Corrosion causes all sorts of problems for enterprises that buy metal alloys for use in various applications – only to find they’re surplus to requirements or they have the wrong alloy.
Leaking bathrooms, broken bridges, damaged oil pipelines and rusty car exhausts are all examples of metal corrosion.
Replacing these alloys is an expensive job, not only do you have to pay for the materials, but there’s also the added cost of the labour required to replace them.
Corrosion presents humungous challenges for engineers and repairs can run into thousands or hundreds of thousands of pounds for one business.
On the other hand, you may lack the experience and knowledge to buy corrosive resistant metals, which means you’ll want to get your first buy right.
We’re going to explain the basics of metallic corrosion and how you can find the correct alloy to help you prevent metallic corrosion in your applications.
What is Metallic Corrosion & What Causes Corrosion in Metals?
Corrosion is the deterioration of a metal or alloy or the gradual destruction of the material which is due to the environment it is present in.
Metals are subject to harsh conditions like extreme temperatures, or even simple elements like wind and water. The gases that come into contact with the metal determine the rate of corrosion, as well as the metal itself.
Gases such as hydrogen and oxygen cause corrosion, as well as dirt and grime, electric currents or when there is too much stress on the metal causing it to crack.
As you can see, there are many factors which can affect the corrosion resistance of metals, so it’s important to get the right one by speaking to a company that has experience with corrosion-resistant metals.
How Does Corrosion Affect Metals?
Corrosion can have either positive or negative effects, but we wouldn’t advise picking a metal that isn’t corrosion resistant when you need it to be. For example, sometimes the green patina that covers metals can prevent damage from the harsh weather conditions. But you have to consider the instability and potential damage corrosion can cause.
The Different Types of Corrosion
There are four different types of corrosion, they are:
General Attack Corrosion
Occurs on the surface of the metal, and is easy to treat and is a common form of corrosion. For example, you may have handled a copper coin that had a green surface.
Localised Corrosion
localised corrosion attacks part of a metal structure, it can be destructive as it’s quite difficult to predict, detect and characterise. There are three types of localised corrosion:
- Pitting – the creation of small holes in the surface of the metal.
- Crevice – an attack on the gap or the area next to the gap between to materials.
- Filiform – when water gets under the surface of a material and causes corrosion.
Galvanic Corrosion
Occurs when two metals join together in a liquid electrolyte such as saltwater. One metal draws the molecules of the other towards it and only one metal corrodes.
Environmental Cracking
In stressful environments, some metals can begin to crack or show signs of damage and fatigue or weakness.
What Metals Rust or Corrode?
Does Alloy Metal Rust?
First, it all depends on whether you mean rust or corrode. Corrosion is a type of oxidisation whereas rusting is a part of corrosion. If an alloy contains ferrous metal (iron), it will rust. All alloys can corrode. Rusting occurs when we expose the metal to air and moisture, creating a layer of iron oxide. Corrosion occurs when we expose metals to air and chemicals, which leaves a formation of oxides of metals or salts.
- Rust = only applies to iron.
- Corrode = applies to all metals.
Does Alloy Steel Rust or Corrode?
Stainless steel is a mix of elements and it does contain iron, so yes it can rust. However, most stainless steels contain around 18% or higher levels of chromium, which forms a protective layer (chromium oxide) on top of the metal, protecting it from corroding while the chromium and molybdenum content prevents rust.
Does Aluminium Alloy Rust or Corrode?
An aluminium alloy won’t rust because there is almost no iron in them. Without iron, the metal cannot rust. However, aluminium allows does oxidize, but when water hits the surface of the metal it forms a protective layer ‘aluminium oxide’, making it more resistant to corrosion.
Does Magnesium Alloy Rust or Corrode?
Because it doesn’t contain iron, magnesium alloys won’t rust. However, magnesium is susceptible to corrosion (particularly galvanic corrosion), which looks like a grey film on top of the metal.
Does Zinc Alloy Rust or Corrode?
Zinc doesn’t rust because it doesn’t have iron in it. When we expose zinc to air it reacts with the carbon dioxide and forms a layer of zinc carbonate. This protects the metal and prevents it from reacting to air and water, which is why we use zinc to galvanise other metals and prevent corrosion.
Does Nickel Rust or Corrode?
Nickel doesn’t rust as it doesn’t contain iron. Pure nickel is very corrosion resistant, especially to a variety of reducing chemicals. Alloying it with chromium gives resistance to oxidation. This leads to a broad variety of alloys, such as ZERON® 100, with optimal corrosion resistance in both reducing and oxidising environments.
Alloys based on nickel can tolerate more alloys than stainless steel and other iron-based materials while maintaining good stability. This flexibility has led to the development of a variety of nickel-based alloys with many alloys, designed to be resistant to a host of different corrosive environments.
Many alloying elements can join with nickel to resist corrosion in a variety of environments, and NeoNickel supplies them all. The exact metal alloy that’s right for you depends on your circ*mstances, and there are many potential options.
Nickel gives alloys metallurgical stability:
- Alloys have more thermal stability and weld better.
- Heightened resistance to reducing acids and caustics.
- Resistance to stress corrosion cracking increases – especially in chlorides and caustics.
Rivalling the benefits of these alloys is impossible. It’s little wonder they are so popular in aqueous applications.
Nickel-based metal alloys have excellent corrosion resistance properties. This makes them the material of choice for the construction of applications for many different industries. Their main use is in aqueous environments, in parts such as pumps, valves and piping systems.
How Does Alloying Prevent Rusting?
Alloying prevents rusting by combining several metals or elements that interact with each other to form a protective layer over the top of the surface of the metal. This barrier limits oxygen and air getting past the surface of the metal and penetrating the inner structure. Any ferrous metals that don’t contain other reactive metals that form this layer are subject to rust.
Different Alloying Elements & Their Corrosion Properties
The different alloying elements have an impressive set of properties, which you can see in our line cards.
Chromium
Resistance to oxidising corrosives and high-temperature oxidisation. Sulfidation is also improved. There is a boost in resistance to pitting and crevice corrosion.
Molybdenum
Improvement in the resistance to reducing acids, pitting and crevice corrosion in environments containing aqueous chloride. It increases high-temperature strength.
Iron
Improved resistance to high-temperature carburising environments, and helps to control thermal expansion. It reduces alloy costs.
Aluminium
Encourages age hardening. Improved resistance to oxidation and heightened temperatures.
Copper
Improved resistance to reducing acid. Notably, non-aerated sulphuric and hydrofluoric acid, and to salts. Added to nickel-chromium-molybdenum-iron alloys, it boosts resistance to hydrochloric, phosphoric and sulphuric acids.
Niobium (Formerly Known As Columbium)
Combines with carbon, which reduces vulnerability to intergranular corrosion caused by chromium carbide precipitation (that stems from heat treatments). It increases high-temperature strength. Improved resistance to pitting and crevice corrosion.
Tungsten
Improved resistance to reducing acids and localised corrosion. Weldability and strength are both boosted.
Nitrogen
Heightened metallurgical stability. It increases high-temperature strength. Resistance to carburisation and sulfidation is also improved.
Combining These Elements With Nickel
Many of these elements can alloy with nickel in different combinations, so a very broad range of corrosion-resistant alloys is available to suit a wide variety of environments.
Fabricating these alloys is easy due to their metallurgical stability and they can be thermally processed without the risk of harmful consequences.
Strengthening high nickel alloys is possible by hardening processes: precipitation hardening, dispersion strengthened powder metallurgy and carbide precipitations.
Which Alloys Offer the Most Protection to General Corrosion?
Things like pitting, crevice and stress corrosion are more localised which means they’re more difficult to predict. Predicting general corrosion is simple. There’s a method of rating each alloy’s ability to resist uniform attacks and determine which materials work better under testing circ*mstances.
Those ratings are ‘Excellent’, where an alloy displays outstanding resistance to general corrosion and can help form in critical parts or components; ‘Satisfactory’, which means the metal is generally suitable for non-critical parts and finally, ‘Not Suggested’, which means the alloy isn’t suitable in the environment in question.
So which alloys offer the best general corrosion resistance? At NeoNickel, we have a range of general corrosion-resistant alloys that remain durable for long periods of time, playing critical roles in demanding environments.
It goes without saying that the alloy you select should depend on the exposure to corrosive environments, so it’s important you are aware of the properties that make up our alloys. Some, like the 600 alloy is ideal in hot, concentrated caustic environments; whereas the AL-6XN alloy is ideal when organic acids are present, such as naphthenic acids used in refineries. There is also alloy 20, which has molybdenum and copper elements and is ideal for areas rich in sulphuric acid.
Whatever the environment, when it comes to supplying general corrosion-resistant alloys, NeoNickel has a huge variety of relentless and ultra-reliable metals available to suit customers working in the most extreme and demanding of environments.
Methods to Prevent Corrosion
Metal Type
The most effective way to prevent corrosion is to get the right metal alloy, which can also reduce the need for further prevention methods.
Protective Coatings
There are two types of paint coatings that prevent corrosion, one of them is paint, which acts in a similar way to the elemental reaction we mentioned above. The other method is powder coating, where the powder spreads on the new metal surface which is then heated to form a protective film.
Chemical Balancing
Corrosion happens when metals react to different chemicals, so controlling these chemicals can help prevent it. For example, you could limit contact with certain chemicals by the placement of the applications or by diminishing the chemical levels in air or water.
Sacrificial Coatings
Sacrificial coating is placing another metal on top of the original surface metal so there’s more chance of that metal corroding than the one beneath it. There are two methods to achieve sacrificial coating:
- Cathodic – coating the metal with a more active metal like zinc (galvanizing), as the zinc corrodes it oxidizes which prevents the metal from rusting.
- Anodic – coating the metal with a less reactive metal like tin, which doesn’t react with the metal below the surface. As long as the surface metal is in place the structure won’t rust.
Design Modification
Modifying designs can help to prevent corrosion and enhance any corrosion preventing methods you’re using. Designs should avoid trapping water and dust, avoid open crevices and promote the movement of air; as well as being easy to service and maintain.
Corrosion-Resistant Alloys for Your Business
Would you like to find out more about corrosion-resistant metal alloys, and whether they are right for you?
The technical staff at NeoNickel will be happy to discuss your specific metal alloy requirements in more detail. Contact us today for more information.
