Anodizing

Sulfuric Anodizing

Sulfuric anodize, commonly referred to as Type II anodizing, is formed by using an electrolytic solution of sulfuric acid at room temperature and a current density of 15 to 22 Amps per square foot. This will produce a generally clear coating, depending on sealing, a minimum of 8µm thick. One third of the coating thickness will build up per surface and 2/3 will be penetration.

Formation

Anodizing is an electrochemical conversion process, not an applied coating. The surface of the aluminum metal is converted to aluminum oxide as a result of reactions occurring at the anode in a acidic solution. The thickness and properties of the anodic coating will vary with alloy, anodizing process utilized and cycle time (ampere-hours).

Properties Type II sulfuric anodizing provides these surface attributes:

• Corrosion Resistance (336+ hours salt spray resistance per ASTM B117)
• Moderate Durability * Excellent Dyability (yielding deep, rich colors)
• Electrical Insulation Hot water seals produce the clearest sulfuric anodize while sodium dichromate yields a yellow-green appearance but is generally a better seal.

Specifications

There are thousands of governmental, industrial, and commercial anodizing specifications today, each with their own method of calling out coatings, seals, dyes, etc. Any finisher should recognize the Type II designation to indicate a sulfuric anodize, Class 1 to specify natural color or no dye, and Class 2 to indicate a dye.

Hardcoat Anodizing

Hardcoat anodize, commonly referred to as Type III anodizing, is formed by using an electrolytic solution of sulfuric acid at approximately 32° F and a current density of 23 to 37 Amps per square foot. This will produce a generally gray coating 10µm to 50µm thick with 50% buildup and 50% penetration.

Properties

Hardcoat anodizing provides these attributes:

• Corrosion Resistance (336+ Hours salt spray resistance per ASTM B117)
• High Durability (file hard, 60-70 on Rockwell C-scale)
• Electrical Insulation (800 V / mil thickness)

Hardcoat anodize coatings may be dyed a variety of colors. However, due to its naturally darker appearance, dyeing does not produce the vibrant colors that a Type II sulfuric anodize will.

Specifications

There are thousands of governmental, industrial, and commercial anodizing specifications in use, each with their own method of calling out coatings, seals, dyes, etc. Any finisher should recognize the Type III designation to indicate a hardcoat anodize, Class 1 to specify natural color or no dye, and Class 2 to indicate a dye.

Materials

Hardcoat anodizing works very well with 6000, 5000, and 7000 series alloys. High-copper 2000 series alloys can present some difficulties and should be avoided if possible. As with all forms of anodizing high-silicon die castings are not recommended. The Chromating Process Chromating coatings, are extremely thin chemical coatings that provide a couple of desirable characteristics to the surface of an aluminum part such as:

• Enhanced corrosion resistance
• Enhanced bonding ability
• Electrically conductive

Compared to anodizing, chromating offers a couple of advantages over Anodizing. For example, it is not an electrical process. This allows for generally quicker fixturing & even bulk processing, usually resulting in a less expensive process. This process is also referred to as Alodine & Irridite in the industry.

Characteristics as applied to:

Wrought Alloys

1100 Series

Most common: 1100 only Bronze-grey in color at .002 inch, alloy is soft and not particularly good for machining. Maximum practical coating thickness .0025 inch.

2000 Series

Most common: 2014, 2017, 2024, 2618 (forgings). Avoid sharp corners on 2011 and 2017; grey-black at .002 inch to blue-grey at .002 inch. Excellent machining characteristics, Maximum practical coating .002 inch; .0025 inch possible for salvage though not as hard as less heavy coats.

3000 Series

Most common: 3003. Good for die work and machining. Grey-black in color at maximum coating of .002 inch.

5000 Series

Most common: 5005, 5052. 5005 best for die work; 5052 not good for die work, except black. Both have good machining characteristics, Maximum practical coating .002 inch. 5052 has excellent dielectric properties when coated to .002 inch.

6000 & 7000 Series

Most common: 6061 6063 and 7075. 6061 forms excellent hardcoat for grinding, lapping or honing. Excellent dimensional stability. 6063 used for extrusions. Maximum practical coating .0025 inch. 7075 for high strength applications