Brief Guide: How Resistant Is Stainless Steel To Corrosion?

The essential property of stainless steel is its resistance to corrosion in any environment. The Chromium alloying element provides corrosion resistance. It is a thin layer covering the metal surface shielding from exposure to the external environment.  Molybdenum and Nickel are also alloying elements that enhance strength and you can consider this factor when looking for stainless steel tube. However, high-resistance doesn’t mean it never corrodes, its corrosion is at a lower rate compared to other metals. Below is a summary of the corrosion types that affect stainless steel.

Shielding a metal like under washers, bolt heads and other fittings exposes it to crevice erosion. The corrosion is more severe in environments containing Chlorides. It results in the formation of areas depleted of oxygen. Oxygen absence produces galvanic effects resulting from the creation of anode at the crevice regions and cathode at the surface. Prevention is through the quality design of products that minimize shielding and applying stainless steel with robust alloying elements. You can start looking for stainless steel tube to deal with this type of corrosion.

This type of corrosion affects the surface uniformly, and its measurement is in mm/year. Its prediction is smooth and has the least impact on the metal. The approximate 304 series corrosion rate is about 0.02 to 0.03mm/year, which is the least compared to other steel types. So you can buy 316 stainless steel tube which is better resistant to corrosion.

With carbon’s high Chromium affinity, both the compounds combine resulting in chromium carbide under high temperatures.  The reaction occurs mainly at the boundaries destroying the chromium layer hence exposing the metal to corrosion.

The use of metal types with alloying elements such as Niobium and Titanium may help in reducing inter-granular corrosion. The high affinity to the formation of carbides by these elements plays a role in preventing the destruction of chromium. Low carbon grades like 316L may also serve the protection purpose.

This type of corrosion mainly occurs in environments containing Chloride. There are oxygen depletion and the formation of small pits on the metal surface where Chloride ions collect. The outer coating serves as the cathode, and galvanic erosion occurs. The corrosion is not selective to basic or acidic solutions, but the impact is more in acidic solutions. Prevention is through the use of stainless steel with high alloying elements. PREN (Pitting Resistance Equivalent Number) indicated the resistance to pitting corrosion. Metals with PREN above 32 are the most suitable in these cases.

This corrosion is the Stress Corrosion Cracking (SCC) caused by high temperatures, solutions with high hydroxides and chloride concentration, and tensile stress. It starts with small crevice followed by the rapid acceleration due to high pressure at the base, causing rapture. Austenitic 304 and 316 grades are prone to this corrosion hence limiting their use, unlike the ferritic grades that are highly resistant. The categories concentrated with Nickel, Chromium, and Molybdenum have high resistance to SCC. Also, the duplex has remarkable resilience but is slightly lower than ferritic and higher than austenitic.

The pharmaceutical industry follows regulations requiring them to use high-quality alloys with nickel, chromium, and molybdenum in all processes. The specified alloy elements provide higher resistance in harsh environments.

Stainless steel has applications in the manufacture of tanks, pipes, orthopedic implants, pressure vessels, and hypodermic needles. The applied material should withstand interaction with high temperature, corrosive chemicals, fierce cleaning, and high pressure. These factors are the main concerns in the industry that could lead to corrosion.

Grade 316L of Austenitic meets most industry requirements and hence its extensive use. It contains molybdenum and nickel that provide high resistance. However, products with high molybdenum are costly. Austenitic grade steel has its uses in the less critical applications where medium strength is desirable. It is easy to clean, sterilize and costs a fair price.

Due to the expensive nature of austenitic containing essential high alloy elements concentration, duplex grade 2205 may take its place. Duplex grade offers a similar service at a lower cost. It has higher levels of nitrogen which gives it more strength that significantly reduces the component’s thickness.

Most equipment made of stainless steel is passivated and electropolished to enhance clean-ability and passivity. This process creates a thin layer on the metal surface which increases its resistance and removes any manufacturing imperfections.

Acquiring pharmaceutical equipment made of steel requires prudence in decision making because they are expensive and highly customized. The use of low-quality material opens the door for contamination. The high performance may be costly upfront, but its long-lasting nature makes them cheaper in the long run. The stated mechanical properties and the long term cost-benefit makes stainless steel a preferable option in the pharmaceutical industry. Pharmaceuticals is a critical sector that requires equipment that ensures health safety.

Stainless steel material minimizes corrosion. It ensures the safety of the equipment and increases durability.

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