Stainless steel selection factors to consider
When choosing stainless steel for environments prone to corrosion, it’s essential to have a thorough understanding of the specific conditions in which the stainless steel will be used. Beyond this, several key factors need to be taken into account, including corrosion resistance, strength, toughness, physical properties, processability, availability, cost, and difficulty. Each of these elements plays a critical role in determining the suitability of stainless steel for your application.
Corrosion Resistance
Corrosion resistance encompasses stainless steel's ability to withstand acids, bases, salts, and other corrosive media, as well as its resistance to high-temperature oxidation, sulfidation, chlorination, fluoridation, and more. Since the choice of stainless steel often addresses specific corrosion challenges in real-world projects, the corrosion resistance of the chosen material is paramount. Corrosion occurs due to chemical or electrochemical interactions between the metal and its surrounding medium, and corrosion resistance refers to the stainless steel's capacity to resist such damage. When evaluating corrosion resistance during material selection, here are some important considerations:
1. The criteria for corrosion resistance are set by humans and should be understood and applied flexibly rather than rigidly adhered to. Specific corrosion resistance standards should be tailored to meet the unique demands of each application. Today, the corrosion resistance of stainless steel is commonly measured using a 10-level scale. The appropriate level depends on the type of equipment, the characteristics of the component (e.g., thinness, size), and the desired service life. For instance, for devices requiring a polished or precise finish, Levels 1–3 might suffice. Components with stricter tolerances or those requiring long-term reliability could fall within Levels 2–5, while equipment needing extended durability may warrant Levels 4–7. Generally, stainless steel should not be used if its annual corrosion rate exceeds 1 mm under typical operating conditions. It’s worth noting that the 10-level scale isn’t suitable for localized corrosion.
2. Corrosion resistance is relative and conditional. We often say "stainless steel rusts," but this refers to relative resistance—under certain conditions such as medium type, concentration, temperature, impurities, pressure, flow rate, etc. No single stainless steel alloy is immune to corrosion in all environments. Therefore, it’s crucial to apply the material according to the specific conditions of use. Even after selecting a grade of stainless steel, users should understand and leverage its unique properties effectively to maximize either its corrosion resistance or its intended functionality.
3. When choosing stainless steel, both general corrosion resistance and localized corrosion resistance should be considered. In water-based or chemical environments, localized corrosion tends to demand more attention. This is because material selection typically prioritizes general corrosion resistance, yet localized corrosion forms like stress corrosion cracking or pitting often go unnoticed until they become problematic. Localized corrosion usually arises in favorable conditions and can lead to sudden failures in stainless steel components, causing significant harm compared to general corrosion.
4. When referencing corrosion resistance data from various technical manuals, remember that much of the information is derived from controlled laboratory experiments and may not fully align with real-world conditions. To ensure more accurate data, it’s advisable to conduct corrosion tests in actual media environments in a lab or perform coupon testing under on-site conditions. In some cases, simulation tests might also be necessary.
5. In applications where stainless steel equipment experiences part failure due to corrosion, it’s important to analyze the root cause of the damage before taking corrective action. Simply discarding corroded parts without investigation can lead to unnecessary waste. Additionally, in industries like nuclear fuel production, pharmaceuticals, or pigments, trace amounts of metallic ions released from stainless steel can impact product quality, affecting factors such as gloss, color, and purity. In such cases, it’s often preferable to select stainless steel free of certain elements or to enhance its corrosion resistance to acceptable levels.
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