Manufacturers of metal parts today enjoy access to a wide variety of industrial finishing processes categorized as surface treatments. These procedures help alter the exterior of a workpiece to achieve desired qualities. For instance, enhanced aesthetic appearance, increased electrical conductivity or improved corrosion resistance may result from the application of a surface treatment.
In some industrial fabrication environments, for example, manufacturers endeavor to coat surfaces of components with protective coatings capable of enhancing a finished part's ability to resist degradation in the surrounding environment. This passivation process typically stems from chemical reactions taking place on the surface of the workpiece immediately after the application of the coating. A finishing process called "blueing" occurs when steel acquires a characteristically blue-to-black surface finish following the application of a protective coating. This process represents one subset of black oxide coatings.
The Blueing Process
Today, several oxidizing processes fall within the scope of "blueing". The resulting color of the surface of steel depends in large measure upon the chemical composition of the specific proprietary coating employed by a manufacturer during a finishing surface treatment and the duration and extent of the part's exposure to the coating substance. The resulting protective patina helps provide better protection against corrosion.
Some manufacturers refer to the blueing process as "black oxidizing." This term recognizes that the protective coating forms from a chemical reaction on the surface of the metal. Essentially, during blueing ferric oxide undergoes a chemical reaction to become magnetic iron oxide. This process conveys a distinctive blackish-blue hue to steel's surface.
Types of Blueing
Manufacturers have developed a number of different ways to achieve blueing. Two of the most popular have gained widespread use in industrial metal parts manufacturing settings: "Cold Blueing" and "Hot Blueing".
As its name suggests, this process does not require elevated temperatures. It usually occurs following parts fabrication, when hobbyists purchase proprietary gels to coat metal surfaces. The proprietary formulations sold for this purpose consist of patented blends of chemicals capable of producing oxidizing chemical reactions on the exterior of finished steel components. These chemicals typically change the color of the metal part, giving it a desired bluish-to-black hue.
This fabrication process generally involves the use of high temperatures between 275 degrees Fahrenheit and 311 degrees Fahrenheit, as well as the use of caustic chemicals. It occurs in some automated manufacturing settings. A manufacturer may submerge a metal part in a heated liquid bath containing chemicals such as sodium hydroxide and potassium nitrate, or hydrochloric and nitric acid, for a designated period of time before rinsing. Metal components undergo a surface chemical reaction which results in the formation of magnetite and black oxide. This surface shell coats the metal part, enhancing corrosion resistance. The Hot Blueing process imposes significant environmental hazards during manufacturing.
Materials And Applications
Both metal parts fabricators and hobbyists utilize blueing as steel finishing surface treatment today. The materials required for this process vary widely based upon the process and application:
In industrial manufacturing environments, the blueing of steel sometimes occurs during sheet metal production. Sophisticated equipment permits the coating of one (or both) flat surfaces of the sheet metal during fabrication at a steel mill. This finishing process lends itself to automation.
However, more frequently, the blueing of metal parts occurs after the production of a component and during the finishing stage of work piece fabrication. Industrialized settings perform blueing more easily when a manufacturer possesses access to a modern blast furnace and acid bath equipment. To complete Hot Blueing accurately, it remains preferable to regulate temperature settings precisely (although this process has reportedly in the past infrequently occurred by trial and error in some "low tech" blacksmith settings).
Significant applications for blueing have occurred in the firearms manufacturing industry in the past. Blueing today sometimes still occurs during the production of gun barrels in munitions plants. (Some firearms owners also perform hobbyist blueing in home workshops as a way of safeguarding guns against environmental corrosion. They typically utilize commercially available proprietary Cold Blueing formulations to accomplish this goal.)
Additionally, blueing as a finishing surface treatment may occur in other limited production environments. For instance, a few modern cutlass and knife blade manufacturers also perform bluing to increase blade corrosion resistance. Manufacturers of steel or ferrous metal parts used in clocks, watches and electronic gear housings may conduct bluing if they anticipate their products will undergo frequent use in particularly harsh environments. For example, bluing permits a steel watch casing to resist damage better in arctic environments. The hazards imposed by Hot Blueing make this fabrication surface treatment less popular than some other, less expensive and less potentially toxic, commercial finishing techniques for widely used steel products.
The Browning Process
Closely related to bluing, a process called browning or "plum brown" obtained use several decades ago as a surface treatment for finishing steel exteriors in a reddish-to-dark-brown hue. Manufacturers employed caustic chemicals such as mercury salts or cyanide to create reddish rusting in steel or ferrous metal parts before subsequently treating the surface with the same chemicals used to cause blueing. The presence of extensive ferric oxide contributed a brown shade to the metal.
Although rarely utilized today due to the harsh chemicals involved, browning contributes to the color of some older firearms barrels. In Europe centuries ago, some blacksmiths referred to this fabrication process as "russetting". Gunsmiths developed a number of formulations for causing rusting in order to perform browning. The process gives a distinctive reddish-brown shade to steel items, such as gun barrels.