a) Sensitization: Ordinary 300 series stainless steel (such as 304, 316) contains high carbon content, belongs to the non-stable state (i.e., does not contain titanium or niobium and other stabilizing elements), the solubility of carbon in austenite at room temperature is very small, about 0.02% to 0.03%, much lower than the actual carbon content of stainless steel, so the supersaturated carbon is solid solution in austenite, when the temperature exceeds 425 ℃ and in 425 ℃ ~ 815 ℃ range to stay for a period of time, supersaturated carbon will continue to austenite grain boundary diffusion, and chromium elements, and the formation of chromium carbide compounds between the crystal, such as six carbide twenty-three chromium (Cr23C6), etc.. Chromium in the grain diffusion rate is smaller than the rate of diffusion along the grain boundary, the internal chromium is too late to diffuse to the grain boundary, the formation of chromium carbide in the intergranular chromium required mainly from the vicinity of the grain boundary, so that the chromium content near the grain boundary is greatly reduced.
When the chromium mass fraction of the grain boundary is less than 12%, the so-called "chromium-poor zone" is formed. The difference in electrochemical properties between the chromium-poor zone and the grain itself makes an activation-passivation cell with a large potential difference between the chromium-poor zone (anode) and the substrate (cathode) in the passivation state. The small anode of the chromium-poor zone and the large cathode of the substrate constitute a corrosion cell. Under the action of the corrosive medium, the chromium-poor zone is rapidly corroded, and the grain boundaries are firstly destroyed, with a significant weakening of the intergranular bond, deterioration of the mechanical properties, and a significant reduction of the mechanical strength, yet the deformation is not obvious. This precipitation of carbide on the grain boundary is generally called sensitization.
For austenitic stainless steels containing stabilizing elements, the welded joint area undergoes multiple heating and cooling cycles, resulting in the precipitation of carbon dissolved in titanium carbide (TiC) or niobium carbide (NbC) in a narrow and specific area, and combining with chromium to form chromium carbide compounds in the intergranular area, such as twenty-three chromium carbide (Cr23C6), also forming a chromium-poor zone, resulting in reduced corrosion resistance.
b) Intergranular Corrosion: Metal materials after sensitization, in the corrosive medium in the grain boundary due to low corrosion resistance and priority corrosion. Or not sensitized material in a specific corrosive medium in the grain boundary or near the grain boundary priority corrosion, so that the loss of bonding between the grains of a local damage process. The occurrence of sensitized austenitic stainless steel is very susceptible to intergranular corrosion.
a) The occurrence of sensitization, the general size, shape no obvious changes and will not occur plastic deformation; intergranular corrosion occurs if the grain shedding is not obvious, visual inspection is not easy to find the damage.
b) Only the sensitized area visible corrosion traces, if the sensitized area is narrow, such as the formation of the sensitized area when welding, generally appear narrow corrosion grooves or cracks.
c) Sensitized parts may still maintain a bright metallic luster, but the plasticity is completely lost, cold bending is prone to cracking, in serious cases, brittle fracture and metal grain shedding, and there is even no metal collision sound when it hits the ground. If there has been intergranular corrosion, sometimes there will be obvious grain shedding, so that the metal surface luster dull, local may appear obvious thinning.
d) Metallurgical microscopy or scanning electron microscopy can be observed under the grain boundaries significantly widened, mostly in the form of a network, and in serious cases can be observed in the obvious grain shedding.
e) Containing stabilizing elements of austenitic stainless steel welded joints when sensitization and intergranular corrosion can be observed in the joint area unique "knife-like corrosion" (or edge-like corrosion).
f) After sensitization of the material in the corrosive medium susceptible to intergranular corrosion, in the high tensile stress area often leads to stress corrosion cracking along the crystal.
3. Affected materials
a) Carbon content: the higher the carbon content, the higher the sensitivity to sensitization, the greater the tendency of intergranular carbide precipitation, but also the more likely to intergranular corrosion.
b) Alloy composition: the addition of titanium, niobium and other elements that can form stable carbide (TiC or NbC) and stabilization treatment, can reduce sensitization and intergranular corrosion susceptibility.
c) Heat treatment: heating to high temperatures for solution treatment, followed by rapid cooling (such as water cooling) to form a single austenite phase can avoid sensitization, but site construction generally can not meet the requirements of solution heat treatment, so generally only used in manufacturing plants.
d) Process conditions: the use of 300 series stainless steel section to reduce the operating temperature to below 425 ℃ can avoid the occurrence of sensitization.
5. Vulnerable devices or equipment
a) 300 series stainless steel equipment for catalytic cracking units used at high temperatures.
b) 300 series stainless steel conversion gas piping for coal gasification units.
c) 300 series stainless steel superheated steam furnace tubes and liquid cracking raw material furnace tubes for the cracking and emergency cooling system of ethylene cracking units operating at temperatures above 400°C in the convection section of the cracking furnace.
d) Stainless steel lining of urea synthesis tower of urea plant.
e) Other equipment and piping of 300 series stainless steel manufactured or installed by welding methods and without solution heat treatment, if the material is more sensitive to non-low carbon grades.
f) The occurrence of σ-phase embrittlement of 300 series stainless steel or 400 series stainless steel made of equipment or lining susceptible to intergranular corrosion (i.e., not sensitized will also occur intergranular corrosion).
a) The use of low carbon content of austenitic stainless steel can effectively reduce the occurrence of sensitization, such as ultra-low carbon austenitic stainless steel series.
b) The addition of certain alloying elements, such as titanium, niobium, etc. to form stable carbides.
c) Solution heat treatment is generally applied only to equipment and pipes made in the factory, and is not recommended for construction sites.
d) adjust the ratio of austenite forming elements to ferrite forming elements in the steel, so that it has austenite + ferrite biphasic organization, this biphasic organization is not easy to produce grain boundary sensitization.
e) For ferritic stainless steel with intergranular corrosion tendency, anneal at 700℃~800℃.
7. Detection or monitoring methods
a) Sensitization generally can not be directly observed, sensitization of the material by the role of the media corrosion or cracking, then may be directly observed.
b) Unsensitized material occurs when the intergranular corrosion of the grains appear obvious shedding, visual inspection can be observed on the surface roughness and unevenness, and even parts of the regional grain shedding to form an obvious corrosion zone, otherwise it is generally difficult to identify through visual inspection.
c) Metallographic analysis or scanning electron microscopy observation.
d) Intergranular corrosion may occur in the parts, set up corrosion pendant to do regular measurement of metal loss.
e) Penetration testing.
