A fog spray test chamber—commonly known as a salt spray chamber—is a controlled lab device designed to evaluate the corrosion resistance of materials and protective coatings. It simulates long-term environmental exposure by using a fine saline mist in a sealed environment, speeding up the corrosion process to assess durability in hours or days instead of years.
Purpose of a Fog Spray Test Chamber
Corrosion Resistance Testing
The most common use is exposing metallic or coated parts (e.g., painted, anodized, or plated surfaces) to a salt fog (typically 5% NaCl solution). This allows manufacturers to observe how materials withstand corrosion over time in a controlled, repeatable setting.
Ingress or Moisture Protection Testing
Some fog spray chambers are configured to test how well enclosures or seals resist moisture penetration. Although not a strict salt spray test, it applies a similar misting approach to simulate humid or wet conditions for components like electronics, gaskets, or sealed assemblies.
Comparative Quality Control
By placing multiple samples inside the chamber simultaneously, manufacturers can perform side-by-side evaluations of different coatings or surface treatments to see which performs better over time.
How Does a Fog Spray Chamber Work?
A salt solution, typically 5% sodium chloride, is atomized into a fine mist using a compressed air nozzle inside a sealed chamber.
The test environment is kept at a regulated temperature (usually 35°C), creating a constant corrosive atmosphere.
Samples are placed at an angle inside the chamber to allow even exposure.
The test duration varies by standard, from 24 to over 1000 hours.
This setup simulates extended environmental stress in a fast-forward format, making it ideal for accelerated corrosion testing.
Common Testing Standards
| Standard | Test Type | Purpose |
|---|---|---|
| ASTM B117 | NSS (Neutral Salt Spray) | General-purpose corrosion testing |
| ASTM G85 | AASS & CASS | Advanced testing with acidified or copper-enriched salt |
| ISO 9227 | NSS/AASS/CASS | International equivalent with strict procedural alignment |
These standards define details like solution composition, temperature, humidity, and test duration, ensuring repeatable and industry-accepted results.
3. Key Chamber Components and Operation
1. Atomizing Nozzle or Fog Generation System
- A specialized nozzle(s) sprays a controlled flow of the salt or water solution into the test space, creating a uniformly dispersed mist.
- The chamber often includes an air compressor or regulated supply of compressed air.
2. Heater and Temperature Controls
- Most salt fog standards require maintaining the internal environment at a steady temperature (commonly around 35°C for neutral salt spray).
- Chambers may also have external specimen heaters or humidity controls for more complex profiles.
3. Solution Reservoir and Collection
- A reservoir holds the solution (e.g., 5% sodium chloride in distilled water for neutral tests).
- To confirm compliance with test standards, the chamber typically includes collection funnels or measuring cylinders to verify spray deposition rates (e.g., 1–2 ml of collected solution per hour per 80 cm²).
4. Racks or Fixtures for Test Samples
- Samples are mounted at a prescribed angle (often ~15–30° from vertical) for consistent exposure.
- Spacing ensures uniform mist coverage and prevents dripping from one specimen onto another.
5. Fog Exhaust System
- An exhaust vent or fan maintains slight positive pressure and expels excess spray to prevent chamber over-pressurization or contamination of the lab environment.
- Some labs use a closed-loop or neutralizing system to handle potentially corrosive effluent.
4. Typical Test Procedures
1. Solution Preparation
- For neutral salt spray, dissolve 5% NaCl in deionized or distilled water and ensure the pH is in the specified range (often 6.5 to 7.2).
- Adjust for special tests by adding acetic acid (AASS) or copper chloride (CASS).
2. Pre-Test Sample Inspection
- Clean and label each sample; note any surface treatments or coatings.
- Avoid fingerprints or contaminants that might alter test results.
3. Chamber Loading
- Place samples on racks inside the chamber at the required orientation.
- Ensure no direct contact between samples.
4. Start-Up and Stabilization
- Heat the chamber to the specified setpoint (e.g., 35°C for NSS).
- Begin fog generation, verifying the spray density and uniform distribution.
5. Ongoing Monitoring
- Check solution levels and pH daily (or at prescribed intervals).
- Confirm spray deposition rates by measuring solution collected in funnel/cylinders.
6. Test Duration
- May range from hours to thousands of hours, depending on standard requirements and the desired level of corrosion stress (e.g., 24 hours, 96 hours, 500 hours, or more).
7. Inspection and Documentation
- Periodically pause or open the chamber (if the standard allows) to inspect sample surfaces.
- At test completion, document visible corrosion, blistering, or coating breakdown.
- Some standards or specifications define rating scales for severity (e.g., ASTM D1654 for paints).
5. Test Result Evaluation
1. Visual Ratings
- Often the first step is a simple appearance check: rust, pitting, or blister formation on paint or plating.
- Standards like ASTM D1654 define how to measure corrosion creep from a scribed line or other specific metrics.
2. Mass Loss / Thickness Reduction
- In some cases, samples are weighed before and after testing to quantify total mass loss or measure thickness loss (for metals).
3. Adhesion or Mechanical Testing
- After the salt spray exposure, some protocols require an adhesion test (e.g., cross-hatch tape test on painted surfaces).
4. Functional Checks
- For parts that must remain operational (e.g., electronics), a functional test can verify if corrosion degraded performance.
Applications Across Industries
Automotive & Aerospace – Fasteners, panels, undercarriage coatings
Electronics – Moisture seal validation
Construction – Railings, fixtures, bolts
Marine & Offshore – Corrosion resistance in salt-rich environments
6. Limitations and Considerations
1. Accelerated but Not Always Perfectly Realistic
- Salt spray or fog tests greatly accelerate corrosion, but real-world environments involve cycles of wet/dry, UV exposure, and temperature fluctuations. Thus, additional cyclic corrosion testing may be needed.
2. Test Reproducibility
- Maintaining consistent solution chemistry, temperature, and spray density is crucial to producing comparable, repeatable results.
3. Interpretation Variability
- Results can vary significantly based on sample preparation, orientation, or small chamber differences. Many companies develop in-house correlations between salt spray hours and expected real-life service.
4. Safety and Handling
- Salt fog is corrosive to lab equipment and can irritate skin/eyes. Proper ventilation, PPE, and regular chamber maintenance are important.