TYPES OF METAL FINISHES FOR FASTENERS: AN INTRODUCTION
Metal finishing refers to the final production processes applied to metal parts and fasteners to produce finished products. In many cases, this involves coating, plating, and other finishing processes that equip the base metal surface with desirable characteristics like corrosion resistance, aesthetic appeal, hardness, durability, abrasion resistance, or lubrication.?
Finishing processes are commonly applied to all different kinds of fasteners, including?pins,?retaining rings,?key stock and machine keys, and more. Depending on the fastener in question, there are many different types of metal finishes that could protect and elevate the fastener’s performance.?
Type of Metal Finishes
There are several types of metal finishes, each utilizing different metal finishing techniques. The most popular types can be broken down into four main groups:
Coating Finishes:?Coating either introduces or creates a new substance to provide a protective layer to the metal fastener. Common coatings include:
Plating Finishes:?Plating is a process that fuses another metal to the base metal surface. The two most common processes are:
Oxidizing Finishes:?This refers to a group of finishes that create an oxide layer to protect the base material. There are three common types of oxidizing finishes:
Mechanical Finishing Processes:?These are processes that physically roughen up the surface of a metal fastener, typically in preparation for a secondary metal finish. They include:
Before introducing the specific types of metal coating, it is important to understand a few key definitions. Substrate refers to the base metal material that will receive the finish. In many cases, the substrate will be sprayed or painted. When an electrical current is used in a finishing process, the substrate will also either be an anode (a positively charged electrode that will dissolve) or, more commonly, a cathode (a negatively charged electrode that will attract anodes).?
It is also important to note that these finishes can be added on top of each other. For instance, most finishing processes start with a deburring process like metal grinding or vibratory finishing. Many electroless plated parts start off with being electroplated, and electroplating often provides a base for additional coatings. However these finishes will be used, a basic introduction is necessary to understand how they function.?
Metal Coating
Coating?is used as a broad term that encompasses most metal finishing processes that add a protective layer to a metal fastener. This can cause some confusion because it also refers to specific types of metal finishes. There are four common types of coating: conversion coating, powder coating, dip-spin coating, and paint.?
Conversion Coating
Conversion coating?is a process that transforms – or converts – the composition of a metal surface into a more protective substance. Unlike other coating and plating options, conversion coating does not add a new substance; rather, it alters the substrate material to protect itself. This is accomplished through either chemical or electro-chemical means.?
Phosphate Conversion Coating
Phosphate conversion coating?takes place in a chemical bath that contains phosphoric acid and other additives – like manganese, iron, or zinc – that either sprays or immerses the metal substrate (typically steel).?
A chemical reaction occurs when these elements interact with each other and increase the pH level near the substrate. This eventually creates crystalline structures which are deposited onto the substrate surface. The substrate is left with a crystal layer that:
A problem that arises with this finish is that the process is high-maintenance, which may result in extra labor and longer processing times.
Zinc and iron phosphate coatings are often used as a paint base, while manganese works well as an oil base. The coating is commonly used for fasteners in military, medical, and electronic equipment.?
Black Oxide Conversion Coating
To achieve a?black oxide coating?, the substrate (typically carbon or stainless steel) is dipped in a boiling alkaline chemical solution with a mixture of sodium hydroxide, nitrites, and nitrates. The solution reacts with the iron in the steel to convert the substrate surface into magnetite, which is an iron oxide mineral. From here, the metal fasteners are dipped in a supplemental protective substance.
The purpose of a black oxide conversion coating is to achieve an aesthetic black color. The coating itself does not provide much protection, so the process is usually followed by a wax or oil dip that fills the black oxide pores and offers corrosion resistance and can repel water.
Black oxide coatings offer several advantages:?
Its main disadvantage is its need for an additional coating if the product requires corrosion resistance.
This coating is common for fasteners in the firearm, medical, automotive, and hardware industries.?
Chromate Conversion Coating
Chromate conversion coating?is a complicated chemical reaction. It occurs when a metal – typically aluminum – is sprayed with or immersed in a chemical bath of chromium salts, chromic acids, and other components. The chemical reaction eats away at the substrate and creates a soft surface that will harden as it dries and potentially change color, depending on the chemicals in the bath. In practice, the process is much more complex and often involves second or third “dips” in additional chromium solutions.?
This coating cannot be discussed without addressing its?significant safety concerns. Chromate conversion coating utilizes hexavalent chromium – which is known to potentially cause lung cancer, severe respiratory irritation, allergic reactions, and skin sores – at multiple stages of the process. For these reasons, the manufacturing industry has been moving away from this coating since 2007.?
EU directives like?RoHS?and?REACH?address these concerns. You can read Huyett’s?RoHS?and?REACH?compliance statements on our?Quality Assurance page.?
Other Coatings
Outside of conversion finishes, coating typically refers to processes that add a protective layer onto a substrate. Common coatings of this nature include powder coatings, dip-spin coatings, and paint.?
Powder Coating
Powder coating?is a popular, cost-efficient technique that involves three steps: pretreatment, application, and curing.?
In pretreatment, the substrate is thoroughly cleaned to remove any particles. In many cases, it is also scratched or etched to roughen it up and help the powder adhere to the surface. The fastener is completely dried before moving on to the next step.
In the application stage, the substrate is either sprayed (which is most common) or dipped in a powder coating. The powder is actually a dry plastic that, when electrically charged, will adhere to the grounded substrate.?
Electrical charge is added to the powder either in the nozzle of the spray gun or in a fluidized powder bed.?
The substrate is almost immediately moved to the curing process, where it is heated to up to 450?F for up to an hour or more, depending on the size of the fastener. If the fastener was dipped instead of sprayed, the curing process may also take place as it is dipped.?
Powder coating offers a range of advantages as a finishing option:
However, it is not without a few drawbacks:?
Dip-Spin Coating
Dip-spin coating?– also called spin coating – is defined more by the process than the materials used. As the name suggests, fasteners are dipped in a coating (zinc coatings, molydisulfides, PTFE, Geomet, Magni, and others) and then spun to create a thin, even coating.?
The main advantage of the dip-spin process is the uniform coating it creates due to the force of the spin. It is also a cost-efficient process for coating small fasteners in bulk. However, it may create significant waste, as a large portion of the coating may be lost when it is spun off.?
Paint
Paint?is a simple, economic choice for metal fasteners. While it does not provide the kind of protection that other coatings or platings would, it can protect against water and light corrosion. Its main advantages are that it offers cost-efficiency, color and texture options, and a simple application process. On the other hand, materials need to be repainted every so often, so maintenance cost and labor may build up over time. Learn more about the?comparisons between powder coating and paint.?
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Metal Plating
Plating?is a common metal finishing process that fuses metal particles onto the surface of the substrate to form a protective layer. The base substrate is typically a lower-cost metal that needs the properties of a more noble metal (like gold, silver, or platinum), which can provide extra hardness, corrosion or heat resistance, electrical conductivity, and aesthetic appeal.
Plating is also referred to as a type of coating because it adds a substance as a protective layer. However, plating is different because it specifically refers to metal fusing to metal.
There are several plating processes that accomplish this fusion, with the two most common being?electroplating?and?electroless plating?.?
Electroplating
Electroplating?is a process that fuses metal particles to the base metal material through electrical currents. It is one of the most common plating processes in industrial metal finishing because it is simple and cost-effective. It offers costs savings in a number of ways:
There are two important definitions to keep in mind when discussing electroplating:
Electroplating begins by thoroughly cleaning the cathode in a multi-step process that removes contaminants and prepares the surface to accept the anode (the plating metal). Then, the cathode (substrate) is placed into a container of water, the anode, and chemicals that increase the electrical conductivity and will help transport the anode to the cathode.?
The anodes connect to a positive electrical terminal, while the cathode connects to a negative electric terminal. When the electrical source is activated, the anode dissolves, attaches itself to the cathode, and fuses to the surface. This can take anywhere from a few minutes to a few hours, depending on the desired plating thickness.?
Anode materials are chosen based on the desired property they will add to the cathode, but the base requirement is that both metals are conductive. Common anodes and their benefits include:?
There are not many restrictions when it comes to which metals can plate which substrates. However, the anode needs to have more negative potential than the cathode. There may also be instances where the substrate needs multiple platings to achieve the desired end goal. For example, silver will not bind with steel. If silver is the desired final plating, the steel will need to be plated in copper first so the copper can bind with the silver.?
Electroless Plating
With?electroless plating?– also called electroless deposition or autocatalytic plating – a metal plating is still fused to a substrate for similar purposes (less expensive metal is coated in noble metals to acquire their superior properties on the surface). However, chemicals, rather than electricity, instigate the fusion.
While electroless plating is a more expensive option, it does provide several advantages over electroplating:
The process of electroless plating involves an in-depth cleaning and surface preparation of the substrate, just as with electroplating. After this, a container is filled with the plating’s metal salt solution before adding the substrate to the bath. Once the substrate is in place, a reduction agent – instead of an electrical current – breaks the plating metal down into atoms, which bond to the substrate surface.
The main drawback to electroless plating is the chemical bath, which must consist of the plating’s salt solution, accelerators, complexing agents, and other elements to ensure the metal is sustainably and evenly distributed. It must also be manually replenished, whereas the electroplating bath replenishes itself as the electrical charge draws out the anode. However, electroless plating produces a higher quality surface.?
Nickel is the most common plating for this process because it reduces friction while providing exceptional corrosion resistance and good ductility. However, copper, palladium, silver, and gold are also used.?
Oxidizing Finishes
There are a few metal finishes that resemble plating and conversion coating but possess a few key differences. These include?anodizing?,?passivation?, and?electropolishing?.
Before delving into the finishes, it is helpful to understand exactly what an oxide layer is:
Oxide Layer:?A thin layer of an?oxide, which is a chemical compound composed of at least one oxygen atom and another chemical element.?
These processes immerse the substrate in chemical baths and use its own metal properties to provide protection by manipulating the base material to protect itself. Each process produces an oxide layer in their own way.?
Anodizing
Anodizing?is an electrochemical process that sounds similar to the electroplating and conversion coating, but it is unique in that it produces an oxide layer on the substrate surface. The most common anodized metal is aluminum, but it is possible to anodize other nonferrous metals.?
There are several steps that usually take place to anodize aluminum:?
Steps 1,2,4, and 5 are standard steps in many finishing processes. The anodizing step consists of lowering the aluminum material into an electrolytic bath with an electric current. What happens next is essentially oxidation in that the bath releases oxygen that will come in contact with the aluminum. The atoms on the surface of the aluminum interact with the oxygen and produce an oxide layer that both integrates with and grows outward from the aluminum surface.?
The final sealing step is essential because the oxide layer is porous. Once sealed, the oxide layer provides corrosion resistance, durability, and a decorative finish.?
Anodizing offers several advantages for aluminum products:
The main disadvantage is that, since the aluminum reacts with itself, it is hard to attain a consistent coloration between batches of fasteners.?
Passivation
Like anodizing,?passivation?is an economical, environmentally friendly process that protects against corrosion and rust. It sounds similar to anodizing because it creates its own protective oxide layer; however, it is specific to stainless steel and requires acids to instigate the reaction.?
The purpose of passivation is to remove free iron – which causes rust– from the substrate surface to accentuate the chromium presence and increase corrosion resistance. This creates a “passive” surface, meaning that the stainless steel is less reactive to environmentally corrosive elements.?
Passivation is a multi-step process that starts with a deep clean of the stainless steel that removes oil and debris. After it is rinsed, it is placed in an acid bath of either citric acid, nitric acid, or nitric acid with sodium dichromate. These are mild acids that will remove the free iron on the surface of the stainless steel but leave the chromium intact.?
After the acid bath, the stainless steel is rinsed and air dried for 1-2 days. The oxygen in the air interacts with the chromium surface to form a protective oxide layer.?
Passivation offers several advantages for stainless steel products:?
Electropolishing
Electropolishing?is often confused with passivation and anodizing because it produces an oxide layer. However, it is actually somewhat of a mix of the two latter processes to achieve superior polishing on stainless steel fasteners. Let’s lay out the differences between the three types of finishes:
Electropolishing uses the stainless steel substrate as the anode rather than the cathode. The surface components of the substrate detach from itself, which leaves the surface open to passivation. As this happens, it also smooths the stainless steel surface by dissolving impurities and settling the substrate’s internal bumps and valleys. The result is a much smoother and aesthetically appealing product than if it had just been passivated.?
Electropolishing does come with a cost. It is typically more expensive than passivation and takes more time to complete. It is also the only process of the three mentioned above that produces hazardous waste. However, it is much better suited to prevent contamination and corrosion.
Mechanical Finishing Processes
In addition to the chemical and electrochemical metal finishes, there are several mechanical finishes that rough up or deburr metal surfaces. In some cases, such a process is done as a solo finish. However, most of the other finishing processes require a pretreatment that prepares the metal surface of the substrate to accept the coating, plating, or release of components. These can utilize the following treatments:?
Mechanical Finishes for Your Fasteners
Huyett offers a broad selection of?industrial fasteners and lifting hardware?available in a wide range of finishes to suit the needs of your application. You can find these finish options in the left hand menu of all our search results pages. If you don’t see what you’re looking for,?contact our Sales Team?to get in touch with Engineering Support.?