Selecting Between Stainless Steel 304 and 316
Figure 1: Stainless Steel Valve
In short, stainless steel 316 offers better protection than type 304 to 99% of common media present in applications. However, type 304 is still commonly used as it offers good protection to most applications and is the more economical choice. Understand what can cause corrosion in your application and speak with a Tameson Engineer to ensure you select the appropriate material for each application.
What is Stainless Steel?
All steels have the basic iron and carbon composition, but the term stainless steel refers to a wide range of steel alloys with a minimum of 10.5% chromium content by mass. These alloys are produced primarily for resistance to corrosion or oxidation, which happens when metals reacts with oxygen (in water or air) and rust. Stainless steels are 100% recyclable.
Chromium (Cr) is a hard metal which reacts with oxygen when exposed to water or air. In stainless steel, the chromium oxide creates a very thin stable surface film, which protects the metal from corrosion by limiting the oxygen’s access to the rest of the metal. This process, which results in resistance to corrosion, is called passivation. The stable film formed on the stainless steel is self-repairing in case it is scratched or temporarily disturbed by an upset environment. Figure 2 shows how the chromium layer (dark black) protects the rest of the alloy from oxidation, even when damaged.
Figure 2: Chromium Oxide Surface Film
Localized Corrosion of Stainless Steel
Despite it’s name, stainless steel is not quite stainless after all! Stainless steel can corrode if its ability to form its passivating film is disturbed. This intervention can have chemical or mechanical origins both of which can activate major modes of corrosion. However, this corrosion is typically localized and isn’t uniform across the entire metal like carbon steel. The main types of corrosion are pitting and crevice corrosion, which form due to a presence of an aggressive species like chloride (Cl-). However, if an environment is known to have increased chloride, different stainless steels can be selected that have higher concentrations of alloys to protect it.
Figure 3: Pitting Corrosion
- Pitting corrosion occurs when a damaged film layer has limited access to oxygen and an aggressive species such as chloride (Cl-) is present. In this scenario, stainless steel is incapable of re-forming the protective metal oxide film and corrosion forms small holes (pits) on the surface of the metal, which can have a sponge-like appearance. These pits are in some cases right under the surface and therefore invisible from the eye. A thin film pierced by a tiny hole can hide a thumb sized pit from view Figure 3 shows pitting corrosion under a microscope.
Figure 4: Crevice Corrosion
- Crevice corrosion is in principle similar to pitting corrosion and occurs in confined spaces (crevices) where access to the environment is limited, as seen in Figure 4. This mode of corrosion happens in contact areas between parts, under gaskets or seals or spaces filled with deposits. In short, the corroding agent (an example being CI-) gets into the crevice interfering with the alloy’s ability to re-form a passivating film, and causing oxidation.
Types of Stainless Steel
Besides chromium, other alloying elements such as molybdenum (Mo), nickel (Ni), titanium (Ti), etc. are added to stainless steel to enhance its properties such as formability, strength and chemical resistance. Therefore, there are numerous stainless steel grades with varying content of chromium and other elements to suit different environments and applications.
Stainless steels are classified by their crystalline structure, with the most widely produced family being Austenitic. Austenitic stainless steel is achieved by alloying nickel, which gives it excellent formability and weldability. This family has two sub-groups of 200 and 300 series. The 300 series which include Type 304 and Type 316 are the most widely used stainless steel.
When selecting a specific grade of stainless steel for a given application, the first and foremost criteria is the corrosion resistance. Other mechanical or physical properties may also need to be considered to meet service performance.
Corrosion Resistance of Type 304 Vs. Type 316 Stainless Steel
Type 304 (EN steel number 1.4301) and type 316 (EN steel number 1.4401, 1.4436) have very similar physical and mechanical properties but their major difference remains in their resistance to corrosion in different environments:
304 Stainless Steel:
- Contains 18% chromium and 8% nickel
- More of an economic choice if high concentrations of chloride aren’t 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 sea water and de-icing salts.
Corrosion resistance of stainless steel in acidic or basic solutions depends on the kind and concentration of acid or base, and the solution temperature.
In general, acids can be categorized into reducing acids such as hydrochloric acid and dilute sulfuric acid, and oxidising acids such as nitric acid and concentrated sulfuric acid. Increasing amounts of chromium and molybdenum contents in stainless steel provide increasing resistance to reducing acids.
Looking specifically at sulfuric acid, Type 316 is more protective. Type 304 is only resistant to up to 3% acid at room temperature while type 316 is resistant to up to 20% acid at temperatures of up to 50° C. This suggests that type 304 is not suitable for use with sulfuric acid solutions. Hydrochloric acid on the other hand is not recommended and will damage all stainless steel.
Generally speaking, type 316 is considered more resistant to solutions of chlorides, bromides, iodides and fatty acids at high temperatures while most oxidising acids such as nitric acid and phosphoric acid can be used with type 304 at room temperature.
In general, organic acids tend to be less corrosive than mineral acids. However, as the molecular weight of organic acids increase, their corrosivity decrease. For example, formic acid has a low molecular weight meaning it has higher corrosivity. A common organic acid that is used commercially is acetic acid. Type 316 Stainless Steel is used for these applications.
Both type 304 and 316 are resistant to weak bases such as ammonium hydroxide in high concentrations and elevated temperatures. However, care must be taken when the environment contains stronger bases at high concentrations and/or temperatures. Higher contents of chromium and nickel in stainless steel will result in a better resistance to stronger bases.
Type 304 is resistant to aldehydes, while type 316 is preferred for use with amines and cellulose acetate. Fats and fatty acids start attacking type 304 and 316 at temperatures higher than 150° C and 260° C respectively.
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