Knowledge
Solutions for Salt Spray Test Failure after Passivation of Stainless Steel Castings
Salt spray testing is a core method for evaluating the passivation effect of stainless steel. By simulating the corrosive environment of marine atmosphere, it verifies the corrosion resistance of the passivation film. However, many enterprises still encounter problems of failing salt spray tests after passivation, manifested as rust spots, white spots or corrosion products on the surface. This article will provide a systematic checklist to analyze the possible causes of test failure step by step, from pre-passivation treatment, passivation process, post-passivation treatment to salt spray test operation, and offer solutions.
I.Pretreatment before Passivation: A Weak Foundation Leads to Collapse
The purpose of pretreatment before passivation is to ensure that the stainless steel surface is clean and free of contamination, providing a good foundation for the formation of the passivation film. If the pretreatment is not thorough, the passivation film will not be evenly distributed, resulting in failure of the salt spray test.
1. Incomplete Oil Removal
- Phenomenon: "Oil spots" appear on the surface after passivation, and these spots are prone to corrosion during the salt spray test.
- Troubleshooting Methods:
- Water film test: Rinse the surface with clean water. If a continuous water film cannot form (contact angle > 10°), it indicates residual oil.
- Infrared spectroscopy analysis: Check for the presence of organic residues (such as grease, cutting fluid additives) on the surface.
2. Incomplete rust removal
- Phenomenon: "Floating rust" appears on the surface after passivation, and rust spots expand during salt spray testing.
- Troubleshooting methods:
- Visual inspection: Use a magnifying glass to observe the surface. If there are unremoved oxide scales or rust products, it indicates insufficient rust removal.
- Blue dot test: Drop potassium ferricyanide solution onto the surface. If blue spots (Fe³⁺ reacts with potassium ferricyanide to form Prussian blue) appear within 30 seconds, it indicates exposed base material.
II.Passivation Process: Parameter Out of Control, Film Quality Not Up to Standard
The core of the passivation process is to control parameters such as the concentration, temperature, and time of the passivation solution to ensure the formation of a dense and uniform passivation film. If the parameters are out of control, the quality of the passivation film will not meet the requirements of the salt spray test.
1. Insufficient Concentration of Passivation Solution
- Phenomenon: The passivation film is thin and uneven, and rust spots appear early in the salt spray test.
- Troubleshooting Methods:
o Titration method: Regularly test the concentration of oxidizing substances in the passivation solution to ensure it meets the process requirements.
o Film thickness measurement: Use an ellipsometer or X-ray photoelectron spectroscopy (XPS) to measure the thickness of the passivation film (typically 5-15 nm).
- Solutions:
o Add passivation agent: Supplement to the specified concentration based on the test results.
o Replace passivation solution: If the passivation solution is aged (such as pH deviation, impurity accumulation), it needs to be completely replaced with a new solution.
2. Inappropriate passivation temperature or time
- Phenomenon: The passivation film is either too thick (prone to cracking) or too thin (insufficient corrosion resistance), which is manifested as local or uniform corrosion in salt spray tests.
- Troubleshooting methods:
- Temperature recording: Use a thermometer or data logger to monitor whether the temperature of the passivation solution is within the process range.
- Time control: Accurately time the passivation period to avoid it being too long or too short.
III.Post-treatment after Passivation: Details Determine Success or Failure
The cleaning, drying, and passivation film strengthening treatment after passivation directly affect the stability and corrosion resistance of the passivation film. If not handled properly, the passivation film may fail prematurely in salt spray tests.
1. Incomplete Cleaning
- Phenomenon: Residual passivation solution causes "white spots" on the surface, which are prone to corrosion first in salt spray tests.
- Troubleshooting Methods:
- pH Paper Test: Wipe the surface with pH paper. If the pH value deviates from neutrality (e.g., pH < 2 after nitric acid passivation), it indicates insufficient cleaning.
- Ion Chromatography Analysis: Check if corrosive ions such as Cl⁻ and NO₃⁻ remain on the surface.
- Solutions:
- Multi-stage Rinsing: Rinse with clean water → Neutralize → Rinse with clean water to ensure the pH value returns to 6-8.
- Ultrasonic Cleaning: For complex-shaped parts, use an ultrasonic cleaner to remove residual passivation solution from dead corners. o
2. Inadequate Drying
- Phenomenon: Residual moisture on the passivation film surface forms "water spots" during salt spray testing, accelerating local corrosion.
- Inspection Methods:
- Visual Inspection: Wipe the surface with a dry paper towel. If the paper towel becomes damp, it indicates insufficient drying.
- Moisture Measurement: Use a Karl Fischer moisture meter to test the surface moisture content (should be <0.1%).
- Solutions:
- Compressed Air Drying: Use oil-free compressed air to blow dry the surface to prevent moisture residue.
- Drying Treatment: For precision components, use an oven (60-80°C) to dry for 10-15 minutes to ensure complete surface dryness.
IV.Salt Spray Test Operations: Standardized Execution to Avoid Misjudgment
The standardization of salt spray test operations directly affects the accuracy of the test results. Improper test operations may lead to false failures (such as insufficient salt spray deposition, temperature deviation, etc.).
1. Insufficient Salt Spray Deposition
- Phenomenon: The degree of surface corrosion after the test is less than expected, but the actual passivation film quality meets the standard.
- Troubleshooting Methods:
- Salt spray deposition measurement: Use a graduated cylinder to collect salt spray and ensure the deposition is 1-2 mL/80 cm²·h (in line with ISO 9227).
- Spray uniformity check: Use test paper or a mirror to observe whether the spray evenly covers the entire test area.
- Solutions:
- Adjust spray pressure: According to the salt spray chamber manual, adjust the spray pressure to the specified range (e.g., 0.07-0.17 MPa).
- Clean the nozzles: Regularly clean the nozzles to prevent salt deposits from clogging and causing uneven spray. o
2. Test Temperature or Humidity Deviation
- Phenomenon: The surface corrosion type after testing does not match expectations (e.g., pitting corrosion instead of uniform corrosion).
- Troubleshooting Methods:
- Temperature Recording: Use a temperature sensor to monitor the temperature inside the test chamber and ensure it meets the standard (e.g., 35 ± 2°C for neutral salt spray testing).
- Humidity Detection: Use a hygrometer to measure the humidity inside the test chamber and ensure the relative humidity is > 95%.
- Solutions:
- Calibrate Equipment: Regularly calibrate the temperature and humidity controllers of the salt spray chamber to ensure accuracy meets requirements.
- Seal the Test Chamber: Check if the sealing strip of the test chamber door is aged and replace it if necessary to prevent external air from entering and affecting the temperature and humidity.
The failure of stainless steel passivation in salt spray tests often stems from a loss of control in one of the following stages: pretreatment, passivation process, post-treatment, or test operation. By using the following systematic checklist, the root cause of the problem can be quickly identified:
1. Pretreatment before passivation:
- Is the oil removal thorough (water film test, infrared spectroscopy)?
- Is the rust removal thorough (visual inspection, blue dot test)?
2. Passivation process:
- Is the concentration of the passivation solution up to standard (titration, film thickness measurement)?
- Are the passivation temperature and time controllable (temperature record, time control)?
3. Post-treatment after passivation:
- Is the cleaning thorough (pH paper, ion chromatography)?
- Is the drying sufficient (visual inspection, moisture determination)?
4. Salt spray test operation:
- Is the salt spray deposition sufficient (collection with a graduated cylinder, spray uniformity)?
- Are the test temperature and humidity within the deviation range (temperature record, humidity detection)?
By systematically addressing and resolving the above issues, the pass rate of salt spray tests for passivated stainless steel can be significantly improved, ensuring that the corrosion resistance of the passivation film meets the requirements. Remember: Salt spray testing is not only a verification method but also a "compass" for optimizing the passivation process.
Vigor team has more than 20 years experience in stainless steel casting production and the robust supply chain of different treatment. If anything we can help or any parts you want to develop, please contact us at info@castings-forging.om
