Stainless steel valves, duplex stainless steel valves, and Hastelloy valves are applied in different anti-corrosion environments. The selection of specific materials is essentially

a dynamic balance between materials science and environmental adaptability. From component design to surface treatment,

and from application scenarios to maintenance strategies, every link is crucial to the final performance of corrosion resistance, affecting the performance and

service life of the valves.

1. Fundamental Differences: Base Materials and Chemical Compositions

Stainless steel valves are alloys based on iron, with chromium (content ≥ 10.5%) as the core element. They achieve corrosion resistance through the

formation of a dense oxide film (Cr?O?) on the metal surface by chromium. According to the combination of alloying elements, stainless steel can be

divided into four major categories:

1. Chromium steel (Cr series): Represented by 430 ferritic stainless steel, with a chromium content of 16%-18%. It has excellent atmospheric corrosion resistance but poor weldability.
2. Chromium-nickel steel (Cr-Ni series): Such as 304 austenitic stainless steel (18Cr-8Ni). It stabilizes the austenitic structure through nickel elements, combining corrosion resistance and processability, and accounts for more than 70% of the global stainless steel output.
3. Chromium-nickel-molybdenum steel (Cr-Ni-Mo series): A typical representative is 316L stainless steel (16Cr-12Ni-2.5Mo). The addition of molybdenum significantly improves pitting resistance, making it widely used in seawater environments.
4. Duplex stainless steel: Such as 2205 (22Cr-5Ni-3Mo), which consists of 50% ferrite and 50% austenite, combining high strength and stress corrosion cracking resistance.

Hastelloy valves, on the other hand, are superalloys based on nickel, with a nickel content usually exceeding 50%. They achieve corrosion resistance in extreme environments through the synergistic effect of elements such as molybdenum, chromium, and tungsten. Taking Hastelloy C-276 as an example, its composition includes:

1. Nickel (57%): Serves as the base material to provide high-temperature strength and oxidation resistance.
2. Molybdenum (15%-17%): Significantly enhances the resistance to reducing media (such as hydrochloric acid and sulfuric acid).
3. Chromium (14.5%-16.5%): Improves the corrosion resistance to oxidizing media (such as nitric acid and wet chlorine gas).
4. Tungsten (3%-4.5%): Inhibits intergranular corrosion and improves the thermal stability of the material.
5. Carbon (≤ 0.01%): The ultra-low carbon design avoids the sensitization phenomenon caused by the precipitation of carbides in the heat-affected zone during welding.

Technical comparison: Stainless steel achieves basic corrosion resistance through chromium elements and relies on the cost advantage of the iron matrix; Hastelloy, however, constructs a "chemical protective shield" through the synergistic effect of the nickel matrix and multiple elements. Its corrosion resistance covers extreme environments with a pH value of 0-14, but the material cost increases by 3-5 times.?

1. Corrosion Resistance?

The corrosion resistance of stainless steel valves is environmentally selective:

1. 304 stainless steel: Performs stably in dilute sulfuric acid and acetic acid at room temperature, but is prone to pitting corrosion when exposed to chloride ions (such as seawater and salt spray).
2. 316L stainless steel: Improves chloride ion resistance through molybdenum elements, but still undergoes severe corrosion in strong oxidizing acids such as 65% nitric acid.
3. Duplex stainless steel: Exhibits excellent performance in mixed media containing chloride ions and hydrogen sulfide. For example, its corrosion resistance in seawater is 5 times that of traditional 316L, but its corrosion resistance decreases sharply at high temperatures (> 300℃).?

Hastelloy valves demonstrate full-medium coverage capability:

1. C-276 alloy: The corrosion rate in boiling 65% nitric acid is only 0.01 mm/year, which is 1/50 of that of 316L; the corrosion rate in 10% hydrochloric acid is 0.02 mm/year, while 316L is completely dissolved under the same conditions.
2. B-3 alloy: Specifically designed for hydrochloric acid environments, with a corrosion rate of 0.05 mm/year in boiling 37% hydrochloric acid, making it the preferred material for hydrochloric acid storage tanks in the chemical industry.
3. G-35 alloy: Performs exceptionally well in phosphoric acid production (containing fluoride ions and chloride ions), and its corrosion resistance is more than 20 times that of 316L.

1. Mechanical Properties

Stainless steel materials have a wide range of strength:

1. Austenitic stainless steel: Tensile strength ranges from 520 to 620 MPa, and elongation ranges from 40% to 60%. It is suitable for deep drawing processing and is applied to most valve bodies or valve plates.
2. Martensitic stainless steel: Such as 420 stainless steel, with a tensile strength of up to 1080 MPa but an elongation of only 15%, suitable for making valve stems.
3. Precipitation-hardening stainless steel: Such as 17-4PH, which can achieve a tensile strength of 1310 MPa through aging treatment while maintaining an elongation of 10%. It is also a commonly used material for high-pressure valve stems.

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The mechanical properties of Hastelloy materials have better temperature adaptability. Taking Hastelloy C-276 alloy valves as an example: the room-temperature tensile strength is 730 MPa, and the elongation is 40%; at a high temperature of 650℃, it still maintains a tensile strength of 520 MPa, while the strength of 316L decreases to 310 MPa at the same temperature.

1. Potential Problems

All valves made of stainless steel or alloy materials must undergo solution treatment as specified by standards. If the solution treatment is insufficient or the valves are used within the carbide precipitation temperature range, resulting in the precipitation of carbides at grain boundaries, it may cause local intergranular corrosion of the material.

Even if the material has undergone qualified solution treatment, corrosion may still occur due to the following reasons:

1. The material contains inclusions, such as sulfide or silicate inclusions, which promote the development of pitting corrosion. To address this phenomenon, it is necessary to strengthen the control of raw material procurement and reduce the content of S and P elements. If rust has occurred due to inclusions, but the inclusions are on the surface and the rust depth is shallow, grinding and removal can be carried out according to the measured depth.
2. The surface of SS304 or SS316L valves has not undergone pickling and passivation surface treatment, resulting in the failure to form a chromium oxide passivation film on the surface; or improper pickling and passivation treatment, leading to incomplete cleaning of residual acid on the surface, which may also result in harsh local corrosion environments on the surface and accelerate corrosion.
3. For stainless steel valves with welding zones, solution treatment is required. Otherwise, chromium-depleted zones may appear, causing intergranular corrosion. At the same time, processing defects such as incomplete weld filling and undercutting can also become corrosion sources.
4. In addition to the acidity and alkalinity of the material itself, the selection of valve materials must also consider the medium temperature. Otherwise, high temperatures may directly dissolve the passivation layer, resulting in surface corrosion. For example, 304 material can be used in dilute sulfuric acid at room temperature, but at high temperatures (> 65℃), concentrated sulfuric acid will damage the diaphragm, leading to uniform corrosion.

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1. How to Select Valves of Different Materials

The selection of stainless steel and Hastelloy valves should follow the "environment-performance-cost" triangular model:

1. Environmental corrosiveness: Chloride ions are the main cause of pitting corrosion in stainless steel. Therefore, it is necessary to consider whether the corrosive environment is compatible with the selected material. When the chloride ion concentration is < 100 ppm and the temperature is < 100℃, stainless steel is preferred. For indoor dry environments, 201/403 can be selected; for urban atmospheric environments, 304 can be considered; for marine environments, 316L material is the minimum recommendation; and for high-temperature and strong acid environments, at least 2205 duplex stainless steel should be selected. When the chloride ion concentration is > 1% or there are strong oxidizing/reducing media, Hastelloy must be used.

2. Mechanical requirements: For high-temperature (> 300℃) or high-stress scenarios, the thermal strength advantage of Hastelloy needs to be evaluated.

3. Economy: Life-cycle cost analysis shows that the long-term economy of Hastelloy in corrosive environments is better than that of stainless steel.

In the future, with the advancement of refining technology, the carbon and silicon contents of Hastelloy can be further reduced to below 0.005%, and the gap in weldability between Hastelloy and stainless steel will gradually narrow. In the field of stainless steel, the development of nickel-saving duplex stainless steel (such as 2002) is expanding its application boundaries in medium and high-corrosion scenarios such as seawater desalination and oil and gas exploitation. The competition and cooperation between these two materials will continue to drive the development of industrial valve materials towards higher performance and lower cost.