How Q-FOG Testers Meet Fog Collection Requirements?

Achieving consistent results in cyclic corrosion testing can be a challenge, especially when trying to meet the fog collection requirements of standards such as ASTM B117 and ISO 9227. While Q-FOG testers are engineered to comply with standards requirements, it can still at times be challenging to simultaneously meet all fog specifications. Understanding the physics behind fog generation and collection in Q-FOG testers can help understand how to obtain proper collection volume, concentration, uniformity, and acidity.

Fog Uniformity and Volume

Q-FOG testers are designed to give users high levels of control over the physical distribution of the salt fog, both the total amount and the uniformity of distribution.

• Fog Uniformity: Through precise air pressure regulation, Q-FOG SSP, CCT, and CRH testers produce even salt fog distribution throughout the chamber. At Q-Lab's recommended air pressure levels (which can vary depending on chamber size and cycle requirements), collection rates at the center and corners of the chamber are nearly identical.
• Collection Volume: The overall volume of collected solution is directly tied to the solution pump speed. Q-Lab's factory default of 30% is recommended to consistently hit the target ranges required by ASTM B117 and ISO 9227, though other standards may require an adjustment to achieve them.

Navigating the Chemistry: Salt Concentration and pH

While Q-FOG testers excel at delivering proper fog quantities and distribution, the chemical composition of the collected solution is often influenced by external factors.

• Salt Concentration: Evaporation within the chamber can cause NaCl concentrations to rise above the required 4–6% referenced in ASTM B117 and ISO 9227. While the bubble tower can help by moisturizing compressed air, users can also offset concentration spikes by mixing their initial solution at a lower concentration (down to 4% in ASTM B117 or 4.5% in ISO 9227) or by insulating the chamber lid in cold environments.
• pH Management: Q-FOG testers do not adjust nor control pH. Deionized water naturally absorbs CO₂, forming carbonic acid. This can make the collected solution more acidic (lower pH), droping pH levels below the acceptable 6.5–7.2 range referenced in ASTM B117. We recommend using freshly-deionized water stored in closed containers or using  CO₂-absorbing filters to maintain pH stability.

Q-PANEL's Corrosion Coupons

Although fog collections are a required verification in many standards, the true measure of a cyclic corrosion test apparatus is how effectively and consistently it generates corrosive chamber conditions. The ultimate way to validate your tester's performance is by measuring actual corrosion rates. While not always mandatory, using mass loss coupons like Q-Lab's CX series every month or two provides a definitive check on your testing environment.

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Frequently Asked Questions

Q: What is the recommended pump speed for ASTM B117 in a Q-FOG 1100 L chamber?

A: We generally recommend a pump speed of ~30% to ensure the average collection volume meets the 1–2 mL requirement.

Q: How does fog pressure affect collection uniformity?

A: Pressure has a significant effect on collection rates near the nozzle (center) but minimal effect on the corners. Operating at manufacturer-recommended pressure helps synchronize these locations for better uniformity.

Q: Why is my collected salt concentration too high?

A: High concentrations are usually caused by evaporation. This is common in cold labs where moisture condenses on the lid. You can mitigate this by increasing bubble tower temperature, insulating the lid, or starting with a lower salt concentration.

Q: Can the Q-FOG automatically adjust the pH of the salt fog?

A: No. The Q-FOG has no internal mechanism to change the solution acidity. pH must be controlled during the solution preparation and storage stages.

Q: Should I use corrosion coupons if my standard doesn't require them?

A: Yes. Even if not required by your test protocol, mass-loss coupons are the most effective way to verify that your tester is delivering the correct corrosive conditions.