1. Stress corrosion:
Stainless steel produces stress corrosion in a corrosive medium environment containing oxygen and chloride ions. The proportion of stress corrosion failure is as high as 45%.
Commonly used protective measures:
Reasonable selection of materials, the use of stress corrosion-resistant materials mainly includes high-purity austenitic chromium-nickel steel, high-silicon austenitic chromium-nickel steel, and high-chromium ferritic steel and ferrite-austenite dual-phase steel. Among them, the ferrite-austenite dual-phase steel has the best stress corrosion resistance.
Control stress: When assembling, minimize stress concentration, and make the part in contact with the medium have the smallest residual stress, prevent bumps and scratches, and strictly abide by the welding process specifications.
Strictly abide by the operating procedures: strictly control the raw material composition, flow rate, medium temperature, pressure, pH value and other process indicators. Add corrosion inhibitor within the range allowed by process conditions. When chromium-nickel stainless steel is used in oxygen-dissolved chloride, the mass fraction of oxygen should be reduced to below 1.0×10 – 6. The practice has proved that in the water containing chloride ions with a mass fraction of 500. 0 × 10 – 6, only a mixture of nitrate with a mass fraction of 150. 0 × 10 – 6 and sodium sulfite with a mass fraction of 0. 5 × 10 – 6 needs to be added. You can get good results.
2. Pitting failure and preventive measures
Pinhole corrosion generally occurs easily in stationary media. The pit usually develops along the direction of gravity or lateral direction. Once formed, the pit will automatically accelerate to the depth. , The oxide film on the stainless steel surface is dissolved in the aqueous solution containing chloride ions, resulting in the formation of small corrosion pits with a pore size of 20 μm to 30 μm on the base metal. These small corrosion pits are pitting nuclei. As long as the medium contains a certain amount of chloride ions, the corrosion nuclei may develop into corrosion pits.
Common preventive measures:
Elements such as molybdenum, nitrogen and silicon are added to stainless steel or the chromium content is increased while adding these elements. Reduce the content of chloride ions in the medium. Adding a corrosion inhibitor can increase the stability of the passivation film or help the damaged passivation film to be passivated again. The external cathodic current protection is adopted to inhibit pitting corrosion.
3. Pitting corrosion
Since any metal material has non-metallic inclusions to varying degrees, these non-metallic compounds will soon form pit corrosion under the corrosion of Cl ions, and the Cl ions outside the pit will migrate into the pit under the action of blocking the battery. The positively charged metal ions in the pit will migrate out of the pit. Among stainless steel materials, the materials with Mo added to have better pitting corrosion resistance than those without Mo. The more Mo content is added, the better the pitting corrosion resistance.
4. Crevice corrosion
The mechanism of crevice corrosion is the same as that of pitting corrosion. It is a corrosion phenomenon caused by the accumulation of Cl ions due to the effect of blocking the battery in the crevice. This type of corrosion generally occurs in the gaps of flange gaskets, lap joints, bolts and nuts, as well as the gap between the heat exchange tube and the tube sheet hole. The crevice corrosion is closely related to the concentration of the static solution in the gap. In crevice corrosion environments, the probability of inducing stress corrosion is very high.
Two Applicable conditions of several stainless sheets of steel in chlorine-containing aqueous solutions
1 Type 304 stainless steel
This is the cheapest and most widely used austenitic stainless steel (such as food, chemical, atomic energy and other industrial equipment). Suitable for general organic and inorganic media. For example, nitric acid with concentration <30%, temperature≤100℃ or concentration≥30%, temperature <50℃; carbonic acid, ammonia water and alcohols with various concentrations with temperature≤100℃. Corrosion resistance in sulfuric acid and hydrochloric acid is poor; especially sensitive to crevice corrosion caused by chlorine-containing media (such as cooling water).
2 Type 304L Stainless Steel
Corrosion resistance and use are basically the same as Type 304. Due to the lower carbon content (≤0.03%), the corrosion resistance (especially the intergranular corrosion resistance, including the weld zone) and weldability are better, and it can be used for semi-welded or full-welded PHE.
3 Type 316 Stainless Steel
Suitable for general organic and inorganic media. For example, natural cooling water, cooling tower water, demineralized water; carbonic acid; acetic acid and caustic lye with a concentration of <50%; solvents such as alcohols and acetone; dilute nitric acid (concentration <20% =, dilute phosphoric acid ( Concentration < 30% = etc. However, it is not suitable for use in sulfuric acid. Since it contains about 2% Mo, its corrosion resistance in seawater and other chlorine-containing media is better than that of type 304, and it can completely replace type 304.
4 Type 316L Stainless Steel
Corrosion resistance and use are basically the same as Type 316. Due to the lower carbon content (≤0.03%), the weldability and post-weld corrosion resistance are also better, and can be used for semi-welded or full-welded PHE.
5 Type 317 Stainless Steel
Suitable for applications requiring longer service life than the Model 316. Since the content of Cr, Mo, and Ni elements is slightly higher than that of 316 type, it has better resistance to crevice corrosion, pitting corrosion and stress corrosion.
6 AISI 904L or SUS 890L type stainless steel
This is a cost-effective austenitic stainless steel that takes into account both price and corrosion resistance. Its corrosion resistance is better than the above materials, especially suitable for general sulfuric acid, phosphoric acid and other acids and halides (including Cl-, F — ). Due to the high content of Cr, Ni and Mo, it has good resistance to stress corrosion, pitting corrosion and crevice corrosion.
7 Avesta 254 SMO Premium Stainless Steel
This is an ultra-low carbon high-grade stainless steel that has improved Type 316 by increasing the Mo content. It has excellent resistance to chloride pitting and crevice corrosion.
8 Avesta 654 SMO Premium Stainless Steel
This is an ultra-low carbon high-grade stainless steel with Cr, Ni, Mo, and N contents higher than 254 SMO. It has better resistance to chloride corrosion than 254 SMO and can be used in cold seawater.
9 RS-2 (OCr20Ni26Mo3Cu3Si2Nb) stainless steel
This is a domestic Cr-Ni-Mo-Cu stainless steel. The pitting and crevice corrosion resistance is equivalent to Type 316, and the stress corrosion resistance is better. It can be used for concentrated sulfuric acid below 80 ℃ (concentration 90~98%), and the annual corrosion rate is ≤0.04mm/a.
10 Incoloy 825(S)
This is Ni(40%)-Cr(22%)-Mo(3%) high-grade stainless steel. Incoloy is a registered trademark of the International Nickel Co. It is suitable for sulfuric acid of various concentrations at low temperatures; it has good corrosion resistance and does not produce stress corrosion cracking in caustic alkali (such as NaOH) solution with a concentration of 50% to 70%. However, it is very sensitive to crevice corrosion caused by chlorides. In addition, the stamping performance is not very good, so it is not a commonly used material for plates.
11 31 Alloy
Improved from 904L (increased Mo, N content), standard 6%Mo high-grade stainless steel (31%Ni-27%Cr-6.5%Mo-32%Fe). The corrosion resistance in many media is better than that of 904L; in sulfuric acid with a concentration of 20%~80% and a temperature of 60℃~100℃, the corrosion resistance even exceeds that of C-276.
12 33 Alloy
A fully austenitized, chromium-based high-grade stainless steel with corrosion resistance comparable to some Ni-Cr-Mo alloys such as Inconel 625. In acidic and alkaline media (including nitric acid, a mixture of nitric acid and hydrofluoric acid), it has good resistance to localized corrosion and stress corrosion cracking; the corrosion resistance in concentrated nitric acid is much better than 304L. For example, it is suitable for sulfuric acid with a concentration greater than 96%~99%, temperature ≤150℃, and sulfur oxide content less than 200 mg/L; hot seawater; a strong corrosive solution with concentration ≤50% and boiling; concentration≤85%, temperature Phosphoric acid of ≤150℃, etc. However, it is not suitable for reducing media (such as dilute sulfuric acid, etc.). The price is about the same as the C-276.
13 C-2000 Alloy
A nickel-based alloy developed in the 1990s, the price is similar to that of C-276, and it is one of the above materials with the best corrosion resistance. In the medium concentration of sulfuric acid, dilute hydrochloric acid and boiling temperature, the concentration of ≤50% phosphoric acid, and hot chloride and other media, its corrosion resistance is better than C-276 and C-22, and there are substituted C-22 alloy trends. However, for sulfuric acid concentration ≥70%, the corrosion resistance is not as good as C-276.
14 59 Alloy
Compared with C-2000, the chemical composition is basically the same except that the Ni content is slightly higher (59%), the Fe content is low, and there is no Cu and W. This is a material with the best corrosion resistance, thermal stability, punch ability and weldability among nickel-based alloys. Since its commercialization in 1990, it has been widely used in sulfuric acid, hydrochloric acid, hydrofluoric acid and chlorine-containing, many media containing oxygen and low pH.
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