Back in Black: Birchwood Casey's black oxide

Black anodize finishes have long been favored for their thick, hard coating properties. It is also these very same characteristics that often make the anodizing unusable for many military and aerospace applications.
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Black anodize finishes have long been favored for their thick, hard coating properties. It is also these very same characteristics that often make the anodizing unusable for many military and aerospace applications.


Consider critical military components, such as inertial guidance systems, ordnance trigger mechanisms, vacuum pumps and piston/cylinder assemblies. Many of these components may sit idle in a desert or winter cold warehouse for years, and are then expected to work perfectly when put into service. Many require a uniform, non-dimensional finish (under 2 microns) that will not reduce hole diameters, change critical dimensions or interfere with critical clearances. Many also require a finish with enough inherent lubricity to prevent galling and allow for smooth initial operation -- without chattering -- in assemblies that do not receive added lubricants.


Or, they may require a coating that can withstand elevated temperatures of 500° F and higher. These requirements often eliminate black anodizing as a process option, but yet they also open the door for a new process development that meets all of the above requirements and more.


Birchwood Technologies Casey from Eden Prairie, MN, has developed a black oxide process it calls Lumiclad for blackening aluminum that operates as a simple dip-tank process to form a durable black oxide finish on most aluminum alloys


“The process develops a coating thickness of just .000060 inches (1.5 micron), and is smooth and durable,” says Mark Ruhland, vice president of Birchwood Casey. “The finish has a slightly porous crystal structure that absorbs an optional topcoat, such as a clear polymer, light oil or dry-to-touch sealant. The resulting finish is an attractive satin-gloss black surface that is well suited for many machine components, tooling and a wide range of decorative aluminum applications requiring scratch resistance and sales appeal.”


Prior to the development of this new blackening process, black anodizing had been the only viable blackening option for manufacturers of aluminum components. While the anodized finish is extremely durable, the process is often so complex that only those who specialize in it can operate it properly and consistently. And while the protective properties of a black anodized finish are quite high, they are often higher than the application requires, and at significant cost.


“Conventional black anodizing is a long and complicated electrolytic process requiring experienced operators for consistent quality,” Ruhland says. “Parts must be manually clamped onto spring-clip racks to ensure electrical contact. For this reason, small parts cannot be processed economically.”


Ruhland adds that, because the Lumiclad process requires no electrical current, small parts can be economically bulk-processed in baskets or rotating polypro barrels. This opens the door to economical blackening of low-value parts, such as stampings, screw machine parts, fasteners and other parts that require lubricity or galling resistance, but are not good candidates for anodizing.


The Lumiclad process utilizes a conventional immersion tank process line. Ruhland says it is easy and safe to operate so that manufacturers can operate it themselves, thereby eliminating the need for outside processing. For most aluminum alloys, the process takes about 30 minutes from start to finish, while aluminum extrusions and heavily corroded parts may require etching or bead blasting prior to finishing.


The Lumiclad coating reaction utilizes zinc/molybdenum chemistry to form the black finish, so the process rinse waters will contain minor concentrations of these metals and must be disposed of properly, Ruhland says.


“Many process lines are configured to utilize ion exchange technology to purify and re-use the rinsewaters,” he says. “In this manner, the process line operates as a self-contained, closed-loop system, with no discharge to the drain.”


Ion exchange systems and services are available in most localities for this type of process line. Birchwood Casey, the manufacturer of Lumiclad, offers an ion exchange system for purchase. In addition, the company has coordinated with Siemens Water, a leader in ion exchange technology, to supply water purification on a lease basis.


Ruhland says the Lumiclad process can be designed to handle any part size and volume requirement at modest cost. Options for installing an in-house aluminum blackening system include retrofitting an existing tank line, installing a new manual tank line or implementing an automated finishing system. The process can be automated through the use of a CNC programmable hoist system, he says.


For more information on Birchwood Casey’s Lumiclad, call 1-800-328-6156; or visit www.birchwoodcasey.com.


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Lumiclad Process Cycle

Step One: Cleaning. Parts must be cleaned and micro-etched to remove metalworking fluids and light surface oxides. 5-8 minutes.

Step Two: Rinsing. A clean water rinse removes cleaning residues. 20 seconds.

Step Three: (Optional) Oxide cleaning. Certain alloys may need further oxide removal. 5 minutes.

Step Four: Rinsing. A clean water rinse removes residues: 20 seconds.

Step Five: Zinc primer coating. Prepares aluminum surface to receive black oxide. 5 minutes.

Step Six: Rinsing. A clean water rinse removes residues: 20 seconds.

Step Seven: Blackening. LumicladÒ Black Oxide: 200° F, 10 -15 minutes.

Step Eight: Rinsing.  A clean water rinse removes residues: 20 seconds.

Step Nine: Sealing. Displaces water and seals out atmospheric humidity. 1 minute immersion; dry.


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