Coil Anodizing Explained

Coil Anodizing is an electrochemical process, not an applied coating. The surface of the aluminum is converted to an aluminum oxide as a result of reactions occurring in an acid solution and electricity. The end result is a technical wonder that provides many inherent benefits. Lorin Industries specializes in coil anodizing as the most useful method for its clients due to the utility, consistency, and affordability it brings.

Continuous Coil Anodizing

In coil anodizing, coils of aluminum product are unwound through a series of anodizing tanks where the process itself takes place, and then rewound into a coil upon completion. The benefits are many.

Steps in the Coil Anodizing Process

Coil Anodizing Line Diagram

Cleaning

The raw aluminum is received containing mill oils, grease, aluminum oxides, and dirt on the surface. These contaminants must be removed prior to the anodizing process.  If these contaminants are left on the surface, the anodic film may be pitted and not uniform in appearance, which can cause serious imperfections in the surface especially, if coloring the anodic film. Lorin Industries uses cleaners that dissolve these contaminants while minimizing any metal removal in the cleaning process. 

Pre-Treatment 

Etching is a chemical process that gives the aluminum surface a very matte and appealing surface quality.  Etching in an acid or basic solution prepares the aluminum for anodizing by chemically removing a thin layer of aluminum.  The removal of a very thin layer of aluminum will clean up minor imperfections in the aluminum alloy itself.  The etching process is also used to control the surface finish of the aluminum allowing for a more consistent panel-to-panel match.

Chemical Brightening is a process that smoothes the surface roughness of the aluminum to achieve a non-directional and highly reflective surface.  The chemical brightening process is used where applications such as lighting fixtures or solar concentrators require a reflective surface to capture and distribute light. 

Electropolishing is an electrochemical process that brightens high purity aluminum alloys. Reflective tests have shown that electropolished aluminum will be 10% to 12% brighter after anodizing.  It will also produce a sharper reflected image on high purity alloys. Electropolishing has been around for a long time, but the only chemical solution that worked was one requiring several pounds of chromic acid.  Chromic acid is more costly to waste treat and is also toxic [carcinogenic].  Lorin Industries' electropolishing solutions have been developed without the use of chromic acid. This reflects Lorin’s ongoing commitment to eliminate toxic materials and help protect the environment.

Anodizing 

The standard anodizing process uses a sulfuric acid electrolyte.  Once the surface is prepared, the anodic film is built from the aluminum itself, and is not a coating.  Electrical current is passed through an electrolyte bath in which the aluminum has been immersed.  The anodize film is built from the aluminum itself, not applied like paint.  The anodic thickness can be tightly controlled and is hard and porous for accepting coloring dyes.

Lorin offers several proprietary anodizing processes designed for specific purposes and aluminum applications.  AnoGrip® Type 4 and AnoGrip® Type 5 anodizing processes were developed for applications requiring an aluminum substrate with superior adhesive bonding characteristics. These processes are environmentally safe unlike chromate conversion coatings that have been known to cause cancer.  ClearMatt® with Adhere® was developed for customers that require a superior anodize quality finish on the top-good side and has an excellent adhesive surface on the other side.

Coloring

The anodizing process provides the opportunity for an artist’s pallet of colors in the final product.  Anodic films are well suited for several coloring methods.

Absorptive Dye Coloring - This coloring process employs organic or inorganic dye stuffs.  The aluminum oxide layer is a very porous structure and will absorb staining materials. For example, any colored fabric dye can be absorbed into the aluminum oxide layer.  These dyes offer vibrant colors with intensities that cannot be matched by any painted metal system in the market.  The color possibilities are endless.

Electrolytic Two-Step Coloring - After an anodize layer is built, the metal is immersed in a bath containing an inorganic metal salt.  Electrical current is applied which deposits the metal salt in the base of the pores. The resulting color is dependent on the metal used and the processing conditions.  The process offers color versatility and is the most technically advanced coloring system available.

Lorin’s ColorIn® Portfolio of products uses the electrolytic two-step coloring process. This process offers exceptional fade resistance and is very suitable for exterior applications.

The unique color matching capabilities of Lorin Industries offers a range of options not available through other coil anodizers.

Sealing  

This is the final step in the anodizing process. It closes the pores in the anodic film, locking in colors and creating a surface resistance to staining, abrasion, and color degradation.