Corrosion Resistance of Stainless Steels – Everything You Need to Know

What Provides Corrosion Resistance?

Corrosion-resistant metals owe their durability to a thin, self-repairing protective oxide layer formed by key alloying elements. This layer prevents the underlying metal from reacting with environmental factors, such as moisture or chemicals.

Key Alloying Elements and the Protective Layer

• Chromium: A crucial element in stainless steel, chromium reacts with oxygen to form the protective oxide layer that shields the metal surface from corrosion. This layer is what makes stainless steel corrosion-resistant.
• Nickel: Improves resistance to acids and enhances the mechanical properties of the metal, providing greater toughness and stability.
• Molybdenum: Helps protect against pitting corrosion and crevice corrosion, especially in environments with chloride exposure, such as seawater.

The Role of Carbon Content in Corrosion Resistance

The carbon content in stainless steel affects its strength and corrosion performance. High carbon levels increase hardness but can also make the metal more susceptible to certain forms of corrosion, such as intergranular corrosion, by disrupting the grain boundary structure. Lower carbon alloys, such as 316L, maintain their integrity better in corrosive environments.

Types of Corrosion in Stainless Steel

Although stainless steel is known for its robust corrosion resistance, it can still experience localized corrosion under specific conditions. Factors such as exposure to chloride-rich environments, high temperatures, or mechanical stress can compromise its protective oxide layer, leading to deterioration in certain areas. Understanding the types of localized corrosion helps in choosing the right grade of stainless steel and taking preventive measures to ensure long-term durability. The following is an overview of the most common types of corrosion.

Galvanic Corrosion

This type of corrosion occurs when two different metals are in electrical contact in the presence of an electrolyte, such as saltwater. The less corrosion-resistant metal becomes the anode and corrodes faster, while the more resistant one remains protected. Using materials with similar electrochemical properties or insulating them from one another can prevent galvanic corrosion.

Pitting and Crevice Corrosion

Pitting is caused by the localized breakdown of the protective oxide layer, often in the presence of chloride ions. Similarly, crevice corrosion happens in confined areas where oxygen access is limited, leading to deterioration of the metal in these spaces.

Stress Corrosion Cracking

A combination of tensile stress and a corrosive environment can cause cracking in stainless steel. This is often seen in high-temperature applications and environments containing chlorides.

Improving Corrosion Protection

Beyond alloying, surface treatments like Bortec’s BORINOX® process further improve corrosion protection by hardening the surface without compromising the protective layer. This is essential for applications where wear and corrosion resistance are both critical.

BORINOX® for Enhanced Surface Durability

The BORINOX® process hardens the metal surface without forming chromium carbides that can weaken corrosion protection. This technology increases surface hardness up to 1700 HV while maintaining the natural corrosion resistance provided by the protective oxide layer. This makes BORINOX® particularly useful in sectors like medical devices, automotive components, and food processing.

Mechanical Properties and Material Performance

The mechanical properties of stainless steel, including its strength and ductility, are influenced by both its carbon content and the alloying elements used. Treatments like BORINOX® improve not only the wear resistance but also the overall durability of the material without affecting its ability to resist corrosion.

Real-World Applications of Stainless Steel

Thanks to its corrosion-resistant properties, stainless steel is used across a wide range of industries:

Food and Beverage

In the food processing industry, stainless steel is favored due to its ability to resist acids and its hygienic surface, which is easy to clean.

Marine Environments

Duplex stainless steel is ideal for use in saltwater applications because it resists pitting corrosion and galvanic corrosion, making it perfect for offshore platforms and ship components.

Medical Devices

The material is used extensively in medical instruments, where both high corrosion protection and durability are required, especially in environments exposed to bodily fluids.

Frequently Asked Questions (FAQs)

1. How does chromium help protect stainless steel?

Chromium forms a protective oxide layer on the surface, which blocks corrosive elements from attacking the metal underneath.

2. Can stainless steel suffer from galvanic corrosion?

Yes, when it comes into contact with less corrosion-resistant metals in the presence of an electrolyte like saltwater, galvanic corrosion can occur.

3. How does BORINOX® improve stainless steel?

BORINOX® enhances the surface hardness while maintaining the material’s corrosion resistance, extending its lifespan in harsh environments.