ASTM B117 — the Standard Practice for Operating Salt Spray (Fog) Apparatus — has been the dominant corrosion test standard for over a century. First published in 1939, it specifies the apparatus, test solution, operating conditions, and procedures for neutral salt spray (NSS) testing of metallic and non-metallic coatings. Despite the rise of more realistic cyclic corrosion tests like ASTM G85 and GM9540P, B117 remains the most-cited salt spray test in specifications across automotive, fastener, coating, and electronics industries.

A salt spray test machine for ASTM B117 compliance is a controlled environment chamber that atomizes a 5% sodium chloride solution into a fine mist, maintains chamber temperature at 35°C ± 1°C, and continuously exposes test specimens to this corrosive fog. The standard does not specify pass/fail criteria — it specifies how to run the test. Pass/fail criteria come from the product specification (e.g., 240 hours with no red rust = ASTM B117 + spec).

Chamber & Air System

Solution & Specimen

Phase 1 — Solution Preparation

Dissolve 5 parts by mass of sodium chloride (ACS reagent grade or equivalent) in 95 parts of distilled or deionized water. The pH of the collected spray (not the solution itself) must be 6.5–7.2. If pH is outside range, adjust with dilute HCl or NaOH. Prepare fresh solution for each test cycle; do not reuse solution beyond 7 days.

Phase 2 — Specimen Preparation & Mounting

Clean specimens per the coating manufacturer's specification or ASTM B117 Section 6. Record initial weight, dimensions, and baseline photographs. Mount specimens on the slotted racks at 15–20° from vertical. Specimens must not contact each other or the chamber walls. Use inert plastic or coated metal supports.

Phase 3 — Chamber Conditioning

Bring the chamber to 35°C ± 1°C and verify spray rate by placing two collection cylinders (10 cm² area each) at any position in the working zone. The collected spray must measure 1.0–2.0 mL per hour per 80 cm² collection area. Adjust atomizer pressure and nozzle position until rate is within spec.

Phase 4 — Continuous Exposure

Maintain chamber temperature at 35°C ± 1°C continuously. The fog is continuous (24/7) for the test duration. Replenish solution tank as needed. Daily checks: chamber temperature, atomizer operation, solution level, pH check on collected spray. Specimens are not removed during the test unless required for intermediate evaluation.

Phase 5 — Final Evaluation

At test completion, remove specimens, rinse gently in deionized water to remove salt deposits, and dry. Evaluate per the product specification — typical criteria include: (1) time to first red rust, (2) percentage of surface area corroded per ASTM D610, (3) scribe creep from a cross-cut per ASTM D1654, or (4) weight loss per ASTM B117 Section 10.

Decision 1 — Chamber Volume

Match chamber volume to your largest specimen. A 270 L chamber fits panels up to 150×300 mm and is the lab workhorse. A 480 L chamber is the most common industrial size. 110 L chambers are entry-level for fastener QC. 800–2000 L chambers are for large automotive body panels and aerospace components.

Decision 2 — Chamber Material

PP (polypropylene) and PVC are the most common. FRP (fiberglass-reinforced plastic) is more durable for continuous operation. Stainless steel is unsuitable (corrodes in salt spray). For ASTM B117 compliance, the chamber material does not affect test results as long as it is corrosion-resistant and does not contaminate the spray.

Decision 3 — Atomizer Quality

The atomizer is the heart of the chamber. Look for: (1) 0.7mm nozzle diameter, (2) borosilicate glass saturator tower to humidify and warm the air, (3) two atomizers for redundancy, (4) adjustable atomizer pressure gauge. Cheap atomizers produce inconsistent droplet size and clog quickly — this is the #1 reason for failed B117 tests.

Decision 4 — Solution Tank & Plumbing

40 L external solution tank is standard. Confirm the tank has a level sensor (alarms when low) and is plumbed to the atomizer via corrosion-resistant tubing (PTFE or PP). For ASTM B117, the solution should be replaced every 7 days maximum, even if the tank is not empty.

Decision 5 — Data Logging

ASTM B117 §11 requires continuous temperature recording. A chamber with built-in data logging (chamber temp, pH, spray rate) creates a defensible audit trail. For ISO 17025 accredited labs, look for calibration certificates traceable to NIST or NIST-equivalent.

Construction Quality

Look for a chamber with a 5–10 mm PP or FRP wall thickness, a sloped lid (so condensation drips back into the chamber rather than onto specimens), a pneumatic lid lift (reduces operator strain), and an observation window with a heating element to prevent fogging. Chamber interior should be smooth and free of crevices that can trap salt deposits and contaminate future tests.

Routine Maintenance

Atomizer nozzles should be inspected weekly for clogging — clean with ultrasonic bath or replace. The saturator tower should be drained and refilled monthly. The solution tank should be drained and refilled with fresh solution every 7 days. The chamber interior should be rinsed and dried between test runs. With proper maintenance, a Derui salt spray chamber runs 3000+ hours continuously without issues.

Safety Considerations

Salt spray is highly corrosive to surrounding equipment. The chamber should be installed in a well-ventilated area with exhaust to the building exterior (not recirculated). The exhaust air contains 5% NaCl droplets that will damage electronics, metal furniture, and HVAC ductwork. Operators should wear safety glasses, nitrile gloves, and lab coats when handling salt solution or removing specimens. Solution disposal must comply with local wastewater regulations (NaCl is not hazardous, but the volume warrants drain permits in most jurisdictions).

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