Material and Seal Selection for Corrosion-Resistant Differential Pressure Sensors in Chemical Acid and Alkali Media

In differential pressure measurement of pressure or liquid level in acid, alkali, salt solutions, and various chemical mother liquors, sensor wetted parts are continuously exposed to multiple factors, including corrosion, permeation, crystallization, temperature fluctuations, and pressure pulses. Improper selection of materials and sealing solutions may result in zero drift, slow response, and increased measurement errors in minor cases, or diaphragm perforation, seal leakage, and process media overflow in severe cases.

The selection of corrosion-resistant differential pressure sensors should not be based solely on a simple classification of “acidic” or “alkaline.” A comprehensive evaluation should also consider medium concentration, operating temperature, pressure range, impurity content, oxidizing properties, chloride ion content, and whether negative-pressure conditions are present. The corrosion mechanisms affecting metals, plastics, and elastomeric seals may vary significantly for the same medium at different concentrations and temperatures.

For applications such as chemical storage tank level measurement, filter differential pressure monitoring, reactor pressure balancing, and pipeline resistance monitoring, differential pressure sensors must not only provide stable pressure measurement performance, but also reduce the risk of direct medium attack on the core sensing element through the proper matching of isolation diaphragms, pressure-guiding structures, fill fluids, and sealing components.

Xi'an Shenghongchuang Instrument Co., Ltd., supported by the manufacturing and testing capabilities of Shaanxi Qinkong Sensor Technology Co., Ltd., can provide targeted configurations for the wetted materials, diaphragm structures, process connections, and sealing forms of differential pressure sensors according to medium parameters, installation methods, and control requirements, delivering suitable measurement solutions for complex chemical operating conditions.

Confirm Medium Conditions Before Determining Wetted Materials

The corrosion resistance of a differential pressure sensor depends mainly on the components in direct contact with the medium, including isolation diaphragms, process fittings, flanges, impulse pipes, valve manifolds, sealing rings, and remote seal assemblies. During selection, priority should be given to obtaining basic data such as the medium name, mass fraction or volume fraction, normal and maximum temperatures, design pressure, and allowable leakage level.

For example, the corrosiveness of low-concentration hydrochloric acid at ambient temperature differs from that of high-concentration hydrochloric acid at high temperature. Sodium hydroxide solution is generally compatible with most stainless steels, but at relatively high temperatures and concentrations, it may still cause stress corrosion or degradation of material properties. For acidic media containing chloride ions, 316L stainless steel is not a universal solution.

When the medium contains suspended particles, crystals, polymers, or high-viscosity components, attention should be paid not only to chemical corrosion but also to deposition blockage and diaphragm wear. For such applications, flush diaphragm structures, flush flange connections, or installation methods with flushing ports may be considered to reduce measurement lag caused by material buildup in pressure-guiding holes.

If the process involves vacuum conditions, frequent start-stop operation, or reverse pressure fluctuations, diaphragm strength and fill system stability should also be evaluated. For low-range differential pressure measurement, it is recommended to keep the normal differential pressure within 30% to 80% of the sensor range to balance resolution, overload resistance, and long-term stability.

Typical Applications of Common Wetted Materials

316L stainless steel offers good mechanical strength, machinability, and general corrosion resistance. It is suitable for mildly corrosive acid and alkali media, general water treatment media, low-chloride-ion solutions, and most conventional auxiliary chemical processes. Its advantages include relatively controllable cost and high structural strength; however, it should not be used indiscriminately for high-concentration hydrochloric acid, wet chlorine gas, or high-chloride-ion environments.

Nickel-based alloys such as Hastelloy C-276 and C-22 provide strong resistance to various reducing and oxidizing acid media, particularly for process environments containing chloride ions, wet chlorine, mixed acids, and certain highly corrosive media. In actual use, the specific grade, upper temperature limit, and medium concentration must still be verified, and “Hastelloy” should not be used as a general term for all corrosive operating conditions.

Tantalum provides excellent corrosion resistance to many strong acids, including hydrochloric acid, sulfuric acid, and nitric acid, and is commonly used for high-value, highly corrosive measurement points. However, tantalum is costly and is not suitable for hydrofluoric acid, strong alkalis, or certain fluorine-containing media. It is therefore better suited as a dedicated diaphragm material after medium compatibility has been confirmed.

Fluoroplastics such as PTFE and PFA offer a broad range of chemical corrosion resistance and are suitable for many strong acids, strong alkalis, and organic solvent media. Their limitations are that their mechanical strength, pressure resistance, and dimensional stability are generally lower than those of metallic materials. Under high-pressure, high-temperature, or frequent pulse conditions, reinforcement through metal frameworks, lining structures, or remote isolation solutions is required.

Seal Selection Determines Long-Term Leakage Risk

Although sealing rings are small in size, they are a critical factor in the reliability of corrosion-resistant differential pressure sensors. Chemical media may penetrate through tiny gaps in sealing surfaces, causing metal corrosion, reduced insulation performance, and even ingress into the housing and electrical chamber. Therefore, sealing materials should be evaluated for medium compatibility together with wetted metal materials.

FKM fluoroelastomer provides good oil resistance, heat resistance, and resistance to most acidic media, making it suitable for many conventional chemical applications. EPDM rubber performs well with hot water, steam, and certain alkaline and water-based media, but is unsuitable for most oils and aromatic hydrocarbon solvents. Neither should be selected independently without considering the specific medium composition.

For highly corrosive, high-purity, or highly permeable media, PTFE-encapsulated O-rings, perfluoroether elastomer seals, or welded sealing structures may be selected. Perfluoroether materials generally offer broader chemical resistance and temperature resistance, but have a higher procurement cost and are suitable for critical measurement points where leakage consequences are severe and maintenance windows are limited.

The sealing form should also take the installation condition into account. Flange gaskets, threaded seals, clamp seals, and diaphragm seals are loaded differently. During field installation, components should be tightened to the specified torque to prevent leakage caused by insufficient compression, while avoiding excessive tightening that may cause permanent seal deformation or damage to flange sealing surfaces.

Key Points for Matching Sealing Materials with Operating Conditions

For media at ambient temperature and low pressure with relatively stable corrosiveness, properly selected FKM or EPDM seals can generally balance cost and maintenance convenience. In low differential pressure applications with measuring ranges of 0kPa to 40kPa or 0kPa to 100kPa, particular care should be taken to prevent seal extrusion deformation from affecting diaphragm loading conditions.

When the medium temperature approaches or exceeds 150℃, the continuous operating temperature of the sealing material should be verified rather than relying only on short-term temperature resistance specifications. For every 10℃ increase in temperature, the aging rate of certain elastomers may increase significantly. Sufficient temperature resistance margin should be reserved for long-term operation.

For concentrated sulfuric acid, fuming nitric acid, fluorine-containing media, strong oxidizers, and mixed solvents, priority should be given to obtaining the chemical composition stated in the medium safety data sheet. If composition fluctuations exist, selection should be based on the most unfavorable concentration, highest temperature, and highest pressure conditions, rather than only on average daily operating conditions.

For equipment requiring frequent disassembly, assembly, and cleaning, sensors with flange, clamp, or sanitary connection structures may be prioritized to reduce sealing surface damage caused by repeated disassembly of threaded connections. For alkaline or saline solutions with a risk of crystallization, regular flushing and zero-point verification intervals should also be established.

Coordinated Design of Isolation Diaphragms and Installation Structures

For differential pressure measurement in direct contact with highly corrosive media, the isolation diaphragm is an important barrier protecting the sensing element. The diaphragm material must simultaneously meet requirements for corrosion resistance, pressure resistance, fatigue resistance, and welding reliability. An overly thick diaphragm reduces the sensitivity of low differential pressure measurement, while an overly thin diaphragm may reduce pressure resistance and impact resistance. Therefore, the design should be balanced according to the measuring range and static pressure rating.

In high-static-pressure differential pressure systems, such as pressure differential measurement across pipeline filters or between pump inlets and outlets, the actual differential pressure may be only 20kPa to 200kPa, while the static pressure on both sides may reach 1MPa, 4MPa, or even higher. In such cases, in addition to having the corresponding static pressure capability, the sensor's one-way overload capacity and positive and negative pressure-side connection arrangements should also be verified.

For operating conditions prone to crystallization, adhesion, or containing solid particles, ordinary pressure-guiding holes can easily become blocked, resulting in distorted differential pressure values. Flush diaphragms, enlarged pressure-guiding passages, remote flange diaphragms, or capillary isolation systems may be used, with flushing ports configured when necessary. The installation location should avoid deposition zones, bubble accumulation zones, and areas of severe vibration as far as possible.

When the measured medium temperature is high, remote diaphragm seals can isolate the high-temperature medium from the transmitter body through capillaries, but it should be noted that fill fluid temperature effects may cause zero-point changes. For measurement points with high accuracy requirements, ambient temperature, process temperature, and capillary length should be specified, and system verification should be performed after installation.

On-Site Control Measures for Reducing Corrosion Failures

Before sensor installation, verify that flange sealing surfaces are flat, impulse pipes are clean internally, and threads are undamaged. Also avoid conditions in which dissimilar metals form significant galvanic corrosion in humid corrosive environments. When used outdoors or in high-humidity environments, cable entries, cable sealing glands, and enclosure protection ratings must not be overlooked.

During operation, an inspection plan can be established according to process risks. For highly corrosive media, it is recommended to regularly check zero-point stability, output fluctuations, leakage traces at connections, and diaphragm surface conditions. If continuous output drift, a significantly slower response, or abnormal pressure signal jumps occur, blockage, corrosion, and seal failure should be promptly investigated.

Calibration of differential pressure sensors should cover commonly used measurement ranges rather than only full-scale endpoint verification. For products with 4mA to 20mA output, zero point, span, linearity, and hysteresis error should be checked simultaneously. For products with digital communication such as RS485, range parameters, unit settings, damping time, and alarm thresholds should also be checked for consistency with process requirements.

During project procurement and technical confirmation, it is recommended to provide the supplier with complete medium information, design temperature, normal pressure, maximum pressure, differential pressure range, process connection dimensions, and explosion-proof requirements. For media whose corrosion rating cannot be clearly defined, material compatibility confirmation or limited trial use may be conducted first, avoiding selection based solely on experience.

Recommendations for Submitting Selection Information

To improve the accuracy of corrosion-resistant differential pressure sensor selection, it is recommended to clearly identify the media in contact with the positive-pressure side and negative-pressure side respectively, particularly in filtration, absorption, washing, and reaction systems, where the media compositions on both sides may not be completely identical. If the corrosion levels on both sides differ, wetted materials and sealing grades should be determined based on the more severe side.

It should also be specified whether configurations such as intrinsic safety explosion protection, flameproof protection, IP65 or IP67 protection rating, local display, smart communication, square-root function, and remote seals are required. The instrument's process connection, installation space, and maintenance method directly affect the final selection of flange dimensions, pressure-guiding direction, and electrical interfaces.

Xi'an Shenghongchuang Instrument Co., Ltd. can assist in confirming wetted-part solutions such as 316L, Hastelloy, tantalum, and PTFE lining based on the concentration, temperature, pressure, and installation conditions of chemical acid and alkali media, and can match sealing structures including FKM, EPDM, PTFE-encapsulated, or perfluoroether seals to reduce the risk of long-term corrosion failure.

For specific project selection, please compile the medium name and concentration, operating temperature, static pressure and differential pressure range, process connection method, and on-site explosion-proof requirements, and submit them to technical personnel for material compatibility and sealing structure verification before determining a corrosion-resistant differential pressure sensor configuration suitable for on-site operating conditions.