Damage of metal under corrosive medium and alternating stress is characterized by corrosion pits and a large number of cracks, which significantly reduce fatigue strength of steel, leading to premature fracture of steel. Corrosion fatigue is different from mechanical fatigue and it does not have a certain fatigue limit.
Phenomenon that metal is gradually damaged under action of external medium is called corrosion, and corrosion basically has two forms, chemical corrosion and electrochemical corrosion. Corrosion encountered in production practice is mainly electrochemical corrosion, no current is generated in chemical corrosion, and only some corrosion products are formed during corrosion process. This corrosion product typically covers a metal surface to form a film that isolates metal from dielectric. If this layer of chemical product is stable, dense, intact and firmly bonded to metal surface layer, it will greatly reduce or even prevent further development of corrosion and protect metal. Process of forming a protective film is called passivation. For example, oxide films such as SiO, AlO, and CrO are formed. These oxide films are dense, complete, non-porous, crack-free, not easily peeled off, can protect base metal and prevent further oxidation. For example, FeO produced when iron is oxidized at a high temperature. Conversely, some oxide films are discontinuous or porous and have no protective effect on base metal. Some metal oxides, such as WO, are volatile at high temperatures and do not cover substrate at all.
Ability to improve chemical resistance of metals is mainly achieved by alloying or other methods to form a stable, complete order dense oxide film bonded to substrate on the surface of metal, also known as a passivation film.
Electrochemical corrosion is an important and common form of metal corrosion, which is caused by difference in electrode potential between different grooves of different metals or metals. This galvanic cell corrosion is generated between microstructures, so it is also called microbattery corrosion. Electrochemical corrosion is characterized by presence of a dielectric, and there is a potential difference between different metals, metal domains or phases, and a corrosion current is generated.

Form of corrosion failure of die steel in industrial production is varied. Under different loads and different medium environments, different steel materials have following types of corrosion:

A relatively uniform corrosion of a large area on bare surface of steel material reduces effective area of member and its service life, but is less harmful than local corrosion.

Refers to corrosion along boundary of product, causing damage to connection of grains. This kind of corrosion is the most harmful, it can make alloy brittle or lose strength, and lose metal sound when struck, which is easy to cause a sudden accident. Intergranular corrosion is main corrosion form of austenitic stainless steel. This is due to difference in grain boundary region and intragranular composition or stress, which causes electrode potential to be significantly reduced in boundary region.

Metals produce cracking under combined action of corrosive media and tensile stress (external stress or internal stress). Fracture mode is mainly along crystal and also through crystal, which is a dangerous low-stress brittle fracture, stress corrosion often occurs in chlorinated and alkaline oxides or other water-soluble media, and accounts for a significant proportion of accidents in many equipment..

Pitting corrosion is a form of corrosion failure that occurs in a localized area of a metal surface. After pitting corrosion, it can rapidly develop deep into ground and finally penetrate metal. Pitting corrosion is very hazardous, especially for a variety of containers. After pitting corrosion, it should be polished or painted in time to avoid deepening of corrosion. Pitting corrosion is caused by local damage of passivation film on metal surface under action of medium. Or in media containing chloride ions, loose surface defects and non-metallic inclusions can cause pitting corrosion.

Damage of metals under corrosive media and alternating stress is characterized by corrosion pits and a large number of cracks. It significantly reduce fatigue strength of steel, leading to premature fracture. Corrosion fatigue is different from mechanical fatigue. It does not have a certain fatigue limit. As number of cycles increases, fatigue strength decreases.
In addition to various forms of corrosion described above, there is also corrosion due to macroscopic battery action. For example, in metal member, rivet is different from riveting material, welding of dissimilar metal, hull is different from spiral material. It can be seen from above corrosion mechanism that focus of corrosion prevention should based on reducing number of primary batteries as much as possible, so that surface of steel forms a stable and complete passivation film which is firmly bonded to steel substrate in the case of forming a primary battery, minimize electrode potential difference between two electrodes.

 

There are reasons for corrosion of steel such as material, as well as use of environment and contact medium, so method of corrosion protection is also focused. Anti-corrosion methods currently used are as follows.

Alloying elements such as chromium, nickel, tin, titanium and copper, which are capable of improving corrosion resistance, are added to carbon steel to form different alloy steels, which can effectively improve corrosion resistance of steel.

Metal with good corrosion resistance is coated on the surface of steel by electroplating or spraying to improve corrosion resistance of steel. Such as galvanizing, chrome plating, copper plating and nickel plating.

A non-metallic material is used as a protective film on the surface of steel to isolate it from environmental medium to avoid or slow down corrosion. Such as spray coating, enamel and plastic.
The most common method used to prevent corrosion from steel structures is surface paint.

red rust anti-rust primer, epoxy zinc-rich paint and iron red epoxy primer. Primer requires better adhesion and rust resistance.

gray lead paint, alkyd enamel and phenolic enamel. Topcoat is to prevent primer from aging and has a good appearance color. Therefore, topcoat is required to have good weather resistance, moisture resistance, heat resistance, good chemical stability, week light sensitivity, it is not easy to be pulverized and cracked.
General anti-rust measures for concrete reinforcement are: to ensure solidity of concrete, to ensure thickness of protective layer of steel and to limit amount of chloride salt admixture or to use rust inhibitors. Steel bars for prestressed concrete are prohibited from being corroded, so use of chlorine salt admixtures should be prohibited.