The Costs and Benefits of Cold Blackening

Metalworking shops face a dizzying array of issues — many of which only indirectly impact the main business of cutting metal. One of those issues is proper finishing of workpieces by blackening.

For years, the only blackening option open to shops required running a hot oxide line. Tightening of environmental regulations and increased concern for operator safety caused many shops to abandon in-house blackening in favor of sending the work out.

Now, in an effort to satisfy customer demands for quick delivery, zero-defect quality standards and to qualify for process certification programs such as ISO 9000, many shops are bringing previously out-sourced operations (like blackening) back in-house to maintain control.

Aside from aesthetics — it does make parts look nice — the black deposit on the workpiece is actually a means to hold a sealant, which is the business end of blackening.

We contacted Birchwood (Minneapolis, Minnesota) to look at their cold blackening process. According to Mark Ruhland, vice president of Birchwood Casey, cold blackening often becomes the system of choice for many shops for in-house blackening operations.

Companies that have established blackening lines using the cold process find the system safe and regulation compliant. Bringing the process in has allowed these shops to take direct control over part delivery schedules and the finishing process.

Here's how cold blackening works, and why it has been able to overcome the two major objections to in-house blackening: worker safety and environmental regulation compliance.

What Is Blackening?

Blackening is a finishing operation that chemically coats the surface of ferrous materials. It creates a strong barrier against humidity and corrosion. Blackening is usually done in a batch operation. Generally, it is less expensive than other finishing options such as painting and plating.

Blackening uses a chemical compound that clings to the surface of machined metal (in all the nooks and crannies). It creates a porous base that bonds chemically with the workpiece surface. In cold blackening, that chemical compound is copper/selenium (CuSe).

Aside from aesthetics — it does make parts look nice — the black deposit on the workpiece is actually a means to hold a sealant, which is the business end of blackening.

The sealant (usually oil) finds its way into the pores of the black oxide coating where it is held in close contact with the metal substrate. It's the oil that prevents corrosion from reaching the workpiece surface. Corrosion protection is a prime reason for blackening a workpiece. The black oxide finish that we see keeps the oil in place so the workpiece doesn't rust.

Why Use Blackening?

In general, metalworking applications that use blackening are parts exposed to corrosion, high accuracy parts, and parts that "show." Blackening is a less expensive process for corrosion resistance than plating and painting.

Another key benefit of blackening for metalworkers is virtually zero dimensional impact of the coating on a workpiece surface. Thickness of the black coating is 0.000030 inch — that's 30 millionths (about a micron). For most workpieces, that additional thickness is negligible. Coverage of the material is uniform over the workpiece surface because the chemical bond between the blackening coat and metal is only a few atoms thick.

There are two alternatives for applying black oxide to workpieces — out-source or in-house. Going out to get blackening done has some obvious advantages, namely not having to fool with it. On the downside, at your local blackening shop, you become only as important as everyone else that vendor does business with.

Because plating and paint finishes often have non-uniform or excessive thickness, they are not suitable for parts that are dimensionally critical--bearings, gears, drives, cutting tools and so on. For these parts, blackening is the coating of choice. It can withstand temperatures of 1,000°F.

Wearability, though not equivalent to titanium nitride and other very tough coatings, is sufficient to stand up to component break-in periods.

Cosmetics is another reason why many part makers use blackening. It makes the workpiece look better. Many applications that use blackening are internal but just as many are parts that are seen on the exterior of a product. Tooling manufacturers use blackening to protect toolholders from corrosion. The fact that the holder looks better is a bonus.

Why Do Blackening Yourself?

Let's assume your shop has a need to blacken parts. It may be for protection from corrosion, to coat a finished surface but maintain dimensional accuracy, or simply to make the part look good.

It's tough for a shop to meet its delivery schedule if the delivery schedule for work sent out is variable. That's a big reason why shops are looking to bring black oxide finishing inside

There are two alternatives for applying black oxide to workpieces — out-source or in-house. Going out to get blackening done has some obvious advantages, namely not having to fool with it. On the down side, at your local blackening shop, you become only as important as everyone else that vendor does business with.

Control is the problem. Many contract blackening shops accumulate large batches of work in order to maximize equipment utilization. It's more profitable. Because most commercial blackening shops use the hot oxide process, larger batches make running the line more efficient. The problem for you is that delivery of your workpieces may take days or even weeks, depending on where your jobs fall in the vendor's schedule.

It's tough for a shop to meet its delivery schedule if the delivery schedule for work sent out is variable. That's a big reason why shops are looking to bring black oxide finishing inside, says Mr. Ruhland. They feel the need to take control of the process to meet the JIT demands of their customers.

How Is Blackening Done?

There are two general methods used in metalworking to apply black oxide. The oldest and most common is a hot oxide process. It's been around for 50 years or more, says Mr. Ruhland. Cold blackening is the second method and, as the name implies, works at room temperature.

These processes are chemically different but performance characteristics for the two are identical, says Mr. Ruhland. Both protect ferrous materials equally. This is determined by standard tests that subject a black oxide finish to 200 hours of neutral salt spray or several hundred hours of humidity.

Basically, the blackening process choice for shops is hot or cold. Hot oxide uses a caustic soda bath operating at 285 to 290°F. Generally, these tanks are gas fired to keep the caustic soda solution at a boil. Obviously, working around these tanks is unpleasant, which is one reason why many shops with hot lines opted to remove them and sent the work out.

The cold black oxide works at room temperature. An attractive advantage for shops using the cold process is safety for the operators.

Cold blackening, on the other hand, has its roots in the energy crisis of the 1970s. To save money on energy (rather than fire up the hot tanks), shops started applying room-temperature activated gunbluing chemicals to small batches of parts.

Mr. Ruhland says at that time Birchwood Casey made much of the gunbluing chemicals used by these shops. When it was discovered that machine shops were using the bluing in place of hot blackening, the company developed a line of products specifically for metalworking shops.

The cold black oxide works at room temperature. An attractive advantage for shops using the cold process is safety for the operators. Gloves and eye protection are required but the danger from boiling hot caustic soda splashing onto a worker is eliminated by the cold process.

Moving work through either process consists basically of taking a dip in five to six tanks for hot oxide or six to eight tanks for cold blackening. These tanks contain, in order, alkaline cleaner, water, blackening compound and sealant. This can be done with automated parts carriers or simply dipping baskets from one tank to the next. Hot blackening takes approximately ten to 30 minutes to complete a batch. Cold is faster, because cold batches require approximately ten to 15 minutes to get through the line.

Zero Sewer Discharge

Few shops can escape the vagaries of local sewer discharge regulations. What's okay to discharge today may change tomorrow. According to Mr. Ruhland, "generally speaking the rinse water discharge from cold blackening lines is within local guidelines."

"But," he continues, "metalworking shops prefer not to deal with sewer districts and effluent regulations at all. These people cut metal for a living. Testing water is non-value added activity."

Because one cannot dump the water down the drain in most areas of the country, it must be treated prior to disposal or filtered in such a way that the water is in compliance. Ion exchange does the latter. It's not a new process, but Birchwood is the first to apply it to the cold black oxide process.

For a cold blackening line, the only sure way to eliminate pollution concerns is not to discharge any effluent at all. Ion exchange is the secret to making this possible on a cold blackening line. It's an option. You don't have to use ion exchange to run a cold blackening line. But, it does allow a shop to disconnect from the drain.

In the blackening process, ion exchange is used to filter the cleaning and rinse water that is used before and after dipping workpieces in the activation and blackening tanks. Chemicals dragged out into the rinse must be removed from the rinse water periodically to keep it clean.

Because one cannot dump the water down the drain in most areas of the country, it must be treated prior to disposal or filtered in such a way that the water is in compliance. Ion exchange does the latter. It's not a new process. It is used in EDM applications and others. According to Mr. Ruhland, Birchwood Casey is the first to apply it to the cold black oxide process.

How Zero Discharge Is Achieved

The ion exchange unit consists of two tanks, sized commensurately with the blackening line capacity. Plastic resin in the tanks removes different ions from the rinse water that is continuously pumped through the tanks. The resin particles are small plastic beads that are electrostatically charged to attract and remove dissolved ions.

The blackening line can be located anywhere in the plant that's convenient to workflow, instead of convenient to a monitored drain.

Positively charged beads attract molecules of copper, iron and sodium and exchange them with hydrogen atoms. Negative beads attract the alkaline residue from the wash station and exchange hydroxide for it. Put the hydroxide molecule (OH) together with the hydrogen atom (H) and you get H₂O, water. It works well.

This system discharges nothing down the drain. The blackening line can be located anywhere in the plant that's convenient to workflow, instead of convenient to a monitored drain.

When the media in the ion exchange canisters are saturated (this is indicated by a sensor), the canisters are exchanged for a fresh set. The saturated canisters are sent out to be regenerated. The cost to regenerate the tanks is approximately $110, and turnaround is usually two to three weeks, says Mr. Ruhland. Depending on use, ion exchange tanks need regeneration from once to several times a year.

The Benefits of Cold Blackening

Cold black oxide works on all ferrous materials except stainless steel. However, the best results come from low carbon steels — cast and ductile iron and powdered metal. Medium carbon steel, 4130 and heat treatable alloys may require some additional time in the activation tank or a stronger mix of blackening chemicals to effectively bond with the material. This is the case on HSS such as A2 and D2 tool steels.

Room temperature blackening systems are becoming more attractive to shops because of their relatively safe operation and lessened environmental impact.

Another consideration for shops, says Mr. Ruhland, is the quantity and shape of workpieces being run through the cold line. Because the immersion time is very short for cold process, flat or overlapping parts may not be completely coated. In some cases racks may be needed to arrange workpieces in ways that ensure complete coverage. This may slow throughput.

Room temperature blackening systems are becoming more attractive to shops because of their relatively safe operation and lessened environmental impact. But it's the need for control of the manufacturing process in light of increasingly narrow delivery windows that's driving shops to explore alternatives.

Making blackening of workpieces an in-house process has many advantages, particularly if a shop experiences delivery problems with outside vendors. Today's machine shop business depends on keeping promises to customers. Getting control of process variables is key to keeping those promises.