Electrolytic Coloring of Hard Anodized Aluminum Alloys

By Leonid Lerner 

INTRODUCTION

The electrolytic coloring (“two-step”) process was first in use in Europe and Asia during the 1960’s (Asada Process), and was introduced in the USA during the 1970’s by ALCAN. The earliest claim to coloring anodized aluminum belongs to Italian scientist Dr. V. Caboni. Dr. Caboni received an Italian patent in 1936 for impregnating aluminum oxide film with finely divided metal from aqueous solutions of the salts of copper, nickel, silver or metals electropositive to aluminum.  The final product of the two-step process is conventional anodized (soft) oxide film with coloring done in a subsequent step, as shown in Fig.1. The coloring is produced by electrochemical action using a proprietary second step electrolyte, which contains the dissolved metal salts of tin, cobalt, nickel or others.

Alternate Current (AC) power is applied to the process tank by means of a counter–electrode. The applied AC power deposits metallic particles (pigment) of salt in the pores of the “first step” aluminum oxide film. The color is developed as a result of the optical effect produced by light scattering of metals, which have been deposited in the pores of oxide film. The electrolytically colored oxide film demonstrates very good weathering and corrosion resistance vs. colored oxide film formed by immersion in organic dye or in an inorganic metal salt. The differences between immersion in organic/inorganic dye and electrolytic coloring are:

  1. In Immersion, the absorption of dye/salts occurs on the top portion of the pores while in two-step coloring metallic colloids deposit at the base of pores. (See Fig.2)
  2. The organic dye may fade or discolor under sunlight while electrolytic coloring has excellent light resistance to fading caused by light.
  3. The organic/inorganic dye will remove if the oxide film partially wears out vs. electrolytic coloring. (See Photos ##1, 2 & 3).

BACKGROUND OF STUDY

According to the recommendations of the Aluminum Association1, the preferred aluminum alloys for “two-step” coloring are AA1XXX, AA3XXX, and AA5XXX & AA6XXX. The aluminum alloys of 2XXX, 7XXX series and casting are not recommended for electro coloring because of poor results.

The purpose of our study was:

  1. To find whether the low voltage DC+AC Power Supply2, used as the first step procedure in “two-step” electrolytic coloring, would demonstrate better results in color acceptance and hardness on different series of aluminum alloys, especially 2XXX and 7XXX series.
  2. To compare the TaberTM abrasion resistance test between the two processes.
  3. To determine the maximum time break between first and second step in the “two-step” coloring process and compare between the two processes.

In our studies we employed 2 different power supplies. For conventional anodizing as first step we used a DC Power Supply (0-15 VDC) and a DC+AC Power Supply2 (0-12VDC). (See Fig.1 & Fig.3) All samples were 4”x 4” squares (32 sq. in) of 2024-T3, 6061-T6 & 7075-T6 aluminum alloys. Prior to this study some “first step” anodized samples, including 2024-T6, were sent to Clariant laboratory for electrolytic coloring. The time break between the “first step” (DC+AC anodizing) and “second step” (electrolytic coloring) was approximately 72 hours. All samples show a good color acceptance. (See Tables 4 & Photos ##6, 7, & 9)

The electrolytic coloring as “second step” was done using an AC power supply (60 Hz) with a maximum constant voltage of 20VAC. The counter–electrode in the plastic process tank was made from stainless steel.  The bus bar was made from aluminum 6061-T6. A filtration pump in conjunction with a 10-micron filter was installed. The pre-mix electrolyte was SandocolorTM supplied by Clariant Corporation. In most cases the coloring cycle was 10 minutes. All samples were sealed for 5 minutes in (185ºF) Nickel Acetate solution.

TABLES

Table 1 displays the different data related to the first step anodizing with the same electrolytic coloring techniques for all samples in the experiment.

  • Column#1 is the name of the process and identifies the part number of each sample.  The power supplies used were a conventional DC Voltage and a proprietary Low Voltage DC + AC
  • Column #2 contains the electrolyte composition.
  • Column #3 contains the electrolyte concentration in grams per  liter.
  • Column #4 contains the temperature of electrolyte in º C during the process.
  • Column #5 contains the temperature of electrolyte in º F during the process.
  • Column #6 shows the time of the hard coating process in Minutes.
  • Column #7 shows the maximum voltage used in the process.
  • Column #8 shows the maximum current density (Amps/sq. ft.) reached during the process.
  • Column #9 shows the thickness of the coating in mils, determined by averaging at least nine measurements from different spots on the samples.
  • Column #10 contains the type of aluminum alloy.

RESULTS AND ANALYSIS

Electrolytic Dyeability

Our study of samples in Photo # 11 shows:

  • Different intensity of dark color on 6061-T6 electrolytic colored aluminum alloys between Conventional DC anodizing sample (CA-2) and DC+AC hard anodizing sample (2S#9)
  • The DC+AC process visually demonstrates deep dark and uniform appearance of color.

Time Break Between First and Second step in “two-step” Electrolytic Coloring Processes

72 hours after the different anodizing processes (DC and DC+AC), the samples (2024-T3, 6061-T6 & 7075-T6 aluminum alloy) were electrolytically colored during 10minutes. The samples run by DC+AC process demonstrate very promising results on absorption and uniformity of color after spending long hours in the open air as compared to the conventional DC process, which showed the different intensity, uniformity and poor dyeability. (See Photos #4 & 5).

Taber Abrasion Tests

Mil-A-8625F specifies that the anodic coating shall have a maximum wear index of 3.5 mg/1000 cycles (or 35 milligrams of weight loss per 10 thousand cycles on aluminum alloys having a copper content of 2 percent or higher) and 1.5 mg/1000 cycles (or 15 milligrams of weight loss per 10 thousand cycles on other aluminum alloys). We cannot compare the abrasion resistance of oxide films after electrolytic coloring to Military Specification numbers, but we try to compare results between the two processes (DC & DC+AC).

Our study of Tables 2 & 3 shows:

  • The abrasion results produced on 2024-T3, 6061-T6 & 7075-T6 aluminum alloys after the DC+AC “first step” process surpassed the results of the Conventional Anodizing DC process.
  • The Wear Index results of coatings produced in DC+AC anodizing process exceed coatings produced by Conventional DC process.

Bibliography:

  1. 1993 PF Directory “Anodizing Aluminum” by J.G. Hecker, Jr.
  2. 1972 Surface “Treatment and Finishing of Aluminum” by S.Wernick & R.Pinner

About Mike Sung

Chemical Engineer
This entry was posted in Aluminum Anodizing - Type II, Aluminum Anodizing - Type III, Newsletters, Post Treatment. Bookmark the permalink.

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