EN
In chemical processing plants, pharmaceutical manufacturing, food production and marine applications, springs can be exposed to the most aggressive materials such as acids, alkalis, solvents, salts, and cleaning solutions. While standard carbon steel springs readily corrode in these environments through pitting, stress corrosion cracking and premature failure. Stainless steel coil springs have been designed for this challenging chemical environments with respect to its mechanical properties. Here are 4 ways that stainless steel coil springs meet the demands of the harshest chemical jobs.
1. Corrosion-Resistant Alloys for Chemical Compatibility
Not all stainless steel alloys provide equivalent resistance to the demands of chemical environments. It is essential to specify the correct alloy to meet the corrosive demands of the application. Grade 302 & 304 stainless steel coil springs are suited for applications that have good resistance to oxidizing acids, organic chemicals, and many alkaline solutions. They are ideal for food processing equipment, pharmaceutical manufacturing equipment, and general chemical environments.
For more aggressive demands involving exposure to chlorides, salt spray and reducing acids, 316 stainless steel coil springs are a better specification. The addition of 2-3% of Molybdenum (Mo) increases pit resistance and offers exceptional resistance to chlorides and acidic conditions. Extremely severe applications involving high concentrations of sulfuric acid, hydrochloric acid and hot chlorides may necessitate specialized alloys such as Hastelloy C-276 or Inconel 625. Hongsheng Spring provides stainless steel coil springs in a variety of grades (302, 304, 316 and 17-7 PH), and specialty alloys, to meet any specific application's chemical demands.
2. Passive Oxide Layer and Self-Healing Protection
The inherent ability for stainless steel coil springs to resist corrosion is due to the passive oxide layer (chromium oxide). The oxide layer is extremely thin (a few nanometers in thickness) but effectively blocks further oxidation of the surface. If the surface of the spring is scratched or abraded in operation the chromium in the alloy will react with the oxygen on the surface to "self-heal" the oxide layer and restore its protection.
This self-healing characteristic of stainless steel coil springs is especially important for those that are likely to encounter a abrasive chemical environment that may cause surface damage during operation. Carbon steel springs need an applied coating (such as a zinc, epoxy or PTFE coating) for resistance. Once this coating is scratched base metal is exposed to the environment and rapid corrosion occurs. Stainless steel coil springs do not require these applied coatings to offer protection to the component, as the base material retains its corrosion resistance after abrasion. For chemical mixing equipment, agitator springs or valve springs, self-healing offers significantly increased service life and less downtime.
3. Resistance to Stress Corrosion Cracking (SCC)
Stress corrosion cracking is particularly damaging in chemical environments for springs. SCC occurs when a stressed material is exposed to a corrosive environment which induces a sudden, catastrophic failure without any major deformation of the material. Examples of commonly seen SCC inducing materials include chlorides, caustic solutions and hydrogen sulfide.
Austenitic stainless steel coil springs (302, 304, 316) may become prone to chloride stress corrosion cracking at higher temperatures (over 60°C). For applications with hot chlorides, the recommendation will be 17-7 PH precipitation hardening stainless steel or a duplex stainless steel grade for enhanced SCC resistance. Hongsheng Spring will work closely with you to determine the best specification stainless steel based on your applications operating temperature, chemical concentrations and mechanical loads. We will also apply necessary low temperature post processes to reduce residual tensile stress in springs which are more prone to SCC.
4. Cleanability and Regulatory Compliance
In the food, pharmaceutical and biotechnology industries it is essential that springs are not only resistant to chemicals, but also have smooth surfaces which prevent bacterial adhesion and allow for easy sanitation. The inherent smoothness of the stainless steel coil springs and electropolishing offer a solution. Electropolishing can reduce the Ra of the surface of stainless steel below 0.4 micrometers creating an extremely smooth surface and eliminating many crevices where contaminants could collect.
Hongsheng Spring supplies stainless steel coil springs that are suitable for clean in place (CIP) and steam in place (SIP) operations. Food applications will meet FDA and EU food contact regulations. For pharmaceutical applications, springs will meet the high standard of control in manufacturing with full traceability of material which is important for validation. With its resistance to chemicals, ease of cleaning and its ability to meet stringent regulatory requirements in food and pharmaceutical industries the stainless steel coil spring is the specification of choice for the demanding chemical jobs.
Summary
The stringent demands placed on springs in a chemical environment can be met by utilizing four characteristics of stainless steel coil springs: corrosion resistance is afforded by utilizing grades of stainless steel such as 302, 304, 316, and 17-7 PH; reliance on coatings is eliminated through the utilization of self-healing passive oxide layers; resistance to SCC is met through grade selection of stainless steel; and, cleanliness of surface is guaranteed through stainless steel's intrinsic properties and optional electropolishing for food and pharmaceutical applications. Hongsheng Spring will manufacture custom stainless steel coil springs to meet your chemical processing, marine, food or pharmaceutical equipment's demanding requirements. Contact us today.