Stainless steels (S.S) are classified into two series according to the American Iron and Steel Institute (AISI): series 200-300 and series 400. Each series includes several types of steel with different behaviors.

Steel from the 200-300 Series

These steels are austenitic and due to their high toughness and ductility, they do not require heat treatment. These characteristics make them suitable for welding, and they do not need annealing under normal atmospheric conditions. Moreover, these steels have excellent corrosion resistance and are usually non-magnetic. They harden only through cold working processes. The carbon content in these steels ranges between 0.08% and 0.25%, chromium content between 16% and 26%, and nickel content between 6% and 22%.

The Difference Between 304 and 316 Steel

One of the key differences between 316 steel and 304 steel is the presence of up to 3% molybdenum in 316. Molybdenum enhances the corrosion resistance of this chromium-nickel alloy against most industrial chemical solvents and also improves resistance to pitting corrosion in chloride-containing environments. For this reason, 316 steel is one of the most important choices for use in marine environments.

Corrosion Factors in Stainless Steels

In stainless steels, two major factors that contribute to corrosion and rusting are sensitization and inadequate oxidation removal after welding. Each of these factors is explained below.

Sensitization

Sensitization refers to the phenomenon where chromium carbides precipitate at the grain boundaries of austenitic stainless steel. This occurs when the steel is heated in the temperature range of 425 to 870°C (800 to 1600°F), especially during welding. The time the steel spends at this temperature determines the amount of carbide precipitation.

As chromium carbides precipitate at the grain boundaries, the surrounding areas are depleted of chromium. If this process is continuous, the steel becomes prone to intergranular corrosion. Sensitization also reduces the steel's resistance to other forms of corrosion such as pitting, crevice corrosion, and stress corrosion cracking (SCC).

Methods to Prevent Sensitization

To prevent sensitization, temperature-time sensitization curves can be used to observe the effects of time and temperature on this phenomenon. Below is an example of a sensitization curve for 304 steel.

Another method to prevent sensitization is to use stabilized steels such as 321 and 347. These stainless steels contain titanium or niobium, which have a strong affinity for carbon and readily form carbides. This prevents chromium depletion, even under long-term exposure to sensitization temperatures, keeping the chromium in solution.