[Haichuan Corrosion Prevention Knowledge] The corrosion mechanism of nitric acid and the selection of industrial equipment as well as material protection

Thread Content

As a strong acid and strong oxidizing agent, nitric acid (HNO3) has a corrosion mechanism and protection strategies that are fundamentally different from those of conventional non-oxidizing acids such as hydrochloric acid or dilute sulfuric acid. In industrial production, especially in processes involving the nitration of aromatics, corrosion control of nitric acid system equipment is one of the key considerations in process design and equipment maintenance. I. Corrosion mechanism of nitric acid: The corrosive properties of nitric acid depend highly on its concentration and temperature. Its dual nature (acidic + oxidizing) determines its special behavior when reacting with metals: 1. Strong oxidizing power and the “passivation” effect. Unlike conventional acid corrosion, which involves the release of hydrogen gas, the dissolution of metals in nitric acid is primarily accompanied by the reduction reaction of nitrate ions (to form nitrogen oxides such as NO and NO2). However, it is this strong oxidizing property that enables certain metals (such as iron, aluminum, chromium, etc.) to rapidly form a dense and stable oxide film on their surface in high-concentration nitric acid. This film prevents further contact between the acid and the base metal, thereby putting the metal into a passivated state and significantly reducing the corrosion rate. 2. Concentration inversion phenomenon: Dilute nitric acid (68%): Oxidizing properties prevail, enabling the passivation of aluminum and certain specific stainless steels. II. Selection of industrial equipment and material protection: For nitric acid process media at different concentrations, the following material protection strategies are typically adopted: ·1. Dilute nitric acid to medium-concentration nitric acid (below an azeotropic point of about 68%): Austenitic stainless steel: This is the most commonly used material. 304L and 316L stainless steels exhibit excellent corrosion resistance in dilute nitric acid and nitric acid at moderate concentrations at room temperature to moderate temperatures. o Why choose “L”: “L” stands for Ultra-Low Carbon. During equipment welding, low carbon content can effectively prevent the formation of chromium carbide in the heat-affected zone, thereby avoiding fatal intergranular corrosion. 304L performs in pure nitric acid just as well as, or even better than, 316L (because the molybdenum content in 316L can actually accelerate corrosion in strong oxidizing environments). ·2. Concentrated nitric acid and fuming nitric acid (>68%) and high-purity aluminum (such as 1060, 3003 grades): It exhibits excellent corrosion resistance in concentrated nitric acid at room temperature (thanks to a perfect passivation layer), and is therefore commonly used in storage tanks or transport tankers for concentrated nitric acid at room temperature. However, once the temperature rises, the corrosion rate of aluminum increases sharply. o High-silicon cast iron: It exhibits excellent corrosion resistance to nitric acid at various concentrations and temperatures. However, due to its extreme hardness and brittleness, it cannot be machined or welded, and is usually used only for casting pump casings or valve components. o Special stainless steels/titanium alloys: For high-temperature, concentrated nitric acid environments, high-silicon austenitic stainless steels (such as Uranus S1, ZeCor, etc.) are usually required. Titanium exhibits excellent corrosion resistance in nitric acid containing trace amounts of water, but it must not be used in fuming nitric acid or completely anhydrous nitric acid, as this can easily lead to spontaneous ignition or violent explosions. ·3. Non-metallic materials and linings: For highly corrosive high-temperature nitration reactors or complex pipelines, glass-lined steel or PTFE/FEP linings are extremely reliable choices. They are virtually immune to nitric acid at all concentrations and temperatures. III. Key Points to Consider in Equipment Manufacturing and Operation In engineering practice, selecting the right materials is only the first step; it is the details that often determine the lifespan of the equipment: Be cautious of the destructive effects of chloride ions (Cl-): Chloride ions present in nitric acid systems are particularly problematic, especially those originating from cooling water leaks or impurities in the raw materials. Chloride ions have a strong penetrating power; they can locally damage the passivation film on the surface of stainless steel, leading to severe pitting. Post-weld heat treatment (PWHT): For stainless steel equipment that is not ultra-low carbon, if welding was performed during manufacturing, solution treatment is necessary to eliminate thermal stresses and restore the intergranular corrosion resistance of the weld area. Flow velocity and erosion corrosion: At the impeller of the pump, pipe elbows, or areas where the diameter narrows, the flow of fluid can strip away the passivation layer on the metal surface, leading to a combined effect of erosion and corrosion. In these high-flow areas, it is usually necessary to increase the wall thickness allowance or use materials of a higher grade.