BS EN ISO 2080
3.1 General Types of Surface-Finishing Processes and Treatments — FAQ
3.1.1 What is Chemical Plating?
Chemical Plating is the deposition of a metallic coating by chemical, non-electrolytic methods.
The term "chemical plating" is a broad category. Its key, defining feature is the absence of an external electrical current, which is what separates it from "electroplating."
In electroplating, you use a power source (a rectifier) to create a circuit. The part you want to plate is the cathode (negative), and a metal "anode" (positive) is also placed in the electrolyte bath. When you turn on the power, metal ions from the bath are forced onto the part.
Chemical plating, by contrast, gets its energy from a chemical reaction inside the bath itself. The plating solution is a complex, unstable mixture that contains:
1. Metal Ions: The metal you want to deposit (e.g., zinc or nickel).
2. A Reducing Agent: A chemical (e.g., sodium hypophosphite) that will provide the "energy" (in the form of electrons) to turn the metal ions into solid metal.
This process is also known as electroless plating.
ISO 2080 further breaks "chemical plating" down into three specific types:
3.1.1.1 Autocatalytic Plating
DEPRECATED: Electroless Plating
Deposition of a metallic coating by a controlled chemical reduction that is catalysed by the metal or alloy being deposited.
This is the most common and commercially important type. The deposition is started by a catalyst (often the base metal itself) and then crucially the freshly plated metal acts as its own catalyst. This means the reaction is self-sustaining and will continue to build up the coating thickness as long as the part is in the solution and the chemicals are replenished.
3.1.1.2 Contact Plating
Deposition of a metal by use of an internal source of current by immersing the work (3.2.218) in contact with another metal in a solution containing a compound of the metal to be deposited.
This is a less common method where the part to be plated is immersed in the solution while in direct physical contactwith a different, less noble metal (like aluminium or zinc). This creates a small, internal galvanic cell (like a battery), which drives the deposition without an external power source.
3.1.1.3 Immersion Coating or Displacement Plating
Metallic coating produced by a displacement reaction in which one metal displaces another from a solution
EXAMPLE: Fe + Cu2+ → Cu + Fe2+
This is a simple displacement reaction where a more active metal (like steel) is dipped in a solution containing ions of a less active metal (like copper). The steel (iron) dissolves, "displacing" the copper, which then deposits onto the part's surface. This process is self-limiting and stops as soon as the entire surface is covered, resulting in a very thin coating.
The main advantage of autocatalytic (electroless) plating over electroplating is its perfect uniformity. Because it's not dependent on electrical current, which favors high points and edges, the chemical deposit builds up at the same rate everywhere—on flat surfaces, in sharp corners, and even deep inside holes or tubes.
The most common industrial standard that directly links to this definition is:
BS EN ISO 4527: Metallic coatings — Autocatalytic (electroless) nickel-phosphorus alloy coatings — Specification and test methods.
This standard governs "electroless nickel" (ENP), which is the primary example of an autocatalytic plating process—itself a sub-type of "chemical plating" as defined in ISO 2080.
3.1.2 What is Chemical Vapour Deposition (CVD)?
Chemical Vapour Deposition (CVD) is the "deposition of a coating by a chemical reaction, induced by heat or gaseous reduction of vapour condensing on a substrate."
In simple terms, it’s a high-temperature process used to "bake" a high-performance ceramic or metallic coating onto a part. Unlike electroplating (which uses a liquid bath), CVD uses reactive gases in a vacuum.
How does the CVD process work?
The process happens inside a sealed vacuum reactor and follows a few key steps:
- Heating: The parts (the substrate) are loaded into the reactor and heated to very high temperatures, typically 900°C–1100°C (1650°F–2000°F).
- Introduction of Gases: One or more reactive "precursor" gases are pumped into the chamber. For example, to create a gold-coloured Titanium Nitride (TiN) coating, the gases might be titanium tetrachloride (TiCl4) and nitrogen (N2).
- Chemical Reaction: The intense heat energizes the gases, causing them to chemically react or decompose only when they hit the hot surface of the parts.
- Deposition: The solid product of this reaction (e.g., TiN) "condenses" onto the substrate, building up atom by atom. This forms an extremely dense, pure, and strongly bonded coating.
- Exhaust: All gaseous by-products (like hydrogen chloride, HCl) are continuously pumped out of the reactor.
What is CVD used for?
CVD is used to create extremely hard, wear-resistant, and heat-resistant coatings. The high temperatures create an exceptionally strong (diffused) bond with the part that is almost impossible to chip or flake.
It's commonly used for:
- Coating cutting tools, drill bits, and milling inserts to keep them sharp.
- Hard-wearing engine and automotive components.
- Biocompatible coatings on medical implants.
What is a standard specification that uses CVD?
A common standard that governs this process is ISO 13179-1: Implants for surgery — Plasma-sprayed, thermal-sprayed and chemical-vapour-deposition (CVD) coatings on metallic surgical implants....
3.1.3 What is a Conversion Coating (or Conversion Layer)?
A conversion coating (or conversion layer) is a coating obtained by a conversion treatment (3.1.4).
In simple terms, it's a protective layer that is chemically grown on the surface of a metal.
How does a conversion coating work?
Unlike paint or electroplating, which adds a new layer on top of the metal, a conversion coating is created by a chemical reaction that converts the top atoms of the metal itself into a new, stable compound.
- A metal part (e.g., steel, aluminium, or zinc-plated steel) is immersed in a specific chemical solution.
- This solution reacts with the metal surface.
- The reaction forms a new, insoluble, non-metallic layer that is chemically bonded to the metal.
This new layer is now an integral part of the part's surface.
What is a conversion coating used for?
Conversion coatings have three main purposes:
- Corrosion Protection:The new layer is more passive (less reactive) than the base metal, which significantly slows down rust and other corrosion. A perfect example is the passivate (clear, yellow, or black) applied over zinc plating.
- Improved Adhesion: The coating creates a uniform, micro-crystalline surface that is an ideal base for paint or powder coating to "grip" onto, preventing it from flaking or peeling.
- Decorative Finish: Some conversion coatings provide a specific aesthetic, such as the deep black of black oxide (bluing) on steel.
What are common examples of conversion coatings?
- Phosphate Coatings (Phosphating): A very common pre-treatment for steel parts before painting, especially in the automotive industry.
- Chromate & Trivalent Passivates:The clear, yellow, or black coatings applied to zinc, zinc-nickel, and aluminium.
- Anodising/Anodizing: An electrochemical process that creates a thick, durable oxide conversion coating on aluminium.
- Black Oxide (Bluing): A black iron oxide coating used on steel tools, firearms, and fasteners.
What is a standard specification for a conversion coating?
A clear example is ISO 9717: Metallic and other inorganic coatings — Phosphate conversion coatings for metals, which specifies the requirements for phosphating steel, aluminium, and zinc.
3.1.4 What is a Conversion Treatment?
A conversion treatment is a "chemical or electrochemical process producing a superficial layer containing a compound of the substrate (3.2.205) metal."
In simple terms, this is a process that changes the very surface of a metal part into a new, protective layer. Unlike painting or plating, which adds a new material on top, a conversion treatment converts the existing metal into a new chemical compound.
How does a conversion treatment work?
The process involves immersing a metal part (the substrate) into a specific chemical solution. This solution reacts with the atoms at the surface of the metal, transforming them into a new, stable, and insoluble layer.
This new layer is chemically bonded and integrated directly into the part, rather than just sitting on top of it.
The main purposes of this new layer are:
- To improve corrosion resistance.
- To provide a key for paint and powder-coating adhesion.
- To provide a decorative finish.
What are common examples of conversion treatments?
The definition provides several excellent examples:
- Passivation coatings on zinc, zinc alloys, and aluminium (e.g., the clear, yellow, or black finishes on zinc-plated fasteners).
- Phosphate coatings on steel (a very common pre-treatment before painting).
- Chromate conversion coatings (the traditional, often hexavalent, passivation method).
- Black Oxide (or "bluing") on steel.
Is Anodizing a conversion treatment?
This is a common point of clarification. The note to the definition states:
"Anodizing, although fulfilling the above definition, is not normally referred to as a conversion coating process."
While anodizing technically is an electrochemical process that converts the surface of aluminium into aluminium oxide, it is such a specialized and significant industrial process that it is almost always classified in its own category.
What is a standard specification for this process?
A clear example given is for phosphating. The relevant standard is:
ISO 9717: Metallic and other inorganic coatings — Phosphate conversion coatings for metals.
3.1.5 What is a Diffusion Treatment (or Diffusion Coating)?
A diffusion treatment is a "process of producing a surface layer (diffusion layer) by diffusion of another metal or non-metal into the surface of the substrate (3.2.205)."
In simple terms, this is a high-heat process that doesn't just lay a coating on a part, but actually soaks the coating material into the part's surface.
How does a diffusion treatment work?
This process uses high temperatures to cause the atoms of one material (like zinc or carbon) to "diffuse" or migrate into the crystal structure of the base metal. This forms a new, hybrid layer called a diffusion layer, which is an alloy of the two materials.
This is different from electroplating, which deposits a distinct, separate layer on top. A diffusion coating is an integral part of the substrate itself.
What are common examples of diffusion treatments?
The definition provides several examples from two categories:
For Electroplating:
- Electroplating (3.1.6): Diffusion treatment to form an alloy coating from two or more different electroplated coatings. (For example, plating a layer of tin and a layer of copper, then heating the part so they diffuse into each other to form a bronze (tin-copper) alloy layer).
For Non-Electroplating:
- Galvanising (3.1.7) (Hot-dip galvanizing forms zinc-iron alloy layers where the molten zinc meets the steel).
- Nitriding (Diffusing nitrogen into steel to harden it).
- Carburizing (Diffusing carbon into steel to harden it).
- Sherardizing (3.1.15) (A process of tumbling parts with zinc dust at high heat to form a zinc-iron diffusion coating).
What is not a diffusion treatment?
The definition makes a key distinction:
Post-coating heat treatment (3.2.128) after electroplating, for example, to remove hydrogen, is not normally designated as a diffusion treatment.
Even though baking a part for hydrogen de-embrittlement involves heat, its purpose is not to create a new alloy layer, so it isn't considered a diffusion treatment.
What is a standard specification for this process?
A clear example given is sherardizing. The relevant standard is:
ISO 17668: Zinc diffusion coatings on ferrous products - Sherardizing - Specification.
3.1.6 What is Electroplating (or Electrodeposition)?
Electroplating (or electrodeposition) is the deposition of an adherent coating of a metal or an alloy upon a substrate (3.2.205) by electrolysis for the purpose of imparting properties or dimensions to a surface different from those of the basis material (3.2.29).
How does electroplating work?
This is a chemical process that uses an external electric current to deposit a coating. In simple terms:
- A part (the substrate) is immersed in a liquid chemical bath (an electrolyte).
- The part is connected to the negative side of a power source, making it the cathode.
- A piece of the coating metal (e.g., pure zinc or nickel) is connected to the positive side, making it the anode.
- When the power is turned on, a current flows. This causes metal ions from the bath to be attracted to the part, where they "plate out" and form a solid, adherent metallic layer.
What is electroplating used for?
The purpose is to change the surface properties of the part. This includes:
- Improving corrosion resistance (e.g., zinc plating).
- Improving wear resistance (e.g., hard chromium plating).
- Providing a decorative finish (e.g., chrome plating on a car bumper).
- Increasing electrical conductivity (e.g., gold or silver plating on contacts).
What is a standard specification for this process?
Standards for electroplating define the required thickness, passivates, and performance. Examples we have already discussed include:
ISO 2081: ...Electroplated coatings of zinc...
ISO 19598: ...Electroplated coatings of zinc and zinc alloys... (e.g., Zinc-Nickel).