Steel Impellers: Grades & Applications
Introduction
Regular steel, also called carbon steel, is an alloy consisting of iron, carbon (< 2%), and other variable elements. Without some kind of protection, the iron will react with oxygen and water to form rust by the process of corrosion, weakening the steel.
Stainless steel is corrosion-resistant due to the addition of chromium (minimum 10.5%) to the alloy. The chromium reacts with oxygen forming a chromium oxide barrier, shielding the iron from environmental oxygen and water. Nickel is also usually present in alloys of stainless steel to improve ductility, making the stainless steel easier to form and shape as needed.
Carbon is an important component that adds mechanical strength to stainless steel. However, when exposed to high temperatures (450°C‑850°C), such as during welding, the carbon could react with chromium to form chromium carbide. This reduces the amount of chromium available to form the protective chromium oxide barrier, risking exposure that leads to corrosion.
Chromium depletion by this process, called sensitization, is avoided by using low carbon steel designated as “L” grade, or by adding titanium or niobium to the alloy.
Common types of impeller stainless steel
There are a wide variety of compositions of stainless steels possible, determined by the properties desired to best fit the application. The most common grades of stainless steel used in impellers are listed in the table below.
As shown below, these grades provide higher levels of corrosion resistance, providing long life to the user. Most shafts and blades, such as Caframo impellers, are durable, able to withstand exposures to many widely used industrial chemicals, solvents, sterilizing solutions, and autoclaving.
| Type of Stainless Steel | Key Components in Alloy* | Corrosion Resistance | Application |
|---|---|---|---|
| 303 | Chromium: ~18%; Nickel: ~8%; Phosphorus Sulphur | Slightly lower than 304 | Suitable for objects requiring machining |
| 304 | Chromium: 18% min.; Nickel: 8% min; Carbon: 0.08% max. | Good resistance to oxidizing acids, food acids, sterilizing solutions and many organic and inorganic chemicals. Risk of pitting in warm chloride environments. | Widely used. Popular for dairy, food & beverage, chemical, textile, mineral and petrochemical processes. |
| 316 | Chromium: 16% min.; Nickel 10% min.; Molybdenum: 2-3%; Carbon: 0.08% max. | Greater corrosion resistance than 304. Resistance to pitting by chlorides or brines, hypochlorite solutions (bleach), and other industrial chemicals and solvents. | Can handle harsher conditions than 304. Popular for inks, paper pulp, textiles, bleaches, rubber, petrochemical, pharmaceutical and chemical processes. |
| 316L | Chromium: 16% min. Nickel: 10% min.; Molybdenum: 2-3%; Carbon: 0.03% max. | L indicates low carbon content. Same corrosion resistance as 316, plus prevention of sensitization. | Suitable for any 316 application. For impellers with blades welded to shaft. |
| 316Ti | Chromium: 16% min.; Nickel: 10% min.; Titanium: 0.7% max.; Carbon: 0.08% max. | Similar to low carbon 316L. The titanium prevents sensitization, and includes the same corrosion resistance as 316. | Suitable for any 316 application. For most conditions, the 316L and 316Ti are interchangeable. Advantage noted for 316Ti in environments over 600°C. |
* Iron makes up the balance of the alloy composition, 50% minimum.
(Information provided courtesy of Caframo.)