Intaglio printing plate electroplating process - Chrome plating: Plating process flow, basic principles of chrome plating!
Release Time:
Sep 08,2017
1. Chrome Plating Process
Engrave the roller → Inspection (Qualified) → Assembly → Roller Cleaning → Chrome Plating → Polishing → Self-Inspection (Qualified) → Handover for Final Inspection (Unqualified, return for re-chroming).
2.Basic Principles of Chrome Plating
Chrome Plating SolutionChromic acid generally exists in the form of dichromate (H2Cr2O7), in high concentration chrome plating solutions, it can exist in the form of trichromate (H2Cr3O10) and tetravalent chromate (H2Cr4O13) as well. When the plating solution contains only chromate without catalysts like sulfuric acid, passing direct current results in only hydrogen gas being released at the cathode, with no chrome layer deposited, equivalent to electrolysis of water. After adding an appropriate sulfuric acid catalyst (CrO3:H2SO4=100:1)
1) The following reactions occur sequentially at the cathode:
Cr2O72-+ 8H++ 6e → Cr2O3+ 4H2O ①
2H++ 2e → H2↑ ②
Cr2O72-+ H2O → 2CrO42-+ 2H+③
CrO42-+ 8H++ 6e → Cr↓ + 4H2O ④
From the above reactions, it can be seen that the cathodic reaction of chrome plating is quite complex. Now, using the theory of colloidal membranes and the polarization curve of chrome plating, we briefly describe the mechanism of chrome plating. At the beginning of electrification, the first reaction that occurs is the reduction of hexavalent chromium to trivalent chromium (reaction formula ①), as shown in segment ab of the polarization curve in Figure 1. As the potential shifts negatively, the current density increases dramatically, and the speed of reaction ① producing trivalent chromium is very fast, with the potential shifting to point b, where the current reaches its maximum value. After point b, the potential reaches the point of hydrogen ion precipitation, thus reactions ① and ② occur simultaneously. Looking at segment bcd of the polarization curve, as the potential shifts negatively, the current density gradually decreases, indicating that the state of the electrode surface has changed. Because reactions ① and ② consume a large amount of hydrogen ions, the pH value at the electrode interface increases, forming a layer of basic chromic acid colloidal membrane (Cr(OH)3·Cr(OH)CrO4), covering the electrode surface, increasing resistance, thus decreasing current density. The increase in pH near the cathode surface creates conditions for Cr2O72-ions to convert to CrO42-ions, leading to reaction ③ proceeding to the right, and the concentration of CrO42-rapidly increases. When the potential shifts to point d, the corresponding potential ψ at this point is the reduction precipitation potential of chromium ions, and reaction ④ begins. Segment de is the true polarization curve of chrome plating, where reactions ①, ②, ③, and ④ occur simultaneously, and as the potential shifts negatively, reaction ④ accelerates rapidly. Under the action of the catalyst sulfate ions, the colloidal membrane covering the electrode surface dissolves: this dissolution first occurs locally and then gradually expands, exposing a small area of the substrate, resulting in a very high real current density and significant polarization, allowing the reduction of chromium to proceed at a certain speed. A colloidal membrane will again form on the surface of the newly formed chromium layer, and the dissolution and formation of the colloidal membrane continue to cycle, playing an important regulatory role. The presence and concentration of SO42-and the trivalent chromium generated at the cathode, although not directly involved in the electrode reactions, are crucial to the quality of the chrome plating layer. Trivalent chromium is an important component of the colloidal membrane; if its concentration is low, the colloidal membrane is difficult to form or is thin and porous after formation, easily dissolved by sulfuric acid. At this time, the exposed substrate area is large, and the low current density areas cannot reach the precipitation potential of chromium, resulting in poor coverage due to low trivalent chromium; if the concentration of trivalent chromium is high, the colloidal membrane is thick and dense, difficult to dissolve by sulfuric acid, and the chromium layer can only grow on the original crystal grains, leading to rough crystallization and a dull, non-lustrous plating layer. High sulfuric acid content easily dissolves the colloidal membrane, resulting in no chromium layer in low current density areas, similar to the situation with low trivalent chromium; insufficient sulfuric acid leads to a situation similar to that with high trivalent chromium, resulting in a rough chromium layer. Therefore, it is essential to strictly control their concentrations in chrome plating, especially the ratio of chromic anhydride to sulfuric acid.铬酸酐与硫酸的比值。
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