1.What is the typical range of thicknesses (expressed in mass)?
Standard Corrosion Passivation:
Range: Approximately 5 - 30 mg/m² (based on total chromium)
Conversion: This equates to a very thin coating, approximately on the order of a few nanometers to tens of nanometers. Films in this range provide good inter-process rust protection with minimal impact on subsequent coatings.
High Corrosion Passivation (e.g., for galvanized sheets, especially when no oiling is required):
Range: Approximately 30 - 100 mg/m² (based on total chromium), or even higher.
Applications: Requires high salt spray test durations (e.g., over 72 hours).
Fingerprint Resistant Passivation:
This involves adding an organic coating (anti-fingerprint agent) to the passivation film. Its film thickness typically comprises two parts:
Underlying chromate conversion film: Approximately 10 - 30 mg/m²
Top layer organic film: Approximately 0.5 - 2 g/m²
The total film thickness increases significantly, but is primarily composed of organic components.
2.What is the difference between "available chromium" and "total chromium"?
The corrosion resistance of a passivation film primarily depends on the content of hexavalent chromium (Cr6+) in the film layer, which possesses excellent "self-healing" capabilities.
The most commonly measured content in testing is the total chromium content (including trivalent chromium Cr3+ and hexavalent chromium Cr6+). Trivalent chromium forms the framework, while hexavalent chromium provides corrosion protection. The ratio of these two components is crucial to performance.
3.What are the measurement methods?
X-ray fluorescence spectrometry: The most commonly used, non-destructive, and rapid online or laboratory method. It directly measures the chromium content per unit area (mg/m²).
Chemical titration: A traditional laboratory method, more cumbersome, but yields accurate results.
Electrolyte thickness gauge: Can be used to measure non-conductive films on conductive substrates, but is more commonly used to measure phosphating films or thicker coatings.
4.What process parameters affect film thickness?
Passivation solution concentration: Higher concentration results in faster and thicker film formation.
Processing temperature: Increased temperature accelerates the reaction and increases film thickness.
Processing time (strip speed): Longer residence time in the passivation tank results in a thicker film.
Passivation method: Roll coating typically yields a more uniform and controllable film thickness than spray coating.
Drying conditions: Insufficient drying leads to an incomplete film layer; over-drying may cause hexavalent chromium reduction failure.
5.What are the differences between environmentally friendly chromium-free passivation?
With increasingly stringent environmental protection requirements, chromium-free passivation (using zirconium salts, titanium salts, silanes, organic resins, etc.) is being used more and more frequently.
The concept of "thickness" for chromium-free passivation films differs from that of chromates. It is usually expressed as the coating amount per unit area (g/m² or mg/m²). While its numerical range may be similar to that of chromates, its corrosion protection mechanism is completely different (primarily a physical barrier effect, with no self-healing capability).