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Aluminum Finishes:


Aluminum finishes can be broken down into several groups of finish types, all of which we will attempt to cover as we complete the work on our site.

The primary groups could be termed:

  1. Anodized Finishes

  2. Painted Finishes

  3. “Pure” Protective / Bonding Finishes

Each of these can be further divided into several sub groups.

When thinking about finishing your aluminum products, you must decide whether your key goal is appearance or surface (corrosion) protection, and/or, in what ratios of the combination of the two. Once this decision has been made, you can more easily decide between the options open to you.


Anodizing Overview:


As an example, the anodized finishes can be categorized by the specific electro-chemical processes employed (as an example at least two different acids can be used as the “base chemical” of the process, by the color to be viewed on the surface, by the type and extent of pretreatment given the metal before anodizing, by the thickness of the coating, and other specifications.

Before we get too deeply into the details of some of these groups and specifications, let’s discuss some of the basics.

Typically, most metal finishes are applied to either protect the metal from corrosion, or to make it look “better” to someone, for cosmetic and / or marketing purposes. In many cases, the combination of the two requirements will determine the process and finishing specifications used, and most of these combinations will, to some extent, provide some level of benefit in both areas.

The anodizing process is one of attaching aluminum parts (usually, though aluminum is not the only metal which can be anodized) to racks, (to which they are solidly clamped, and electrically connected). These racks are then dipped in a number of tanks of acids, clean water, dyes, etc. for varying lengths of time, and with varying amperages of electricity flowing from the tank walls, through the liquids, into the parts, and then to the racks and back out of the tanks. In some tanks there is no electrical current flow, depending on the content of the tanks.

When thinking of anodizing, one usually thinks of the following attributes (not necessarily in this order): Color, Intensity (darkness) of color, “Shininess”, Surface texture, Surface hardness, and Corrosion protection.

Let’s talk a bit about each of these, in turn:

Anodizing Colors:  

The theoretical range of colors rivals the rainbow (and, maybe, beyond). However, the range of colors available from a particular anodizer is usually fairly limited (often severely so). This is particularly true when one is discussing anodizing long and / or otherwise large items, such as long aluminum extrusions or large fabricated parts. In these cases, the most commonly available “color” is “Clear”, though it is often called “Silver” by those not “in the business”.

Anodizing plants serving the architectural metals markets nearly all have “Bronze” (a brown, usually with some green and / or yellow tones), and usually offer a “Black”. This is often not a true black, but usually is simply a very dark brown (or “Bronze”). It is usually close enough to fool the eye into thinking it is black. A “true” black is also possible, but is often not a very stable color in exterior applications, due to fading. It is usually also a fair amount more expensive than the extra dark bronze version of “Black”.

When one visits “small parts” anodizers, one can usually find greens, blues, reds, maroons, purples, yellows, “golds”, and a host of other “primary” colors.

All of these anodized colors can be varied with regard to their intensity, darkness, or “depth” of color, from the very lightest mist of a hue, to, in most cases, so dark one can barely tell what the color is intended to be. The intensity of the color is largely controlled by the time the material is in the dye tanks, though the temperature of the bath and current flows also have their effect.

Because these factors have so much to do with the intensity of the colors, the very light shades are hard to “match”. One tank load may get a very small additional period of time in the tanks than another, causing that load of metal to have a noticeably different depth of hue than the previous or next batch of metal. For this reason, it’s usually best to try to stay away form the lighter hues, if possible.

“Shininess” and Surface Texture:

“Shininess” and surface texture are often interrelated, in that surface texture will heavily impact ones impression of “shininess”.

Surface texture, itself, is predominately determined by three or four factors. When discussing extrusions, the first to come to the fore is a phenomenon called “die lines”. These can be best thought of as fine (often nearly microscopic) lines, or scratches in the surface of the metal, largely caused by roughness of the die, or foreign materials stuck to it during the extrusion process. These “die lines” can often be sufficiently defined that one can literally feel them with their finger nails. They can virtually always be seen with the naked eye. Polishing the die before the extrusion process is started will usually reduce these lines to a bare minimum, often where they cannot be felt, though their microscopic cousins can be seen in the surface of the new shape as an interesting “sheen”. The longer the “caustic etch” process, which is one of the first steps of the anodizing process, the less likely the die lines will be seen after the material is anodized, though this cannot be used when a “Bright Dip” finish is being used.

For the smoothest, shiniest anodized surface finish (as in the case of a high quality “Bright Dip” finish, which we will discuss later), one would insure that a well polished die is used, and then, after the extruding processes are completed, literally mechanically polish or buff the surface, blending the remainder of the surface imperfections as smooth as possible.

Another way the smoothing process can often be accomplished is to use a “deep etch”, where the surface of the metal is literally eaten away by an extra long “caustic etch” as a part of the anodizing process. It will give a fairly even surface with many thousands of “pock marks” per square inch in it.

This is fine for a good surface finish on most parts, but can be counter-productive if a “bright dip” finish is desired. In that case, the mechanical polishing process will probably have to be used. After the mechanical finishing, a special combination of electro-chemical chemicals and processes will provide the true “bright dip” finish, which can be almost smooth enough to give a mirror like look to the surface. Because of the various processes used, it is a fairly expensive finish, however.

For the preponderance of anodized finishes, a fairly well polished die, a medium caustic etch, and a “normal” anodizing process will provide a very acceptable anodized finish for most applications.

You’ve no doubt seen aluminum patio doors, storefront doors, curtain walls, etc., which were either clear anodized or anodized with a “bronze” dye, making them look brown or dark brown. These are typical anodized finishes, in the two most common basic colors.

Surface Hardness and Protection:  

Barring a surface that has been sand or shot blasted, etc. for special effects prior to anodizing, we’ve pretty well covered all of the main criterion other than surface hardness and surface protection, which go hand–in–hand.

There are two common anodizing processes – “Two Step”, which is somewhat “soft” and “Hardcoat”, which, as is implied, is a harder surface. Hardness is relative, however, as both finishes are second in hardness only to a diamond. They are the same chemically as aluminum oxide and carborundum, and just as hard.

The variations in apparent hardness are largely based on the thickness of the coating, which are often designated more by “Class” than the scientific hardness scales in the US . “Class III” is the thinnest normal anodizing thickness, and is about .0003” thick. “Class II” is the next most expensive, thicker, and “harder”, and is about .0004” thick. “Class I” is the most expensive, thickest, and “hardest”. It is about .0007” thick.

All of this, is, of course, based on conversational designations, rather than scientific criterion. All of these “Classes” are merely ranges of anodic coating thisknesses, and the primary determinating factor in creating the various Classes is the time “in the tank” where the oxide coating is created.

Accordingly, coatings of less than half the thickness and durability of “Class III”, and others of over 50% thicker than “Class I” are also possible. The costs of creating these finishes will vary from the “Standard” thicknesses.

The thicker the anodizing thickness, the greater the corrosion protection, and the longer the protection is likely to last in adverse conditions, such as a salt air, salt water, or other or corrosive environment.

The corrosion resistance needs of the project, its environment and the use of the parts should determine the thickness, or “Class” of anodizing to be used. The different colors are merely for cosmetic purposes and have nothing to do with the levels of protection offered.

Painted Finishes:  

Contrary to anodized finishes, painted finishes provide an almost infinite range of available colors. The range of anodized finishes which may be available to you is limited by the hues “stocked” by the chosen anodizer, and varied by the shade of that hue (the “time in tank” issue). Contrary to this, virtually any painter can provide virtually any color of the rainbow (and many more not normally seen in the rainbow). If you can provide them with a “color chip” (sample), they can probably match it.

There is argument as to whether anodizing or the better painted finishes provide the best protection against corrosion, with people taking opposing sides and being vehement about their choice. This writer is in the “quality anodizing is better than quality painting” camp, but there are others who argue the opposing side with equal vehemence. There is virtually no argument about the color range option question, however.

There are two primary types of painted finishes used on aluminum – Liquid and Powder. As the terms imply, liquid paints are put on wet (and usually baked to dry them). Powder paints are applied in a powder form and baked to melt the particles together to form a (hopefully) impermeable surface.

Powder coat painted finishes are usually much thicker than liquid painted finishes, and are probably somewhat more resistant to limited abrasion, though their surfaces are probably only very little (if any) harder, and, thereby probably do not provide much (if any) significant abrasion resistance. There are, however, some special powder coat painting formulations which are specifically formulated to have extra mar resistance.

One of the problems often found with powder coated painted surfaces is that the finished surface is often somewhat “stippled” in appearance, I.E. having a surface similar to that of an orange, with “pock marks” in it, though they are somewhat more like dimples. Some find this appearance to be unsightly, though it is not usually visible at a distance of over about 4-6 feet. Conversely, painted finishes, because the paint is fairly cohesive (as well as adhesive), and is put on in the liquid form (rather than comparatively large granules), are usually considerably smoother looking. As a case in point, the writer is building a large sunroom onto his house during the period of writing this, and has chosen a high quality white liquid paint for the finish. Conversely, he might well choose a powder coat painted finish, partly BECAUSE of the irregular surface finish on an industrial part, as it might show abrasive damage to a lesser extent.

Aluminum is not an inherently chemically stable element. It has a high affinity to join with oxygen. It begins to form a “skin coat” of aluminum oxide immediately after any processing. As an example, one can take an aluminum extrusion, which was produced less than a day before, wipe one’s hand along the surface, and see a noticeable dark gray or black smudge on it. This is one of the three aluminum – oxygen compounds, and is a corroded form of aluminum.

Interestingly enough, the aluminum surface is somewhat self healing, in that the coating (which is extremely thin) effectively seals the aluminum off from the atmosphere in a few days, thereby preventing further corrosion of that type.

As an aside, you’re no doubt familiar with “carborundum” or “aluminum oxide” grinding wheels or sanding paper grits. These are based on another form of aluminum – oxygen compound, one which is chemically stable, and, which, is, in fact, is chemically the same as an anodized aluminum finish.

Getting back to our major train of thought, however, the black “smut” we discussed is a problem when you want to paint the aluminum. It must be washed off immediately before painting, or the paint will stick to it, rather than the aluminum, and, therefor, easily flake off later on. Accordingly, the painter’s pretreatment processes are of the utmost importance. The smut need not be considered with anodized finishes, as the anodizing process itself will remove it.

Another factor in one’s choice of which finish to use may well be what finish is offered by the extruder, casting plant, fabricator, etc. producing the parts or products. It is often quite logical to use their services for the finishing, as they will usually offer the painting on a lower cost per square foot basis than another firm. They are already making a profit on their other services. Therefor, they don’t need to charge as much for the painting, as they can afford to make less on it. Additionally, going this way cuts the total wrapping / packaging costs in half, and eliminates the transportation costs between the two facilities, thereby offering an overall lower cost finished product cost.

In the interests of trying to get this site as complete as possible in a wide variety of areas, we will not address the different types of paint formulations in detail. Let it suffice to say, for the moment, that there are a number of different formulation bases, such as enamels, acrylics, polyesters, and the Kynar type products. Not all are available in both types of finishes. Some of them may only carry a year or two of warranty, and others will be warranted for twenty or more years, under normal circumstances.  

“Pure” Protective / Bonding Finishes

There are several finishes, which are typically used only for either protection against corrosion and the elements, or as a good bonding surface for painted finishes, or a combination of the two.

A couple of key examples of these finishes are the alodines and chromates. If you’ve ever had an opportunity to look into the “unoccupiable” areas of nearly any aluminum aircraft, you may remember seeing a strange semi-transparent looking greenish or yellowish surface. You probably saw a chromate finish at that time.

Please feel free to contact us with any technical questions that we have not covered here.