Problems with Color of Hardcoat Anodize

A. Stan,

It is a tough job.

What kind of material are the parts you want to anodize? Hard coatings on aluminum alloys usually have intrinsic colors, so they are generally dyed in black color. If you want a medium grey color, you need a clear hard coating using special process with proprietary additives. Although raising temperature might help a little bit, it will reduce hardness and could cause burning problem, especially for 2000 series and 7000 series.

Ling

A. The factors you mention are important and along with tank temperature, part temperature, and agitation are all inter-dependent. Note the term "part temperature". Agitation must be uniform throughout the tank to maintain the parts at the same temperature. This is especially true if you are using high current density, e.g., 36 asf. Hopefully your ramp up is automatic.

You didn't mention the alloy but there is one factor you may or may not have any control over: alloying elements affect color, and the content of the alloying elements of a particular alloy can vary quite a bit. For example, the silicon content of 6061 is listed as 0.4-0.8%. Silicon is one element responsible for turning hardcoat grey. We could also talk about the importance of consistent temper, but you get the idea.

Those of us who have been involved in color anodizing for many years learned very early to establish not one color standard, but two- an acceptable light and an acceptable dark standard. AAMA has established this as standard practice in their specifications. Considering all the variables you need to control, and the likelihood for color variation, you may need to establish this practice.

Phil Johnson
- Madison Heights, Michigan

A. One of the hardcoat anodize processes we do here produces only a slight, if any, color change on 6061. The thinner the coating, the less the color change.

? Why does a clear coating have any color at all?

Chris Jurey, Past-President IHAA
Luke Engineering & Mfg. Co. Inc.

International Hard Anodizing Association - Wadsworth, Ohio

A. Lucy,

One possible problem is that not all 7075 is created equal. Each lot probably has some variances (allowable) in the alloying constituents (I've griped about the inconsistency of American-made AL in this forum before). Unless all of your 7075 came from the same LOT (not vendor), some variances will probably continue to occur. Have you thought about having these parts dyed black? That way, at least all of your parts will be consistent. Other possibilities include varying current densities, anodizing bath temperatures, or different methods of sealing. All of these have a bearing on the final appearance of the coating.

If you did a search on type 7075 on this site, you will note that its not the easiest of alloys to anodize, it can be quite tricky.

A. Hard coat color will vary with very minor differences in alloy, temperature, current density (How accurate is the anodizers surface area measurement? I have a 1956 paper which has the very best gimmick for always getting the same current density), dissolved aluminum, rack contacts, time, thickness, and on and on. Tighten up all parameters.

A. 7075 after hardcoating is a gold to a milky gold green.

All color after hardcoating is 100% dependent on alloying materials.

6061-T6 will give you gray color.

A. Your anodizer is giving you worst case, but it is well established that dye and seal will reduce the hardness and the wear resistance of hard coat. I question your stated thickness of the hardcoat. Anything over 0.003 per side is questionable or requires extremely high voltage. Do a search of this site for hard anodizing and hardcoat. It will be educational.
Teflon will reduce the wear on other parts, but it will wear over time when compared to the hardcoat.

A. Scott

If you hard anodize parts from different heat lots of material, different 'colors' will be a possibility. Dyeing the parts black should ensure you get uniform color.

To determine if the dyeing steps will degrade the quality of your product, runs tests on both dyed and non-dyed parts, that simulates the conditions for which the hard anodize is required, be it wear resistance, hardness, or adhesion for your coatings.

Your hardcoater may be sealing the dyed hard anodize, which will soften the coating but help prevent fading of the color/dye.

Thanks. You're right to question my thickness. Typo on my part. Need to shift the decimal point. It was 0.0010 - .0012" thickness.

A. The color of 50 micron (0.002 inch) thick hard coat anodizing can vary in color as functions of alloy, anodizing temperature, free acid, dissolved aluminum, current density, additive, heat treatment, work hardening, and other causes. 6061 would be dark gray, 2000's would be darker almost black at 180 gm/L, 32 °F, 24 ASF. Both alloys would be lighter in shade with higher free acid, higher temp, and other causes. Change any variable and the color shade changes but the generally accepted industry definition of "hard coat" does not require a color requirement. If he wants a color, then charge extra and be sure his alloy and heat treatment does not change and your parameters do not change.

A. Hi, in my job I look at many similar cases. These chromatic aberrations are often caused by material problems, e.g., material inhomogeneities or use of different batches. 6064 is a free cutting alloy with Bi and Pb, inhomogeneities or different compositions can affect the color. But anodization errors are not excluded. For a clear clarification, additional investigations are often necessary. Also important are the material certificates and measurements. According to the pictures, only an initial assessment is possible.

1) Probably no error from the material or surface treatment.
An influence of the machining is possible.
e.g., insufficient tools can solidify the surface easily. As a result, the layer thickness is thinner in these areas and the color brighter.
A investigation (metallography) could help further.

2) Possible is a different composition. Either different batches or inhomogeneity within the batch.
Analysis and microstructure could be investigated for further clarification.

3. Maybe are inhomogeneities of the chipbreaker (Bi, Pb)
Also, an investigation for the clarification is necessary (metallography).

Differences can often be detected by measuring the electrical conductivity.

I would measure the conductance and layer thicknesses at the different positions and check the material certificate (different batches?) so a better analysis is possible.

A. The color variation could be due to contact UNreliability where some parts get more current thru the sharp contact points. Send us digital pictures of the racking and pay attention to the area of the sharp contacts.

With so many parts in the tank, is there a pattern related to the distance from the cathodes, if so, increase time 30 minutes and see if the color is more uniform.

If I understand your coating thickness, if it as low as you state, then contact resistance is even more probable.