Corrosion Resistant Coatings
Corrosion can be described as the gradual chemical or electrochemical attack of an alloy or metal by agents in its surrounding environment. This attack of the metal or alloy causes disintegration of the surface as well as material loss due to one of two reasons:
- The conversion of the material into a much less adherent material (such as sulfide or oxide).
- The dissolution of the material into its surrounding environment.
In cases where wear mechanisms are present, this material loss is worsened.
Corrosion resistant coatings help protect against these gradual attacks and aids in increasing the lifespan of a part, in reducing its maintenance as well as aiding in reducing its replacement cost.
Types of corrosions will differ based on how a part is used as well as the conditions it is exposed to.
There are five general types of corrosion:
- General Corrosion
- Localized Corrosion
- Stress-Corrosion Cracking
- Galvanic Corrosion
- Caustic Agent Corrosion
This type of corrosion occurs as a result of rust. In cases where metals, and more specifically steel, is exposed to water, the surface of the metal is oxidized and a thin layer of rust appears.
It is an electrochemical corrosion and, in order to prevent oxidation, there has to be a preventative coating which interferes with the metal’s reaction to water.
This type of corrosion occurs in cases where a small part of the component experiences corrosion or is exposed to stresses that result in corrosion.
This small area now corrodes at a faster rate than the rest of the component, with localized corrosion working alongside processes such as fatigue and stress resulting in a much larger problem than fatigue and stress would have caused on its own.
A component can experience Stress-Corrosion Cracking (SCC) along the grain boundary when it is subjected to extreme tensile stress. This causes cracks to form, become a target for further corrosion.
SCC can be caused by a number of things ranging from thermal treatments and cold working, to welding. Combining these factors with the component’s environment (which in many cases can increase and intensifies stress-cracking, results in a component undergoing irreversible damage and experiencing failure.
Note that this type of corrosion is capable of damaging a component beyond repair.
Galvanic corrosion is a common corrosion type and occurs in cases where two metals, both with different electrochemical charges, are linked via a conductive path. Corrosion now occurs when the metal ions from the anodized metal moves to the cathodic metal.
In cases where electrochemical charges causes corrosion, manufacturers will apply a corrosion resistant coating to either:
- Prevent the condition that causes the transfer of ions, or
- Prevent the transfer of ions.
Note that this type of corrosion can also occur in cases where an impure metal is present. In instances where a metal that contains a combination of alloys (which in turn possess different charges), one of the metals stands the chance of being subject to corrosion.
Here, the anodized metal is less resistant and weaker. In turn, this weaker metal loses ions to the stronger, positively charged cathodic metal.
In cases where no electrical current is present, corrosion will take place uniformly. This corrosion is then known as general corrosion.
Caustic Agent Corrosion
This type of corrosion occurs in cases where a liquids, impure gases or solids wear a material down. Note that although impure gases do not damage the material in dry form, gases, like hydrogen sulfide, dissolve when it is exposed to moisture, in turn forming corrosive droplets.
Types of Corrosion Resistant Coatings
The corrosion resistant coating used will depend on:
- The type of metal, and
- The type of corrosion prevention needed.
Galvanic corrosion in iron and steel alloys can be prevented by using coatings that are made of aluminum and zinc. Iron and steel fasteners, bolts and threaded fasteners can be coated with a thin layer of cadmium. This is layer of cadmium is applied in order to help block the absorption of hydrogen as it can lead to stress cracking.
Because of low levels of porosity and high moisture resistance, both cobalt-chromium and nickel-chromium is also used. These coating aid in preventing the development of rust.
Furthermore, ceramic metal mixes and oxide ceramics coatings offer both a resistance to corrosion as well as a resistance to wear.
Chromium, copper, nickel and zinc are corrosion resistant metals and is widely used within the industry as protective coatings in order to control corrosion. Today, advancements in vapor deposition, anodizing, thermoplastics, thermal spray and other engineered polymers offer manufacturers an assortment of solutions to protect metals against corrosion.
Listed below are a few corrosion resistant solutions/coatings:
The process of anodizing involves the creation of a corrosion resistant film on the surface of the material, and is seen in both the aluminum and magnesium finishing industries. These electrolytic-based films do not only offer protection against atmospheric corrosion, but is highly stable as well.
Enhancements here are made by filling the coating’s micro porosity, making it non-absorptive. Note that these types of coatings are suited for use in marine environments and atmospheric conditions.
Plating, making use of Electroless Nickel Plating (EN), also referred to as electroless plating, offers surface protection against chemicals, has the ability to penetrate deep internal or recessed surfaces, as well as cover complex geometries. This process, unlike electrolytic processes, uses an aqueous bath without electrical energy.
With plating, base metal selection plays an important role. Homogenous metals are preferred over alloys as alloys are known to accelerate corrosion processes.
This corrosion resistant coating is ideal for applications that involve large structural components such as bridges, ship structures that are subject to salt water corrosion, large expanses of pipes (such as those in petrochemical plants or refineries), wind turbines and water towers.
With thermal spraying, a zinc or aluminum coating is applied as ‘sacrificial coatings’. These sacrificial coatings serves to protect the underlying substrate from corrosion. Note that these coatings are designed to deteriorate, but at a much slower pace than the substrate.
Sacrificial coatings both attract and capture oxygen molecules so that it cannot reach the underlying substrate, thus ‘sacrificing’ itself to protect the substrate. Coatings are capable of lasting for decades, making it effective for long-term corrosion protection in conditions where it is exposed to continuous corrosive attacks.
These coatings can also be used on smaller metal components that are subject to salt water or atmospheric corrosion, including parts such as electrical enclosures, exhaust pipes, pump housings, generator mufflers and valve bodies.
Note also that ceramic coatings can also offer an effective resistance against corrosion. These types of coatings can be used for a number of parts and applications such as sleeves, pump shafts or seal rings (found in the harsh environments of the mining, pulp and paper, and chemical industries).
This type of coating offers an excellent resistance to corrosion, especially for parts operating in environments where a hard, thin film is required.
Diamond-like coatings and some nitride-based and carbide materials are commonly used here, as it provides both a chemical corrosion stability as well as general oxidation stability. Note that vapor materials can be combined, multi-layered deposits that offer an enhanced protection.
The use of vapor coatings will rely on its ability to form ‘pin hole free structures’ (microscopic voids found in the coating which exposes the bare metal), as well as the coating’s chemical compatibility with its environment.
Chemical Vapor Deposition (CVD) forms the densest structures – structures formed using CVD has the least amount of microscopic voids that expose the bare metal. Note however that the process’ temperature range is between 1500°F (± 815.5°C) and 2200°F (± 1204.4°C), while Physical Vapor Deposition (PVD) has a process range of between 750°F (± 398.8°C) and 900°F (± 482.2°C).
These temperature ranges must be kept in mind, especially when it comes to the base metal being coated.
Advancements in vapor deposition includes Plasma Enhanced Vapor Deposition (PECVD) and Plasma Assisted Vapor Deposition (PACVD). Both the PECVD process and PACVD process has operating temperatures below 550°F (± 128.7°C), while still being able to deliver coatings that are uniform, resistant to corrosion and has excellent adherence.
Vapor deposition coatings are used in environments where moving parts are present or where abrasion is high.
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