18-8 Stainless Steel vs 316: Understanding the Differences in Corrosion Resistance and Applications

Identifying your project’s suitable grade of stainless steel starts with knowing and discerning material grades. Among the most popular grades are 18-8 and 316 stainless steels. Each of them is well known for their utility and remarkable resistance to climatic corrosion, yet they are tailored for very different uses. This article aims to clarify the most important aspects of differentiation between 18-8 and 316 stainless steel in regard to their chemical constituents, ability to resist corrosion, and preferred application. If you are in manufacturing, construction, or any other industry that requires high strength-to-weight ratio materials, this guide is designed for you. Continue reading to learn the benefits and drawbacks of the two most commonly used types of stainless steel.

What is 18-8 Stainless Steel?

Primarily consisting of 18% chromium and 8% nickel, 18-8 stainless steel is an austenitic stainless steel with a permissive scope of use. Its structure gives it strong durability, ease of fabrication, and excellent corrosion resistance. With such versatility and balance of several properties, it is widely used in food processing, kitchen utensils, chemical equipment, and many more. Also, its moderate resistance to the environment and non-magnetic nature make it a practical option for many standard industrial and commercial purposes.

Understanding the 18-8 Composition

The term ”18-8” suggests an austenitic stainless steel with a traditional mixture of roughly 18% chromium and 8% nickel. Chromium aids in the prevention of corrosion through passive oxide layer formation, whereas nickel improves the strength and ductility of the material. This combination guarantees structural integrity, as well as protection from corrosion, ease of fabrication, or rusting in most environments. Due to the combination of factors and properties that are mechanical and chemical, this material is appropriate for industrial and consumer products; thus, its global application is well known.

Applications of 18-8 Stainless Steel

Stainless steel grade 18-8, which primarily includes iron, chromium, and nickel, is used in a variety of industries because of its exceptional characteristics. Below is its li of key uses grouped by different industries:

Food and beverage industry

• Application: Used widely in making kitchen appliances, cookware, cutlery, and food processing equipment.
• Reason: Its surface being non-reactive aids in avoiding contamination and meeting hygienic as well as cleanliness standards.
• Data: It has been proven to retain strength and resist corrosion, even with prolonged exposure to acidic or alkaline food materials.

Medical and Pharmaceutical Sector 

• Application: Instruments and surgical tools, as well as sterilizable containers.
• Reason: Due to its biocompatibility and ease of sterilization, it is ideal for medical applications.
• Data: Studies carried out suggest that 18-8 stainless steel does not suffer corrosion when exposed to hospital disinfectants.

Automotive Industry  

• Application: Trims and structural parts of the vehicles, as well as exhaust systems.
• Reason: Vehicle life is increased due to high corruption resistance in combination with high heat.
• Data: The material is able to hold up its mechanical properties in high temperature environments (up to 800°F/427°C).

Construction Sector

• Application: Expansion joints, structural supports, fasteners, and fittings.
• Reason: Aesthetic appeal combined with strength and longevity in hostile conditions.
• Data: It is widely used in coastal buildings because of its resistance to corrosion from salt water over an extended period.

Petrochemical and Energy Sector 

• Application: Used in pipes, valves, and tanks for storage and processing of chemicals and fuel.
• Reason: The steel is highly resistant to stress corrosion cracking and chemical attack in harsh environments.
• Data: Laboratory stress tests indicate its reliability in environments high in chloride and sulfide concentration.

Marine Applications 

• Usage: Underwater cables, anchors, and boat fittings.
• Reason: Outstanding corrosion resistance for both fresh and saltwater.
• Data: Due to the protective layer of chromium oxide, it performs better underwater than above.

Household Items  

• Usage: Sinks, appliances, countertops, and faucets.
• Reason: Durable, stain resistant, and aesthetically pleasing for daily use.
• Data: A study on market trends shows that 18-8 stainless steel is used in more than 60 percent of households worldwide.

The remarkable combination of strength, resistance to corrosion, and ease of fabrication makes 18-8 stainless steel arguably the most versatile type of stainless steel.

Comparing 18-8 to 304 and 316 Stainless Grades

The 18-8 grade of stainless steel is closely associated with the 304 and 316 grades since all of them are inclusively part of the austenitic stainless steel family. In particular:

• 18-8 and 304: The nomenclature  “18-8” is frequently abused in commerce instead of “304.” The reason being “304” is amongst the most ordinary 18-8 alloys, with 18% of chromium and 8% of nickel. On the flip side, 18-8 denotes a wider class and may encompass variances within standard specifications.
• 18-8 and 316: Even though 316 belongs to austenitic steels, it has an addition of molybdenum and thus offers greater resistance to corrosion on account of chlorides and some chemicals, which makes it more useful for marine or chemically severe environments than 18-8. However, for ordinary applications, 18-8 stainless steel should be sufficient.

In conclusion, the specified application and environmental factors are determinant of the best grade of steel to use amongst all these options. 18-8 or 304 is more reliable for most general uses, and 316 is best where superior corrosion resistance is needed.

How Does 316 Stainless Steel Compare?

The Role of Molybdenum in 316

Molybdenum significantly improves the effectiveness of 316 stainless steel. The addition of roughly 2-3% molybdenum gives 316 stainless steel an exceptional ability to resist pitting and crevice corrosion in chloride and marine environments, making it ideal for seawater, de-icing salt, and industrial chemical exposure.

Moreover, molybdenum enhances strength retention at elevated temperatures which allows 316 stainless steel to be used in high temperature applications such as heat exchangers and chemical processing equipment. Research indicates that molybdenum enriched steels show much greater resistance to stress corrosion cracking and intergranular attack than steels lacking molybdenum.

Molybdenum’s contribution further improves the alloy’s life in harsher conditions, reducing maintenance and replacement expenditures in the long term. This additive is crucial to the performance of 316 and differentiates it from lower-grade stainless steels such as 304.

Why 316 Stainless Steel is Preferred in Corrosive Environments

The enhanced resistance of 316 stainless steel to corrosion can be understood in regard to its sophisticated chemical makeup. 316 stainless steel contains 16-18% chromium, 10-14% nickel, and 2-3% molybdenum, making it ideal for resisting pitting and crevice corrosion in chloride-prone environments like marine and chemical processing industries. For instance, research indicates that molybdenum improves resistance to chloride ion attack in 304 stainless steel. Unlike 304 stainless steel, 316 contains molybdenum.

Furthermore, the effectiveness of 316 stainless steel in such environments is reinforced by its high PREN (Pitting Resistance Equivalent Number) value. 316 stainless steel has a typical PREN value of 23 to 28, impressively outperforming 304 stainless steel which has a value of 18. This further reinforces the idea that 316 is preferred for purposes where corrosion is highly localized, as pitting is difficult and allows for durability and productivity over time.

Apart from that, it can be seen in the machinery of desalination plants, pharmaceutical production, and offshore oil rigs. These settings expose the materials to salt, moisture, and corrosive chemicals. Because the alloy is able to sustain mechanical and visual properties under such severe conditions, it is less prone to downtimes, making it cost-effective. This increases and reinforces its standing as the material of choice for extreme environments.

Common Uses of 316 Stainless

The 316 stainless steel, as is evident, is widely used in various industries because of its durability, ability to withstand high temperatures and corrosion resistance. Here are the most remarkable applications, along with the details and important figures as provided:

Marine Equipment

• Used for the production of fasteners, marine-grade propeller shafts, and boat fittings.
• Used in the boat industry because of its unrivaled 316 stainless seawater corrosion resistance which makes it ideal for saltwater environments.
• In addition, prevents pitting and crevice corrosion in high salinity conditions.

Food and Beverage Processing

• Misleadingly regarded for its use in tanks, pipelines, and conveyors for storing food and handling activities.
• Meets sanitary requirements due to its non-reactive nature and high ease of cleaning.
• FDA approval guarantees the safety of their use in foods.

Pharmaceutical and Medical Devices, 

• surgical instruments, implants, and equipment for the production of medicine are used on the same level it is used in medicine.
• Enthrones chemical sterilization processes and body fluids, so resistance ensures long-period performance, thus lowering bio-compatibility issues.
• Pureness is so high that it makes sure no contamination occurs during the production or the use of the process.

Chemical Processing Plants

• Reactors, storage tanks and heat exchangers which deal with corrosive things such as acids, alkalis as well as solvents makes use of it.
• It is an excellent candidate for the strong acid traditional sulfuric, hydrochloric in addition to chlorides which brought prolong use of equipment life.

Building and Architectural Applications

• Developed in handrails, facades, and exterior cladding because of its appealing look and resistance to the weathering process.
• It is resistant to atmospheric corrosion which is recommended as suitable in urban and coastal area environments.

Desalination Plants

• Involved in high-pressure piping, evaporators, and brine tanks.
• It is resistant to corrosion from high saline water and elevated temperatures during the desalination process.

Automotive and Aerospace

• In the manufacturing of components that are subject to high thermal and mechanical loads, such as exhaust manifolds, catalytic converters, and structural components.
• Being non-magnetic is of great value in some aerospace applications where accuracy and dependability is required.

Offshore Oil and Gas Industry

• Important in the construction of subsea oil pipelines, valve parts, and platform body structures.
• Is able to resist severe pressures and exposure to aggressive drilling fluid and seawater chemistry.

By taking advantage of their oxidization resistance, mechanical strength, and flexibility to extreme changeable conditions, 316 stainless steel is becoming more popular due to its reliability and low operational costs. The broad range of applications emphasizes the role of 316 stainless steel in global technological progress and infrastructure development.

Understanding the Corrosion Resistance of 18-8 vs 316

Analyzing Chromium and Nickel Content

18-8 stainless steel has around 18% of chromium and 8% of nickel, which gives it its corrosion resistance and strength. 316 stainless steel, however, has similar chromium constituents as 18-8 but has a relatively higher nickel concentration of 10% and contains 2-3% molybdenum. Precisely added molybdenum in 316 improves resistance to corrosion of chloride and acidic environments. Both alloys have impressive corrosion resistance, but the composition of 316 makes this alloy with more extreme corrosion conditions more suitable.

Comparing Resistance to Corrosion in Different Environments

The considerations of different environments still maintain that the chemical composition of the stainless steel grade is of high importance. For example, 304 stainless steel is effective with moderate conditions like mild chemicals or exposure to broad atmospheric elements. Yet, marine environments with high salinity, like seawater, can expose 304 stainless steel to pitting or localized corrosion due to chlorides.

On the other hand, 316 stainless steel has superior performance in marine environments due to its molybdenum content. Research indicates 316 stainless steel outperforms 304 in pitting resistance equivalent numbers (PREN), with 316’s higher pitting resistance ranging from 23 to 28 while 304’s sits between 18 to 20. The increased resistance is the explanation for the preference of 316 in industries concerning seawater systems, chemical processing, and coastal infrastructure.

For cases such as desalination plants or chemical tanks where there is extremely high chloride concentration, 316 stainless steel shows better structural integrity over time than 304. Other factors like temperature have a significant impact as well. With an increase in temperature, the probability of corrosion also increases, which, in turn, makes the case for 316 to be more useful in high-temperature chloride environments. Studies have shown that adding 2-3% molybdenum in 316 reduces the likelihood of crevice corrosion, which is common among tight-fitted parts subjected to corrosive environments.

In the end, it comes down to chloride concentration, temperature, and the specific requirements of the industry to select the correct stainless steel, with 316 being recommended in instances where 304’s resistance might prove to be too much of a challenge.

Impact of Alloying Elements on Corrosive Resistance

Alloying Elements is crucial for improving the corrosive resistance of stainless steel. The primary element for passive oxide layer formation is Chromium, as it assists in protecting the steel from oxidation and corrosion. Nickel, besides improving ductility, is also beneficial in overall corrosion resistance, particularly in acidic mediums. In some grades, such as 316, Molybdenum is present, which is useful for the prevention of crevice and pitting corrosion in chloride ion-containing environments. With the proper combination of these elements, stainless steel is able to resist certain environmental conditions and thus can be utilized in various industries.

Why Choose 18-8 vs 316 for Your Applications?

Factors Influencing the Selection of Stainless Fasteners

Resistance to Corrosion

In the context of using stainless steel fasteners, the degree of required corrosion resistance is a critical consideration. In ordinary environments, stainless steel, especially grade 304 type 18-8, has a high nickel and chromium content which provides adequate prevention against rust and mild corrosion. However, grade 316 is favored in harsh environments such as those with industrial chemicals or marine environments due to the addition of molybdenum, which provides resistance to pitting and crevice corrosion. For example, 316 stainless has proven to perform much better than 18-8 variations in environments with high concentrations of chlorine, such as saltwater.

Mechanical Strength

The remaining aspects of concern regarding stainless fasteners are their mechanical properties particularly, the tensile and yield strength. Corrosion resistance grades 18-8 and 316 are both noted for their weight-to-strength ratio, although they differ slightly. Fasteners made from 18-8 stainless typically exhibit higher initial tensile strength due to minor variations in composition. However, grade 316 fasteners are more reliable under prolonged stress and extreme environmental conditions because of their superior enhanced metallurgical structure.

Cost Implications

One of the most important factors of a project is the cost, particularly for those that need a large amount of fasteners. In general, 18-8 stainless steel is less expensive than 316 stainless because the latter uses molybdenum, which is a costly alloying constituent. The differences in pricing can be more favorable in regions where there is less focus on moisture and corrosion resistance. For instance, in 18-8 fasteners used in inland construction projects or in low-moisture environments, cost efficiency is achieved.

Temperature Resistance

For applications dealing with high or changing temperatures, the working range of the fastener’s material should be examined. Both 18-8 and 316 stainless steel have moderate heat resistance, but 316 also has additional scaling resistance and stays strong at higher temperatures because of the molybdenum. Because of this, 316 is best suited for use in industrial heat exchangers and furnace equipment where the environments are harsher.

Magnetic Properties

Both types are austenitic and, hence, mostly non-magnetic, but some uses necessitate very low magnetic responsiveness. 316 stainless fasteners contain molybdenum and have lower magnetic permeability than 18-8, making them useful in places where low magnetism is crucial, such as very delicate electronic or medical equipment.

Governmental and Industry Regulations

Meeting designated industry standards assists in narrowing down the options for fasteners. For example, food and pharmaceutical manufacturing sectors tend to require the 316 grade of stainless steel because it is more difficult to contaminate and easier to sterilize. On the other hand, the construction and automobile industries might favor the 18-8 grade for its better value compared to other general-use stainless steels.

An evaluation of all these variables – the level of corrosion, mechanical strength, cost, how much heat a material can withstand, its degree of magnetism, and adherence to the industry documents – enables engineers and designers to choose the best stainless fastener suited for their applications.

Cost Considerations: 18-8 Stainless Steel vs 316

Cost considerations are always necessary in comparing two materials, in this case 18-8 stainless steel vs 316 stainless steel. Generally, 18-8 stainless steel will be less costly because it is more readily available and simpler in composition. It is therefore the material of choice for uses where cost is important and exposure to extreme conditions is not excessive.

On the contrary, 316 stainless steel is more costly due to its greater content of molybdenum, which increases its corrosion resistance. Its increased cost is warranted in applications where there is a need for greater durability in marine or chemical processing environments that are corrosive or very extreme.

As with any decision, the trade-off between budget limitations and requirements for performance to do the intended work must always be struck.

What are the Advantages and Disadvantages of Each Type?

Understanding Tensile Strength and Durability

Due to its relatively good chemical resistance, 18-8 stainless steel performs satisfactorily in comparison to 316 stainless steel. Its lower chemical resistance when compared to 316 should result in less performance in very corrosive environments for extended periods of time. 18-8 stainless steel’s tensile strength and durability gives it versatility for use in many general applications.

316 stainless steel has superior tensile strength and durability, especially when there is exposure to chlorides, acids, and high temperatures. Its molybdenum rich composition enables better performance under extreme environments which benefits demanding industrial and marine applications, which makes it ideal for extreme conditions.

Evaluating High Corrosion Resistance Needs

316 stainless steel is often the best choice because of its increased composition. Its molybdenum content of 2-3% provides higher resistance to both pitting and crevice corrosion, especially for chloride-laden regions. This enables its usability in marine, chemical processing, and medical facilities, which frequently face aggressive agents.

Recent studies of materials reveal that 316 stainless steel can withstand up to one thousand ppm of chloride ions without losing its integrity, as opposed to 18-8 stainless steel, whose integrity starts to deteriorate above two hundred ppm. In addition, 316 stainless steel has also shown higher resistance to oxidation when heated, allowing him to work in an environment of twelve hundred degrees Fahrenheit (649 degrees Celsius) and above, leading the 18-8 grades to be used in hot environments.

Another aspect that needs to be considered while choosing materials for high corrosion resistance is the lifetime cost of the material. 316 stainless steel might be expensive at first, but long term cost efficiency after reduced maintenance needs outweighs the upfront cost, especially in scenarios where operational downtimes from material corrosion pose a safety hazard.

Maintenance Requirements and Longevity

Like other alloys, 316 stainless steel needs very little attention because of its remarkable corrosion resistance; however, it does require some cleaning periodically to maximize its life expectancy. In environments where there is constant exposure to chlorides or moisture, surface contaminants should be cleaned regularly on 316 stainless steel. Its base durability means it needs little maintenance for structural integrity for several decades. This further reduces the need for replacements and repairs. However, to ensure optimal performance in high-stress conditions, regular inspections are required. All in all, 316 stainless steel is a cost-effective choice for applications that are long-term due to its robust nature and minimal attention.

Frequently Asked Questions (FAQs)

Q: What sets 18-8 apart from 316 stainless steel?

A: The difference is found in their chemical structures and the degree to which they can resist corrosion. Known as 304 stainless steel, 18-8 is composed of 18% chromium and 8% nickel. On the other hand, 316 stainless steel has 16% chromium, 10% nickel, and 2% molybdenum, which gives the alloy better resistance to corrosion, especially in regard to corrosive chemicals.

Q: In terms of corrosion resistance, what is the difference between 18-10 and 316 stainless steel?

A: 18-10, also known as 304-grade stainless steel, has adequate resistance to corrosion but is outperformed by 316 stainless steel, which is more resistant to corrosion, especially in tough environments with corrosive chemicals.

Q: In what ways does 316 differ from 304 in terms of cost?

A: The price difference for 316 stainless steel is associated with the increased corrosion resistance. Its costs are also higher due to the chemical composition which contains molybdenum. This allows 316 stainless steel to be used in applications that require a high resistance to corrosion.

Q: What kind of stainless steel is used for marine applications?

A: For use in marine applications, 316 stainless steel is popular due to its superior resistance to corrosion, as well as strength in salty and corrosive environments.

Q: What is the difference in applications for 304 and 316 stainless steel?

A: 304 stainless steel is used in general-purpose applications but 316 stainless steel is used in specialized applications with stronger demands in regard to corrosion resistance, like chemical processing, oil and gas industries, and marine applications.

Q: Can 18-8 stainless steel be considered as a replacement for 304 ss?

A: Yes, 18-8 stainless steel is a grade of stainless steel known as ‘304 stainless steel’, and they are one and the same and therefore interchangeable. They have almost the same percentage of chromium and nickel composition which gives them good resistance to corrosion and makes them easy to machine.

Q: What is the difference in the strength of 18-8 stainless steel, a2, and a4 stainless steel?

A: Both 18-8 stainless and a304 stainless have good strength and resistance to corrosion as well. A2-grade stainless is roughly the same as a304, whereas a4-grade stainless, which is a higher grade similar to 316, has much greater strength as well as resistance to ccorrosion making it able to with stand harsher environments.

Q: Does 316 stainless steel require certain conditions in which it will perform best?

A: Yes, the conditions that surround 316 steel will affect its performance. The best performing conditions are those which have high resistance to corrosion, particularly to chlorides and acidic conditions.

Q: In what situations would alloy steel be preferred over 316 stainless steel?

A: Alloy steel may be preferred in certain construction and machinery applications with stainless steel’s uses, where higher tensile strength and hardness yield greater value than corrosion resistance.

Q: Who should I talk to if I have further inquiries about the differences between 18-8 and 316 stainless steel?

A: To obtain more information, you may reach out to us or a qualified metallurgist or materials engineer who can issue specific advice on the 18-8 and 316 stainless steel selection.

Reference Sources

1. Corrosion Behavior of 18-8 Stainless Steels in Hot Concentrated Caustic Soda Solutions under Heat-Transfer Conditions

• Authors: M. Yasuda et al.
• Journal: Corrosion
• Published: December 1st 1985.
• Key Findings: This work evaluates the corrosion behavior of AISI 304 (18-8) and AISI 316 stainless steels in corroding media of hot, concentrated caustic soda solutions. AISI 316 was found to be more prone to corrosion than AISI 304 due to the breakdown of the passive film on the heated surface (Yasuda et al., 1985, pp. 720-727).

2. The Changes of Structure and Mechanical Properties of 18-8 Series Stainless Steels after Prolonged Aging

• Authors: Tohru Mimino et al.

• Journal: Isij International
• Publication Year: 1969
• Key Findings: This paper explores the aging effects on mechanical properties of 18-8 stainless steels at elevated temperatures for long periods of time, emphasizing the damage to the structure and its consequences on the mechanical behavior (Mimino et al., 1969, pp. 472-482).

3. Corrosion Resistance of SS 18/8 Alloy in Artificial Saliva of a Patient 

• Authors: Chandrasekaran Priya Jane et al.
• Journal: International Journal of Corrosion and Scale Inhibition
• Year of Publication: 2021
• Key Findings: The paper studies the corrosion resistance of the alloy SS 18/8 in artificial saliva, which suggests the action of certain factors, like the presence of some Digene Tablet juice, which weakens its corrosion resistance. It does not evaluate it against Type 316 directly but does shed some light on how 18-8 stainless steel behaves(Priya et al., 2021).

4. Effects of ENO Juice on The Corrosion Resistance of SS 18/8 Parts of a Denture Framework in Artificial Saliva 

• Authors: C. J. Priya et al.
• Journal: International Journal of Corrosion and Scale Inhibition
• Release Date: September 22, 2021
• Key Findings: This study similarly traces the effect of ENO juice on the corrosion resistance of SS 18/8 alloy in artificial saliva, this time without a direct comparison with Type 316(Priya et al., 2021).

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