When testing the salt spray test chamber, large rust spots always appear on the surface of the samples, but when the same batch of samples is sent to an external laboratory for testing, it is considered qualified. Is there a problem with your equipment?

1. Analysis of the Cause of the Problem

The situation you encounter—"Severe corrosion in your own box, sent to the lab as qualified"—in most cases, it's not a test chamber fault, but rather differences caused by sample cleaning, placement, and determination methods, or uncontrolled pH and sedimentation of the salt solution during testing. Of course, it cannot be ruled out that there are indeed deviations in the atomization system and temperature control of the test chamber.

The three most easily overlooked key points:

1. Sample pretreatment: Before placing the sample into the test chamber, is it thoroughly degreasing with anhydrous ethanol or petroleum ether? Fingerprints left by direct touch on the sample surface with fingers will cause corrosion in the salt spray environment, forming large rust spots.

2. Specimen placement angle: The standard requires the test surface to be at a 15°-30° angle to the vertical direction. If placed horizontally or at an excessively large angle, salt spray droplets can accumulate on the surface and form a buildup, leading to accelerated local corrosion.

3. Salt solution pH value: The pH value of newly prepared 5% sodium chloride solution should be between 6.5 and 7.2. If long-term storage of salt solutions is used, the pH may drop (become acidic) due to absorption of carbon dioxide from the air, and corrosiveness increases sharply.

2. Detailed Inspection and Operational Methods

Step 1: Conduct equipment self-inspection—verify with standard test pieces

Purchase or manufacture standard reference specimens that comply with GB/T 10125 (such as carbon steel specimens with known corrosion rates). Two sets of tests are conducted simultaneously: one in your own equipment, and the other in a known qualified laboratory (or using a portable pH meter and salt spray deposition measurement tube for self-testing). Compare the corrosion levels and weightlessness per unit area of the two sets of test pieces.

Operating method: After weighing the standard test piece (accuracy 0.1mg), place it into the equipment and run for 24 hours. After removal, wash and dry according to standard methods, then weigh again. Calculated corrosion loss weight. Qualification range: 24-hour weightlessness between 20-50g/m². If your equipment's weight loss far exceeds this value, it indicates that the corrosion conditions for the equipment are excessively harsh.

Step 2: Measure and adjust the settlement amount and pH value

1. Salt spray sedimentation amount: At least two standard collection funnels (100mm diameter) are placed in the test chamber to collect the amount of salt spray liquid collected for 24 hours. Standard requirement: per 80cm² collection area, sedimentation rate of 1-2mL per hour (i.e., 24-48mL collected over 24 hours). If the collection volume is too large (for example, >60mL), it indicates excessive spray volume, and the pressure valve of the spray tower should be adjusted or the nozzle replaced; If it is too small, the nozzle clog or pressure increase should be cleared.

2. pH measurement: The collected salt solution is immediately measured with a pH meter (accuracy 0.1). If the pH is below 6.5, it indicates the solution is somewhat acidic. Possible reasons include: a) The pure water used for preparation contains CO₂—it needs to be boiled and cooled before preparation; b) Compressed air contains acidic substances—install air purification filters (activated carbon + molecular sieves) on the air supply pipes. Adjustment method: Slowly adjust the pH to 6.8-7.0 using 0.1 mol/L NaOH or NaHCO₃ solution.

Step 3: Standardize sample preparation and placement

• Cleaning: Before the test, wipe the sample surface with absorbent cotton dipped in anhydrous ethanol, then rinse with distilled water and blow dry. Wear clean rubber gloves at all times and do not touch the test surface with bare hands.

• Placement: Use a clean glass rod or plastic stand to support the specimen, with the test facing upward at a 20° angle to the vertical direction. Specimens must not touch each other, nor touch the chamber wall. Ensure that salt spray can freely settle on all test surfaces, and that droplets do not drip from the upper sample onto the lower one.

• Avoid liquid accumulation: If the sample has grooves or blind holes, place them face down to prevent salt solution buildup.

Step 4: Check the uniformity of the cabinet's temperature

Place five thermometers in different positions inside the chamber (upper, middle, lower, left, right). After stabilizing for 30 minutes, read the temperature at each point. The standard requires 35°C ± 1°C. If the temperature difference exceeds 2°C, it may indicate aging heating tubes or poor fan circulation. Contact the manufacturer for repairs.

3. Arbitration methods when results are inconsistent with third-party laboratories

If after confirming all the above steps are correct, your device's results still differ significantly from those in the third-party lab, perform a "cross-test": immediately send the sample being tested in your device along with the collected salt solution (at maintaining temperature) to the third-party lab and place it in their test chamber for the remaining 24 hours, or take the third-party lab's sample to your device for testing. This eliminates batch differences in samples and allows direct comparison of the corrosion severity of the two devices.