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Stainless Steels

The story began during World War I when a large number of guns were needed. However, due to the limitation of technology growth, the British army was suffering from critical barrel rust and wear problems. To overcome this issue, the ordnance factory entrusted metallurgical expert Harry Brearley to develop wear and rust-resistant alloy. Brearley tried adding various elements to steels and never got expected results. Once, he mixed chromium to steelmaking raw material and obtained a new type of material with a shiny surface, unfortunately, when he made it into a gun for the shooting test, the material was too brittle for guns. Later someday, Brearley found several fragments of steel that were mixed with chromium still bright and shinny among rusted iron pieces. Experiments found that these chromium steels were not easy to rust in the sun and rain, and they were not as weak as ordinary steels were when placed in an acid or alkali environment. Although this new material was too expensive and brittle to make gun barrels, it was good for making knives, forks, and tableware, etc.

Stainless steel is classified into chromium, chromium-nickel, and chromium manganese nitrogen stainless steel based on the chemical composition; It can be categorized into free cutting, non-magnetic, low-temperature, and high-strength stainless steel according to its functional characteristics; It is itemized into the martensitic, ferritic, austenitic, duplex, and precipitation hardening stainless steel based on the metallographic structure.

Martensitic Stainless Steel

High carbon proportion brings martensitic stainless steel a higher strength, hardness, and wear resistance, but weaker regarding anti-corrosion. It’s majorly used to make parts and components for meeting high mechanical property standards but general anti-corrosion requirements. Such as spring, turbine blade, and hydraulic press valve, etc.

Ferritic Stainless Steel

Ferritic stainless steel refers to stainless steel which chromium composition is between 15% – 30%. Toughness is directly proportional to the number of chromium, and the ability of anti-chloride stress corrosion is better than other types. High chromium has made it greater in anti-corrosion and anti-oxidation but weaker in mechanical property and processability. Widely applied for acid-resistant structures with less stress and used as anti-oxidation steels. For example, it’s used to make equipment for producing nitric acid and for food factories, or make parts that can operate at high temperatures.

Austenitic Stainless Steel

We call stainless steels with chromium over 18%, about 8% nickel, and slight aluminum, titanium, and nitrogen austenitic stainless steels, which can resist corrosions from multiple mediums. Austenitic stainless steel has very good ductility, toughness, weldability. It’s non-magnetic or weakly magnetic, and very commonly used for making corrosion-resistant containers, equipment lining, pipelines, anti-nitric acid parts, etc.

Duplex Stainless Steel

Duplex stainless steel has the nature of both austenitic and ferritic stainless steel, and superplastic. It is tougher, with better weldability and intergranular corrosion resistance than ferritic stainless steel. No brittleness at room temperature and with higher strength, chloride stress corrosion resistance than austenitic stainless steel. It also has better anti-pitting performance.

Precipitation Hardening Stainless Steel

The primary structure of precipitation hardening stainless steel was made of austenitic and martensitic organizations. This type of stainless steel can be strengthened and hardened through precipitation hardening treatment.

Characteristics of Stainless Steels

  • weldability
  • Anti-corrosion
  • Heat-resistance
  • Polishing

Stainless steels are applied widely and commonly in construction, food processing, catering, brewing, chemistry, and medical field, etc. To the site furniture industry that Canaan serves in, outdoor stainless-steel furniture is often scribbled and must stand the test of changing weather at the same time, so the easy cleaning feature has made stainless steel the most preferred material. 

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Metal Product Corrosion

The deterioration of metals caused by the environment and medium is very common. The destruction of most metal articles is related to corrosion factors except for a few precious metals such as Au and Pt. Metal corrosion causes huge losses to society, therefore, anti-corrosion has become a critical issue that must pay attention to.

Anti-corrosion

Corrosion is one of the major causes of damage to metal products, specialized anti-corrosion solutions should be applied to prolong the life cycle of metal products according to the feature that metal products are easily corroded. Chemical, physical, and electrochemical protection are the most common methods.

Corrosion can be categorized into five types, general corrosion, pitting, intergranular corrosion, selective corrosion, and stress corrosion cracking. General corrosion affects the entire surface of a metal, commonly known as rust. Pitting occurs in the small holes on the surface of the material, and these holes become larger over time. Intergranular corrosion is almost invisible on the surface as it takes place inside. Selective corrosion erodes only components of complex materials. This type of corrosion starts from the surface and then penetrates to the interior and corrodes at different speeds. Stress corrosion cracking happens when the material bears the ultimate tensile strength.

Four Common Anti-Corrosion Methods:

Structural Reforming

Some heavy metals are low chemical active and not easily corroded, so the service life of the compound metal is extended with a great margin when adding heavy metals to other metals. People mix nickel, chromium, etc. with steel to make stainless steel, for instance.

Protective Layer

There are two types of protective layers metal and non-metal. Common non-metal layers include oil paint, ceramic, and plastic coatings. Electroplating and hot plating generally use non-corrosive metals, such as zinc, tin, chromium, or nickel to form an oxide film on the surface.

Electrochemical Protection

Electrochemical protection is a method based on the galvanic cell theory by eliminating the galvanic cell reaction which is the cause of chemical corrosion. The cathodic method is more widely used than the anodic method.

Corrosive Medium Treatment

This method is achieved by eliminating corrosive medium. In other words, keeping the metal machinery dry, to name a couple, wiping off the moisture on the equipment, or adding a corrosion inhibitor to corrosive medium.

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Powder-coating

Powder-coating is to coat powdered paint on the workpiece with an electrostatic spray molding machine. The powder will be evenly attached to the surface in the electrostatic field. Final layers vary in effects according to different paint. The thickness reaches up to 60 microns after high-temperature drying, leveling, and curing, which makes the product surface flat and smooth with strong acid resistance, alkali resistance, and wear resistance, and can withstand long term UV radiation and acid rain without coating pulverization, fading, and falling off.

Powder coating is better than paint spraying in terms of mechanical strength, adhesion, corrosion resistance, aging resistance. The cost is also lower for the same effect. Powder-coating characteristics

  • The final film is flat and smooth with very strong adhesion and excellent decoration performance, under the influence of electrostatic effect.
  • Electrostatic adsorption reduces paint mist pollution and improves the workshop’s hygiene condition.
  • Reduces cost by increasing paint utilization.

Powdered paint

The paint is produced by processing hot extrusion, grinding, and sieving with a mix of specialized resin, pigment, fillers, curing agents, and other additives in certain proportions. It is a solid fine powder that is completely different from coatings in general, very stable at room temperature.