EN
Corrosion, a silent enemy, is all pervasive — found in the sea, in chemical plants, in outdoor hardware, medical implants and internal machines. Even normal steel coil springs cannot last beyond its normal intended life cycle when subjected to moisture, salts, acids, and cleaning fluids it would rust, pit and break while stainless steel coil springs inherently resist its' corrosive nature. The following four areas demonstrate how stainless steel coil springs maximize service life and minimize maintenance expense through their unique corrosion resistant characteristics.
1. Chromium Oxide Passive Layer: The Self-Healing Shield
Stainless steel coil springs' resistance to corrosion starts with its high chromium content. Each grade of stainless steel contains at least 10.5 percent chromium. With oxygen present, chromium immediately forms a thin, clear, adherent chromium oxide layer on the surface of the spring. Even though it is only nanometers thick this passive film still acts as a protective shield that stops the oxygen, water and corrosives from reacting with the base metal. While coatings like paint or plating are easily scratched and chipped off, and can chip away or be worn down, the passive layer is essentially self-healing. In the event of an accidental scratch during installation or during operation, exposed chromium immediately reacts with ambient oxygen to restore the protective layer. Carbon steel springs depend on an applied coating for protection; any scratch or breach in that coating allows rust to form and spread rapidly underneath. Stainless steel coil springs maintain corrosion protection in spite of surface abrasion.
2. Alloy Grades for Targeted Corrosion Resistance
Stainless steel coil springs are not created equal in terms of how they will perform against corrosion. The key to long service life is choosing the correct alloy to suit the surrounding environment. General use stainless steel coil springs of grade 302 and 304 will offer an appropriate degree of corrosion resistance in humid or indoor environments where there is little direct exposure to the elements, food production or in fresh water exposure. If the springs are exposed to chlorides-for example, at coastal installations, on marine vessels, or wherever de-icing salts may be present-you must choose 316 stainless steel coil springs. Molybdenum content of 2-3 percent in this grade significantly enhances the steel's resistance to pitting and crevice corrosion in salt water and chlorinated environments. For more stringent uses (hot chlorides, or acids and alkalis) special alloys like 17-7 PH, Duplex 2205 or Hastelloy might need to be used. Hongsheng Spring manufactures stainless steel coil springs in a number of grades, working closely with customers to select the appropriate alloy considering the temperatures and chemical make-up of the environment.
3. Pitting Resistance and Fatigue Life Preservation
Rust that shows on the surface is easy to detect, but corrosion pitting is much more dangerous because the damage can be localized and hidden. A pit penetrates the passive layer and creates stress concentrations in the spring's surface. A fatigue crack begins at a pit whenever the spring is cyclically loaded, causing the spring to fracture suddenly without significant plastic deformation. A high PRE rating will maintain the resistance of a molybdenum added grade 316 spring to pitting in moist conditions or salt water. Salt spray testing (ASTM B117) shows that even after 2,000 hours exposure in a salt spray cabinet, the 316 springs are able to maintain more than 90 percent of their original fatigue life compared to less than 50 percent for carbon steel.
4. Eliminating Coating Failure and Maintenance Costs
In order to protect the carbon steel spring, it is necessary to coat it with zinc plating, powder coating, epoxy or PTFE. These coatings can add to cost, prolong delivery time, and are not foolproof. A mere scratch which may occur either during shipment or application is all it takes for the bare metal to be exposed. Corrosion may then start immediately on the exposed surfaces, or unseen attack will commence under the coating until the spring structure is compromised. Regularly repainting or replacing rusted carbon steel springs results in a repetitive expense for the end-user. Stainless steel springs are an option that bypasses any need for coating, thereby increasing service life and decreasing maintenance expenses. This leads to shorter lead times, lower costs on the assembly line for manufacturers, and fewer warranty issues. For springs that cannot easily be replaced such as in a buried pipeline, subsea application or a sealed device, this can mean substantial long-term value.
Summary
Stainless steel coil springs offer advantages by utilizing four factors in their resistance to corrosion; a self-healing layer of chromium oxide eliminating reliance on coatings, grade-specific resistance depending on environmental factors (grade 304 for general applications, grade 316 for use in chlorides), preservation of fatigue strength by preventing pitting, and elimination of corrosion driven maintenance costs. Hongsheng Spring is capable of producing custom stainless steel coil springs to your specifications in grades 302, 304, 316, and 17-7 PH for applications involving marine, chemical, food production or outdoor uses. Contact us for your specialty needs and determine the ideal spring for your environment.