The Anodizing Process
Aluminum anodizing is the electrochemical process by which aluminum is converted into aluminum oxide on the surface of a part. The anodized surface provides increased corrosion and wear resistance, can be easily dyed a wide variety of colors, provides electrical insulation, and is an excellent basecoat or primer for secondary coatings.
Sulfuric Anodizing … MIL A-8625 – Type II
Overview – This form of anodizing typically yields coatings under 1 mil thick. It offers mild abrasion resistance and like most anodized finishes, the corrosion resistance is excellent. The most desirable feature of this form of anodizing would be the excellent results from dyeing which yields deep, rich colors. Type II sulfuric anodizing provides for several desirable qualities such as: Corrosion Resistance (336+ hours salt spray resistance per ASTM B117), moderate durability, and electrical insulation.
Classes, thickness, and sealing – Class 1 is used to specify a natural color or no dye, and Class 2 is used to indicate a secondary dye that is used for coloring. One third of the coating thickness will build up per surface and two thirds will be penetration. Sulfuric anodize coatings are often sealed to enhance corrosion resistance, lock in dyes, or both.
Materials – Sulfuric anodizing is generally tolerant of most aluminum alloys for anodizing with the exception of high-silicon die-cast alloys such as 380 (about 8% silicon). The less alloying elements that are present, the higher the clarity and depth of color of the anodize coating. Series 5000 and 6000 are the most ideal while Series 2000 containing copper will require testing before proceeding. Casting alloys 518 and C443 are also acceptable but also require testing before anodizing to insure the desired results.
Hardcoat Anodizing … MIL A-8625 – Type III
Overview – Also using a sulfuric acid electrolyte, although at a lower temperature, hardcoat anodizing makes it an excellent candidate for many high wear applications. Hardcoat anodizing provides for several desirable qualities such as: Corrosion Resistance (336+ Hours salt spray resistance per ASTM B117), high durability (60-70 on Rockwell C-scale), excellent 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.
Classes, thickness, and sealing – Hardcoat anodize will typically produce a greenish gray coating 10µm to 50µm thick with 50% buildup and 50% penetration. Additionally, the required sealing process after dyeing can slightly reduce the coating hardness.
Materials – Hardcoat anodizing works very well with 5000, 6000, and 7000 series alloys. However, high-copper 2000 series alloys can present some difficulties and should be carefully evaluated before proceeding.
Chemical Films .. MIL C-5541E
Chromating, or chromate conversion coatings, sometimes known as Alodine, are extremely thin chemical coatings that provide a couple of desirable characteristics to the surface of an aluminum part such as: enhanced corrosion resistance and improved bonding ability while remaining electrically conductive. Compared to anodizing, it is not an electrical process and contact does not have to be made with the part. This allows for generally quicker and easier fixturing or even bulk processing. TK Anodizing offers trivalent and non chrome chemical film processes.
Passivation … ASTM A-967-99 (QQ-P-35)
TK Anodizing offers stainless steel passivation in accordance with ASTM A-967-99 (QQ-P-35) Nitric acid Type II and Type VI are available.
Value Added Services
TK Anodizing provides a broad range of in-house services that reduce our customer’s material handling logistics and production costs. Precision masking, pad and screen printing, sub-assembly operations, packaging, and part identification are a few of he value added services we offer. Please consult your sales representative with any inquiry of this type.
We look forward to sharing our knowledge and expertise in assisting you with your specific powder coating application.