Corrosion in metals can be defined as the degradation of the material due to chemical reactions between it and the environment. Different metals exhibit different rates of corrosion, depending on the chemical composition of the metal and the environmental conditions.
In many applications, it is important to utilize corrosion resistant steel (often known as stainless steel) to ensure the long life and efficient use of the metal, although corrosion resistant steel can still be susceptible to some forms of corrosion.
This blog post will outline some of the types of corrosion that may still effect corrosion resistant steel, and what measures can be taken to reduce corrosion levels.
Types of Corrosion
Corrosion resistant steel is known for its high level of resistance to corrosion and is often used in applications where metal structures are exposed to harsh environments. This type of steel is characterized by a thin passive film on the metal substrate surface, which provides protection against the electrochemical reactions that cause corrosion.
In instances where corrosion resistant steel becomes susceptible to corrosion, it is due to the passive film becoming permanently damaged. Different factors can cause different levels of corrosion for corrosion resistant steel.
Pitting and Crevice Corrosion
Pitting corrosion is highly localized and occurs if the passive layer is damaged or locally weak. If pitting corrosion occurs the small area becomes the anode and a pit is formed. Crevice corrosion occurs in small crevices, such as joints. As these areas often experience limited oxygen, the pH is lowered and chloride ions migrate into the area causing corrosion.
In order to increase a steels resistance to pitting and crevice corrosion, molybdenum and nitrogen can be added as alloying elements. Molybdenum can be employed as it significantly increased the resistance to localized corrosion and this is further increased with the combination of nitrogen.
Galvanic Corrosion
Galvanic corrosion occurs if two dissimilar metals are in contact with each other and an electrolyte (e.g. water). While this type of corrosion doesn’t effect corrosion resistant steel directly, it can be a big problem for metals in direct contact with them. This is because corrosion resistant steel is a nobler metal in it’s passive state to most other construction metals. So, when corrosion resistant metals are in contact with less noble metals, the corrosion rate of these less noble metals is increased.
To resolve this issue a non-metallic insulator can be used to separate the corrosion resistant steel and the less noble metal.
Oxidation
The form of rust on the surface of a metal is an oxidation reaction. It occurs when the metal encounters water and oxygen and it can seriously impact the ability of a corrosion resistant steel to meet expectations. Steels that exhibit oxidation can be more prone to cracking, and it can considerably reduce the strength of the material.
Adding chromium and molybdenum as alloying elements to form ‘Chrome-Moly Steel’ has been a popular method to reduce the risks associated with oxidation.
Chrome-Moly steel still forms a ‘rust’ but instead of damaging the metal, it provides a protective layer against further oxidation. The addition of chromium provides the outstanding resistance to oxidation, and the molybdenum provides increased strength and temperature resistance.
Corrosion Resistant Steel from Masteel
While corrosion resistant steel may not be fully resistant to corrosion, as we’ve discussed there are different measures that can be taken to ensure these types of steels are as corrosion resistant as possible.
Masteel supplies a large range of corrosion resistant steel, and we are also able to help you when decided which material is suitable for your application. If you would like any more information about corrosion resistant steel, please contact us.