What is corrosion?
Corrosion consists of a sequence of electrochemical reactions, characterized by oxidation processes (where electrons are lost) at the anode and reduction processes (where electrons are gained) at the cathode. As an illustration, iron present in piping undergoes oxidation, shedding two electrons and dissolving into water as Fe2+ ions. Concurrently, the electrons emitted from the oxidizing iron can engage in a reduction reaction, utilizing oxygen dissolved in water to generate OH- ions. The outcome of such oxidation reactions is the formation of rust. When iron, which is part of stainless steel, comes into contact with water and oxygen, it results in the production of hydrated iron oxide, commonly referred to as rust. Typically, the primary reason for stainless steel to rust is the compromise of its surface’s protective oxide layer.
Why corrosion occurs?
Metal components in industrial fluid systems often have to operate in harsh conditions. If left unchecked, adverse conditions can trigger and aggravate the corrosion of these components and cause physical and financial harm to an operation.Here are some situations that may cause corrosion:
The damage of the equipment
The thin protective layer, which is formed through the reaction between chromium and oxygen, can be compromised by mechanical damage like scratches. Once the protective oxide layer is damaged, stainless steel becomes exposed to the external environment, making it highly prone to rusting. This susceptibility is particularly acute in low-oxygen conditions where the oxide protective layer fails to re-form swiftly.
Harsh environments
In highly corrosive environments, particularly those containing extremely strong chlorides, chloride ions pose a threat by breaking down the chromium oxide protective layer on stainless steel. This results in pitting and crevice corrosion, thereby increasing its vulnerability to rust. For this reason, 304 stainless steel is not advisable for offshore applications where chlorine levels are high. Since molybdenum enhances stainless steel’s resistance to chloride, it is advisable to opt for 316 stainless steel, which contains molybdenum, or 6-mo alloy with a higher molybdenum content, in offshore settings.
Contact with different material
Galvanic corrosion can occur when stainless steel comes into contact with dissimilar materials in an electrolyte. If the potential difference between the two materials in the electrolyte is large, the oxide layer on the surface of the stainless steel will begin to decompose. In this case, the stainless steel will rust faster due to galvanic corrosion. To avoid galvanic corrosion, it is recommended to combine materials with the same potential or a small potential difference.
In addition, galvanic corrosion may indirectly cause stainless steel to rust. Carbon steel particles are one source of contaminants during stainless steel manufacturing and application. When stainless steel comes into contact with carbon steel or tools stained with carbon steel particles, these particles can become embedded in the surface of the stainless steel, damaging its oxide layer. Due to galvanic corrosion between stainless steel and carbon steel, carbon steel particles on the surface of stainless steel will rust quickly. If left untreated, this can lead to rust and pitting of the stainless steel.
Influence of high temperature
When stainless steel is exposed to high temperatures, the reaction between chromium and oxygen is accelerated, leading to the precipitation of chromium on its surface as it reacts with oxygen to form a thicker chromium oxide layer. Consequently, the chromium content in the stainless steel’s surface area decreases, creating chromium-depleted zones. Austenitic stainless steel, due to its high thermal expansion rate, may experience cracking and peeling of its thick chromium oxide protective layer during thermal cycling, exposing the chromium-depleted zones to the environment and promoting rust formation. Furthermore, high temperatures cause chromium to precipitate at the grain boundaries of stainless steel, where it combines with carbon to form carbides, resulting in chromium-depleted zones around the grain boundaries and ultimately causing intergranular corrosion. For instance, during welding processes, it is crucial to maintain proper temperature control to prevent the formation of a thick oxide layer on the surface and the precipitation of carbides at the grain boundaries. To minimize the risk of intergranular corrosion, the use of low-carbon stainless steels and shielding gases to prevent excessive oxide layer formation is recommended.
How to mitigate the damage of corrosion?
Corrosion occurs when metal atoms oxidize, causing the metal’s surface to lose material and the thickness of the component wall to shrink. This could ultimately lead to premature mechanical failure. In addition, the chemical composition of the fluid being transported may have an impact on a component’s corrosion resistance. An emphasis on quality at each step must prevail, from bar stock qualification to the component’s final inspection.
Seeking Higher-Strength Materials
Before selecting a component for specific applications, its mechanical properties should undergo examination, with considerations encompassing tensile strength and ductility. The expected performance of the component, especially under high pressures, is more likely to be achieved if the materials used in its construction are of higher quality.
Ensuring Component Compatibility
Understanding where corrosion is likely to occur and catching it early can help to lower repair costs over time. If system designers choose the right materials at the outset, then system integrity, component life cycles, consistent performance, and safety all improve. The following steps reduce the impact of corrosion on applications:
• Identifying types of corrosion – what it looks like, where it occurs, and why it happens, • Selecting materials resistant to corrosion
• Minimizing locations where corrosion can occur and reducing contact with non-compatible metals
• Specifying everything from the supports and clamps to the tubing itself to reduce the potential for corrosion.
Working With Reliable Suppliers
A supplier should always provide the chemical composition of a material and indicate compliance with standards published by the International Standards Organization (ISO) and ASTM International. As a member of International Semiconductor Association (SEMI), we , Kavya Group have certificates such as TS, ASME BPE, 3A, ISO9001, ISO14001, ISO45001 etc. We can Meet your needs at anytime and anywhere.
Conclusion
Stainless steel rusts. Good design, fabrication, and maintenance practices can minimize the risk of corrosion in stainless steel. If the working environment is harsh or the medium is corrosive, Kavya Group can provide products made of super duplex stainless steel and other high-strength materials to meet your requirements.
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