Shandong Jiugang Tisco Steel Co., Ltd.
Types of corrosion faced by stainless steel
At Tisco steel, we specialise in the import and commercialisation of stainless steel for the global market. We have extensive experience and can provide the necessary advice and instructions on the use and characteristics of this metal.
Therefore, in the following article we would like to share with you some of the types of corrosion faced by stainless steel that can deteriorate it in an irreversible way.
What is corrosion?
Let us first define what corrosion is. It is a natural process of metals whose physical and chemical properties are deteriorated by exposure to and direct contact with the environment in which they are found.
Some metals age much more rapidly than others, a clear example being iron, which is very susceptible to corrosion. In turn, carbon steel is also quite susceptible to corrosion in the various environments in which it is found. These problems make it necessary to apply certain protection mechanisms to certain steel surfaces.
Levels of resistance of stainless steel
Stainless steel has a myriad of characteristics and properties that make it more resistant to corrosion than other metallic materials. However, it is essential that the appropriate type and design of steel is selected without neglecting the environment to which the material will be exposed.
In addition, good maintenance is essential. With proper and consistent cleaning, the metal will have a longer and better service life.
Types of corrosion
Despite all its excellent corrosion protection properties, stainless steel is still not completely immune to the effects of corrosion processes. Basically, there are two types of corrosion, depending on the environment, that can damage stainless steel
Dry corrosion: this type of corrosion only occurs in environments where there are high temperatures.
Wet corrosion: this is an electrochemical process that can be generalised or localised. When we talk about generalised, this means that deterioration is present on the surface and in a continuous manner. On the other hand, when we refer to localisation, deterioration occurs only on a specific part of the surface.
The different types of degradation that metals may experience under these conditions are.
Homogeneous corrosion
Uniform corrosion is considered to be a uniform attack on the surface of a material and is the most common type of corrosion. It is also the most benign as corrosion and the effects it causes can be assessed fairly easily. This type of corrosion usually occurs on relatively large areas of the material surface.
You may also be interested in: How to protect stainless steel from environmental corrosion?
Focal corrosion
Focal corrosion may be caused by different factors, such as.
Pitting corrosion
The type of corrosion to which stainless steel is exposed
This is one of the most destructive types of corrosion as it can be difficult to predict, detect and characterise. It is a localised form of corrosion where a localised anode spot, or more commonly a cathode spot, forms a small corrosion cell with the surrounding normal surface.
Once a pockmark starts to appear, it grows into a ‘hole’ or ‘cavity’ that takes on a variety of different shapes. The cause of this corrosion is usually a local breakdown or damage to the protective oxide layer, or an inherent inhomogeneity in the metal structure.
Crevice corrosion
Crevice corrosion is also a localised form of corrosion and is usually caused by a microenvironment where there is a difference in ion concentration between two areas of the metal.
It occurs in protected areas, such as areas beneath gaskets, bolt heads, spacers etc., where oxygen is mainly restricted. These smaller areas allow the corrosive agent to enter, but do not allow sufficient internal circulation, which depletes the oxygen content and the natural passivation process takes place.
Intergranular corrosion
Intergranular corrosion can be caused by impurities present at grain boundaries, or by depletion or enrichment of alloying elements in these areas. This severely affects the mechanical properties of the metal, while the majority of the steel remains intact.
An example of intergranular corrosion is carbide precipitation, a chemical reaction that can occur when the metal is subjected to very high temperatures. In stainless steels, during these reactions, the carbon ‘consumes’ the chromium, forming carbides and resulting in an alloy with less than the 11% of chromium remaining required to create the passivation layer.
304L stainless steel sheet and 316L stainless steels sheet are improved alloys of the 304 and 316 types as they contain a lower carbon content and offer the best resistance to carbide precipitation corrosion.
Stress corrosion cracking
Stress corrosion cracking is the result of a combination of tensile stress and a corrosive environment, usually at high temperatures. This corrosion can be caused by external stresses such as actual tensile loading on the metal or expansion/contraction due to rapid temperature changes.
In addition, it can be caused by residual stresses imparted during manufacturing processes such as cold forming, welding, soldering, machining, grinding, etc. In stress corrosion, most of the surface usually remains intact; however, small cracks appear in the microstructure, which can make corrosion difficult to detect.
Galvanic corrosion
Types of corrosion faced by stainless steel
Electrochemical corrosion is the degradation of the metal in the vicinity of a joint when two electrochemically dissimilar metals come into electrical contact in an electrolytic environment; for example, when copper comes into contact with steel in a brine environment.
However, even when these three conditions occur, there are many other factors that influence the likelihood and amount of corrosion, such as temperature and the surface finish of the metal. If care is not taken at the design stage, projects with multiple types of metal in construction can be susceptible to galvanic corrosion.
In an aqueous environment, metals are not only subject to uniform corrosion, but also to various localised types of corrosion. In areas where corrosion is a concern, stainless steel products offer value and protection against these threats.