Chloride Ion Corrosion and Protection Technical Guide
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1. The core mechanism of chloride ion corrosion Chloride ion (Cl-) has a small radius and strong penetrating power, and can easily destroy the passivation film on the metal surface. It is one of the most common and destructive corrosive media in chemical production. Its main corrosion forms are divided into the following three types:: 1. Pitting Corrosion Chloride ions preferentially adsorb on the defects of the passivation film on the metal surface (such as inclusions and grain boundaries), crowding out oxygen atoms, causing the local passivation film to dissolve, forming the "large cathode-small anode" structure of the microbattery. The metal in the pores rapidly dissolves, and local acidification occurs along with the hydrolysis reaction (the pH value can be reduced to 1-2), further accelerating corrosion. 2. Stress Corrosion Cracking (SCC) is a brittle fracture that occurs under the combined action of tensile stress (residual stress or working stress), specific environment (containing chloride ions) and temperature (austenitic stainless steel is usually particularly sensitive above 60°C). SCC is extremely concealed and sudden, often leading to catastrophic failure of reactors or high-pressure pipelines. 3. Crevice Corrosion often occurs on the flange connection surface, the bottom of the gasket, bolt holes or dead corners in the equipment. After the oxygen in the gap is exhausted, chloride ions are enriched into the gap to maintain electrical neutrality, resulting in high chlorine and low pH value of the solution in the gap, thus triggering severe corrosion.
2. Core evaluation indicators: Pitting Resistance Equivalent Number (PREN) When selecting metal materials, the Pitting Resistance Equivalent Number is a key indicator for measuring the ability of stainless steel and nickel-based alloys to resist localized corrosion in chlorine-containing environments. The calculation formula is as follows: PREN = %Cr + 3.3%Mo + 16%N (Note: The formula of some high tungsten alloys will add tungsten element, that is, PREN = %Cr + 3.3%Mo + 0.5%W) + 16%N) PREN < 32：在含氯离子的高温或酸性环境中存在明显的点蚀风险（如304、316L）。 PREN > 40: Generally considered as "super stainless steel", it is suitable for environments with higher concentrations of chloride ions (such as 254SMO, 2507). 3. Chemical material selection ladder For chloride ion media with different concentrations, temperatures and pH values, material selection needs to be upgraded in a ladder-like manner.: 1. Basic austenitic stainless steel (304 / 316L) applicable working conditions: Normal temperature, neutral, very low chloride ion concentration (usually requires Cl- < 50 ppm 对于304，Cl-< 200 ppm 对于316L）。 工程限制：严禁用于温度 > A chloride ion environment of 60°C and tensile stress is likely to cause SCC. 2. Duplex Stainless Steels - 2205 / 2507 Characteristics: It has the metallographic structure of both austenite and ferrite, has high yield strength, and its resistance to stress corrosion cracking (SCC) is significantly better than that of 300 series austenite. Selection reference: o2205 (PREN=35): Suitable for medium concentration chloride conditions. o2507 (Super duplex steel, PREN > 40): Suitable for harsh chloride environments, such as high-salt wastewater treatment or specific separation tower equipment. 3. Super austenitic stainless steel (904L / 254SMO / AL-6XN) characteristics: High in molybdenum (Mo above 6%) and high in nitrogen, it has strong resistance to pitting and crevice corrosion. Applicable working conditions: Harsh environment with high concentration of chloride ions, low pH value or certain oxidizing ions. 254SMO is often used in condensers or heat exchanger tubes containing chloride ions. 4. Nickel-based alloy (Hastelloy C-276 / Inconel 625) characteristics: The ultimate line of defense. It is not only resistant to extremely high concentrations of chloride ions, but also can withstand extreme corrosion containing chlorine gas, sodium hypochlorite, and strong acids (such as mixed acid systems). Applicable working conditions: Core components of complex organic synthesis reactors (involving halogenation reactions) and pump valves for extremely corrosive materials. 5. Non-metallic and lining materials (PTFE / PFA / titanium) applicable working conditions: In strong corrosive conditions where metal materials are extremely cost-effective or incompetent (such as high temperatures and high salts aggravating corrosion), use carbon steel lined PTFE/PFA pipelines (similar to the AA1E grade design ideas of highly concentrated alkali or concentrated acid pipelines), or use titanium (Ti Gr.2, please note that titanium will spontaneously ignite in dry chlorine, but has excellent corrosion resistance in wet chlorine or aqueous solutions).
4. Structural design and process protection measures In addition to material upgrades, engineering design and operation management are equally critical: Eliminate dead corners and fluid accumulation: Equipment structural design should avoid stagnant areas. The sewage outlet should be set at the lowest point, and high-quality gaskets should be used at flange connections to reduce gaps. The pipeline design must ensure sufficient flow velocity (usually > 1.5 m/s) to prevent sediment adhesion and cause underscale corrosion. stress relief: For austenitic stainless steel equipment, the overall solution treatment or stress relief annealing should be carried out as much as possible after the manufacturing is completed, and the heat input in the weld area needs to be strictly controlled and polished. 3. Process parameter control: temperature control: Reduce operating temperatures as much as possible to break the temperature threshold that induces SCC. Dissolved oxygen control: Removing dissolved oxygen from the media (such as through a deaerator or adding hydrazine) can significantly reduce the driving force for chloride pitting. 4.Electrochemical protection: In large storage tanks or long-distance pipelines, cathodic protection can be performed by applying external current or sacrificial anodes (such as zinc, aluminum, magnesium alloys) to make the metal potential lower than its pitting corrosion rupture potential.
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