Alodine vs Anodize: Aluminum Finish Comparison Guide

Overview: Two Ways to Protect Aluminum

Aluminum is one of the most popular CNC machining materials. Grades like 7075, 6061 and 2024 are everywhere in aerospace, defense and consumer products. But raw aluminum corrodes over time. It needs a protective finish.

The two most common finishes for aluminum CNC parts are Alodine (chromate conversion coating) and anodizing. They both protect the surface, but they work in very different ways. Picking the wrong one can cause electrical failures, corrosion problems, or wasted money.

This guide breaks down each process, compares them side by side and tells you exactly when to use each one.

How Each Process Works

Alodine (Chromate Conversion Coating)

Alodine is a simple chemical dip. The part goes into an acid bath that contains chromate compounds. The bath reacts with the aluminum surface and creates a very thin film. The whole process takes just 1 to 5 minutes.

The film is only 0.00001 to 0.0001 inches thick. You can barely see it. On most parts it has a gold or iridescent color. Clear versions also exist for cosmetic parts. The process meets MIL-DTL-5541, the military spec for chromate conversion on aluminum.

Anodizing (Type II and Type III)

Anodizing is an electrochemical process. The part is dipped in a sulfuric acid bath and connected to a power supply. Electricity forces the aluminum to grow a hard oxide layer on its surface. This layer is much thicker and harder than what Alodine creates.

Type II anodize grows a layer 0.0002 to 0.001 inches thick. It meets MIL-A-8625 Type II. This is the most common anodize for general parts. It can be dyed many colors, including black, blue, red and gold.

Type III anodize (hardcoat) grows a layer 0.001 to 0.003 inches thick. It meets MIL-A-8625 Type III. This coating is extremely hard and wear resistant. It is used on parts that see heavy sliding, abrasion, or harsh environments. For a deeper dive into the differences, see our anodizing type comparison.

Side-by-Side Comparison Table

Electrical Conductivity

This is the biggest difference between the two finishes. Alodine keeps the aluminum fully conductive. Anodize creates an insulating oxide layer that blocks current.

If your part needs to carry electrical current or serve as a grounding path, use Alodine. This is why most electronic enclosures, RF shields and EMI housings use Alodine instead of anodize. MIL-DTL-5541 Class 3 is specifically designed for electrical contact areas where conductivity must stay high.

If your part should NOT conduct electricity, anodize is the better pick. The oxide layer acts as a natural insulator. Many engineers use anodize on heat sinks and mounting brackets to prevent short circuits.

Corrosion Resistance

Both finishes protect aluminum from rust and pitting. But anodize wins by a wide margin.

Alodine passes about 168 hours of salt spray testing per ASTM B117. That is enough for indoor parts and mild environments. For aerospace parts that need paint over the top, Alodine works great as a primer layer.

Type II anodize passes 336 or more hours of salt spray. Type III hardcoat can exceed 1,000 hours. For parts that face rain, salt air, chemicals, or outdoor weather, anodize is the clear winner.

Cost Comparison

Alodine is the cheapest aluminum finish. A small batch of parts typically costs $5 to $15 per part. The process is fast and uses simple equipment. Many shops can turn around Alodine work in 1 to 2 days.

Type II anodize runs $10 to $30 per part. Type III hardcoat costs $20 to $50 per part. Anodize takes longer and needs more expensive equipment. Lead times are usually 3 to 5 days.

For large production runs, the per-part cost drops for both. But Alodine stays the cheaper option at every volume level.

Dimensional Impact

Alodine does not change the size of your part at all. The film is so thin it has zero impact on fit or tolerance. This makes it perfect for tight-tolerance parts where every thousandth matters.

Type II anodize grows into the surface and out from the surface. Roughly half the thickness goes each way. On a 0.001" coating, expect about 0.0005" growth per side. For a bore or shaft with a tight fit, you must account for this in your DFM design.

Type III hardcoat causes even more growth. A 0.002" coating adds about 0.001" per side. On precision parts, the machine shop may need to cut the part slightly undersize before sending it to the anodizer. This adds cost and lead time. Always call out the post-anodize dimension on your drawing.

When to Use Each Finish

Choose Alodine When:

• The part must stay electrically conductive (grounding paths, RF shields, EMI enclosures)
• You plan to paint the part and need a primer coat
• Tight tolerances leave no room for dimensional growth
• Budget is limited and mild corrosion protection is enough
• The part lives indoors or in a controlled environment
• Fast turnaround is critical, Alodine takes hours, not days

Choose Type II Anodize When:

• You need better corrosion protection without paint
• The part needs a specific color (black, blue, red, etc.)
• Moderate wear resistance is needed
• The part faces outdoor weather or mild chemicals
• Consumer-facing cosmetic appearance matters

Choose Type III Hardcoat When:

• Extreme wear resistance is the top priority (sliding surfaces, valve bodies)
• The part sees abrasive contact, grit, or sand
• Maximum corrosion protection is needed without paint
• Operating temperature is high, hardcoat withstands up to 2,000 degrees F
• Defense or aerospace specs require MIL-A-8625 Type III

At RivCut, we machine aluminum parts from 6061, 7075, 2024 and other grades every day. We work with trusted finishing partners to deliver Alodine, Type II and Type III anodize. Upload your CAD file and we will quote the machining and finishing together.

Frequently Asked Questions

RivCut Engineering Team

Reviewed by Jimmy Ho, Founder & CEO

Our team combines 30+ years of CNC machining expertise across aerospace, defense, medical and automotive industries. We write what we know, from the shop floor.