Stainless properties of stainless steel, special steel plate surface passivation film, first briefly explain the corrosion mechanism of stainless steel, the passive film theory.
So-called passive film on stainless steel surface layer of Cr2O3-based film. Stainless steel substrate due to the existence of this film corrosion blocked in various media, this phenomenon is called passivation. This passive film formation of the two cases, one stainless steel itself, there is self-passivation ability to self-passivation ability with the increase of chromium content is accelerated. Another wider range of formation conditions of stainless steel in various aqueous solutions (electrolytes), leaving the corrosion of passive film formed in the corrosion process blocked.
General corrosion of the stainless steel types are divided into two categories: uniform corrosion, localized corrosion, with the popularity of stainless steel in people's lives, derive a new type of corrosion - "rust".
Uniform corrosion
uniform corrosion refers to the bare metal surface in the corrosive environment of all the electrochemical or chemical reaction, even subject to corrosion. Such corrosion can also measure the speed of corrosion also can predict the future, set the safety factor, set the period of use of the material, so it is in many types of corrosion most dangerous corrosion, usually uniform corrosion, corrosion in accordance with weight, thickness reduction measured by the number. In addition to the special environment of the uniform corrosion rate of stainless steel is very low, long life, low maintenance costs.
Required to maintain in the course of the mirror or size of sophisticated equipment should be used in 1-3 grade stainless steel; require long-term leakage or require the useful life of equipment should be used in 2-5 grade; maintenance convenience or life without a long device can choose 4-7 grade stainless steel. For the corrosion rate of more than 1mm generally do not use.
2, local corrosion
localized corrosion is a form of corrosion under the action of corrosive media World pipe network reported that the world's steel pipe network to provide the world's steel pipe network editor , the steel substrate in specific parts of the rapid corrosion. Such a great threat to corrosion of the equipment must therefore be made of stainless steel according to the medium conditions. Localized corrosion of the main types: intergranular corrosion, pitting corrosion, stress corrosion and corrosion.
● intergranular corrosion
crystalline intergranular corrosion occurred in the acidic medium to the medium concentration of sulfuric acid, high concentrations of nitric acid and organic acids. The form of corrosion of the stainless steel substrate grain boundaries accelerated corrosion. This corrosion is caused by the grain boundary chromium depletion.
Order to prevent grain boundary chromium depletion to improve the resistance to intergranular corrosion, there are mainly two ways: First, to reduce the ultra-low carbon stainless steel carbon content ≤ 0.03%; the steel add titanium or niobium.
● the pitting
pitting is a very dangerous localized corrosion occurred in the aqueous solution containing chlorine, bromine, iodine, etc., resulting in holes and then a sharp corrosion phenomenon, and in severe penetrating steel plate, and generally can not be reduced by weight to evaluate the extent of corrosion.
Measures to improve the pitting resistance there are two main aspects, first is to enhance local resistance to pitting corrosion, thereby reducing the inclusions in the steel, especially the sulfur content; pitting corrosion resistance of the steel substrate, the influence matrix resistance The eclipse of the alloying elements chromium, molybdenum, nitrogen three elements world pie network information . The
● crevice corrosion
produce the crevice corrosion the main reason the equipment within the gap, such as riveting, pads or equipment dead space and other reasons, the media in these places does not flow, so the chlorine ion concentrate and accelerate the corrosion. Prevent crevice corrosion,
should first try to avoid the design gap, or gap open; ability to improve the resistance to crevice corrosion, where the influence of alloying elements and pitting. The
● The appearance of stress corrosion
stress corrosion is along the vertical direction of the device thickness was dendritic corrosion, equipment cracking. Stress corrosion conditions in addition to the medium conditions, the tensile stress generated during the manufacturing process and equipment has a direct relationship. The main equipment of this corrosion of heat exchangers, coolers, steam generators, blowers, dryers and boilers, etc..
Measures to improve the stress corrosion resistance of stainless steel: One is to increase resistance to stress corrosion index △ of Ni; the equipment for heat treatment to eliminate residual stress.
3, corrosion
stainless steel corrosion resistance of the corrosion resistance of stainless steel, widely used as decorative materials in recent years a new index. Stainless steel plate, pipe and other materials used in construction at the same time decorative and aesthetics. Factors and factors of resistance to pitting corrosion of stainless steel resistant to rust performance is identical to that depends largely on the content of the resistance to rust and corrosion of the substrate source (inclusion).