Pavonado del acero

El pavonado es un acabado para piezas de acero, de gran duración, efecto decorativo y resistencia a la corrosión. El ácido ferroso-ferrico que se forma protege el hierro subyacente de su ulterior alteración.

El pavonado atrae y retiene los aceites lubricantes. El revestimiento no aumenta ni disminuye las dimensiones de los metales tratados, por lo que las tolerancias para el ajuste de piezas no se ven afectadas. Además, las superficies tratadas pueden ser soldadas, enceradas, barnizadas o pintadas.

Los colores que se obtienen varían del negro al azulado, según la clase de aleación tratada. A veces el tono es marrón. Los tonos negros y azules se dan con pinturas de sales de diversos metales y el colorante correspondiente.

Se obtiene un revestimiento mate cuando se aplica sobre una superficie tratada con chorro de arena o con un mordiente químico, y un revestimiento brillante sobre una superficie pulida o lisa.

El pavonado se puede aplicar a herramientas de corte, resortes, castillos de válvula, portaherramientas, fijadores, moldes, componentes y bloques de máquinas hidráulicas, cadenas, ruedas dentadas, engranajes, tornillos, herramientas manuales y de máquinas, componentes automotores en general, etc.

Procedimientos de pavonado

El pavonado se puede aplicar de forma sencilla, económica y segura, aunque también hay procedimientos peligrosos. Los procesos de pavonado se pueden dividir en dos grandes grupos: en caliente y en frío. La mayoría de las recetas se refieren al pavonado en caliente, que corresponde con los métodos tradicionales.

El pavonado en frío se realiza a temperatura ambiente (20 a 30ºC) y utiliza soluciones líquidas concentradas (p. ej. de la marca Blackfast) que se disuelven en agua y a través de reacciones químicas producen un acabado negro, adherente y uniforme en la superficie de las piezas tratadas. Se trata de un proceso ecológico, extremadamente sencillo y seguro. Se pueden tratar piezas de acero y hierro fundido con tenor de cromo inferior al 12%. Estos procedimientos suelen ser más ornamentales que protectores. Para aumentar la protección es necesario aplicar al final una capa de aceite hidrófugo que sella los micro-poros.

El pavonado en caliente puede realizarse a baja (135-140ºC) o a alta temperatura (250-1000ºC).

El pavonado en caliente a baja temperatura utiliza distintos productos químicos (normalmente sales cáusticas) y a veces se lo conoce como pavonado químico o pavonado con sales. Pueden conseguirse distintos tonos desde marrones y azulados a negros. Algunas fórmulas desprenden gases o salpicaduras que son muy venenosas o corrosivas y deben manejarse con extremado cuidado. Pueden no funcionar en acero con contenido de cromo superior al 12%.

El pavonado en caliente a alta temperatura suele ser sencillo pues sólo requiere de un horno o una llama y algún producto fácil de conseguir como cuero, aceite, etc. Pero tiene el principal inconveniente de que la temperatura puede afectar al temple de la pieza. También puede producir deformaciones, pues es difícil conseguir una temperatura homogénea, por lo que suele usarse para piezas pequeñas o en trabajos artesanales, donde puede dedicarse más tiempo a cada pieza.

En todos los casos hay que tener en cuenta que una de las principales características del pavonado es que la capa protectora es tan fina que no se modifican las dimensiones de las piezas. Esa misma es la causa de que el pavonado no oculta los defectos, por lo que para un acabado correcto los trabajos de pulido, limpieza, desengrase y aclarado previos son incluso más importantes que el pavonado en sí mismo.

En general, la pieza debe estar libre de grasa, y en muchos casos, también de óxido. Esto puede requerir algún proceso previo al pavonado propiamente dicho. Igualmente, según el tipo de acabado que se desee, mate o brillante, puede ser necesario aumentar la rugosidad de la superficie o, contrariamente, un pulido previo.

Clasificación

Material necesario

Antes de comenzar hay que tenerlo todo preparado. Además de los elementos de seguridad personal (guantes de goma, gafas de protección, mono de trabajo o ropa vieja, etc., podemos necesitar las vasijas y recipientes de vidrio o acero inoxidable, un suministro de agua corriente fría y caliente, alguna vasija de plástico para el desengrase previo, un termómetro que pueda medir temperaturas de 150º centígrados o más, alicates o pinzas para manipular las piezas calientes, una balanza que permita pesar con una exactitud de un gramo, una probeta o vaso graduado para medir líquidos, alambres para preparar un sistema de cuelgue de las piezas para que no toquen el fondo y las paredes... y todo lo que permita trabajar cómodo y seguro.

Si se van a usar productos químicos corrosivos o venenosos el lugar debe estar bien ventilado y disponer de salida de emergencia. Hay que haber leído las medidas de seguridad de cada producto.

Iron oxides and rust

Throughout history, manufacturers have explored different metal finishing processes to protect their carefully crafted products from oxidisation. Their solution to the problem was to oxidise the metal surface and render it inactive from further attack.

Rust

Rusting is the common term for corrosion of iron and its alloys, such as steel. In colloquial usage, the term is applied to red oxides, formed by the reaction of iron and oxygen in the presence of water or air moisture. Other forms of rust exist, like the result of reactions between iron and chloride in an environment deprived of oxygen which generates green rust. Many other metals undergo equivalent corrosion, but the resulting oxides are not commonly called rust.

Strictly speaking, rust consists of hydrated iron(III) oxides Fe₂O₃·nH₂O and iron(III) oxide-hydroxide FeO(OH)·Fe(OH)₃. Several forms of rust are distinguishable visually and by spectroscopy, and form under different circumstances.

When impure (cast) iron is in contact with water, oxygen, or other strong oxidants, or acids, it rusts. Iron metal is relatively unaffected by pure water or by dry oxygen. If salt is present, the iron tends to rust more quickly, as a result of electrochemical reactions.

As with other metals, like aluminium, a tightly adhering oxide coating, a passivation layer, protects the bulk iron from further oxidation, but the combined action of two agents, usually oxygen and water converts the passivating ferrous oxide layer to disintegrative rust.

In comparison with a tightly adhering oxide coating, rust, the hydrated red oxide of iron (Fe₂O₃), undergoes an extremely large volume change upon hydration; as a result, the oxide easily flakes off causing the typical reddish rusting away of iron and provides no protection to the underlying iron.. Given sufficient time, oxygen, and water, any iron mass will eventually convert entirely to rust and disintegrate.

Iron oxides

Among the many known oxides, hydroxides and oxyhydroxides of iron , there are three iron oxides (chemical compounds composed only of iron and oxygen):

Deoxidizing and cleaning

Es muy importante que la pieza debe estar libre de grasa, y en muchos casos, también de óxido. Esto puede requerir algún proceso previo al pavonado propiamente dicho. Igualmente, según el tipo de acabado que se desee, mate o brillante, puede ser necesario aumentar la rugosidad de la superficie o, contrariamente, un pulido previo.

Rust protection

There are many processes to get a protective coating to steel and, among them, those based in creating an oxide passivating shell are the oldest and cheapest. Blued finish options are offered as the least expensive finish, and this finish is also the least effective at providing rust resistance, relative to other finishes such as Parkerizing or hard chrome plating or nitriding processes like Tenifer.

When exposed to air, many metals naturally form a hard, relatively inert surface. The reduction of the rate of corrosion will vary, depending on the metal and its environment, and is notably slowed at room-temperature air for aluminium, chromium, zinc, titanium, and silicon (a metalloid); the shell inhibits deeper corrosion, and so is the key factor of passivation. The inert surface layer, termed the ‘’native oxide layer‘’, is usually an oxide or a nitride, with a thickness of a monolayer (1-3Å) for a noble metal like platinum, about 15 Å for silicon and nearer to 50Å for aluminium after several years.

Rust protection on steel based in creating an oxide passivation coating varies depending on which the oxide is:

Many older browning and bluing formulae are based on corrosive solutions (necessary to cause metal to rust), and often contain cyanide solutions that are especially toxic to humans.

Comparison of Coatings for Iron and Steel

	Black Oxide	Cold Blackener	Cold Blackener (gel)	Aluminum Oxide	Zinc Phosphate

Colour:	Satin Black	Satin Black	Satin Black	Satin Silver/Gray	Matte Gray
Coating:	Magnetite	Copper Selenide	Copper Selenide	Aluminum Oxide	Zinc Phosphate
Temp:	93 °C	20 °C	20 °C	65 °C	65 °C
Application:	Immersion	Immersion	Brush-On	Immersion	Immersion
Time:	30 min	20 min	2-10 min	20 min	20 min

Browning

Browning is controlled red rust Fe₂O₃and is also known as pluming or plum brown.

One can generally use the same solution to brown as to blue. The difference is immersion in boiling water for bluing; the rust then turns to black-blue Fe₃O₄.

Blackening

Black oxide or blackening is an electrochemical process for ferrous materials in which the surface is converted into an oxide coating resulting from an oxidizing chemical reaction with iron on the surface selectively forming magnetite (Fe₃O₄), the black oxide of iron. It is used to add mild corrosion resistance, for cosmetic appearance, to minimise light reflection and to helps to maintain the metal finish by resisting tangential scratching.

Traditionally, blackening was called bluing and now the later designates only the traditional procedures to distinguish them from some other more modern black oxide coatings, although bluing is a subset of black oxide coatings.

Blackening and bluing are passivation processes (in which steel is partially protected against rust), and is named after the blue-black appearance of the resulting finish. One of its advantages over other coatings is its minimal build-up.

Black oxide provides small protection against corrosion, unless also treated with a water-displacing oil or wax to reduce wetting and galvanic action.

Bluing, being a chemical conversion coating, is not as robust against wear and corrosion resistance as plated coatings, and is typically no thicker than 2.5 micrometers. For this reason, it is considered not to add any appreciable thickness to precisely-machined parts.

Se obtiene un revestimiento mate cuando se aplica sobre una superficie tratada con chorro de arena o con un mordiente químico, y un revestimiento brillante sobre una superficie pulida o lisa.

El pavonado en caliente puede realizarse a baja (135-140ºC) o a alta temperatura (250-1000ºC).

The caustic soda of the blackening compound bonds chemically to the surface of the metal, creating a porous base layer on the part. Oil is then applied to the heated part, which seals it by "sinking" into the applied porous layer. It is the oil that prevents the corrosion of the workpiece. There are many advantages of blackening, mainly:

Cold bluing

Modern cold bluing is not an oxide conversion coating, but rather a deposited copper selenium dioxide based compound that colours steel black, or more often a very dark grey.

There are compounds difficult to apply evenly which tends to rub off easily, offers minimal protection and is generally best used for small fast repair jobs and touch-ups to prevent a small scratch from becoming a major source of rust over time.

There are also Copper Selenoid Phosphate solutions which provide corrosion protection that surpasses that of the traditional hot black oxide method. The corrosion protection is afforded due to the holding ability of the microcrystalline converted surface. The bound microcrystalline black surface adheres applied corrosion inhibitors to the surface for a longer period of time.

Hot bluing or Traditional Caustic Blackening

The 'hot' process may be applied, for example, by immersing the steel parts to be blued in an alkali salt solution (solution of potassium nitrate and sodium hydroxide in water, and even some nitrites) heated to the boiling point, 135°C to 154°C, depending on the recipe. Water must be periodically added to the bath, with proper controls to prevent a steam explosion.

Similarly, stainless steel parts to be blued are immersed in a mixture of nitrates and chromates, heated to the boiling point.

Hot blackening involves dipping the part into various tanks. These tanks contain, in order, alkaline cleaner, water, the hot blackening solution, and finally the sealant, which is usually oil.

This method was adopted by larger firearm companies for large scale, more economical bluing. It does provide good rust resistance which is improved with the use of oil.

This process is not safely used to re-blue vintage shotguns. Many double barrelled shotguns are silver brazed together and many of the parts are attached by that method also. The higher temperatures of the 'hot' processes as well as their caustic nature can weaken the brazed joints and make the gun hazardous to use.

Large scale industrial hot bluing is often performed using a bluing furnace. This is an alternative method for creating the black oxide coating. In place of using a hot bath (although at a lower temperature) chemically-induced method, it is possible through controlling the temperature to heat steel precisely such as to cause the formation of black oxide selectively over the red oxide.

There are several other methods of hot bluing. The oldest and most widely used specification for hot black oxide is MIL-DTL-13924 and MIL HDBK 205A, which cover four classes of processes for different substrates. Alternate specifications include AMS 2485, ASTM D769, and ISO 11408.

For a good black oxide

Además de los elementos de seguridad personal (guantes de goma, gafas de protección, mono de trabajo o ropa vieja, etc., podemos necesitar las vasijas y recipientes de vidrio o acero inoxidable, un suministro de agua corriente fría y caliente, alguna vasija de plástico para el desengrase previo, un termómetro que pueda medir temperaturas de 150º centígrados o más, alicates o pinzas para manipular las piezas calientes, una balanza que permita pesar con una exactitud de un gramo, una probeta o vaso graduado para medir líquidos, alambres para preparar un sistema de cuelgue de las piezas para que no toquen el fondo y las paredes... y todo lo que permita trabajar cómodo y seguro.

Si se van a usar productos químicos corrosivos o venenosos el lugar debe estar bien ventilado y disponer de salida de emergencia. Hay que haber leído las medidas de seguridad de cada producto.

Mid temperature caustic black oxide

Mid temperature black oxide blackens at a temperature of 104-118 ºC, significantly less than hot black oxide. This is advantageous because it is below the boiling point, meaning there are no caustic fumes produced. This makes the process much safer for the finishing line operator. Also, the user of mid temperature black oxide can see a significant reduction in energy costs.

Since mid temperature black oxide is most comparable to hot black oxide, it also can meet the military specification MIL-DTL-13924 as well as AMS 2485.

Mid temperature acid black oxide

Rust Bluing and Fume Bluing processes are between hot and cold bluing and use acids instead of alkalis. The difference between them is the way to apply the acid.

They were originally used by gunsmiths in the 19th century to blue firearms prior to the development of hot bluing processes, but they were later abandoned by major manufacturers as it often took parts days to finish completely, and was very labour intensive. It is still sometimes used by gunsmiths to obtain an authentic finish for a period gun of the time that rust bluing was in vogue, analogous to the use of browning on earlier representative firearm replicas.

These processes are safely used to re-blue vintage shotguns where there are soldered parts, as hot bluing solutions would dissolve the solder during the bluing process.

One of the reasons why rust and fume bluing tend to be more rust and corrosion resistant than any other method is because the process continually converts any metal that is capable of rusting into magnetite (Fe3O4). Treating with an oiled coating enhances the protection offered by the bluing.

Rust bluing

The process was to coat the gun parts in an acid solution, let the parts rust uniformly, then immerse the parts in boiling water to stabilize the rusting process by removing any remaining residue from the applied acid solution. Then the rust was carded (scrubbed) off, using a carding brush or wheel. A carding brush is a wire brush with very soft, thin (usually about .05 mm thick) wires. This process is repeated until the desired depth of colour is achieved or the metal simply will not colour any further.

Fume bluing

In fume bluing, instead of applying the acid solution directly to the metal parts, as in rust bluing, the parts are placed in a sealed cabinet with a moisture source, a container of nitric acid and a container of hydrochloric acid. The cabinet is then sealed. The mixed fumes of the acids will produce a uniform rust on the surface of the parts (inside and out) in about 12 hours.

The parts are then boiled in distilled water, blown dry, then carded, as with rust bluing.

Furnace bluing

Bluing can also be done in a furnace, for example for a sword or other item traditionally made by a blacksmith or specialist such as a weaponsmith.

Heat / Oil Bluing

This system implies to heat the piece between 320-480 degrees and is a carburising method. It has to be done carefully because the temperature could draw the temper out of certain steels.

Puede producir deformaciones, pues es difícil conseguir una temperatura homogénea, por lo que suele usarse para piezas pequeñas o en trabajos artesanales, donde puede dedicarse más tiempo a cada pieza. The process is only recommended for parts where tolerances are unimportant.

Colouring

Even thought some rust protection is provided by the following methods, they are more suitable only to colour or harden the metal

Niter bluing

This is not a chemical means of bluing. Parts to be niter blued are steel which has been polished and cleaned, then immersed in a bath of molten salts; typically potassium nitrate and sodium nitrate (sometimes with 1⁄3 ounces (9.4 g) of manganese dioxide per lb of total nitrate). The mixture is heated to 590 to 610 °F (310 to 321°C) and the parts are suspended in this solution with wire. The parts must be observed constantly for colour change. The cross section and size of parts will affect the outcome of the finish and time it takes to achieve.

This method is generally employed on smaller parts such as pins, screws, sights, etc. The colours will range through straw, gold, brown, purple, blue, teal, then black.

Examples of this finish can be seen commonly on older pocket watches whose hands exhibit what is called "peacock blue", a rich iridescent blue.

Colour case hardening

This is the predecessor of all metal colouring typically employed in the firearms industry. Contemporary heat-treatable steels did not exist or were in their infancy. Soft, low-carbon steel was used, but strong materials were needed for the receivers of firearms. Initially case hardening was used but didn't offer any aesthetics.

Colour case hardening occurs when soft steels were packed in a reasonably airtight crucible in a mixture of charred leather, bone charcoal and wood charcoal. This crucible was heated to 1,350 °F (730 °C) for up to 6 hours (the longer the heat was applied the thicker the case hardening). At the end of this heating process the crucible is removed from the oven and positioned over a bath of water with air forced through a perforated coil in the bottom of the bath. The bottom of the crucible is opened allowing the contents to drop into the rapidly bubbling water.

The differential cooling causes patterns of colours to appear as well as hardening the part.

Patina

Copper, copper based alloys, zinc, powdered metals, and silver solder can also be protected with an coating of their respective oxides (patina).

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Browning, Bluing and Blackening

Pavonado