1. Stainless steel is not incapable of rusting, and it will also generate an oxide on the surface.

The stainless mechanism of all stainless steels on the market today is due to the presence of Cr. The root cause (mechanism) of stainless steel corrosion resistance is the passivation film theory. The so-called passivation film is a thin film mainly composed of Cr2O3 on the surface of stainless steel. Because of the existence of this film, the corrosion of the stainless steel substrate in various media is hindered. This phenomenon is called passivation.

There are two cases of the formation of this passivation film. One is that stainless steel itself has the ability to self-passivate. This self-passivation ability accelerates with the increase of chromium content, so it has rust resistance;

Another broader forming condition is that stainless steel in various aqueous solutions (electrolytes) forms a passivation film during corrosion, which inhibits corrosion. When the passivation film is damaged, a new passivation film can be formed immediately. The reason why stainless steel passivation film has the ability to resist corrosion has three characteristics:

a. The thickness of this passivation film is extremely thin. Generally, it is only a few microns under the condition of chromium content> 10.5%; It is also dense, so this passivation film is difficult to be penetrated by the corrosive medium to quickly etch the substrate;

c. The chromium concentration of this passivation film is more than three times higher than that of the substrate; therefore, the passivation film has high corrosion resistance. 2, stainless steel will also be corroded under certain conditions.

The application environment of stainless steel is extremely complicated, and the simple chromium oxide passivation film cannot meet the requirements of high corrosion resistance. Therefore, it is necessary to add molybdenum (Mo), copper (Cu), nitrogen (N) and other elements to the steel according to the different use conditions to improve the composition of the passivation film and further improve the corrosion resistance of stainless steel.

Adding Mo, because the corroded product MoO2- is close to the substrate, it strongly promotes the collective passivation, preventing the corrosion of the substrate;

The addition of Cu makes CuCl contained in the passivation film on the surface of stainless steel, which improves the corrosion resistance because it does not interact with the corrosive medium;

Adding N, because the Cr2N is enriched in the passivation film, the Cr concentration in the passivation film is increased, thereby improving the corrosion resistance of stainless steel.

The corrosion resistance of stainless steel is conditional. A grade of stainless steel is resistant to corrosion in one medium, but may be damaged in another medium. At the same time, the corrosion resistance of stainless steel is also relative. So far, no stainless steel is absolutely non-corrosive in all environments.

3. Sensitization phenomenon

Stainless steel forms chromium oxide film on the surface due to Cr, which loses its chemical activity and is called a passivated state. However, if the austenite system passes through the temperature range of 475 ~ 850 ° C, C will combine with Cr to form chromium carbide (Cr23C6) and precipitate in the crystal Boundary, so the Cr content near the grain boundary is greatly reduced, becoming a Cr-depleted region. At this time, its corrosion resistance is reduced and it is particularly sensitive to the corrosive environment, so it is called sensitization.

Sensitization is most likely to corrode in the environment where oxidizing acid is used. In addition, there are welding heat affected zones and heat bending processing zones. The prevention methods are:

a. When it rises and falls in the temperature range of 475 ~ 850 ℃, it passes quickly, so that Cr does not have enough time to combine with C, and chromium carbide cannot be precipitated.

b. Apply high temperature solid solution treatment to the stainless steel that has undergone sensitization, heat it to a temperature sufficient to decompose chromium carbide (1040 ~ 1150 ℃), and then rapidly cool it to make chromium carbide too late to precipitate.

c. Select low carbon stainless steel and add 304L, 304LN, 316L, 316LN, 316J1L, 317L, 317LN, so that Cr cannot be combined with C. Generally, the C content is less than 0.05%, and no grain boundary corrosion will occur during welding. However, special attention must be paid to cleaning to avoid contamination by C-containing substances such as oil. Used in concentrated hot nitric acid or mixed acid containing hydroxide, the C content is preferably below 0.03%.

d. Use stable stainless steel containing titanium (Ti) or niobium (Nb), such as 316Ti, 321, 347, 348, because the affinity of Ti, Nb and C is greater than Cr, and the precipitation temperature of titanium carbide and niobium carbide is more than carbonized The chromium is high, so the precipitation of chromium carbide can be avoided.

4. So under what circumstances does stainless steel corrode?

In fact, stainless steel is not necessarily non-rusty, but the corrosion rate is much lower than other steels in the same environment, and sometimes it can be ignored. Conditions for corrosion include:

a. Characteristics of materials: physical and chemical properties of materials

b. Environmental conditions: temperature, humidity, fluid speed, oxygen content and pHPHc. The media that cause corrosion: acid, alkali, salt, oxide, organic matter and microorganisms According to the form of corrosion, it is divided into eight categories: a. comprehensive Corrosion, uniform corrosion

As the simplest type of corrosion, a uniform layer of corrosion products appears on the metal surface. Generally, the rust of steel in the atmosphere belongs to this type. Corrosion in this form causes a layer of corrosion products to be uniformly formed on the metal surface, and the thickness of the corrosion reduction is uniform, which often occurs in atmospheric corrosion or acid corrosion. B. Galvanic corrosion or contact corrosion of dissimilar metals

Placing the structure of two different metal combinations in a corrosive environment (electrolyte), the chemical side with more active or lower potential will accelerate corrosion, and the more inert (positive) or higher potential metal will slow down corrosion. For example: if the steel pipe and the copper pipe are connected, the steel pipe with a low potential will accelerate the corrosion. This potential difference is called a Galvanic battery, and the corrosion formed is called dissimilar metal corrosion or Galvanic corrosion. C. Interstitial corrosion

Gap corrosion is corrosion that occurs inside the structure structure gap. Poor convection at the gap is easy to produce attachments or deposits. The dissolved oxygen is thinner than the outside and the anode is formed. The outer oxygen-rich region is the cathode, which forms a kind of oxygen concentration The battery began to corrode. D. Hole Erosion

Pitting occurs only on materials with a passivation film, such as stainless steel or aluminum alloy. At the beginning of the recess or surface defect of the material, due to the poor convection of the liquid, the lack of oxygen area is the anode, and the oxygen around the recess is sufficient as the cathode, and the two become concentrated batteries and begin to corrode. There are many similarities between pitting corrosion and crevice corrosion, but crevice corrosion requires the existence of gaps, and pitting corrosion will generate gaps (holes) by itself. E. Grain boundary corrosion

When the components or impurities in the alloy are desorbed at the grain boundaries, a potential difference is generated between the grain boundaries of the material, so that a metal with a relatively negative potential will corrode at the metal contact. This corrosion occurs along the grain interface. , Causing corrosion like cracks. For example, the presence of polysulfuric acid on the surface of 304 stainless steel causes sensitization, which causes the material to rupture in the presence of stress. F. Select corrosion or separation corrosion

A certain metal element in the alloy is selectively eroded and precipitated. The alloy composition changes and loses the original alloy characteristics. Such damage is called selective erosion. The metal elements that are precipitated are usually chemically active components or metal elements with lower electrochemical potential. The most common example is brass (30% Zn +

Zinc in 70% Cu) loses zinc due to corrosion. The surface of the zinc-losing part shows the original red color of copper. The naked eye can distinguish red and yellow, so it is also called zinc loss effect. G. Stress corrosion

Stress corrosion cracking is a fracture phenomenon caused by the coexistence of tensile stress and stress in a metal under a specific corrosion environment. The fracture form starts from the surface. The corrosion extends along the crystal lattice of the metal and cracks, but sometimes it crosses the grain Rupture, leading to serious damage. The stress may be the stress left during cold work or processing. H. Erosion

Corrosion is a chemical action, but if there is relative friction between the metal and the corrosive environment, there must be mechanical friction or collision between the two, and the rate of corrosion damage to the metal must be accelerated. This phenomenon is called erosion. Such corrosion often occurs at the direction of fluid change. If the fluid contains dissolved oxygen or the pH value is low, the corrosion will be accelerated, and most of its shapes are erosion and corrosion in the shape of grooves.