Understanding Sensitization and Intergranular Corrosion

Sensitization occurs when standard 316 stainless steel is exposed to temperatures between 425∘C and 815∘C (approximately 800∘F to 1500∘F), such as during welding. At these temperatures, the carbon within the steel quickly combines with chromium to form brittle chromium carbides at the grain boundaries. This process effectively depletes the surrounding area of the crucial chromium content necessary to maintain the passive oxide layer. The result is a steel that, while chemically 316, is functionally weak at its grain boundaries, making it highly susceptible to intergranular corrosion—a destructive form of localized attack in the heat-affected zone (HAZ) adjacent to the weld.

The maximum carbon content of 0.03% in 316L virtually eliminates this risk. The trace amount of carbon is insufficient to combine with and deplete the chromium, meaning the material retains its full corrosion resistance even after extensive welding, eliminating the costly and complex requirement for post-weld annealing.

Due to this superior welding performance and near-identical mechanical properties, 316L sheets are frequently dual-certified as 316/316L by premium ss 316 sheet suppliers. This practice simplifies inventory and procurement for industrial clients, guaranteeing they receive a material that meets the requirements of either grade specification.

In-Depth: The Corrosion Resistance Mechanism of SS 316

The high-performance nature of SS 316 sheets is not accidental; it is rooted in a meticulously engineered chemical composition that optimizes the steel’s defense against aggressive media. For procurement specialists, understanding why this grade works is as important as knowing that it works.

The Passive Layer and the Role of Molybdenum

All stainless steels rely on a microscopically thin, self-repairing passive layer of chromium oxide (Cr2​O3​) to resist corrosion. This layer forms instantly when chromium interacts with oxygen. In SS 316, the addition of $2-3\%$ Molybdenum significantly enhances the stability and impermeability of this passive layer, especially in the presence of chlorides. Molybdenum works by promoting the formation of a more resistant and less soluble oxy-hydroxide film on the surface, which is critical for defending against two primary forms of attack:

1. Pitting Corrosion: Occurs when chloride ions break down the passive layer at isolated points, creating microscopic pits that deepen over time. Molybdenum drastically increases the critical pitting temperature (CPT), making the steel much harder to break down.
2. Crevice Corrosion: A more severe form of attack that occurs in confined spaces (under gaskets, bolts, or laps) where oxygen supply is restricted. The resulting acidic, high-chloride environment is highly corrosive, but SS 316’s composition, again thanks to Molybdenum, provides significantly better resistance than 304.

Mechanical Properties, Heat Treatment, and Cold Forming

Beyond corrosion resistance, the mechanical properties of SS 316/316L sheets are vital for ensuring structural integrity and optimizing fabrication processes. Reputable ss 316 sheet suppliers provide detailed certifications confirming these properties adhere to ASTM standards.

Strength and Ductility

As an austenitic stainless steel, 316 exhibits excellent strength and ductility. Its mechanical properties are achieved through the initial annealing process (often performed at the mill), which softens the material for subsequent cold working.