Corrosion Resistant Steel Grades have unique properties that enable them to perform optimally despite operating in a corrosive environment. This article reviews the types of corrosion-resistant steel grades, their composition, and how to select the right grade for an application.
What is Corrosion Resistant Steel
Corrosion resistant steel, or stainless steel, is a type of steel that is able to resist corrosion or rust at an efficient rate. To achieve this resistance, steel is alloyed with other metals to form a compound with chromium being a key element. When the chromium content exceeds 10.5%, a protective passive oxide film forms on the steel’s surface. Thus, shielding it and preventing oxygen and other corrosive agents from attacking the underlying metal and effectively rendering it ‘stainless’.
However, corrosion resistance in steel extends far beyond simply adding chromium. The interplay of diverse alloying elements and the influence of manufacturing processes contribute to a diverse array of grades. In addition to chromium, nickel plays an essential role in enhancing corrosion resistance and improving other characteristics like ductility and toughness. Other elements such as manganese, silicon, aluminum, copper, molybdenum, tungsten, and titanium are added in varying amounts to fine-tune the steel’s properties. These variations are what constitute the different stainless steel grades in the industry today.
Grades of Stainless Steel
There are several categories of corrosion-resistant steels, each with unique properties and applications.
Austenitic Stainless Steels
Austenitic stainless steels are the most common type of stainless steel because of their excellent corrosion resistance and high ductility. They constitute mostly iron, chromium (typically 16-26%), and nickel (6-22%), which form a non-magnetic structure. This makes them suitable for a variety of applications, especially in acidic and chloride environments. There are different grades of austenitic stainless steel with the most common being the 304 and 316.
- Grade 304: This all-purpose stainless steel contains approximately 18% chromium and 8% nickel. It offers good resistance to a broad range of corrosive environments including food processing, kitchen equipment, and architectural applications.
- Grade 316: This grade adds 2-3% molybdenum to enhance resistance to chlorides. As a result, this grade is a common choice in maritime applications and chemical processing.
Ferritic Stainless Steels
Ferritic stainless steels contain little or no nickel and are primarily composed of iron and chromium (10.5-30%). These steels possess good corrosion resistance but are less ductile in comparison to austenitic grades. Also, ferritic steels are magnetic and have good resistance to stress corrosion cracking. Common grades of this type of corrosion resistant steel include:
- 430 Stainless Steel: Often used in automotive trim and appliances, 430 offers good corrosion resistance and is more affordable than austenitic grades.
- 444 Stainless Steel: Enhanced with molybdenum and titanium, 444 provides better resistance to pitting and crevice corrosion.
Martensitic Stainless Steels
Martensitic stainless steels, with carbon content ranging from 0.1-1.2%, offer high strength and moderate corrosion resistance. Moreover, they can be hardened by heat treatment, which makes them suitable for tools and applications requiring high wear resistance. Some of its grades include:
- 410 Stainless Steel: Common in cutlery and surgical instruments, 410 offers a balance of strength and moderate corrosion resistance.
- 440C Stainless Steel: With the highest hardness among stainless steels, 440C usually serves in bearing and tool applications.
Duplex Stainless Steels
Duplex stainless steels combine the desirable properties of both austenitic and ferritic steels, due to a balance in chromium, nickel, and molybdenum content. Generally, duplex compositions are around 18-28% chromium, 4.5-8% nickel, and 0.5-5% molybdenum. This unique microstructure provides strong resistance to pitting and crevice corrosion, hence, making them suitable for high-chloride or high-oxygen environments. These features make them a popular material choice in the oil and gas industry. In specific environments that require superior corrosion resistance, the super duplex grades, with additional elements like copper and tungsten can serve. Common grades of duplex alloys include 318L, 2205, 2507, and 4501.
How to Select the Right Corrosion Resistant Steel Grade
Choosing the appropriate corrosion-resistant steel grade for your application involves several considerations.
Environment
The working environment of the steel is the most critical factor. Consider the presence of moisture, chemicals, temperature variations, as well as exposure to salts or acids. For example, marine environments require steels with high resistance to chlorides such as 316 stainless steels.
Mechanical Properties
Evaluate the mechanical property requirements for your application including tensile strength, impact resistance, and hardness. Martensitic stainless steels offer high strength and hardness, thus, making them suitable for applications like cutlery and tools.
Fabrication Requirements
Consider the ease of fabrication, including welding, forming, and machining. Austenitic stainless steels are generally more formable and weldable in comparison to other types. However, for applications requiring heat treatment, martensitic steels may be more appropriate.
Cost
When thinking about this, it is important to consider balancing performance with cost. While higher-grade materials provide superior corrosion resistance, they may also come with increased costs. Assess the potential for long-term maintenance costs and equipment failures associated with weaker materials.
Standards and Specifications
Finally, it is wise to make sure to refer to industry standards and specifications. Identifying the certification requirements for the application helps ensure that the material selection process meets safety and performance benchmarks.
Corrosion Resistance Chart for Stainless Steel Grades
Corrosion resistance charts serve as a quick reference for industry professionals during the material selection process. A typical chart provides a visual representation of the resistance levels or properties of different grades of corrosion resistant alloys. Moreover, there are charts for standard materials, as well as custom corrosion resistant steel grades developed by material manufacturers. An example is the chart below, which is a snippet of the properties of different grades of Ferro-Tic alloys.
| Grade | Carbide (Vol%) | Matrix Alloy Type | Hardness (HRC) | Maximum Working Temp (℉) | Compressive Strength (×103psi) | Impact Strength (in-lb/in2) | |
| Ann. | Hard | ||||||
| C | 45 | Medium Alloy Tool Steel | 44 | 71 | 375 | 520 | 323 |
| SK | 35 | Impact Resistant Tool Steel | 35 | 68 | 800 | 381 | 422 |
| CS-40 | 45 | Martensitic Stainless Steel | 47 | 70 | 400 | 453 | 148 |
| MS-5A | 41 | Age Hardenable Martensitic SS | 52 | 63 | 840 | 415 | 343 |
| PK | 42 | Maraging Steel | 53 | 64 | 840 | 417 | 421 |
Corrosion Resistant Alloys from Ferro-Tic
At Ferro-Tic, our corrosion-resistant steel grades are engineered to deliver exceptional durability and performance in the harshest environments. Our advanced steel bonded titanium carbide components offer superior resistance to wear and corrosion, thus, ensuring long-lasting reliability and reduced maintenance costs. When choosing Ferro-Tic’s innovative materials, you can trust in the quality and expertise that come with over two decades of industry experience. Learn more about our steel alloy grades by taking a look at our grade selection guide.