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HaiGang Yunyi
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Tianjin Haigang Steel Coil
Service first, quality assured. More dedicated and focused on crafting even better boards.
07
2021
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02
Analysis of the Current Status of Application of Hot-Dip Galvanizing Chromium-Free Passivation Technology
Chromium-free passivation agent technical types
At this stage, commonly used chromium-free passivators can be broadly categorized into three types based on their composition: inorganic passivators, organic chemical passivators, and composite passivators combining both inorganic and organic components. Inorganic compound passivation films exhibit excellent temperature resistance but slightly weaker corrosion resistance; organic compound passivation films offer good corrosion resistance but somewhat lower temperature resistance; while composite passivators that combine inorganic and organic components leverage the advantages of both.
Chromium-Free Passivation Agent Processing Technology Standard
Passivation time: The passivation time is related to the thickness of the passivation film. Therefore, the duration of the passivation process directly affects the corrosion resistance of the passivation film. If the passivation time is too short, the passivation film will be thin and have poor corrosion resistance; if the passivation time is too long, the passivation film will become excessively thick and loose, resulting in poor physical properties and reduced corrosion resistance.
Passivation temperature: The passivation temperature is correlated with the demulsification rate of the passivation film. When the passivation time is fixed, the thickness of the passivation film is also relevant, and it can adversely affect the corrosion resistance of the passivation film. Under otherwise identical conditions, different passivation temperatures are selected for passivation, and the corrosion resistance of the passivation films is compared using the sodium thiosulfate drop-test method. Ultimately, the temperature range required to form a passivation film with excellent corrosion resistance can be determined.
Passivation solution pH value: The pH value of the passivation solution is related to the corrosion resistance of the passivation film. Under the condition that other parameters remain unchanged, different pH values were selected for passivation, and the corrosion resistance of the passivation films was compared using a dropwise test with sodium thiosulfate. Ultimately, the pH range required to form a passivation film with excellent corrosion resistance was determined.
Chromium-Free Passivation Agent Property Testing
Appearance inspection The surface morphology of the passivation film was characterized using a scanning electron microscope (SEM). The SEM examination primarily detects secondary electrons—numerous electrons emitted from the sample surface after being excited by the ion beam's negative electrons—which provide a three-dimensional view of the sample’s surface topography.
Sodium thiosulfate drop-by-drop test The electrode potential of copper is higher than that of zinc; therefore, zinc can displace copper from CuSO4. When a passivation film is present, it physically blocks the reaction between CuSO4 and the underlying zinc substrate, thereby prolonging the time it takes for the sample surface to turn black. The duration of this blackening process serves as an indicator of the passivation film’s corrosion resistance. At room temperature, use a burette to drop a single drop of 3% CuSO4 aqueous solution onto the sample surface, observe the changes in the surface color, and record the time it takes for the surface to turn black using a timer. For each sample, the blackening time is reported as the average value obtained from five measurements taken at different locations on the surface.
Sodium Chloride Solution Immersion Test At room temperature, in a solution containing chloride ions, zinc ions will react with the chloride ions to form a compound, thereby accelerating the corrosion of zinc. By immersing the test specimen in a 5% NaCl aqueous solution (with a pH around 7), observe the changes on the specimen’s surface and record the time at which white spots first appear on the surface. This method provides a consistent assessment of the specimen’s corrosion resistance. The results obtained from this method are consistent with those from salt-spray tests.
Photoelectrocatalytic detection Photoelectrochemical measurements were performed on samples treated with chromium-free passivation, blank hot-dip galvanized samples, and samples treated with chromic oxide passivation. All photoelectrochemical measurements were conducted using a three-electrode system, with the test sample serving as the working electrode, a platinum sheet as the auxiliary electrode, and a saturated calomel electrode (SCE) as the reference electrode. The effective detection area was 25 cm², and the electrolyte used for the measurements was a 3.5% NaCl solution at room temperature with a pH of 7.
Polarization curve The potentiostatic scanning method is a precise technique for measuring polarization curves in strongly polarized regions under steady-state conditions. In this method, the polarization current serves as the primary independent variable, and the corresponding electrode potential is measured by varying the applied current. By accurately measuring the polarization curves of passivation films and hot-dip galvanized coatings in different systems, we can investigate their kinetic characteristics within the measured potential range and highlight the differences from their respective passivation curves.
Challenges in the Marketing and Promotion of Chromium-Free Passivators
At this stage, marketing and promotion of chromium-free passivators face certain difficulties. Many companies lack sufficient understanding of chromium-free passivation, and there are two key reasons for this:
First, the company lacks sufficient understanding of the processing technology for chromium-free passivators. The chromic acid passivation process has been widely adopted and is now commonplace. This long-standing, widespread familiarity has led to a kind of ingrained mindset, causing people to remain skeptical toward new technologies, new processes, and emerging innovations. Consequently, whenever problems arise, the first thing that comes to mind is often a flaw in the chromium-free passivation product itself—rather than considering potential issues with their own operational practices or process controls.
Second, there is insufficient awareness among people about the adverse effects of chromic acid environmental pollution. Right now, air pollution in many regions has become quite severe. Companies must feel a sense of mission and responsibility, and actively embrace the use of chromium-free passivation—rather than passively waiting for administrative orders from government authorities. If all companies proactively address their own environmental pollution issues, the time required for overall environmental improvement will be significantly shortened, and clear blue skies, lush green meadows, and fresh air will soon become part of our everyday living environment.
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