Intergranular corrosion is a common local corrosion, corrosion along the metal or alloy grain boundaries or its adjacent areas of development, and grain corrosion is very slight, this corrosion is called intergranular corrosion, this corrosion between the grains of bonding greatly weakened. Severe intergranular corrosion, can make the metal lose strength and ductility, in the normal load cracking. Modern intergranular corrosion theory, the main chromium-poor theory and grain boundary impurities selective dissolution theory.
1.1 Chromium-Poor Theory
Commonly used austenitic stainless steel, in oxidizing or weakly oxidizing medium why intergranular corrosion, mostly due to processing or improper use of heat caused by. The so-called improperly heated steel is heated or slowly cooled through 450 ~ 850 ℃ temperature zone, the steel will be sensitive to intergranular corrosion. So this temperature is the use of austenitic stainless steel dangerous temperature.
Stainless steel materials in the factory has been solid solution treatment, the so-called solution treatment is the steel heated to 1050 ~ 1150 ℃ after quenching, the purpose is to obtain a homogeneous solid solution. Austenitic steel contains a small amount of carbon, carbon in austenite solid solution degree is decreasing with the temperature. Such as 0Cr18Ni9Ti, at 1100 ℃, the solid solution degree of carbon is about 0. 2 %, at 500 ~ 700 ℃, about 0. 02 %. So the solid solution treatment of steel, carbon is supersaturated.
1.2 Selective Dissolution of Grain Boundary Impurities Theory
In production practice, we also learned that austenitic stainless steel in strong oxidizing media (such as concentrated nitric acid) can also produce intergranular corrosion, but the corrosion and in oxidizing or weakly oxidizing media in the case of different. Usually occurs in the solid solution treatment of steel, after the sensitization of steel generally does not occur.
When the solid solution contains phosphorus impurities such as 100ppm or silicon impurities of 1000 - 2000ppm, they will be segregated on the grain boundaries. These impurities in the strong oxidizing medium under the action of dissolution, resulting in intergranular corrosion. While the steel is sensitized, because carbon can be generated with phosphorus (MP) 23C6, or because the first carbon segregation limits the diffusion of phosphorus to the grain boundaries, both of which will eliminate or reduce the impurities in the grain boundary segregation, eliminating or reducing the susceptibility of steel to intergranular corrosion.
2 Commonly Caused by Austenitic Stainless Steel Intergranular Corrosion Media
The "Corrosion Data Chart" prepared by G. A. Nelson lists the common media that cause intergranular corrosion of austenitic stainless steels: acetic acid, acetic acid + salicylic acid, ammonium nitrate, ammonium sulfate, chromic acid, copper sulfate, fatty acids, formic acid, ferric sulfate, hydrofluoric acid + ferric sulfate, lactic acid, nitric acid, nitric acid + hydrochloric acid, oxalic acid. phosphoric acid, seawater, salt spray, sodium bisulfate, sodium hypochlorite, sulfur dioxide (wet), sulfuric acid, sulfuric acid + copper sulfate, sulfuric acid + ferrous sulfate, sulfuric acid + methanol, sulfuric acid + nitric acid, sulfite, phthalic acid, sodium hydroxide + sodium sulfide.
3 Measures to prevent and control intergranular corrosion
According to the corrosion mechanism, the prevention and control of austenitic stainless steel intergranular corrosion measures are as follows:
(1) The use of ultra-low carbon stainless steel to reduce the carbon content to 0. 03% or less, so that the steel does not form (Fe, Cr) 23C6, does not appear in the chromium-poor area, to prevent the generation of intergranular corrosion.
(2) Stabilized stainless steel selection of steel containing titanium and niobium stainless steel, smelting steel by adding a certain amount of titanium and niobium two components, their affinity for carbon, so that the formation of TiC or NbC steel, and TiC or NbC solid solution degree and much smaller than (Fe, Cr) 23C6, in the solid solution temperature is almost insoluble in austenite. In this way, although after the sensitization temperature, (Fe, Cr) 23C6 does not cause a large number of precipitation at the grain boundaries, to a large extent to eliminate the austenitic stainless steel tendency to produce intergranular corrosion.
(3) Re-solidification treatment when welding austenitic stainless steel, the arc pool temperature up to 1300 ℃ or more, the temperature on both sides of the weld decreases with the increase in distance, where there is a sensitization temperature zone. Should try to avoid austenitic stainless steel in the sensitization temperature range of heat and slow cooling, if found to have a tendency to intergranular corrosion, generally for non-stabilized stainless steel heated to 1000 ~ 1120 ℃, holding temperature by 1 to 2 minutes per millimeter, and then rapid cooling; stabilized stainless steel to 950 ~ 1050 ℃ is appropriate. After solid solution treatment of steel should still be prevented in the sensitization temperature heating, otherwise chromium carbide will precipitate again along the grain boundaries.
(4) The selection of the correct welding method welding, if the operation is not skilled or welding material is too thick, the longer the welding time will stay in the sensitized temperature zone, the more opportunities, the result of the weld on both sides of the base material produced on the sensitivity of intergranular corrosion. In order to reduce the sensitivity of the welded joint, welding should be minimized in the input of line energy. General argon arc welding than arc welding input line energy is low, so welding and welding repair should be used argon arc welding. For the welded parts should be selected ultra-low carbon stainless steel or stainless steel containing Ti, Nb stabilizing elements, for the electrode should be selected ultra-low carbon electrodes or electrodes containing Nb. Argon arc welding, in order to avoid overheating of the welded joint, the operation should be fast, the weld should be quickly cooled, to minimize the base material on both sides of the weld in the sensitization temperature range stay time.
4 Post-solder Treatment
Weld area does not necessarily emphasize post-weld heat treatment, general solution treatment to 1100 ~ 1150 ℃ range of insulation for a certain period of time after acute cooling, three minutes to complete the cooling of 925 ~ 540 ℃ temperature range, in the continued fast cooling to 425 ℃ or less; stabilization treatment to 850 ~ 880 ℃ temperature range within a few hours of insulation after air cooling. The expected post-weld heat treatment effect, with the whole process of heat treatment of each key process parameters (such as furnace temperature, heating speed, temperature difference between parts of the workpiece in the heating process, furnace atmosphere, holding time, holding process temperature difference between parts, cooling speed, furnace temperature, etc.) is closely linked.
For austenitic stainless steel vessels that may cause intergranular corrosion environment, the general parts of the solution treatment or stabilization treatment can be achieved. The whole vessel (mostly heat exchangers) post-weld heat treatment of the weld will face a lot of difficulties. This type of treatment is not a local post-weld heat treatment, but the entire welded parts or the whole vessel post-weld heat treatment. Due to the complex shape of the structure of most chemical vessels (such as our commonly used shell and tube heat exchangers) .
5 Some Perspective
Chromium-nickel austenitic stainless steel is the most commonly used corrosion-resistant materials, and intergranular corrosion is the most common form of failure of chromium-nickel austenitic stainless steel containers. Intergranular corrosion greatly weakened the bond between the grains, and in severe cases, the mechanical strength can be completely lost. Suffering from this corrosion of stainless steel, the surface seems to be very bright, but can not withstand a light knock will break into fine grains. Because intergranular corrosion is not easy to check, so the sudden destruction of equipment, it is very harmful, should cause us to pay sufficient attention.
Chromium-nickel austenitic stainless steel containers are basically formed by welding, and both sides of the welded joint is the intergranular corrosion-sensitized area, which is always damaged by corrosion before the base material. Through post-weld heat treatment, to improve the weld area resistance to intergranular corrosion, and the base material to the same extent, this is our goal, is our original intention of post-weld heat treatment.
For intergranular corrosion resistance of chromium-nickel austenitic stainless steel vessel weld area for solid solution treatment or stabilization treatment, can not simply generalize, should be a specific analysis of the structure of the shape of the vessel, analysis can guarantee the effect of heat treatment, otherwise even if we put forward the requirements of post-weld heat treatment, but often contrary to expectations, not only does not achieve the desired effect, but will affect the structure of the base material.
In order to improve the ability of chromium-nickel austenitic stainless steel containers against intergranular corrosion, must be for the specific corrosive environment, according to the corrosion mechanism, the first selection of materials optional ultra-low carbon stainless steel, stabilized stainless steel, welding the right welding method, the appropriate combination of the above-mentioned prevention and control measures, in order to achieve good results, can not simply rely on post-weld solution or stabilization treatment.
