Blackening of Ferrous Metals

Article From: Products Finishing, from Birchwood Technologies

Posted on: 11/1/2000

The reasons for installing an in-house cold blackening system are many and varied.

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Cold blackening

Cold blackening on machined steel parts produces a non-dimensional finish that enhances the precision appearance of the components.

Precision parts

Precision parts are corrosion protected with the room temperature blackening process.

The reasons for installing an in-house cold blackening system are many and varied. For most new users it means achieving better control of finishing quality, production scheduling, ISO compliance, not to mention the improved cost efficiencies.

These pre-engineered systems make an important contribution to the quality of a product, but just as important, to the control of the process. By bringing this step of the manufacturing process in-house, companies take direct control over part delivery schedules and the finishing process.

Costs to install and operate a system are minor when compared with the process advantages that are gained to meet ISO 9000 compliance and just-in-time delivery.

The corporate mandate for ever-improving profits, worker safety and better control of quality, have been the driving force behind a parade of innovations in the cold blackening process.

The "insourcing" of part blackening—doing it in-house—meets all of these new corporate mandates. More and more, blackening of ferrous metal components is viewed as a better alternative to electroplated or painted finishes when performed as an in-house process. Also when manufactured parts are made and/or coated on the outside, ISO 9000 compliance and just-in-time inventory control is difficult, if not impossible. In-house blackening, on the other hand, provides complete control of part quality and on-time delivery needs.

As for the cost of an in-house blackening system, the investment is surprisingly low. This contrasts to paint systems and plating lines which are usually much larger and therefore cost much more.

Cold blackening lines can be installed with zero drain pollution. They are safe and easy to operate and are very compatible with ISO, just-in-time and total quality management programs.

Why Blacken?

In-house blackening is attractive because:
1. Cold blackening processes are simple, and safe to operate and produce high quality results.
2. ISO and JIT programs are easier to administer when inventory stays in the plant.
3. No pollution problems. Process lines can be set up using all water-based solutions and can be fitted with ion exchange to eliminate hazardous effluent to the drains.
4. Pre-engineered systems available. Cold blackening lines can be purchased as completely pre-engineered processing systems containing all of the components needed for startup, or customized systems are available to handle any volume and any size parts.
5. Low cost; and quick payback. In the following example, a typical 100-gal blackening system might process approximately 200,000 lbs of parts per year. Sending this volume of parts outside for black oxide finishing at an average rate of $.30 a pound would cost $60,000 a year. Additional hidden cost factors such as freight, part sorting labor, lost/damaged parts, etc, can add another $15,000 a year to the total cost. This does not include the cost and inconvenience of having part inventories outside the plant.

By contrast, a 100-gal in-house blackening system carries an initial capital cost of about $25,000, including hoist and eyewash station and installation. Operating costs include:

Chemicals $6,000/year
Energy 500/year
Labor 6,000/year
Ion exchange/
waste disposal 5,000 year
Total operating cost $17,500/year
Annual savings $57,500/year
Equipment payback is less than six months!
Even with lower work volume than shown in the above example, the equipment payback is usually less than one year.

Corrosion Resistance. A black oxide finish will withstand 100-200 hrs of neutral salt spray (ASTM B117) or several hundred hours of humidity (ASTM D1748), depending on the sealant used. The sealant is primarily responsible for the corrosion resistance while the underlying black coating acts as an absorbent base, holding the sealant in contact with the metal substrate.

Product Appeal. The satiny smooth black oxide finish is attractive as well as durable, a combination required by more companies in highly competitive markets.

Dimensional Uniformity and Stability. Tooling, machine components and other parts manufactured to precise dimensions cannot tolerate the variable thickness of electroplating or the excessive thickness of paint. Black oxide finishes have a uniform, negligible thickness.

Economical. Compared to nearly all other finishes, black oxide finishes can be applied at significantly lower operating costs.

A black coating can be formed on steel parts in certain types of heat treating processes. However, most blackening is done after the heat treat cycle in order to give the best control over the results on a variety of alloys. The four types of blackening processes in common use are:

1. Copper/Selenium Blackening. This process uses a room-temperature, acidic bath to form a black copper selenide (CuSe) coating. Reaction times are generally one to two min.
2. Caustic Black Oxiding. A caustic soda bath of 6-8 lb per gal and operating at 285F produces a black iron magnetite coating in 10-20 min.
3. Steam Bluing. This method utilizes a pressurized reaction vessel. The parts are sealed inside, heated to 600-800F and steam is injected into the vessel. The chemical reaction forms a black iron magnetite Fe3O4 on the surface in about 10-20 min.
4. Black Dyed Oxalate Finishing. It provides high quality blackening and corrosion resistance without utilizing hot solutions or EPA-regulated chemicals. A grey ferrous oxalate coating is applied to the steel and is then dyed black (similar to black anodizing of aluminum). This process operates at room temperature and does not require waste treatment because there is no hazardous effluent.

Each of the four processes has widely varying heat and chemical requirements that must be considered when designing a blackening system.

Though the exact chemical compound on the surface varies between the four methods described, all deposit a coating that chemically becomes a part of the steel surface. The coating thickness is a negligible 0.000030 inch. However, the property that is essential to the performance of any black coating is its porosity and ability to absorb a subsequent coating of oil or other corrosion inhibitor.

Process Design. Though the basic sequence of the process line is usually very predictable, individual steps can be influenced by factors related to part design and work flow patterns. Some of the more common factors are:

•Mix of alloys used, hardness and reactivity.
•Condition of the surface prior to blackening (for example, type of metalworking fluids used, or presence of oxides such as rust, heat treat scale or machining fines).
•Volume of parts produced per shift.
•Method of parts handling (racks, baskets, conveyors).
•Requirements of the final finish in terms of appearance, gloss and level of corrosion resistance.

The tank layout can take several forms to accommodate any of the factors listed above. Because conditions vary widely from one plant to the next, individual steps in the blackening process can be quite different. For example, many parts are not rusty prior to blackening and may require only degreasing in order to be ready for blackening. If there is pre-existing rust or other oxides, the blackening line should contain a provision for removing them. Many hardened or high-alloy steels require chemical activation prior to blackening in order to ensure a good bond between substrate and finish. If the parts are difficult to clean due to the presence of heavy grease or other deep-drawing compounds, this should be reflected in the line in the form of heavy-duty cleaning stations.

By taking all these various aspects into consideration, the blackening line can be designed to do several jobs and produce high-quality finishes.

Let's look at the individual steps in a blackening line.

1. Cleaning. Parts must be cleaned of metalworking fluids prior to blackening. Acceptable methods include alkaline detergent soak cleaning, spray cleaning, electro-cleaning, solvent degreasing and grit blasting. Each has its own advantages. Water soluble coolants and cutting oils, for example, are removed in an alkaline detergent soak. Insoluble particles such as carbon soot or machining fines come off best in electrocleaners, spray cleaners or agitated soak cleaners. Abrasive blasting is best for removing rust, heat treat scale or mill scale. The detergent soak cleaner is preferred because it does a thorough cleaning job and is not hazardous or costly to operate.
2. Rinsing. A clean water rinse between two adjacent chemical tanks is a must in any finishing line. Ordinary tap water is all that is needed. Successful operation depends on proper chemical balance of each bath with a minimum of contamination by previous baths. Most baths in the line can absorb some carryover without a loss in performance, but gross contamination must be avoided for best results.

The recommended rinse is provided by a bottom-fed water tank with an overflow trough at the top, fed by one to three gpm. The flow rate will depend on the type of cleaner used and the amount of cleaning solution the parts carry out. Higher cleaner concentrations require higher flow rates. Likewise, high-density or bulk-handling methods such as basket or barrel processing may require higher flow rates than racked parts or multiple rinses.

The condition of the rinse water can be monitored by pH or conductivity and the flow rate adjusted accordingly.

3. Activation. The steel surface must be in a reactive state, so that it is chemically receptive to the blackening reaction. Then it is possible to deposit a black coating that is uniformly adherent and of acceptable quality.

The main function of activators is minor oxide removal and adjustment of the pH of the surface. There is always a small amount of residual alkaline salt and wetting agent left in the microscopic pores of the metal after cleaning.

Neutralizing these salts with mild acid aids in initiating the blackening reaction, since the blackening solution itself is a mild acid.

4. Rinsing. The final rinse is very important. Because the blackening solution depends on chemical activity and balance, significant contamination of the blackening bath can result in poor quality coatings. It is important that this rinse tank be kept as clean as possible.

5. Blackening. This is usually a timed operation, especially in cold blackening lines, in which the solutions react very rapidly—30-90 sec. Hot oxide solutions usually require dwell times of 10-30 minutes. Longer times may be beneficial in some cases, if confirmed by testing.

6. Rinsing. A clean cold water rinse is necessary to remove residues of the blackening solution. Ion exchange works well on this rinse.

7. Sealing. Usually, a light oil or dry-to-touch sealant is used to displace water and seal out atmospheric humidity. Many different types are manufactured. Most are available in either solvent or water-borne versions. It is important to choose the sealant appropriate to the intended service life of the component.

Depending on the processing steps used, the entire cycle usually takes 10-30 min.

Even though blackening uses acidic solutions at times, hydrogen embrittlement does not occur.

Hydrophobic Sealants. Recent developments in the technology of rust preventive topcoatings make these options even more attractive. For example, some of the new rust preventives are water-based products that do not contain VOC's. However, they are formulated with a special process that makes them resist water once they are dried on the metal surface (similar to a latex paint). Unlike conventional water-soluble oils, which re-emulsify very easily, these new "hydrophobic" sealants are removed only by strong detergents. Consequently, these coatings resist atmospheric humidity, fingerprints and other corrosive elements as well as solvent based oils do, thereby eliminating the fire-hazard and VOC problems of these other materials with no loss of corrosion protection.

Guidelines For Installing Your Own In-House Blackening System

1. Choose a supplier who can provide the entire installation-tank line, chemical products, ion exchange system, as well as provide operator training and troubleshooting services.
2. Send that supplier a box of parts which are representative of the alloys you work with.

Include details on your specific finish requirements. The supplier will process them in a way that satisfies your requirements and return them to you for your evaluation. Usually, a report is included detailing the best process sequence along with other options you may want to consider.
3. Ask for a design proposal. Once you have tested and approved the finish itself, the supplier can prepare a proposal for a system that accommodates your part size, mix, volume and handling requirements. Describe unusual production flow patterns in order to optimize efficiencies. Consider constructing a new manufacturing cell, utilizing the in-house blackening line as one element in the cell. It may be possible for one operator to operate a machining center or automatic screw machine along with the blackening line!
4. Once the proposal is received, you can begin your analysis of operating costs and capital cost pay back. The supplier can assist in this regard if necessary, to help identify all pertinent costs that must be taken into account: equipment, chemicals, labor, utilities, etc.
5. After installation of the process line, ask the supplier to send in a representative to inspect the equipment assemblies, mix the chemical products, train the operator and establish the maintenance schedule for you.
6. Once the line is operating on a normal basis, stay in touch with the supplier and his representative to take advantage of continued analysis and troubleshooting services as needed.
7. Keep a log of all maintenance performed on the blackening line. This practice makes it possible to monitor costs and establish a statistical data base with respect to production lot tracing and ISO documentation.


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