Stainless Steel Pipes: Stainless steel is a versatile material comprised of a steel alloy and a small percentage of chromium—the addition of chromium adds to the material’s corrosion resistance, a trait that earned stainless steel its name. Because stainless steel is also low-maintenance, oxidation resistant, and doesn’t affect other metals it comes in contact with, it is frequently used in a large array of applications, especially in piping and tubing manufacturing. Let’s know more about Stainless Steel Pipes. What is Stainless Steel? Stainless steel, also known as inox steel and is an alloy with a minimum of 10.5%chromium content by mass. Stainless steel does not readily corrode, rust or stain with water as ordinary steel does. Stainless steels contain sufficient chromium to form a passive film of chromium oxide, which prevents further surface corrosion by blocking oxygen diffusion to the steel surface and blocks corrosion from spreading into the metal’s internal structure. Passivation occurs only if the proportion of chromium is high enough and oxygen is present. Stainless steel is used where both the properties of steel and corrosion resistance are required. How to select Stainless Steel? For selecting stainless steel for a precise application, the fundamental properties should be considered. These properties vary for all five basic types of alloys, such as precipitation hardening, martensitic, Ferritic, duplex and austenitic.
|Required Property||Alloy Groups and Grades Likely to be Selected|
|Corrosion resistance||Selection depends upon environment.|
|Heat resistance||Austenitic grades, particularly those high in chromium, often also with high silicon, nitrogen and rare earth elements (e.g. grades 310 and 253MA). Stabilised ferritics are used in less extreme conditions. High chromium ferritic grades have high oxidation resistance (e.g. 446), but have lower hot strength.|
|Cryogenic (low temperature) resistance||Austenitic grades have excellent toughness at very low temperatures.|
|Magnetic response||Austenitic grades have low magnetic permeability; higher nickel grades (e.g. 316 or 310) are more likely to be non-magnetic if cold worked.|
|High Strength||Martensitic and precipitation hardening grades. Duplex grades can be useful. Cold worked austenitic grades also have high strength|
Can stainless steel corrode and what forms of corrosion can occur? Stainless steel does not readily corrode, rust or stain with water as ordinary steel does. However, it is not fully stain-proof in low-oxygen, high-salinity, or poor air-circulation environments. There are different grades and surface finishes of stainless steel to suit the environment the alloy must endure. The most common forms of corrosion in stainless steel are:
- Pitting corrosion - It is a form of localized corrosion, which produces attacks in the form of spots or pits. The cause of this type of corrosion is due to the presence of chloride ion Cl- which is commonly found.
- Crevice corrosion – Stainless steel requires a supply of oxygen to make sure that the passive layer can form on the surface.
- General corrosion – This type of corrosion is caused due to some chemicals, notably acids, the passive layer may be attacked uniformly depending on concentration and temperature and the metal loss is distributed over the entire surface of the steel. Hydrochloric acid and sulphuric acid at some concentrations are particular aggressive towards stainless steel.
- Stress corrosion cracking (SCC) – This is a relatively rare form of corrosion which requires a very specific combination of tensile stress, temperature and corrosive species, often the chloride ion, for it to occur. Typical applications where SCC can occur are hot water tanks and swimming pools. Another form known as sulphide stress corrosion cracking (SSCC) is associated with hydrogen sulphide in oil and gas exploration and production.
- Intergranular corrosion – This is now quite a rare form of corrosion. If the Carbon level in the steel is too high, Chromium can combine with Carbon to form Chromium Carbide. This occurs at temperatures between about 450-850 deg C. This process is also called sensitisation and typically occurs during welding. The Chromium available to form the passive layer is effectively reduced and corrosion can occur.
- Galvanic corrosion – If two dissimilar metals are in contact with each other and with an electrolyte e.g. water or other solution, it is possible for a galvanic cell to be set up. This is rather like a battery and can accelerate corrosion of the less ‘noble’ metal.
What are the benefits and applications of stainless steel?
Benefit: All stainless steels have a high resistance to corrosion. Low alloyed grades resist corrosion in atmospheric conditions; highly alloyed grades can resist corrosion in most acids, alkaline solutions, and chloride bearing environments, even at elevated temperatures and pressures. Applications: Below you will find some of the sector where applications of stainless steel are been used frequently.
- Chemical Industries
- Thermal & Nuclear Power Plants
- Fertilizer Plant
- Food & Dairy Plants
- Paper Industries
- Oil & Gas Exploration
- Desalination Plant
- Pharmaceutical Plant
- Automobile Plants
- Sugar Plants
- Petrochemical Industries
How do I choose which stainless steel to use? Selecting the right stainless steel grade for a specific application is vitally important in order to achieve a sustainable solution, while cost aspects must also be taken into consideration. There are several stages when material selection needs to be made:
- New applications and equipment
- New processes and process changes
- Exchange of material due to poor performance
- Exchange of existing material to minimize cost
To make the right material selection it is important to get a full picture of the service conditions the steel will face. The first aspects to consider could be material related, for example:
- Corrosion resistance
- Mechanical strength
- Fabric ability
- Physical properties
- Surface aspects
Other factors that may influence the decision include:
- Possibilities for weight reduction
- Life Cycle Cost (LCC)
- Legislation, standards, and approvals
- Previous experience
What are the varieties of stainless pipes widely used? The most known stainless steel pipe varieties which widely used by various industries are as follows:
- Stainless Steel Seamless Pipes : 304 / 304H/ 304L /310 / 310S / 316 / 316L / 317 / 317L / 321 / 321H / 347 / 347H / UNS S32305/S31803/2205/ UNS S32750 /1.4410 /2507 Pipes.