1. Stress corrosion:
Common protective measures:
① Reasonable selection of materials
The stress corrosion-resistant materials mainly include high-purity austenitic chromium-nickel steel, high-silicon austenitic chromium-nickel steel, high-chromium ferrite steel and ferrite-austenitic dual-phase steel. Among them, ferrite-austenite dual-phase steel has the best resistance to stress corrosion.
② Control stress
When assembling, try to reduce stress concentration as much as possible, and make the parts in contact with the medium have minimal residual stress to prevent bumps and scratches, and strictly abide by the welding process specifications.
2. Piting corrosion failure and preventive measures
Pinhole corrosion generally occurs easily in stationary media. Corrosion pits usually develop along the direction of gravity or transverse direction. Once pit corrosion is formed, it will automatically accelerate to depth. The oxide film on the surface of stainless steel is dissolved in an aqueous solution containing chloride ions, resulting in small corrosion pits with a pore size of 20 μm to 30 μm on the base metal. These small corrosion pits are pitting corrosion nuclei. As long as the medium contains a certain amount of chloride ions, corrosion nuclei may develop into corrosion holes.
① Add molybdenum, nitrogen, silicon and other elements to stainless steel or add these elements while increasing the chromium content.
② Reduce the content of chloride ions in the medium.
③ Add corrosion inhibitor to increase the stability of the passivation film or facilitate the re-passivation of the damaged passivation film.
④ Use external cathode current protection to inhibit pitting corrosion.
3. Pointing corrosion
Since any metal material contains non-metallic inclusions to varying degrees, these non-metallic compounds will quickly form pit corrosion under the corrosive action of Cl ions. Under the action of the blocked battery, the Cl ions outside the pit will migrate into the pit. The positively charged metal ions in the pit will migrate out of the pit. Among stainless steel materials, Mo-added materials have better pit corrosion resistance than materials without Mo. The more Mo content is added, the better the pit corrosion resistance.
Applicable conditions for several stainless steels 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 a concentration of <30% and a temperature of ≤100°C or a concentration of ≥30% and a temperature of <50°C; various concentrations of carbonic acid, ammonia and alcohols with a temperature of ≤100°C. It has poor corrosion resistance in sulfuric acid and hydrochloric acid; it is especially sensitive to crevice corrosion caused by chlorine-containing media (such as cooling water).
② 304L stainless steel.
Corrosion resistance and usage are basically the same as Type 304. Due to the lower carbon content (≤0.03%), it has better corrosion resistance (especially resistance to intergranular corrosion, including the weld area) and weldability, and can be used for semi-welded or fully welded PHE.
③ Type 316 stainless steel.
Suitable for general organic and inorganic media. For example, natural cooling water, cooling tower water, softened water; carbonic acid; acetic acid and caustic lye with a concentration of <50%; solvents such as alcohols and acetone; dilute nitric acid (concentration <20%=) with a temperature of ≤100°C, 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 Type 304, and it can completely replace Type 304.
④ 316L stainless steel.
Corrosion resistance and usage are basically the same as Type 316. Due to the lower carbon content (≤0.03%), the weldability and corrosion resistance after welding are also better, and it can be used for semi-welded or fully welded PHE.