1.What are the methods of direct/quantitative analysis?
X-ray photoelectron spectroscopy (XPS) Principle: X-rays are used to irradiate a sample, and the energy of electrons emitted from the surface is analyzed to determine the chemical state and content of surface elements.
Auger electron spectroscopy (AES) Principle: Using an electron beam to excite and analyze Auger electrons to characterize the elemental distribution of an extremely thin surface layer.
Elliptic polarization spectroscopy
Principle: By measuring the change in polarization state of polarized light after reflection from the sample surface, the thickness and optical constants of the thin film can be inferred.
2.What are some indirect/qualitative/performance testing methods?
Copper sulfate titration test (most classic and commonly used)
Principle: Utilizes the principle of a displacement reaction between copper ions in solution and active iron (not passivated chromium). A copper sulfate-sulfuric acid solution of a specific concentration is dropped onto the sample surface, and the time it takes for a red copper plating spot to appear is observed.
Salt spray test
Principle: The sample is placed in a salt spray chamber to simulate a harsh corrosive environment, and the time it takes for rust to appear is observed.
Potassium ferricyanide-nitric acid test (blue spot test)
Principle: Detects surface contamination with free iron or iron ions. Potassium ferricyanide in solution reacts with iron ions to form a blue precipitate.
Electrochemical Testing
Principle: Measuring the open-circuit potential, potentiodynamic polarization curve, or electrochemical impedance spectroscopy of a material in an electrolytic cell.
3.What detection methods are available for specific passivation types?
X-ray fluorescence spectrometry (XRF): This method allows for non-destructive and rapid measurement of chromium content per unit area, thereby indirectly controlling film thickness. It is the most commonly used method for online control in continuous production lines.
Spectrophotometry: This method determines the chromium content in the passivation film by measuring the absorbance of the solution after a specific colorimetric reaction.
4.What is the preferred method for daily quality control of passivation films on cold-rolled stainless steel coils?
The preferred method is the "copper sulfate titration test". It is simple to operate, conforms to international standards, and can effectively determine whether the passivation film is qualified.
5.What is the core conclusion?
When inspecting the passivation film on cold-rolled coils, the industry rarely measures its physical thickness directly. Instead, it comprehensively assesses its composition, integrity, and corrosion resistance to determine whether its quality meets standards. You should choose an appropriate combination of methods based on your testing objectives (quality control, process research, problem analysis) and available resources.