2026-05-08
The selection of wetted part materials—principally the Bourdon tube and socket—is a critical decision that dictates a pressure gauge's service life, accuracy, and safety. While brass offers a cost-effective solution for benign media, 316L stainless steel provides the corrosion resistance necessary for a broad range of industrial processes. Understanding the specific failure mechanisms of each alloy in different chemical environments is essential for proper instrument specification and preventing premature failure, process contamination, or catastrophic leaks. This analysis provides a technical basis for choosing between these common materials and identifies when higher-performance alloys are required.
Wetted parts are any components of the pressure gauge that come into direct contact with the process medium. For a typical Bourdon tube gauge, this includes the socket (process connection) and the Bourdon tube itself. The integrity of these components is paramount, as their failure results in a loss of containment.
Brass, typically alloy C36000 (Free-Cutting Brass) for sockets and C26000 (Cartridge Brass) for tubes, is a copper-zinc alloy. It offers good mechanical properties and machinability at a low cost. Its use is generally restricted to non-corrosive media such as instrument air, fresh water, and hydraulic oils.
316L Stainless Steel (UNS S31603) is an austenitic chromium-nickel stainless steel containing molybdenum. The 'L' designation indicates low carbon content (≤0.03%), which minimizes carbide precipitation during welding, preserving corrosion resistance at the heat-affected zone. This alloy is the default choice for chemical, petrochemical, pharmaceutical, and food processing applications due to its broad chemical resistance. All Manogauge instruments are compliant with relevant international standards, including ASME B40.100 (Pressure Gauges and Gauge Attachments) and EN 837-1 (Pressure gauges - Part 1: Bourdon tube pressure gauges), which dictate requirements for materials, construction, and safety.
Material performance is entirely dependent on the chemical composition, concentration, and temperature of the process fluid. The following outlines general compatibility:
For applications involving chlorides, a quantitative measure of pitting resistance is useful. The Pitting Resistance Equivalent Number (PREN) is an empirical formula used to predict the pitting corrosion resistance of stainless steels.
PREN = %Cr + 3.3 * %Mo + 16 * %N
A higher PREN value indicates greater resistance. For 316L stainless steel, the typical PREN is ~24. This provides adequate resistance for many industrial environments but may be insufficient for seawater or aggressive chemical processing brines, where duplex stainless steels (PREN > 40) are often required. Brass does not have a meaningful PREN value, as its primary failure mechanism in saline solutions is dezincification, not pitting.
Compatibility with Clean-in-Place (CIP) and Steam-in-Place (SIP) protocols is non-negotiable in sanitary industries. These processes use aggressive cleaning agents (e.g., sodium hydroxide, nitric acid, peracetic acid) at elevated temperatures. 316L SS is fully compatible with these standard cleaning regimes, allowing for effective sterilization without material degradation. Brass would be rapidly corroded by CIP chemicals and cannot be used in any application requiring such protocols.
When the process media is too aggressive for 316L stainless steel, higher-specification alloys are necessary. It is often more cost-effective to use these materials in a diaphragm seal, which isolates a standard gauge from the process, rather than constructing the entire gauge from an exotic alloy.