Stainless Steel

Stainless steel—also known as rustproof or corrosion-resistant steel—is a high-alloy steel designed to resist chemical and electrochemical corrosion. Its exceptional durability stems from a process called passivation, where a thin protective oxide layer forms on the surface of the steel, preventing further degradation.

While stainless steel offers excellent resistance to general corrosion in a variety of environments, certain specific conditions can still trigger localized corrosion—including crevice corrosion, pitting, intergranular corrosion, and stress corrosion cracking. To improve resistance against these types, other alloying elements such as nickel, molybdenum, or manganese are added alongside chromium.

Types of Stainless Steel

Stainless steels are classified into three main groups based on their chemical composition and microstructure:

• Ferritic stainless steels – magnetic and highly corrosion-resistant; typically used in automotive and architectural applications.
• Martensitic stainless steels – hardenable and strong; used in tools and turbine blades.
• Austenitic stainless steels – non-magnetic with excellent formability and corrosion resistance; widely used in kitchenware, piping, and medical instruments.

There are also transitional types, such as duplex steels (ferritic-austenitic) and super-ferritic or super-duplex variants, which offer improved performance in aggressive environments.

Applications Across Industries

Thanks to their unique properties, stainless steels are widely used in:

• Chemical and food industries
• Architecture and construction
• Automotive and aerospace sectors
• Shipbuilding and marine environments

Material Composition and Corrosion Resistance

To achieve corrosion resistance, stainless steels typically contain 12–30% chromium and up to 30% nickel. Chromium plays a vital role in forming the passive chromium oxide layer. This layer is usually only a few microns thick (1–2×10⁻⁶ mm), yet it offers significant protection in many environments—though not in extremely aggressive ones like hydrochloric or sulfuric acid.

The minimum chromium content for effective passivation is generally accepted as 10.5–13% by weight, depending on the application and exposure.

Development and Innovation

The first duplex stainless steels were developed in Sweden in the 1930s for use in the pulp and paper industry. Further advancements came in the 1950s when the metallurgy was standardized, followed by the introduction of oxygen-argon decarburization in the 1960s—an innovation that enabled the production of low-carbon steels with controlled nitrogen content.

Today, super-austenitic and super-duplex stainless steels are used in some of the most demanding environments, offering a balance of exceptional corrosion resistance, strength, and weldability.

Predicting Steel Structure

To predict the microstructure of stainless steel, engineers use diagrams like Schaeffler and De Long. These show how elements like chromium and nickel influence whether the resulting steel will be primarily ferritic, austenitic, or martensitic.

Stainless steel’s wide range of types and properties makes it one of the most versatile and valuable materials in modern engineering.