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The corrosion resistance of stainless steel

中国石油公司 — Fri, 26 Aug 2022 08:19:44 +0000

The corrosion resistance of stainless steel

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The corrosion resistance of stainless steel

Whether for the consumer market or industrial use, one of the most popular reasons for choosing stainless steel is its corrosion-resistant properties. While stainless offers excellent resistance to corrosion and staining, it is not immune to these effects.
However, the exact resistance levels and weaknesses of a stainless product, sheet, or bar will depend on both its metallurgic composition and how it was made.
This guide covers the various types of corrosion to be wary of while working with stainless steel and other concerns that might impact either the visual appearance or performance of popular stainless steel grades.
But before we dig into the details, let’s take a look at what makes stainless so resistant in the first place…

How does stainless steel resist corrosion?

While various stainless steel grades might differ in their metallurgic composition and alloying or manufacturing processes, one trait they all share is a passivation layer.
This layer of chromium oxide forms when the surface of the metal is exposed to oxygen. The layer is too thin to observe directly and under most circumstances remains clear. This allows the finished texture of a stainless steel product to show through and provides a variety of appearance options from high gloss to matte. Impervious to water, the passivation layer is self-generating and serves to protect the metal. While it might discolor under extreme temperatures, it’s still working. However, stainless steel is not invincible. It’s still susceptible to corrosion under the right situations.

Stainless steel and chemical compounds: what to consider

One of the most common scenarios in which stainless steel corrodes is through sustained contact with liquids or chemicals.
Known as general corrosion, this type is typically uniform across the surface of the steel. Fortunately, most stainless steels respond in a predictable manner to a range of chemicals.
If you’re planning to use stainless steel, consider exposure to the following to gauge general corrosion risks.

Acids

Stainless steel is generally resistant to acidic corrosion. However, exact resistance levels will depend on the steel in use, concentration, types of acid, and environmental temperature. For example:

Grades 316 and 317 offer resistance to phosphoric acid at most concentrations and sulfuric acid at concentrations below 10 percent.
Grade 904 offers resistance to high concentrations of sulfuric acid.
Types 304L and 430 offer resistance to nitric acid.
Hydrochloric acid is an exception to the general acid resistance of stainless steel and should be avoided.

Bases

Stainless steel offers strong resistance to weak bases in both high concentrations and high-temperature environments. However, stronger bases will cause cracking or etching corrosion. Particular care must be taken with chloride solutions such as sodium hypochlorite.

Organics

Resistance to organics differs. In general, 300-series steels perform best with these solutions. However, exact recommendations vary based on environmental factors such as temperature and oxygen availability.
If you’re planning to use stainless steel to store organics, consulting a professional, such as Unified Alloys, can help to highlight concerns and identify options to create a safe, long-lasting stainless steel solution.

Specialized types of stainless steel corrosion and damage

Apart from general corrosion, stainless steel corrosion can take on several specialized forms. Fortunately, there are ways to address most forms or avoid them entirely. The following are common forms of specialized corrosion.

1. Galling

This corrosion occurs when stainless steel parts are forced together. Commonly seen with nuts, bolts, and other fasteners, corrosion isn’t visible until the parts are separated or disassembled.
The process uses repeated passes to adjust the dimensions of the steel slowly. In most cases, this will involve rolling through multiple mills over time to achieve the desired thickness.

2. Galvanic

Also known as bimetallic corrosion, this corrosion occurs when stainless steel is used with other metals in a moist environment. Rain, condensation, or other moisture sources can act as an electrolyte in the presence of an electrical current causing one metal to corrode at an accelerated rate.
The exact characteristics of galvanic corrosion vary based on metals used, temperatures, areas of contact, and other factors. For more information on the topic, consulting the British Standards Institution’s PD 6484 is recommended.

3. Pitting

This corrosion is typically a result of exposure to chemicals in the environment or poor aeration. It is also more common in steels with unwanted inclusions or manufacturing defects. For example, Manganese (II) Sulfide often initiates pitting.
Adequate oxygen availability can help to reduce the rate of pitting corrosion or halt it entirely depending on the cause. Steels with molybdenum also have an increased ability to both resist pitting and slow their progress. Other considerations include pH, chloride concentration, and temperature of the environment in which the steel is used.

4. Crevice Corrosion

This corrosion occurs when a crevice between the stainless steel and another material allows chlorides to concentrate or prevents proper oxygen levels to regenerate the steel’s oxide layer. While typically found near or within the gap between two metals, crevice corrosion can also occur between metallic and non-metallic surfaces.
Welding, ensuring proper drainage, and using proper gaskets can also help to prevent crevice corrosion as these all help to decrease access to crevices or eliminate them altogether.

5. Weld Decay

Also known as intergranular corrosion, this corrosion results from heating stainless steel to a specific temperature range—often between 550C and 850C. This causes chromium to precipitate from the stainless steel and reduces the ability of the passivation layer to regenerate.
Using low-carbon steels—such as 304L and 316L—when welding can help to reduce the impact of weld decay. Additives such as titanium or niobium can further stabilize the steel and reduce precipitation while welding. In all cases, using a post-weld heat treatment is recommended to minimize corrosion.

Final thoughts

Keeping stainless steel looking and performing great is all about protecting the chromium layer that gives it stainless properties. This also improves safety by ensuring that your stainless steel structures, containers, and equipment stay structurally sound for as long as possible.
By considering the environment in which your stainless steel is used—and matching the grade to your intended application—you can optimize corrosion resistance and ensure long-lasting results for your investment. Though this guide offers a good starting point on different corrosion types and prevention measures, consulting with a professional is always recommended to ensure an ideal product fit and safety.
China Tisco offers more than 20 years of expertise in serving some of the biggest industries worldwide. Have a question about stainless steel corrosion or applications? Call us and we’d be happy to discuss your needs and find the perfect solution.

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Popular for its exceptional strength, corrosion resistance, and machinability, stainless steel has become a staple in the metals industry. Also to a complete inventory of virtually all shapes, sizes, and grades, China Tisco also offers custom stainless steel supply chain solutions and is able to deliver products to each customer’s exact specifications. Contact us to get a custom stainless steel supply chain solution!

Stainless Steel Properties

中国石油公司 — Fri, 26 Aug 2022 06:53:28 +0000

Stainless Steel Properties

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Stainless Steel Properties

Stainless steel is the name of a family of iron-based alloys known for their corrosion and heat resistance. One of the main characteristics of stainless steel is its minimum chromium content of 10.5%, which gives it its superior resistance to corrosion in comparison to other types of steel. Like other steels, stainless steel is composed primarily of iron and carbon, but with the addition of several other alloying elements, the most prominent being chromium. Other common alloys found in stainless steel are nickel, magnesium, molybdenum, and nitrogen.

What are the properties of stainless steel?

Stainless steel has many desirable properties that contribute greatly to its widespread application in the making of parts and components across many industrial sectors. Above all, because of its chromium content, it is extremely resistant to corrosion. The 10.5% minimum content makes steel approximately 200 times more resistant to corrosion than steels without chromium. Other favorable properties for consumers are its high strength and durability, its high and low-temperature resistance, increased formability and easy fabrication, low maintenance, long-lasting, attractive appearance and it is environmentally friendly and recyclable. Once stainless steel is put into service, it does not need to be treated, coated, or painted.

Corrosion resistant
High tensile strength
Very durable
Temperature resistant
Easy formability and fabrication
Low-maintenance (long-lasting)
Attractive appearance
Environmentally friendly (recyclable)

Grading systems for stainless steel

There are many numerical grading systems for stainless steel, designated according to their composition, physical properties, and applications. Each type of stainless steel is classified by its serial number and then assigned a numerical grade. The most popular series numbers are 200, 300, 400, 600, and 2000. The most common grades are type 304 and 316 which consist of austenitic chromium-nickel alloys. Cutlery-grade stainless steels are found in the 400 Series, which is derived from ferritic and martensitic chromium alloys. Type 420 is known as surgical steel, and type 440 is known as the razor blade steel.

Stainless Steel Classifications

The family of stainless steels is primarily classified into four main categories based upon their crystal microstructure.

Ferritic

Ferritic steels are the 400 Grade stainless steels noted for their high chromium content, which can range from 10.5% to 27%. They have magnetic properties, too, and offer good ductility, tensile-property stability, and resistance to corrosion, thermal fatigue, and stress-corrosion cracking.
Typical applications for ferritic stainless steel include automotive components and parts, the petrochemical industry, heat exchangers, furnaces, and in durable goods like appliances and food equipment.

Austenitic

Perhaps the most common category of stainless steel, austenitic grade steels are high in chromium, with varying amounts of nickel, manganese, nitrogen, and some carbon. Austenitic steels are divided into the 300 series and 200 series subcategories, which are determined by which alloys are used. The austenitic structure of the 300 series is distinguished via the addition of nickel. The 200 series primarily uses the addition of manganese and nitrogen. Grade 304 is the most common stainless steel.
Sometimes referred to as 18/8 because of its 18% chromium and 8% nickel, it is used in kitchen equipment, cutlery, food processing equipment, and structural components in the automotive and aerospace industries. Grade 316 is another common stainless steel. It is used in the making of a wide range of products such as food preparation equipment, laboratory benches, medical and surgical equipment, boat fittings, pharmaceutical, textile, and chemical processing equipment.

Martensitic

Martensitic stainless steels are in the 400 Grade series of stainless steels. They have a low to high carbon content and contain 12% to 15% chromium and up to 1% molybdenum. It’s used whenever corrosion resistance and-or oxidation resistance is required along with either high strength at low temperatures or creep resistance at elevated temperatures. Martensitic steels are also magnetic and possess relatively high ductility and toughness, which make them easier to form.
Applications for martensitic stainless steels include a wide range of parts and components, from compressor blades and turbine parts, kitchen utensils, bolts, nuts and screws, pump and valve parts, dental and surgical instruments, to electric motors, pumps, valves, machine parts sharp surgical instruments, cutlery, knife blades, and other cutting hand tools.

Duplex

As the name implies, duplex stainless steels possess a mixed microstructure of ferrite and austenite. The chromium and molybdenum content is high, with 22% to 25%, and up to 5%, respectively, with very low nickel content. The duplex structure gives stainless steel many desirable properties. For starters, it offers double the strength of ordinary austenitic or ferritic stainless steels, with excellent corrosion resistance and toughness.

Designated in the 2000 Grade series, duplex stainless steel is ideal for applications in demanding environments such as in chemical, oil, and gas processing and equipment, marine, high chloride environments, pulp, and paper industry, cargo tanks for ships and trucks, and bio-fuels plants, chloride containment or pressure vessels, transportation, heat exchanger tubes, construction, the food industry, desalination plants, and components for FGD systems.

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What’s the difference between 304 and 316 stainless steel?

中国石油公司 — Thu, 25 Aug 2022 08:13:39 +0000

What’s the difference between 304 and 316 stainless steel?

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What’s the difference between 304 and 316 stainless steel?

When looking at 304 stainless steel vs 316, it is hard to notice the difference between them. However, when manufacturing with 304 vs 316 stainless steel, the difference can be huge. Keep reading to learn more about the two most popular types of stainless steel.

304 vs 316: What Makes Them So Popular?

The high levels of chromium and nickel found in 304 and 316 stainless steel provide them with a strong resistance to heat, abrasion, and corrosion. Not only are they known for their resistance to corrosion, but they are also known for their clean appearance and overall cleanliness.
Both types of stainless steel appear in wide-ranging industries. As the most common grade of stainless steel, 304 is considered the standard “18/8” stainless. 304 stainless steel is widely used because it is durable and easy to form into various forms such as stainless steel sheet, stainless steel plate, stainless steel bar, and stainless steel tube. 316 steel’s resistance to chemicals and marine environments makes it a popular choice among manufacturers.

How Are They Categorized?

The five classes of stainless steel are organized based on their crystalline structure (how their atoms are arranged). Out of the five classes, 304 and 316 stainless steel are in the austenitic grade class. The structure of austenitic grade stainless steels makes them non-magnetic and prevents them from being hardenable by heat treatment.

Properties of 304 Stainless Steel

Chemical Composition

Cr	Ni	C	Mn	Si	P	S	N	Other
18–20	8–10.50	0.08	2	0.75	0.045	0.03	0.1	-

What are the Physical Properties of 304 SS?

Physical Property	Melting Point	Density	Thermal Expansion	Modulus of Elasticity	Thermal Conductivity
Value	1450℃	8.00 g/cm^3	17.2 x10^-6/K	193 GPa	16.2 W/m.K

Mechanical Properties of 304 Stainless Steel

Tensile Strength	Elongation A50 mm	Hardness (Brinell)
500-700 Mpa	45% min	215 Max HB

304 Stainless Steel Applications

The medical industry commonly uses 304 SS because it endures powerful cleaning chemicals without corroding. As one of the few alloys that meet the Food and Drug Administration’s sanitary regulations for food preparation, the food industry often uses 304 SS.

Food preparation: Fryers, food prep tables
Kitchen equipment: cookware, silverware
Architectural: siding, elevators, bathroom stalls
Medical: trays, surgical tools

Properties of 316 Stainless Steel

Chemical Composition

316 contains many similar chemical and mechanical properties as 304 stainless steel. To the naked eye, the two metals look the same. However, the chemical composition of 316, which is made up of 16% chromium, 10% nickel, and 2% molybdenum, is the main difference between 304 and 316 stainless steel.

Physical Properties of 316 SS

Melting Point	Density	Modulus of Elasticity	Thermal Expansion	Thermal Conductivity
1400℃	8.00 g/cm^3	193 GPa	15.9 x 10^-6	16.3 W/m.K

Mechanical Properties

Tensile Strength	Elongation A50 mm	Hardness (Brinell)
400-620 Mpa	45% min	149 max HB

Applications of 316 Stainless Steel

The addition of Molybdenum in 316 makes it much more corrosion resistant than similar alloys. Due to its superior resistance to corrosion, 316 is one of the staple metals for marine environments. 316 stainless steel is also utilized in hospitals because of its durability and cleanliness.

Water-handling: boilers, water heaters
Marine parts- boat rails, wire rope, boat ladders
Medical Equipment
Chemical processing equipment

304 vs 316 Stainless Steel: Heat Resistance

Heat resistance is an important factor to consider when comparing the different grades of stainless steel. The melting range of 304 is around 50 to 100 degrees Fahrenheit higher than 316. Although the melting range of 304 is higher than 316, they both have good resistance to oxidization in intermittent service up to 870°C (1500℉) and in continuous service at 925°C (1697℉).

304 SS: Handles high heat well, but continuous use at 425-860 °C (797-1580 °F) may cause corrosion.
316 SS: Performs best in temperatures above 843 ℃ (1550 ℉) and below 454 ℃ (850°F)

Price Difference of 304 Stainless Steel vs 316

What makes 316 more expensive than 304 stainless steel? The increase of nickel content and the addition of molybdenum in 316 makes it more expensive than 304. On average, the price of 316 stainless steel is 40% higher than the price of 304 SS.

316 vs 304 Stainless Steel: Which is Better?

When comparing 304 stainless steel vs 316, they both have pros and cons to consider when deciding which one to use for different applications. For instance, 316 stainless steel is more resistant than 304 to salt and other corrosives. So, if you are manufacturing a product that will often face exposure to chemicals or a marine environment, 316 is the better choice.
On the other hand, if you are manufacturing a product that does not need strong corrosion resistance, 304 is a practical and economical choice. For many applications, 304 and 316 are actually interchangeable.