How Does SS.304-316L Material Affect Corrosion?

 

  Stainless Steel is a type of steel known for its resistance to corrosion, primarily due to the presence of the main element, chromium (Cr), in concentrations of at least 10.5% and above. The corrosion resistance of stainless steel is attributed to the chemical reaction between the chromium and oxygen, forming a protective layer known as a "Passive Film Layer." This layer is often not visible to the naked eye but serves to protect the surface of stainless steel from corrosive substances and can repair itself when damaged.

 

  While stainless steel exhibits good resistance to corrosion, it can still be susceptible to corrosion under certain conditions. Key factors that can increase the risk of corrosion of stainless steel include exposure to high chloride environments, elevated temperatures, contact with other metals, inadequate cleaning, and improper maintenance.

 

Corrosion of stainless steel can be categorized into

five main types:

 

  1. Pitting Corrosion:

   Pitting corrosion typically results from the presence of chloride ions in a solution or the atmosphere. It manifests as small, localized pits or cavities on the surface, making it a dangerous form of corrosion due to its ability to extend deep into the material.

 

  2. Crevice Corrosion:

   Crevice corrosion occurs in areas where stainless steel surfaces are confined or shielded from air, directly contacting corrosive substances. The corrosive agent attacks the chromium oxide layer, preventing its regeneration within the confined area. Proper design and installation are crucial to preventing crevice corrosion.

 

  3. Galvanic Corrosion:

   Galvanic corrosion occurs when two dissimilar metals with different electrical potentials come into contact in the presence of an electrolyte. This results in the flow of electrical current, causing the metal with lower electrical potential (anode) to corrode, while the one with higher potential (cathode) remains protected. The severity of galvanic corrosion depends on the difference in electrical potentials between the metals.

 

  4. Intergranular Corrosion:

   Intergranular corrosion happens along the grain boundaries of stainless steel, primarily due to the loss of chromium, which combines with carbon to form chromium carbides. This phenomenon, known as sensitization, occurs in certain types of austenitic stainless steel when used within a temperature range of 450-850°C.

 

  5. Stress Corrosion Cracking:

   Stress corrosion cracking occurs when stainless steel is exposed to a corrosive environment with stress acting upon it. This can result from mechanical stress, combined with elevated temperatures and high chloride environments.

 

  When comparing the properties and corrosion impact of SS.304 and SS.316L materials, it's evident that they have distinct chemical compositions:

- SS.304 comprises 18% chromium and 8% nickel, offering good general corrosion resistance.

- On the other hand, SS.316L contains 16% chromium, 10% nickel, and 2% molybdenum, providing higher corrosion resistance and better performance in harsh environments.

Composition (%)
grade	Carbon ( C) ≤	Manganese (Mn) ≤	Phosphorus (P) ≤	Sulfur (S) ≤	Silicon (Si) ≤	Chromium (Cr)	Nikel (Ni)	Molybdenum (Mo)
304	0.08	2.00	0.045	0.030	1.00	18.0-20.0	8.0-11.0	-
316L	0.035	2.00	0.045	0.030	1.00	16.0-18.0	10.0-14.0	2.0-3.0

 

  Products made from SS.316L have chemical compositions similar to SS.304 but exhibit higher resistance to corrosion and staining due to the increased molybdenum content. This higher molybdenum content enhances resistance to acidic environments, corrosion, saltwater, and heat, making SS.316L a preferred choice for various applications, such as chemical industries, high-salinity environments, shipbuilding, and medical equipment.

 

  In conclusion, stainless steel materials, particularly SS.316L, offer superior corrosion resistance in a wide range of conditions, making them suitable for demanding industrial, marine, and medical applications. Proper material selection is essential to ensure the longevity and performance of stainless steel components in various environments.