In these days of lean manufacturing and competition from overseas suppliers, in-house blackening is becoming a popular way to finish tooling and machine components. Though there are several reasons why in-house blackening is more attractive than sending parts out to a plater, they all come down to one simple concept: Effective control of quality, scheduling and costs. Specifically, manufacturers need fast turnaround on part finishing to satisfy customers’ shipping requirements.

Compared with painting or electroplating, an in-house black oxide system is very inexpensive to install and operate. Capital costs are normally low (depending on work volume) and operating costs for chemicals and labor are usually below $0.10 per pound of finished parts.

Tank systems for low-temperature black oxide finishing come in all sizes and can be supplied with spill containment and pollution-free features.

Why Blacken?
Black oxide is attractive because:

1. Black oxide adds value and sales appeal to many fabricated parts, without altering dimensions or interfering with part assembly or operation. In addition, black oxide finishes are quite durable and offer long-term corrosion resistance in storage or in service.
2. In-house black oxide lines are supplied as pre-engineered systems, complete with operator training. They are easy to install and operate and can be adapted to accommodate any part size or volume.
3. ISO and lean manufacturing programs are easier to administer when inventory stays in the plant.
4. For most companies, the main benefit of an in-house black oxide system is the ability to offer fast track service for key shipments without raising inventory levels. Some report that they can receive a purchase order in the morning, machine and blacken the parts, then ship the same day or next day.

Corrosion Resistance. A black oxide finish will normally withstand 100–200 hours of neutral salt spray (ASTM B117) or several hundred hours of humidity (ASTM D1748), depending on the sealant used. Black oxide finishes do require a sealant of some type because the black finish itself has a porous crystal structure. Though the sealant is primarily responsible for the corrosion resistance, the underlying black coating acts as an absorbent base, holding the sealant in contact with the metal substrate.

Toolholders that require blackening for corrosion protection are excellent candidates for the low temperature blackening finishing process.

Dimensional Uniformity and Stability. Black oxide finishes have a uniform thickness of about 20 µ in., making them ideal finishes for precision manufactured components that cannot tolerate the variable thicknesses.

Blackening Processes
There are three types of blackening in common use:

1. Caustic Black Oxidizing. Using a boiling caustic soda bath operating at 290°F, this system forms a blackiron magnetite finish in 20–30 minutes. Overall finish quality is usually quite good, except on cast iron or metal injection molded parts (red coatings are common) or parts with blind holes or recesses (white salt leaching). In addition, the high temperature and concentration of the bath cause significant splattering and boil-over hazards, and must be operated and maintained very carefully. This finish does satisfy the prevailing military specifications (MIL-DTL13924 & AMS2485).
2. Room-Temperature Blackening. Using copper/selenium chemistry, this system operates without heat and forms a black conversion coating in 2–5 minutes. These finishes can replace black oxide in many applications, but the finish is not quite as durable. Because copper and selenium are both regulated by EPA, these process lines are often fitted with ion exchange to purify and recycle the rinse waters for zero-discharge operation.
3. Low-Temperature Black Oxide. Using a unique, patented chemistry, this system operates at 200°F and forms a durable, black magnetite finish in a 10-minute blackening time. The basic chemistry is the same as that of caustic black oxide above, but the reaction proceeds at a lower temperature because it blackens in two steps. Since the process contains no EPA-regulated metals, the rinse waters are non-hazardous and are generally sewerable without waste treatment. Its low temperature makes it easy and safe to operate. The process also avoids the red coatings and white salt leaching commonly seen with conventional black oxide, because the overall concentration of caustic soda is 80% lower than that of regular black oxide.

Up to 70 toolholders per rack are processed with up to three toolholder racks in the finishing cycle at one time. From start to finish, the process takes just 34 minutes.

Each of these processes has different heat and chemical requirements that must be considered when designing a blackening system. It is important to choose the black oxide system that offers the best overall combination of coating performance and system operation for each application. Though the details of the chemistry are somewhat different, all deposit a coating that is chemically bonded to the steel surface, with a porous crystal structure that absorbs a rust preventive topcoat.

Process Design. Though many installations operate with a standard process sequence, the design of the process line can change to accommodate different production conditions:

• The volume of parts per shift will determine the overall size of the blackening tanks.
• The method of parts handling required (racks, baskets, rotating barrels) affect the way in which the parts are processed through the line.
• The condition of the surface prior to blackening may call for special cleaning or descaling provisions. Parts that carry heavy oils or oxides (rust, heat treatscale or hot-rolled mill scale) often require aggressive cleaning or descaling treatment using chemical or grit-blasting methods.

The performance requirements of the final finish will indicate which sealant is needed to achieve the necessary appearance, gloss and level of corrosion resistance.

A well-designed tank layout can accommodate all the factors listed above, and the supplier of the system can offer guidance in these areas. Most standard process lines contain the following stations:

1. Cleaning. Parts must be cleaned of machining fluids, coolants and other soils, most of which are water-soluble materials. In most cases, a heated detergent soak tank is preferred because it does a good cleaning job and is safe and economical to operate. Insoluble particles such as carbon soot or machining fines are best removed by power washers or agitated soak cleaners. Abrasive grit blasting is best for removing rust, heat treat scale or mill scale.
2. Rinsing. A clean water rinse (10-30 sec) after cleaning is important, but ordinary tap water is all that is needed for most lines. Since the rinse waters are gradually getting dirty as loads of parts are processed, they must also be continually cleaned somehow. The most flow fresh water through the rinse tanks by means of a low-level fresh water inlet and an overflow trough at the top, fed by 1-3 gpm water flow. The flow rate will depend on the type of cleaner used and the amount of cleaning solution the parts carry out.
3. Surface Conditioning. The clean steel surface must be in a reactive state, so that it is chemically receptive to the formation of a high-quality black finish with good adhesion. The mildly acidic surface conditioner removes minor oxides and deposits a type of primer coat that aids in formation of the final black oxide finish. Typical immersion time: 3-8 minutes.
4. Rinsing. As noted in Step 2, this rinse station removes any residues of the surface conditioner, leaving the part in a clean and reactive condition, ready for blackening.
5. Blackening. High temperature (290°F) black oxides form an adherent coating in 20-30 minutes, in most cases. Low temp (200°F) black oxide typically requires only 10 minutes. Once the coating is formed, the reaction shuts itself off. Any additional immersion time has no further effect on the coating. Room temperature blackeners utilize a timed reaction (usually 2-5 minutes), as determined by test. Regardless of the chemistry, the blackening solution carries out a chemical reaction with the metal surface to form a tightly adherent black coating, bonded to the substrate.
6. Rinsing. This rinse removes any blackening residue from the parts, and contains small amounts of the chemicals in the blackening bath, some of which may be EPA regulated. For high temperature (290°F)black oxide, the rinse water has a high pH and must be neutralized before discharge. Low temp (200°F) black oxide requires no treatment and is sewerable as non-hazardous discharge because the pH is usually within acceptable limits. Room temperature blackening systems contain copper and selenium which are both EPA regulated. The rinse water cannot be discharged, in most cases, but can be purified and recycled by Ion Exchange at relatively low cost.
7. Sealing. The black oxide finish actually has little inherent corrosion resistance because of its porous crystal structure. This porosity acts as a very effective absorbent base, however, for a rust preventive topcoat. There are several different types of rust preventives available, each with different properties. The best corrosion protection comes from solvent-based products that form either a slightly oily or dry-to-touch film.

The finish produced by low temperature blackening is equivalent to or better than hot oxide with a deep black uniform appearance and high corrosion protection. The per part cost is less and the finish is done in 34 minutes compared to three or more days when sending parts outside for hot oxide finishing.

Water soluble rust preventives are also available. Their protection levels may be a bit lower, but they are more economical to operate. It is important to choose the sealant appropriate to the intended service life of the component.

Areas of Opportunity for Manufacturers
Lean Manufacturing. To paraphrase one company’s motto: zero defects, zero waste, zero incidents. This goal equates to maximum customer satisfaction at minimum cost, and is achieved by streamlining part movement and controlling each phase of the entire manufacturing process. In-house black oxide can play a key role in this program.

Controlling Quality & Costs. Sending parts to an outside plater for black oxide requires part sorting and packing, freight costs there and back, 2-5 day turnaround, then paying the finisher a profit to do the work. This practice requires higher inventory levels, complicates ISO controls and makes it more difficult to offer quick response for key customers.
In contrast, in-house black oxide offers turnaround times of less than one hour. This single fact lowers costs, reduces inventory levels and streamlines operations by keeping the parts inside the plant. In addition, it enhances ISO accountability and allows for better control over finished part quality.

Enhancing Part Value. With modern machinery, fabricating the finished parts often takes only seconds and contributes the largest portion of the part’s value. Outside black oxide may take several days, yet adds only a small portion to the part’s value. Add to this the fact that customers carry lower inventory of these incoming parts and often demand quick delivery. Sending parts outside for finishing is potentially a costly and avoidable production bottleneck, particularly where just-in-time deliveries are required.

In summary, in-house black oxide has proven to be an integral part of modern manufacturing because it enhances the ability to meet objectives while reducing costs. PFD