Black anodize finishes have long been favored by general industry for their thick, hard coating properties. Ironically, it is 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 and open the door for a new process development that meets all of the above requirements and more. The process is called Lumiclad® electroless black oxide for aluminum alloys.
This new Lumiclad process develops a coating thickness of just 0.000060 inches (1.5 micron) and is smooth and durable. 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 so complex that only those who specialize in it can operate it properly and consistently. And, though 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. Parts must be manually clamped onto spring-clip racks to ensure electrical contact. For this reason, small parts cannot be processed economically. 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. It is easy and safe to operate so that manufacturers can operate it themselves, thereby eliminating the need for outside processing.
Step One: Cleaning. Parts must be cleaned and micro-etched to remove metalworking fluids and light surface oxides. Utilize Safe Scrub® Etch Cleaner: 70-130°F, 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. Utilize Deox-All™ Oxide Remover: room temperature, 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. Utilize Lumiprime® Primer/Prep: room temperature, 5 minutes.
Step Six: Rinsing. A clean water rinse removes residues: 20 seconds.
Step Seven: Blackening. Lumiclad® 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. Utilize Dri-Touch® DS Dewatering Sealant: room temperature, 1 minute immersion; dry.
For most aluminum alloys, the Lumiclad process takes about 30 minutes from start to finish. 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. Process rinse waters will contain minor concentrations of these metals and must be disposed of properly. Many process lines are configured to utilize Ion Exchange technology to purify and re-use the rinsewaters. 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 Technologies, the manufacturer of Lumiclad, offers an Ion Exchange system for purchase. In addition, Birchwood Technologies has coordinated with Siemens Water, the industry leader in Ion Exchange technology, to supply water purification on a lease basis.
|Type II Black Anodizing||Lumiclad Black Oxide|
|Finish Quality||Finish Quality|
|Durable, satin black aluminum oxide||Durable, satin black molybdenum oxide|
|0.0004" - 0.001" thick (10-25 microns)||0.000060" thick (1.5 microns)|
|Smooth, clean, scratch resistant.||Smooth, clean scratch resistant|
|Very hard, durable finish.||Durable finish: excellent handling properties|
|Finish harder than base metal.||Finish same hardness as base metal|
|Interior or light exterior applications.||Interior or light exterior applications|
|336+ hours salt spray resistance||40-100 hours salt spray resistance|
|Electrically non-conductive||Electrically conductive|
|Good base for paint, adhesives||Good base for paint, adhesives|
|Wide range of colors available||Black available only|
|Process Line||Process Line|
|Long process cycle: 60-120 minutes||Short process cycle: 30 minutes|
|20 tanks: several heated and/or chilled||7 tanks: one heated|
|Electrolytic process. Spring clamp racks needed.||Electroless. No current flow needed.(no electrical contact needed)|
|Heavy coating. Masking difficult/impossible.||Non-dimensional coating.|
|Basket/barrel processing impossible||Basket/barrel processing easy, economical|
|Difficult to learn. Experience needed.||Easy to learn and operate effectively|
|Full Waste Treatment needed.||Uses Ion Exchange to recycle rinsewater for Zero-discharge operation.|
|Operating Costs||Operating Costs|
|High cost: $5.00-8.00 per square foot plus freight both ways||Low cost – less than $1.00 per square foot|
|Corporate Concerns||Corporate Concerns|
|Sending work out for anodizing:
||In-house Lumiclad line:
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. Whatever design is chosen, the new process offers savings in finished part cost and part turnaround time while providing a high quality black finish to most grades of aluminum.