Stainless steel coil and its corrosion

Introduction

Now we can see stainless steel everywhere in our lives. For example, kitchens and buildings have stainless steel products. They look the same. However, they are all different types. People carry out various treatments and processing on stainless steel as needed. This gives them different functions. And the stainless steel coil is the base material of these products.

If people ignore the types and nature of stainless steel products when choosing stainless steel products, it may be harmful to the human body. Moreover, it may not achieve the desired effect.

Therefore, it is important to understand stainless steel.

Next, let’s take a look at what a stainless steel coil is.

What is stainless steel coil？

First of all, what is stainless steel?

What is stainless steel?

Stainless steel is an alloy of iron. Moreover, its Cr content is at least 10.5%. This makes Cr produce a layer of oxide on the surface of the steel. This is the “Passivation layer”. Moreover, this passivation layer can prevent external forces from corroding its surface. Moreover, with the increase of chromium, the corrosion resistance of stainless steel will also increase.

In addition, stainless steel has many amounts of carbon, silicon and manganese. These all make stainless steel have different properties.

Besides, people can also add other elements to stainless steel, such as nickel and molybdenum.

What is stainless steel coil?

People make stainless steel wires into coils according to customer requirements. This is the stainless steel coil. Moreover, this term often refers to a coil of steel in the steel industry. Therefore, it is ambiguous to some extent. Here we are introducing stainless steel coils.

How to form？

People initially produced stainless steel in slabs. Then, people use Z rolling mill to make it into coils. And people need to roll this coil again. This forms a stainless steel coil.

Moreover, its width is usually around 1250mm, sometimes slightly wider

Then, people can process this coil. For example slitting processing: People cut a wide coil into many strands. After slitting, there are many styles of stainless steel coil: strip coil, slit coil, strip coil, etc.

Besides, the winding method of the coils is different, and their names will be different: pancake coil, winding.

Sometimes, people physically wind the coil onto a plastic spool. And it can produce larger coils. Moreover, this improves stability and yield.

The way the coils are wound may cause them have different names:

The most common type is “pancake coil”, named after the appearance of the coil when it is laid flat;

“winding” is another name for this winding method.

The other type looks like a cotton spool. And people can  physically wound onto a plastic spool. Moreover, producing coils in this way allows larger coils to be produced. Thereby improving stability and increasing yield.

Types of stainless steel coil

Ferrite stainless steel coil

This ferrite stainless steel has Cr and a small amount of carbon. Moreover, its structure is similar to that of carbon steel and low alloy steel.

Features of stainless steel coil

1. However, ferrite stainless steel lacks welding toughness. Therefore, people usually use it when people need thin stainless steel. But, people widely use it when there is no need for welding.
2. Moreover, people cannot do heat hardening treatment on it.
3. For the advantages, they are very resistant to cracking.
4. People cannot harden it through heat treatment.
5. Not magnetic. Therefore, people will not use molybdenum-containing stainless steel under strong corrosive conditions such as sea water.

Austenitic stainless steel coil

Austenitic steel is the most common stainless steel.

People added nickel, manganese, and nitrogen to stainless steel. And this is Austenitic stainless steel.

Features

1. Its structure makes it weldable and formable.
2. Secondly, Cr, molybdenum and nitrogen in Austenitic steel can enhance its corrosion resistance.
3. Similarly, people cannot heat-harden it. However, under high-strength work hardening conditions, people can harden it.
4. Meanwhile, it has ductility and toughness.
5. It is non-magnetic. However, after people harden it according to the composition of steel, it will have certain magnetic properties.

Martensitic stainless steel coil

Martensitic stainless steel is similar to ferrite stainless steel. However, it has a higher carbon content.

This allows people to quench and temper it, just like carbon steel and low alloy steel.

Generally, people use it in places with moderate demands such as strength and corrosion resistance.

Duplex stainless steel coil

Its fiber structure is 50% ferrite and 50% Austenite. So, this makes them stronger than ferrite or Austenitic stainless steels.

Moreover, It is different from the above two types. Duplex stainless steel can resist cracking under external corrosion conditions.

Moreover, on the basis of duplex steel, people have enhanced the strength and corrosion resistance, and developed super duplex stainless steel.

Features

1. It has weld-ability. However, people need to pay attention to the heat input and the choice of welding.
2. Besides, it has form-ability.
3. It is magnetic. However, because it has 50% Austenitic structure, its magnetic properties are not as good as ferrite, Martensitic and PH stainless steel.

So, what is PH stainless steel? Let’s take a look at it together.

PH stainless steel coil

People add copper, niobium and aluminum to stainless steel to produce PH stainless steel. Moreover, people usually call PH stainless steel precipitation hardening stainless steel.

Features

1. After proper ageing heat treatment, very fine particles will form in the rigid matrix. This gives stainless steel a very high strength. Therefore, the strength of PH stainless steel is very high.
2. It has form-ability, and people can process it into various complicated shapes. Moreover, people’s conventional quenching and tempering treatment of Martensitic steels can easily deform them. But，when people process PH stainless steel, the deformation has little effect.
3. Its corrosion resistance is also very good.
4. For another type of stainless steel coil, you can check on Introduction of 201 Stainless steel coil

Surface treatments of stainless steel

Rough surface finish can effectively reduce the corrosion resistance of stainless steel. Therefore, people usually perform surface treatment on stainless steel.

Moreover, stainless steel has many different types of surface finishes. Some of them are processed by people from mills.

Besides, there are surface treatment methods such as polishing, wire drawing, sandblasting, timing, and coloring.

However, if people excessively pursue the surface finish of stainless steel, it will affect the corrosion resistance of the stainless steel surface.

However, many suppliers have their own finishes. Therefore, people cannot fully realize the standardization of the surface finish of stainless steel.

After understanding the basic information of stainless steel, let’s take a look at what is the use of stainless steel.

Usages of stainless steel

Stainless steel has a wide range of uses. And usually we use the following more daily:

For household use and tableware

Sink, pan, washing machine tub, microwave oven, razor blade

About the corrosion of stainless steel

Stainless steel has better corrosion resistance than ordinary steel or alloy steel.

Moreover, in normal atmospheric or water-based environments, stainless steel will not corrode household sinks, cutlery, and pans.

However, more severe conditions will still corrode it.

Next, let’s take a look at the forms of corrosion of stainless steel.

Corrosion forms of stainless steel

Generally, stainless steel has the following corrosion forms:

Pitting

Chloride ions are found in many places in life, such as in salt and bleach. If stainless steel is in contact with these substances for a long time, they will form a passivation layer on the surface of the stainless steel. This is pitting.

Moreover, people can calculate the pitting resistance according to its alloy content.

Crevice corrosion

Stainless steel needs oxygen to avoid a passivation layer on its surface.

However, in a very tight gap, the surface of stainless steel is exposed to very little oxygen. Therefore, corrosion is prone to occur in the gap.

However, one can seal the gap with sealant. So, this can avoid crevice corrosion of stainless steel.

General corrosion

Generally, for stainless steel, uniform corrosion does not occur across the board.

However, certain chemicals, especially acidic chemicals, may corrode stainless steel uniformly, especially under the influence of concentration and temperature.

For example, certain concentrations of hydrochloric acid or sulfuric acid are highly corrosive to stainless steel.

In addition, some metal is lost on the surface of the steel.

Stress corrosion cracking

This is a rare form of corrosion. It only occurs when tensile stress, temperature, and corrosive substances are combined.

For example, hot water tanks and swimming pools often experience this kind of corrosion.

In addition, hydrogen sulfide produced in oil and gas exploration can also cause this corrosion of stainless steel.

Inter-granular corrosion

If the carbon content in stainless steel is relatively high, Cr will combine with carbon to form carbide Cr.

Between 450-850 degrees Celsius, this kind of inter-granular corrosion occurs. It also has another name called sensitization.

Usually when welding, the Cr that can form the passivation layer is reduced, and the inter-granular corrosion of stainless steel will occur.

However, people choose stainless steel with low carbon content, or stainless steel with preferential combination of titanium or niobium with carbon to avoid inter-granular corrosion.

Galvanic corrosion

If two different metals come into contact with each other and with the electrolyte, there will form a galvanic battery. So, it will accelerate the corrosion of metal like a battery.

However, people can use insulators such as rubber to separate the metal to avoid this.

Conclusion

Stainless steel is an alloy of iron. Moreover, its Cr content is at least 10.5%. This makes Cr produce a layer of oxide on the surface of the steel. This is the passivation layer. Moreover, this passivation layer can prevent external forces from corroding its surface.

Then, people use Z rolling mill to make it into coils. Moreover, people need to roll this coil again. This forms a stainless steel coil. Moreover, its width is usually around 1250mm.

Besides，there are many different types of stainless steel. This mainly depends on the metal element content. Generally, the following types of stainless steel are common:

• Ferrite stainless steel
• Austenitic stainless steel
• Martensitic stainless steel
• Duplex stainless steel
• PH stainless steel

Because of its different types with different properties, it has many uses: tableware, construction, transportation, chemistry, medical and pharmaceutical, petroleum, food, sewage treatment and common accessories.

Normally, stainless steel will not corrode in a normal atmospheric environment.

However, more severe conditions will still corrode it.

Moreover, its corrosion forms are bit corrosion, crevice corrosion, general corrosion, stress corrosion, intergranular corrosion, galvanic corrosion and so on.

That’s all for stainless steel coil.

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