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Corrosion Resistance

Corrosion Reaction on Metal (Definition / Types)
Definition of Metal Corrosion
Metal corrosion, as chemical or electrochemistry reaction, which happens between metal and circumstance, results in damage 
      to physical property of material and reducing useful life

Classification table
Temperature Corrosion at high temperature, Corrosion at low temperature
Environment Atmospheric corrosion, seawater corrosion, soil corrosion, microbiologically influenced corrosion
Types 1. Uniform Corrosion (General Corrosion)
2. Galvanic Corrosion (Two-metal corrosion)
3. Intergranular Corrosion
4. Crevice Corrosion
5. Pitting
6. Erosion Corrosion
7. Stress Corrosion
8. Selective Leaching
9. Hydrogen Damage

Diagram for Corrosion Type

Diagram for Corrosion Type
Uniform Galvanic Intergranular Crevice Pitting
Uniform Diagram Galvanic Diagram Intergranular Diagram Crevice Diagram Pitting Diagram

Corrosion of Stainless Steel
There is thin protective film that is hard to recognize on the stainless steel surface. As the protective film is an oxidized substance of oxygen and chrome, it provides corrosion resistance and is passive film. Therefore, corrosion resistance of stainless steel is different to usage condition and environment of the passive film. It generally shows excellent corrosion resistance under the strong oxidization and, even though the passive film on the surface is damaged, it returns to passive condition and preserves the corrosion resistance.

Main Corrosion Types of Stainless Steel
There are three corrosion types for stainless steel; Galvanic, Intergranular, and Pitting.

Main Corrosion Types of Stainless Steel table
Classification Cause Preventive Measure
It is caused by the potential difference between two contacted metals · Use neighbored metals from galvanic series: Reduction of potential difference
· Make big and thick metal
· Coating
· Use more corrosive metal than two metals (Sacrificial Anode)
When performing the heat treatment under the sensitive temperature range between 450°C and 850°C for austenite, Cr23C6 carbide is educed to intergranular. It causes lack of Cr around intergranular and corrosion and erodes the passive film and corrosion · Solution heat treatment: Educed carbide is fully solidified at high temperature
· Reduction of carbon content : Less than 0.03% (304 L)
· Add the stabilizer: Ti, Nb (347, 321)
Pitting · Effect of CI ion : The passive film is partly destroyed according to the action of CI and its area is liquefied first
· Effect of temperature : Reaction with CI is accelerated at higher temperature
· Effect of extraneous matter : The extraneous matter on the product surface increases CI density
· Prevent the CI ion and adhesion
· Surface treatment (grinding)
· Heat treatment to welded part
· Use steel that is strong to CI: Add Mo (316, 316L)

Corrosion Type The corrosion of stainless steel is gradually progresses at the damaged time of passive film surface.

Pitting corrosion - The pitting corrosion is sectional corrosion type that makes small hole on metal caused by the small damage of passive film on the surface of the metal.The pitting corrosion of stainless steel is mainly caused by CI ion but exact mechanism is not established. When the corrosion is widely generated by pitting, it is hardly seen with eyes because of its small-sized hole but it may result into critical so the pitting corrosion resistance of structural frame us very important. Mimetic diagram of pitting corrosion to Fe surface

Intergranular corrosion - The intergranular corrosion is hardly generated under the general circumstance.However, it may educes reactive impurity and passive element like Cr can be exhausted because the intergranular has strong reactivity under the certain condition. As a result of it, the corrosion is seriously generated to intergranular first because corrosion resistance of intergranular and its neighbor region are reducing and it is called intergranular corrosion. The most general intergranular corrosion is when austenite stainless steel is heated and chrome reacts with carbon, the chrome in neighbor region of intergranular exhausts and corrosion resistance decreases. Sensitization that progresses when chrome carbide is educed from intergranular

Surface temperature of welded area of STS304 stainless steel When stainless steel is heated by the temperature between 450°C and 800°C, the chrome carbide is educed to intergranular with Cr23C6. And the chrome around intergranular and its neighbor region is exhausted because the volume of chrome out of carbide reaches to about four times of carbon and diffusion speed of chrome is slower than carbon. When the volume of chrome around intergranular reduces less than 12%, the corrosion resistance drops in sharp and it is sensitization. To reduce the effect of sensitization, it has to reduce the volume of carbon of alley or add Ti or Nb that reacts with carbon. When weld the stainless steel, the HZA (Heat Affected Zone) nearby welded area becomes sensitive and intergranular corrosion is generated. It is because the temperature of HZA reaches from 400°C to 700°C and it is exposed to heat for longer time than other area. So, the corrosion resistance of HZA is much weaker then welded area and basic material. It shows discoloration with red or blue-black color around and backside of welded area, it is the discoloration caused by high-temperature oxidization at 400°C and it is HZA. When it is exposed to corrosive environment with high temperature, the corrosion may generate before basic material or welded area.

Crevice Corrosion Progress of crevice corrosionChemical electric cell formation based on damaged coating film on metal surface The crevice corrosion is partly serious corrosion of certain crevice or hidden area on metal surface that is exposed to electrolyte and its progress is similar to pitting corrosion. It is difficult to observe because the crevice corrosion is generated under the deposit of metal surface or crevice or corner between metal and non-metal. It is normally generated to passive metal and crevice corrosion of stainless steel under the seawater is typical. The corrosion progresses when the corrosion product of iron, chrome and nickel that are the main ingredient of stainless steel is accumulated into crevice and it makes acidic chloride liquid.

Stress Corrosion Cracking (SCC) Stress Corrosion Cracking Diagram The SCC is a type of corrosion when it receives environmental influence and mechanical stress at the same time and cracks and its impact transfers. The stress corrosion of stainless is mainly generates from the liquid including chloride like a pitting and crevice corrosions and it is caused more than 50°C. As its density of chloride is low, it is generated to the environment where pitting and crevice corrosion are not generated. Above figure shows generation and propagation processes of SCC on austenite stainless steel with various factors that affects SCC. The pitting corrosion is generated when the film is broken by chlorine ion or slip step, the pitting corrosion grows to crack when the volume of hydrogen ion in pitting corrosion increases and crack grows according to continuous increase of hydrogen ion and its reduction reaction. It is big problem because SCC forms passive film and it is generated from the material with excellent corrosion resistance under the lower stress than designed stress. Even though there is no external stress, residue stress from material manufacturing and processing such as molding and welding can cause stress corrosion. The chloride that causes stress corrosion exists in the water with various densities under the natural environment and it is caused by gasket or insulating material that includes chloride. In case of water pipe, intergranular-stress corrosion cracking is largely generated because it becomes sensitive to intergranular corrosion based on residue stress from welding and sensitization of HZA. To prevent it, residue stress has to be removed with heat treatment under appropriate temperature and it is better to use STS604L or STS316L that reduce the content of carbon.

Atmospheric & Microbiologically Influenced Corrosion Atmospheric Corrosion As it is caused when metal meets atmospheric environment, more than 50% of expenses to anticorrosive agent and system is related to atmospheric corrosion. Because there is metal, oxygen, water, and pollution gas or dust particle in atmosphere, it continuously interrupts the stability of passive film and it continuously causes atmosphere corrosion because it interrupts. Microbiologically Influenced Corrosion (MIC) There is life in water and soil from single-celled lower organism to mammals and their life directly and indirectly affects to the corrosion of metal. The MIC works alone or together according to the type of life related to environment or reaction. The microorganism is classified to aerobic-microorganisms that needs oxygen and anaerobic-microorganism that does not oxygen. It is decided by environmental factor such as temperature, pressure, flux and oxygen. Corrosion protection method To effective control of MIC, it is preferable to check and examine the organism related to corrosion. It can be controlled by using chemicals to organism that generates corrosion, removing the foreign substance on metal surface and replacing the soil if it is related to earth.