Stainless Steel Fastener Grades Explained

The term stainless steel is often used as if it describes a single, uniform material, but it actually refers to a range of alloys available in multiple grades with different chemical compositions and performance characteristics. When selecting stainless steel fasteners, it’s important to remember that the grade directly affects corrosion resistance and overall performance in the assembly. 

At Earnest, we typically supply 18/8 stainless steel, often referred to as Grade 18/8 or Grade 304, along with Grade 316 and metric fasteners designated as Class A2 or Class A4.

But What Exactly Does “18/8” Stainless Steel Mean?

The term 18/8 refers to a stainless steel composition containing approximately 18% chromium and 8% nickel. These elements are what give stainless steel its corrosion resistance and durability. 

Strictly speaking, 18/8 is not a formal grade designation. It describes a chemical composition range rather than a specific alloy defined by a published material standard. In fastener specifications, however, 18/8 is commonly treated and understood as a grade across the industry. In other words, 18/8 defines the stainless steel’s chemistry rather than a single standardized alloy designation.

In practice, most fasteners sold as 18/8 are manufactured from Grade 304 stainless steel or a closely related alloy that falls within the same composition range. 

How 18/8 Relates to Grade 304 Stainless Steel

Grade 304 is the most common stainless steel alloy used for fasteners and is defined by ASTM material specifications. It falls within the 18/8 composition range and provides good corrosion resistance across a wide range of indoor and outdoor environments.

Grade 304 performs well in applications where fasteners may be exposed to moisture, humidity, or mild chemicals, but not aggressive chlorides or saltwater. It is frequently used in general manufacturing, enclosures, food equipment, and structural assemblies.

Because machining and forming processes can leave free iron particles on the surface of the alloy, Grade 304 fasteners are often passivated to improve corrosion resistance and long-term performance in the assembly.

When Grade 316 Is the Better Choice

Grade 316 is a stainless steel alloy similar to Grade 304 but with added molybdenum, which improves resistance to chlorides and more aggressive corrosive environments. Like Grade 304, it is defined by ASTM material specifications.

Grade 316 is widely used in marine, coastal, and chemical processing applications where exposure to salt or chemicals would exceed the corrosion resistance of Grade 304. It is typically chosen when long-term durability and corrosion resistance are required.

As with other stainless steel grades, proper surface treatment such as passivation can help promote consistent corrosion resistance and reliable performance in demanding environments.

Understanding Metric Classes A2 and A4

In metric systems, stainless steel fasteners are commonly specified by property class rather than alloy number.

Class A2 fasteners are generally equivalent to Grade 304 stainless steel and offer similar corrosion resistance. They are suitable for most general-purpose applications where environmental exposure is moderate.

Class A4 fasteners are typically manufactured from 316 stainless steel. This alloy includes molybdenum, which improves resistance to chlorides and harsh environments. A4 fasteners are commonly used in marine applications, coastal installations, and chemical processing environments where corrosion risk is higher.

Choosing the Right Stainless Steel Grade for Your Application

Selecting the right stainless steel grade depends on the operating environment, expected exposure, and service life requirements.

Grade 18/8, Grade 304, or Class A2 fasteners are often suitable for indoor use and many outdoor applications where salt exposure is limited. Grade 316 and Class A4 fasteners are better suited for marine, coastal, or chemically aggressive environments.

While higher-alloy grades offer increased corrosion resistance, they often come at a higher cost and are generally used only when the application demands that added performance. Understanding the differences between grades helps ensure the correct material is supplied, even when application details are limited.

How Grade and Surface Treatment Work Together

Stainless steel relies on a thin passive oxide layer to resist corrosion, but manufacturing processes such as machining or cold forming can disrupt this layer and leave surface contaminants behind.

As discussed in our previous article, Understanding Passivation in Stainless Steel Fasteners, passivation helps remove free iron particles from the surface of the fastener and restores the protective chromium oxide layer. This process improves corrosion resistance and performance consistency across all stainless steel grades.

Choosing the right grade is important but pairing that material with proper surface treatment is often what ensures long-term fastener reliability.