Salt-spray corrosion testing is the primary accelerated method for validating the corrosion resistance of materials and their protective coatings. Whether a chamber can continuously generate neutral (NSS), acetic-acid (AASS) or copper-accelerated acetic-acid (CASS) salt fog for 48 h–1 000 h determines the repeatability and reproducibility of the test. The brine supply system—functioning as the “liquid source”—must guarantee stable pressure, temperature, concentration and flow. Its engineering quality directly governs whether the chamber can pass third-party calibrations such as CNAS or NADCAP. This paper systematically describes the working principles, key parameters, material selection, control logic and typical failure modes of the system, and is intended for equipment manufacturers, end-users and metrology institutes.
System Positioning and Functional Breakdown

2.1 Positioning
In the air–liquid coupled architecture of a salt-spray chamber the brine supply system is located at the very front end of the liquid circuit, delivering clean salt solution at 35 °C ± 2 °C, 5 % ± 1 % (m/m) NaCl and 1.0–2.0 L h?1 (for a 0.8 m3 chamber). Its output stability shall meet:
a) 24 h concentration drift ≤ ±0.2 %;
b) 24 h temperature drift ≤ ±1 °C;
c) Level-induced spray drift ≤ ±5 %.
2.2 Functions
① Preparation: automatic or manual blending of NaCl with ASTM D1193 Type IV water to 5 % (m/m), followed by 5 μm filtration and 10 min de-aeration at ≤ –0.07 MPa.
② Storage: ≥ 72 h storage at 20–25 °C without crystallisation or microbial growth.
③ Delivery: magnetically coupled or diaphragm pump supplies brine at 0.05–0.15 MPa to the header tank atop the spray tower.
④ Levelling: overflow–siphon principle maintains the siphon head between 200 mm and 500 mm with ≤ ±5 mm error.
⑤ Temperature control: PID-controlled heater jacket or plate heat exchanger keeps brine within 2 °C of the test temperature (35 °C).
⑥ Return: non-atomised brine is collected, dual-filtered and returned to the storage tank, giving ≥ 80 % recycle.
⑦ Rinse: after testing RO water is automatically injected for 15 min to prevent salt blockage.
System Configuration and Material Selection
3.1 P&ID
Storage tank → pre-filter → magnetic pump → header tank → level sensor → temperature module → siphon overflow assembly → nozzle dip tube → return filter → storage tank.
3.2 Key Components
Storage tank
Volume: 600 L (safety factor 1.5 on 2 L h?1 × 168 h continuous spray per GB/T 10125).
Material: 10 mm PVC/PP laminate, 2 mm 304 stainless steel frame, 30 mm PU foam insulation (k ≤ 0.5 W m?2 K?1).
Fittings: redundant low-level alarms (float + capacitance), CIP ball valve, 350 mm man-way, 0.2 μm PTFE vent filter.
Magnetic pump
Type: seal-less magnetic-drive centrifugal, eliminating seal wear debris.
Wetted parts: ETFE or PFA, resistant to 5 % NaCl and pH 3.1–3.3 AASS.
Rating: 4 L min?1, 15 m head, VFD controlled via 4–20 mA.
Header tank
Volume: 5 L vertical cylinder; overflow weir set at 300 mm ± 2 mm (DN15).
Material: transparent PVC with 3 M heating film (200 W) and Class-A PT100.
Level sensor
Type: corrosion-proof hydrostatic, 0–600 mmH?O, 4–20 mA, ±0.25 %FS.
Diaphragm: Hastelloy C276 flange-mount, long-term resistance to chloride pitting.
Temperature module
Heating: external plate heat exchanger, 35 °C water circulation, 0.2 m2 area; brine-side ΔT ≤ 1 °C at 2 L min?1.
Control: PID, 100 ms scan, ±0.5 °C accuracy.
Siphon-overflow assembly
Construction: 8 mm glass dip tube 25 mm above tank bottom; overflow weir height adjustable 200–500 mm via precision thread (±1 mm).
Principle: 0.7–1.2 bar air creates 15–25 kPa vacuum, drawing brine up the dip tube; overflow maintains constant head.
Control Logic and Software Interface
4.1 Architecture
Three-level: PLC + HMI + SCADA.
① PLC: Siemens S7-1200 for real-time loops of level, temperature and flow;
② HMI: KTP700 for graphics, alarms and manual mode;
③ SCADA: WinCC, Ethernet-linked to plant MES, 10-year data retention, FDA 21 CFR Part 11 compliant.
4.2 Sequence
Power-on → level self-test → pump start → fill header to 300 mm → heat to 35 °C → overflow stable 2 min → spray enable → spray running → return filtration every 30 min → end of test → 15 min RO rinse → 5 min air purge → stop.
4.3 Alarms
Low-low level: 5 mm hysteresis, 3 s delay – pump stop + beacon;
Temperature: > 37 °C or < 33 °C, 30 s delay – heater off + alarm; Pump overload: > 1.2 × I rated – instant stop;
Filter ΔP: > 0.05 MPa – replace-filter warning.
Performance Verification & Metrology
5.1 Acceptance Criteria (JJF(軍工) 156-2019)
a) Fog collection: 1.0–2.0 mL (80 cm2 h)?1, centre vs. corners ≤ ±10 %;
b) Temperature: 35 °C ± 2 °C, spatial gradient ≤ 1 °C;
c) pH: 6.5–7.2 (NSS), collected vs. sprayed ΔpH ≤ 0.2;
d) Concentration: collected NaCl 5 % ± 1 % (m/m).
5.2 Calibration by CNAS-accredited body
① Level: digital level gauge, overflow-to-dip-tube height ≤ ±2 mm;
② Temperature: six-point Pt100 matrix, AMS 2750 calibrated, U ≤ 0.1 °C (k = 2);
③ Flow: electromagnetic meter (0.5 class), 10 min sample, ≤ ±3 % deviation;
④ Concentration: argentometric titration (GB/T 1266), triplicate RSD ≤ 0.5 %.
Failure Modes & Maintenance Strategy
6.1 Typical failures
a) Crystallisation: inadequate post-test rinsing causes NaCl to precipitate in 0.8 mm dip tube → spray rate drops > 30 %;
b) Dry-fire heater: level sensor fault, film heater burns through, PVC tank warps;
c) Pump cavitation: clogged filter, inlet < –0.05 MPa, impeller pitting; d) Siphon-height drift: chamber relocation tilts overflow weir → collection rate out of spec. 6.2 Maintenance Daily: immediate 15 min RO rinse after test, visual nozzle leak check; Weekly: remove dip tube, 10 % citric acid 30 min soak + 10 min ultrasonic; Monthly: zero-level calibration, replace 5 μm cartridge, check heater insulation ≥ 50 MΩ; Annually: third-party metrology, certificate archived. Although the brine supply system accounts for only 8–10 % of total chamber cost, it drives > 80 % of test failures. An integrated design—corrosion-resistant materials, steady pressure & level, temperature loop, closed-loop control—can raise the salt-fall Cpk to ≥ 1.67, meeting the stringent repeatability demands of aerospace, automotive and rail industries. Future papers will address spray-tower aerodynamics, saturated-tower temperature chains and salt-fog collection calibrators—stay tuned.