The chromium part of stainless steel must provide oxygen to form a protective oxide layer. The atmosphere usually provides sufficient oxygen for stainless steel to maintain this passive oxide layer. When stainless steel is submerged in water, it may not be able to obtain the oxygen required to remain passive (non corrosive).

Any reduction in surface oxide protection of stainless steel will result in it becoming anodized or active. At the same time, the active part of the metal surface still contains sufficient oxide protection around the area to act as the cathode and remain passive. Freshwater or seawater serve as electrolytes, providing a pathway for the transfer of ions from the anode region to the cathode region around the metal.

The electric shock lasts for a period of time until a pit or many pits are formed, while the surrounding metal remains almost non corrosive. At the same time, due to the increase in hydrogen and chloride ion concentrations, the water in the cavity pit becomes slightly acidic, which leads to a more aggressive corrosion rate. These pits can range in size from pinholes to large and shallow depressions.

Gap corrosion occurs in a similar way. This can occur in any place where oxygen is depleted, whether it is dirt or grime, or in areas with seams, such as fasteners.

The chromium part of stainless steel must provide oxygen to form a protective oxide layer. The atmosphere usually provides sufficient oxygen for stainless steel to maintain this passive oxide layer. When stainless steel is submerged in water, it may not be able to obtain the oxygen required to remain passive (non corrosive).

Any reduction in surface oxide protection of stainless steel will result in it becoming anodized or active. At the same time, the active part of the metal surface still contains sufficient oxide protection around the area to act as the cathode and remain passive. Freshwater or seawater serve as electrolytes, providing a pathway for the transfer of ions from the anode region to the cathode region around the metal.

The electric shock lasts for a period of time until a pit or many pits are formed, while the surrounding metal remains almost non corrosive. At the same time, due to the increase in hydrogen and chloride ion concentrations, the water in the cavity pit becomes slightly acidic, which leads to a more aggressive corrosion rate. These pits can range in size from pinholes to large and shallow depressions.

Gap corrosion occurs in a similar way. This can occur in any place where oxygen is depleted, whether it is dirt or grime, or in areas with seams, such as fasteners.

About stainless steel
Stainless steel is essentially a low-carbon steel with a chromium content of 10% or more. It is the addition of chromium that gives steel unique stainless steel corrosion resistance.

The chromium content of steel allows for the formation of a rough, adhesive, invisible, and corrosion-resistant chromium oxide film on the surface of the steel. If mechanically or chemically damaged, this film is self repairing, provided that oxygen is present, even in very small amounts. Enhance the corrosion resistance and other useful properties of steel by increasing chromium content and adding other elements such as molybdenum, nickel, and nitrogen.

Stainless steel has over 60 grades. However, the entire group can be divided into five categories. Each type is identified and named by the alloy elements that affect its microstructure.

Why choose stainless steel over other materials?
The many unique values provided by stainless steel make it a powerful candidate for material selection. Engineers, specification makers, and designers often underestimate or overlook these values because stainless steel has a higher initial cost. However, throughout the entire lifecycle of a project, stainless steel is often the most valuable choice. The many advantages of stainless steel are as follows:

·Durable

·Low alloy grades with corrosiveness are resistant to corrosion in atmospheric and pure water environments, while high alloy grades can resist corrosion in most acidic, alkaline solutions, and chlorine containing environments. These characteristics are used in processing plants.

·Strength to weight advantage

·The work hardening performance of austenitic materials can be significantly enhanced by cold working alone, while high-strength dual phase materials are smaller than traditional materials, thus saving costs.

·Long term value

·When considering the entire lifecycle cost, stainless steel is usually the cheapest material choice.

·Anti

·The austenitic microstructure of the impact 300 series has high toughness from high temperature to far below freezing point, making these steels particularly suitable for low-temperature applications.

·Fire resistance and heat resistance

·Special high chromium and nickel alloy grades can resist scaling and maintain strength at high temperatures.

·Stainless steel is a "green" material

·To ensure high-quality service life, the materials we use as consumers and manufacturers must not only meet technical performance standards, but also have a long service life, be suitable for various applications, and be environmentally friendly. Once their services are completed, they should be 100% recyclable to complete the lifecycle of reuse. Stainless steel is such a material.

·The lifespan of stainless steel is the result of its alloy composition, therefore it has natural corrosion resistance. There is nothing on the surface that can add additional materials to the environment. It does not require additional systems to protect base metals, as the metal itself will continue to be used.

·Stainless steel requires less maintenance, and its hygiene characteristics mean that we do not need to use irritating cleaning agents to obtain a clean surface. Almost nothing or anything can be dumped into drainage pipes that may have an impact on the environment.

·Stainless steel products complete their service life. Due to the fact that this material is 100% recyclable, there are fewer concerns about disposal. In fact, over 50% of new stainless steel comes from old melted stainless steel waste, thus completing the entire lifecycle.

·Hygiene

·The easy cleaning ability of stainless steel makes it the preferred choice for strict hygiene conditions in hospitals, kitchens, slaughterhouses, and other food processing plants.

·Beautiful appearance

·The bright and easy to maintain stainless steel surface provides a modern and charming appearance.

·Easy to manufacture modern steelmaking technology means that stainless steel can be easily cut, welded, formed, processed, and manufactured like traditional steel.

Stainless steel corrosion
Corrosion on stainless steel is not as easy or obvious as on carbon steel. Sometimes it is believed that stainless steel will not corrode. Indeed, certain types of stainless steel have corrosion resistance, but when exposed to certain conditions, it corrodes.

Stress corrosion: The three fundamental causes of stress corrosion and/or cracking are mechanical, metallurgical, and environmental. Mechanical cracking may occur due to the lifting load or vibration of the parts during use. In corrosive environments, high stress applied to metal (in this case stainless steel) components can accelerate cracking. Metallurgical cracks may be caused by problems with the grain structure of the material itself, improper heat treatment, or problems with the material's casting, forging, stretching, or other manufacturing methods.

Environmental reasons may be corrosive conditions such as salt water, extreme heat, and extreme cold.

Intergranular corrosion may be caused by metallurgical issues in the grain structure, improper forging procedures, heat treatment, or exposure to compounds such as ammonia in the environment. Once cracking occurs, corrosion will spread throughout the entire material and may not be noticeable to the naked eye until a malfunction occurs.

Preventing vibration, loading, and unloading stress can help reduce such problems. Wherever possible, seams should be avoided. If not, coating or electroplating may be helpful. Testing according to ASTM A262 to detect sensitivity to intergranular corrosion can also help prevent the occurrence of stress corrosion.

Electrochemical corrosion: A simple explanation for galvanic corrosion is an electrochemical process that causes metal deterioration through a very slow stabilizing effect. Part or all of the metal transitions from a metallic state to an ionic state, and then becomes a compound in the electrolyte (water). The scientific explanation is that when two different conductive materials that come into contact with each other are exposed to electrolytes (such as water), a current called current flows from one to the other. Natural electrolytes are saltwater and freshwater. Due to the high salt content, the conductivity of saltwater is stronger, resulting in faster occurrence of galvanic corrosion. In freshwater, the reaction rate is slower and not widely distributed among the affected metals. One material is anode, and the other is cathode. Different grades of stainless steel materials will have different galvanic properties. The less corrosion of cathode materials, the faster the anodic corrosion. Therefore, if aluminum parts and stainless steel parts are in the same environment, the corrosion rate of aluminum parts will be much faster than that of stainless steel parts. Usually, stainless steel performs better under galvanic corrosion conditions.

Grain structure and intergranular corrosion
Grain structure is the relationship between individual crystals in a metal or alloy. The space between grains is grain boundaries. The term "particle" refers to a single crystal in a metal.

Intergranular corrosion (IGC), also known as intergranular erosion (IGA), is a form of corrosion in which the boundaries of material microcrystals are more susceptible to corrosion than their interiors. Intergranular corrosion occurs at the grain boundaries of austenitic stainless steel, which has undergone heat treatment between 850 ° F and 1450 ° F.

This situation may occur in other corrosion-resistant alloys, and when the grain boundaries of corrosion inhibiting elements (such as chromium) are depleted through some mechanism, it is called grain boundary depletion. In nickel alloys and austenitic stainless steels, chromium is added to improve corrosion resistance. The mechanism is that chromium carbide precipitates at grain boundaries, leading to the formation of chromium poor areas near the grain boundaries (this process is called sensitization). Approximately 12% chromium is the minimum requirement to ensure passivation, which forms an ultra-thin invisible film on the surface of stainless steel through this mechanism, called a passivation film. This passivation film can protect metals from corrosive environments. The self-healing properties of the passivation film make steel and stainless steel. Selective leaching typically involves grain boundary depletion mechanisms.