Batch & Coil Anodizing

BATCH & COIL ANODIZING

HELPFUL NOTES ABOUT SAF BATCH & COIL ANODIZING

Both batch and coil anodizing share five carefully controlled, calibrated, quality-tested anodizing stages:

1. Cleaning. Alkaline and/or acid cleaners remove grease, and surface dirt.
2. Pre-Treatment.
   1. Etching. An appealing matte surface finish is created with hot solutions of sodium hydroxide to remove minor surface imperfections. A thin layer of aluminum is removed to create a matte or dull finish.
   2. Brightening. A near mirror finish is created with a concentrated mixture of phosphoric and nitric acids which chemically smooth the aluminum’s surface.
3. Anodizing. The anodic film is built and combined with the metal 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.
4. Coloring. Coloring is achieved in one of four ways:
   1. Electrolytic Coloring (The two-step-method)-After anodizing, the metal is immersed in a bath containing an inorganic metal salt. Current is applied which deposits the metal salts in the base of the pores. The resulting color is dependent on the metal used and the processing conditions (the range of colors can be expanded by overdyeing with organic dyes). Electrolyte colors can be specified from any AAAC member. Commonly used metals include tin, cobalt, nickle and copper. This process offers color versatility and the most technically advanced coloring quality.
   2. Intergal Coloring-This so-called one-step process combines anodizing and coloring to simultaneously form and color the oxide cell wall in bronze and black shades while more abrasive resistant than conventional anodizing. It is the most expensive process since it requires significantly more electrical power.
   3. Organic Dyeing-The organic dyeing process produces a wide variety of colors. These dyes offer vibrant colors with intensities that cannot be matched by any other paint system in the market.They can also provide excellent weather-fastness and light-fastness. Many structures built with these finishes have lasted more than 20 years. The color range can be broadened by over-dyeing the electrolytic colors with the organic dyes for a wider variety of colors and shades. This method is relatively inexpensive and involves the least amount of initial capital of any other coloring process.
   4. Interference Coloring-An additional coloring procedure, recently in production, involves modification of the pore structure produced in sulfuric acid. Pore enlargement occurs at the base of the pore. Metal deposition at this location produces lightfast colors ranging from blue, green and yellow to red. The colors are caused by optical-interference effects, rather than by light scattering as with the basic electrolytic coloring process. Further development will produce a greater variety of colors.Here is detailed information comparing two of the most common coloring processes used:(note – these two types of processes will not produce identical colors; both can be overdyed – Source: Aluminum Anodizers Council Technical Bulletin #1-94, issued January, 1994.) See Below.
5. Sealing. This process closes the pores in the anodic film, giving a surface resistant to staining, abrasion, crazing, and color degradation.

Quality control. Through the entire anodizing process, AAC members like SAF monitor the process and quality of the product. The application of electrical power and color is preprogrammed and verified on all batches and coils.

This quality control ensures uniformity to end product specifications for film thickness, density, abrasion resistance, corrosion resistance, color uniformity, fade resistance, reflectivity, image clarity, insulative properties, adhesion and sealing.

In many cases anodizers use Statistical Process Control (SPC) methods to meet rigorous quality assurance standards.

COMPARISON OF A32/A42 AND A34/A44 COLORED ALUMINUM ANODIC FINISHES

This information used by permission of the AAC

For more information about SAF’s coil anodizing capability click here!