Valve Housing Material - Selecting Between Stainless Steel 304 and 316

When choosing the housing material for a valve, it is crucial to understand the distinctions between stainless steel 304 (EN steel number 1.4301) and stainless steel 316 (EN steel number 1.4401, 1.4436). The decision between the two usually hinges on the corrosion resistance needs of the application. This article explores the characteristics of each steel type, the criteria for selecting between them, and the methods to decide which is appropriate for a stainless steel valve.

Table of contents

• What is stainless steel?
• Types of stainless steel
• Stainless steel 304 vs 316
• Other factors
• Stainless steel valves
• FAQs

What is stainless steel?

Steel, in general, consists of iron and carbon. Stainless steel, on the other hand, encompasses various steel blends containing at least 10.5% chromium by weight. These blends are primarily made to withstand corrosion or oxidation, which occurs when metals react with oxygen (in air or water) and form rust. Stainless steels are 100% recyclable.

When chromium (Cr), a durable metal, comes into contact with water or air, it undergoes a chemical reaction with oxygen. In stainless steel, chromium oxide forms a thin, stable film on the surface (Figure 2 labeled A), protecting the metal from corrosion by limiting oxygen exposure. This corrosion resistance process is known as passivation. The protective film can self-repair (Figure 2 labeled C) if scratched or disturbed (Figure 2 labeled B).

Types of stainless steel

Varying the ratios of different elements has led to many kinds of stainless steel. Beyond chromium, other alloying elements are added to stainless steel, for example, molybdenum, nickel, and titanium. These elements enhance stainless steel's formability, strength, and chemical resistance.

Austenitic stainless steel is the most widely produced family of stainless steel. It is created by alloying nickel, which gives it excellent formability and weldability. Stainless steels 304 and 316 are members of this family.

Stainless steel 304 vs 316

When selecting a specific grade of stainless steel for a given application, the first and foremost criterion is corrosion resistance. Other mechanical or physical properties may also need to be considered to meet service performance.

Corrosion resistance

Stainless steel 304 (EN steel number 1.4301) and stainless steel 316 (EN steel number 1.4401, 1.4436) have very similar physical and mechanical properties, but their primary difference remains in their resistance to corrosion in different environments:

• 304 Stainless Steel
  • Contains 18% chromium and 8% nickel
  • Cost-effective choice if high concentrations of chloride are not present.
• 316 Stainless Steel
  • Contains 16% chromium, 10% nickel, and an additional 2% molybdenum
  • The added molybdenum provides greater corrosion resistance to acids and localized pitting attack by chloride solutions such as seawater and de-icing salts.

The corrosion resistance of stainless steel in acidic or basic solutions depends on the kind and concentration of acid or base and the solution temperature.

Corrosion resistance to acids

Stainless steel type	Resistance to acids
Stainless steel 304	Good resistance to moderately aggressive organic acids, such as acetic acid and formic acid Prone to corrosion (e.g., pitting and crevice corrosion) by strong acids such as sulfuric or hydrochloric acids Resistant to most concentrations of phosphoric acid and nitric acid but may have stress corrosion cracking in hot concentrated solutions
Stainless steel 316	Better acid resistance than stainless steel 304, especially if chloride ions are in the system Better resistance to sulfuric acid than stainless steel 304. Can experience corrosion at concentrations above 20% or temperatures above 50 °C (122 °F) Prone to corrosion by hydrochloric acids Better resistance to phosphoric, acetic, formic, and tartaric acid solutions at a wide range of concentrations and temperatures

Table 1: Stainless steel 304 vs 316 resistance to acids

Corrosion resistance to bases

Stainless steel type	Resistance to bases
Stainless steel 304	Good resistance to alkalis Can withstand exposure to weak bases, such as sodium hydroxide or potassium hydroxide May experience localized corrosion in the presence of strong bases at high temperatures
Stainless steel 316	Higher resistance to bases than stainless steel 304 Can maintain corrosion resistance to strong bases, such as sodium and potassium hydroxide, even at elevated temperatures

Table 2: Stainless steel 304 vs 316 resistance to bases

Other factors

As mentioned in the above section about corrosion resistance, most of the other properties of stainless steel 304 and 316 are similar. Understanding the following properties can help select the right stainless steel, but does not need a lot of focus unless the application has very precise demands:

• Mechanical properties: 304 and 316 have similar tensile strength, yield strength, and elongation. 316 has slightly higher strength and toughness due to molybdenum.
• Cost: 304 is less expensive than 316 due to the absence of molybdenum. The lower cost can be significant for large materials or projects.
• Heat resistance: 316 has a slightly higher heat resistance than 304.
• Fabrication and formability: 304 is slightly easier to work with than 316 when fabricating, welding, and forming.
• Weight: 316 is slightly denser than 304 due to molybdenum.

Stainless steel valves

Stainless steel valves are widely used primarily due to their corrosion resistance. With everything else equal, 304 and 316 valves have a negligible difference in temperature and pressure rating. Look for a stamped number on the valve's body to determine if it's 304 or 316, similar to the objects in Figures 3 and 4. The following table describes factors to consider when selecting between stainless steel 304 and stainless steel 316 valves.

Property	Stainless steel 304 valve	Stainless steel 316 valve
Corrosion resistance	Good resistance but inferior to 316	Superior to 316 due to the addition of molybdenum
Applications	Food processing, water treatment, general plumbing and other applications that require moderate corrosion resistance	Marine environments, chemical processing, pharmaceutical industries, and other applications with high corrosion resistance demands
Cost	Less expensive	More expensive

Table 3: Selecting between stainless steel 304 and stainless steel 316 valves

FAQs

How do I tell the difference between stainless steel 304 and stainless steel 316?

Unless stamped, the only way to tell the difference between these two types of stainless steel is to test them chemically. There are no visual markers or mechanical properties that make them easy to differentiate.

How do I choose between stainless steel 304 and stainless steel 316?

Choosing between these two stainless steel types typically comes down to understanding how much corrosion resistance is necessary. Stainless steel 316 has superior corrosion resistance but is also more expensive.