Understanding Anodizing and Chem Film Surface Finishes

Anodizing and chem-film surface conversion processes convert a workpiece’s surface in preparation for secondary coating processes. The workpiece’s surface finishes before conversion coating will affect the quality of finish of the conversion coating.

What is anodizing?

Anodizing converts a metal surface into a durable, decorative, corrosion-resistant anodic finish through an electrochemical process. Aluminum and other nonferrous metals like magnesium and titanium are ideally suited to anodizing.

Unlike painting and plating, anodizing is fully integrated into the workpiece’s surface, creating a chip and peel-resistant surface. As the anodized finish is integrated into the surface and not applied onto the surface, anodizing will not change the dimensions of the workpiece and provides great corrosion resistance.

What is chem film?

Chemical conversion coating (also referred to as chromate coating, chem film, or yellow chromate coating) applies chromate to a metal workpiece by dipping, spraying, or brushing it. Chem film creates a durable, corrosion-resistant surface that is electrically conductive.

Chem film improves the adhesion of paints and other secondary coatings like primers, is a good base for organic coatings, and is an effective pretreatment for powder coating. Chem film does not change the dimensions of the workpiece.

Surface Finish Options

Many different mechanical finishes can be applied to a workpiece’s surface before surface conversion. Each finish will have a different effect once the conversion process is applied. Some of the surface finishes available are:

• Unidirectional and nondirectional scratch brushing
• Buffing, for a smooth finish
• Brushed using “Scotchbrite”
• Hand sanding with various grits
• Milled finish
• “As machined” finished

All of the above surface finishes are equally good if corrosion resistance and aesthetics are the primary concerns.

The roughness of the surface will affect the workpiece’s optical properties. A rougher surface will result in a duller appearance. A highly polished surface will result in a shinier finish. Bead blasting can be used to create a uniform surface finish but will result in a duller appearance after conversion coating.

Surface Defects That Remain Visible

Neither anodizing nor chem film will hide surface imperfections. If a defect is visible before conversion coating, it will most likely be visible after. The following examples of surface imperfections will be visible after anodizing or applying chem-film surface conversion coatings:

• Die lines on extruded sections
• Mold split lines and ejector pin imprints on high-pressure die castings
• Machining marks
• Surface scratches

It is possible to anodize and achieve a dark finish color, which may hide certain surface inclusions.

Surface Finish Preparation Don’ts

To achieve the best result, take care to avoid the following actions before starting a surface conversion process:

1. Do not put adhesive tapes or stickers on the workpiece surface. The adhesive may leave a residue that will show on the converted surface.
2. Do not handle parts with bare hands. Moisture from the skin can cause surface corrosion, affecting the quality of the surface conversion.
3. When using abrasive blasting to prepare surfaces, ensure it is carried out evenly across all surfaces. Use larger beads at lower pressures.
4. Remove swarf from tapped holes as it may trap processing chemicals.
5. Use separate linishing belts for aluminum and other non-aluminum metals to avoid cross-contamination of the workpiece surface.

Applications for Both

Anodizing is often used in commercial and residential building projects, like coating aluminum windows and door frames. It can also be used to coat furniture, appliances, and jewelry. Chem film on the other hand is used in a wide range of applications – from shock absorbers to specialty applications like aircraft hulls.

The metal used for the workpiece will be a key driver when deciding on the conversion coating. Chem film is suitable for a range of metals including zinc, aluminum alloys, magnesium, and steel. Anodizing is suitable for a broader range of metals and can be used on specialty metals like niobium, tantalum, and titanium.