Defined

Aluminum coil anodizing was created as an alternative to piece part anodizing. Customers demanded improved products and cost containment by anodizers in the mid 1930's. The response was a new concept: The concept of pre-anodizing aluminum coil and strip prior to fabrication. Eventually, manufacturers were able to produce and assemble products without having an intermittent finishing step. The aluminum received into their plants and fed into their production lines was already anodized. Savings in time, material handling, and finishing costs were immediate and significant.

Anodizing Basics

Aluminum is anodized for corrosion resistance, abrasion resistance, insulation from electricity, adhesion, or aesthetic enhancement.

Definition

Anodizing is the successful development and control of a natural oxidation process that occurs when aluminum is exposed to the atmosphere. Electricity and chemicals are used jointly to produce a hard, transparent surface that is integral with base aluminum.

Characteristics

Hard, comparable to a sapphire
Transparent, similar to glassInsulative and static-resistantWide variety of colors and finishes
Integral with aluminum surfaces, non-flaking

Cross Section of Sealed AnodicOxide Cross Section of Unsealed Anodic Oxide

The Anodizing Process

Batch (or piece), sheet, and coil anodizing all consist of three processing stages:

Pretreatment

The aluminum surface is first cleaned then chemically treated. Etching results in a satin matte appearance. Various degrees of etching can be specified (i.e. light, medium, heavy). Bright -dipping will enhance an already bright aluminum surface, and result in varying degrees of reflective finishes.

Anodizing

Once the surface is prepared, the anodic film is built. 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. It is a hard and porous film. The coating thickness may be tightly controlled, based on the end use product.

Post Treatment

The porous anodic film can be colored in this stage. Organic dyes can be used to fill pores with color, or metal salts can be electrochemically deposited at the base of the pores to create a broad spectrum of colors. Many of the colors are fade-resistant. Sealing the anodic film normally consists of a hot water bath that basically swells the pores shut.

Methods of Anodizing

Regardless of the method, the anodizing process is basically the same.

Continuous Coil Anodizing	Sheet Anodizing	Batch or Piece Anodizing
Uses High volume Coiled sheet Foil Products with less severe forming.	Uses Wide widths Plate Large fabricated products.	Uses Extrusions Castings Parts with severe forming
Advantages Wide range of metal and film thicknesses Less material handling Precise color control and uniformity Cost effective	Advantages Small runs Thicker films Anodized edges	Advantages Small runs Thicker films Anodized edges
Disadvantage Bare edges on stamped parts Crazing when severely formed Limited to sheet and foil	Disadvantages Color variance High costs Film thickness variance Crazing when severely formed	Disadvantages Color variance Excessive handling High costs

*Coil anodizing involves continuous unwinding of coils through a series of anodizing tanks and then rewinding.	*Sheet anodizing involves racking or framing of sheets and immersing them in large tanks.	*Piece anodizing involves racking parts and immersing them in a series of treatment tanks

Reprinted with permission of Continuous Coil Anodizers Association.

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