microhardness of aluminum hard coatings.
Wear Resistance. Wear resistance is measured by the weight of an abrasive required for a controlled pressure jet to blast through the coating. Hard coatings exhibit a wear resistance greater than 10 times that of ordinary anodized aluminum. In the standard Taber abrasion test, hard-anodized aluminum exhibited only half the wear that cyanide case hardened steel showed after 50,000 cycles. When the test was extended to 100,000 cycles, the surfaces of 4130 steel, mild steel and chrome hardened steel showed greater wear than hard-anodized aluminum (see Fig. 2).
Appearance. As coating thickness increases, color varies from colorless to light brown on pure aluminum at 1-5 mils thickness. Alloys vary in color depending on alloy composition, usually ranging from tan to jet-black for 1-3 mils thickness.
Uniformity. In the hard anodizing process, the coating follows the contour of the part. Hard coat builds up in holes to the same thickness as on the part’s exterior. This allows for precise control of very close tolerances in critical applications.
|Diecast part before (above) and after (below) treatment in the Metabrite process. The process brings contaminants to the surface where they can be mechanically removed, leaving a shiny, bright part.|
Heat Resistance. The inert nature of the anodic coating provides excellent heat resistance. Hard-anodized parts show no effect after short exposure to temperatures as high as 2,000C. Although coating thickness is not a major factor in heat resistance, exposure to direct flame has shown that thicker coatings provide a longer life for the exposed part.Surface roughness, which increases depending on individual applications, alloy and coating thickness can be honed smooth. Due to excellent throwing power of the process, it can effectively coat unusually shaped parts.
Thermal Properties. Hard-anodized coatings exhibit low thermal conductivity and expansion. Pistons for internal combustion engines are hard anodized to minimize the amount of thermal expansion in relation to possible thermal expansion of the engine block.
Electrical Properties. Aluminum oxide and its alloys can be used as electrical insulators. The high temperature stability of the coating permits operation up to 500C. Anodic coatings typically exhibit a voltage breakdown of 2,000-3,000 v/mil, a dielectric constant of 7.4-7.6 and resistivity from 1014 ohm cm to 200C. This resistivity is of the same order of magnitude as glass and porcelain.
These properties make the coating excellent insulation for mounting electronic components.
Lubricity. Any hard-anodized surface has a high degree of lubricity. In certain applications, hard-anodized surfaces can run against other hard-anodized surfaces without lubrication. Impregnated with solid lubricants such as PTFE, hard-anodized aluminum surfaces have an even lower coefficient of friction.
FWA offers vacuum impregnation of diecastings and sand castings. Vacuum impregnation is a way of sealing the porosity in the casting, since untreated porosity can lead to fluid and gas leaks, plating defects, reduced machine tool life and a high scrap ratio.
During vacuum impregnation, a vacuum system is used to draw air and volatile materials from the castings’ porosity. Once the vacuum is drawn, a liquid sealant is drawn into the pores of the casting, creating an airtight seal. Impregnation is the fastest and most effective method of eliminating porosity and interior corrosion in metal castings, forgings, powdered metal parts and plastics. FWA claims it can recover up to 95% of rejects using this method. The company has both dry (vacuum and pressure) and wet (vacuum only) processes.
The answer to your problem can be applied across any number of situations involving the adhesion of bonding agents or organic coatings (paints) over anodized aluminum. Anodizing can be an excellent surface for these applications, but the anodizing must be done with this in mind. The solution to your problem involves the method of rinsing and sealing of the anodic oxide after anodizing.
It is quite common to seal anodic coatings on so-called “proprietary” solutions that contain certain wetting agents (surfactants). This is done primarily to help prevent the formation of smut on the surface of the part. Smut detracts from the appearance of the product and makes it look dirty or hazy. If it is known that the anodic coating is to be used as a base for paint, or that adhesives are going to be used (caulking around windows in an architectural application, for example), the anodized parts may be sealed in either near-boiling deionized (DI) water or a dilute solution of commercially available nickel acetate. Sealing with room temperature nickel fluoride is also acceptable in this case. All three of these methods are free of surfactants. It also helps if the parts can be thoroughly rinsed in clean DI water before and after the sealing step. This will give a clean, “non-slippery” surface (no wetting agents) to which paint and most adhesives will bond. (Anodized aluminum that is to be painted is sometimes left unsealed altogether.) It would also be advisable to prime the anodized surface prior to applying the adhesive by wiping with a highly volatile solvent such as methyl ethyl ketone (MEK) or acetone to remove all dirt, fingerprints, and other possible contaminants.
|Figure 2. Taber abrasion test comparing the wear resistance of various coatings.|