Knowledge

 

When the salt spray test report shows a red "fail" mark, manufacturing enterprises often face delays in product launches, trust crises with customers, and even recall risks. At this point, scientifically and systematically tracing the root cause is more crucial than simply "reworking and retesting". This article will construct a "five-stage, twenty-step" traceability model, which, through a reverse reasoning chain from results to causes, achieves a penetrating analysis from surface problems to essential roots.

 

Stage 1: Result Analysis - Decoding Corrosion Codes

The failure manifestations of salt spray tests are typically classified into three categories: local corrosion (such as pitting and crevice corrosion), uniform corrosion (overall thinning), and functional failure (such as coating blistering and substrate perforation). The first step is to conduct high-precision 3D imaging analysis of the failed samples - using a laser confocal microscope to capture the morphology of corrosion pits and an energy dispersive spectrometer (EDS) to detect the composition of corrosion products. For instance, if a dendritic distribution of chloride ion-rich areas is found, it suggests possible local concentration differences or electrochemical heterogeneity; if abnormal elements (such as zinc and aluminum impurities) are detected, it is necessary to trace back to the raw material supply chain issues.

 

Stage 2: Process Reversal - Reconstructing the Test Scenario

This stage requires a complete reenactment of the entire test process, with a focus on verifying three key control factors:

1. Solution System Validation: Check whether the pH value, concentration, and ion purity of the salt spray solution meet the standards (e.g., ISO 9227 requires a NaCl concentration of 5±0.5% and a pH value of 6.5-7.2). Use a conductivity meter to test the purity of the solution; if the conductivity abnormally increases, it may indicate contamination by foreign ions.

2. Environmental Parameter Calibration: Verify the temperature uniformity within the salt spray chamber (standard requires ±2°C), the spray deposition rate (1-2 ml/80 cm²·h), and the uniformity of spray particle distribution through an array of temperature and humidity sensors. Pay special attention to whether the condensation water at the top of the chamber causes fluctuations in the solution concentration.

3. Sample Preparation Audit: Check the sample pretreatment process - has it undergone sufficient degreasing, pickling, and neutralization? Is the surface cleanliness up to standard (which can be verified through a water film rupture test)? Are the sample support angles and spacing in line with the standards (typically requiring an angle of 15-30° from the vertical and a spacing of ≥2 cm)?

 

Stage 3: Material Tracing - Dissecting the Failed Matrix

The inherent properties of the material are the core variable influencing corrosion behavior. Multi-scale material analysis is required:

- Base Material Composition Analysis: Use an optical emission spectrometer (OES) to detect whether the chemical composition of the base material complies with design specifications, with particular attention to the control of trace elements (such as sulfur and phosphorus) content.

- Microstructure Inspection: Observe the grain size and distribution of secondary phases through a metallographic microscope and analyze the morphology of precipitated phases using a scanning electron microscope (SEM). For example, improper sensitization treatment of austenitic stainless steel can lead to a sharp increase in intergranular corrosion sensitivity.

- Surface Treatment Evaluation: Conduct nanoindentation hardness tests and film thickness measurements (such as X-ray fluorescence) on coatings and passivation films to assess the adhesion strength between the film and the base material. If holes or cracks are found in the film, trace back to the parameters of the electroplating and passivation processes.

 

Stage 4: Equipment Diagnosis - Eliminating Systematic Errors

The reliability of the salt spray test equipment directly affects the credibility of the test results. A systematic equipment diagnosis is necessary:

- Spray System Inspection: Disassemble the nozzles to check for blockages and use a particle counter to verify the particle size distribution of the spray (the standard requires that 80% of the particles have a diameter of ≤7 μm).

- Control System Verification: Validate the accuracy of the temperature controller, timer, and alarm device through simulation tests. Pay special attention to the humidity control of the compressed air - insufficient dryness can lead to a low salt spray deposition rate.

- Chamber Sealing Test: Use a helium mass spectrometer to test the sealing performance of the chamber to ensure no external air infiltration that could affect the test environment.

 

Stage 5: Comprehensive Modeling - Constructing a Causal Network

After completing the data collection in the first four stages, statistical tools (such as fishbone diagrams and the 5Why analysis method) should be used to construct a causal relationship network. For example, if edge corrosion is found in multiple batches of samples, the "sample support method" and the "solution edge effect" should be correlated; if abnormal elements are detected in the corrosion products, the "pretreatment solution contamination" or "cross-contamination in the workshop environment" pathways should be traced.

Ultimately, the main dependent variable is determined through multivariate regression analysis. For instance, in a case of salt spray test failure of an automotive component, a systematic root cause analysis revealed that the failure was due to the lack of chamfering at the sample edge, which led to stress concentration, and the presence of residual organic matter in the pre-treatment solution, which formed a local battery effect and ultimately accelerated corrosion. The improvement plan formulated based on this includes: optimizing the edge design of the sample, upgrading the filtration system of the pre-treatment solution, and adding an ultrasonic enhancement step to the sample cleaning process.

 

The root cause analysis of salt spray test failure is not merely about "finding faults", but a key link in building a quality prevention system. Through a systematic five-stage root cause analysis model, enterprises can not only solve current problems but also establish a data-driven knowledge base for corrosion failure. The accumulation of such a knowledge base will transform into anti-corrosion genes in product design, an immune system in process control, and a warning radar in quality management, ultimately achieving a leap from "problem repair" to "risk prevention" in quality management. When root cause analysis becomes a norm, failures will become precious samples for technological progress rather than obstacles to development.

 

Vigor has more than 20 years of experience and a professional team in castings and forging, as well as the post of treatment processes. If you have any question or products need to do, please feel free to contact us at info@castings-forging.com