Electroplated electrode process
The chrome plating solution widely used in industry is composed of chromic anhydride supplemented with a small amount of anion. The form of Cr6+ in the plating solution varies depending on the concentration of chromic anhydride. In general (Cr03200~400g/L), chromic acid is mainly used. CrO42-) and dichromic acid (Cr2O72-) are present. When the pH value is less than 1, Cr2072- is the main form; when the pH is 2~6, the following balance exists between Cr2O72- and CrO42-, ie Cr2072-+H20====2H2Cr04-+2CrO] one ten 2H+
When the pH is greater than 6, CrO42- is the predominant form. It can be seen that the ions present in the chrome plating electrolyte are Cr2072-, H+, cro42-, and S042-. Practice has proved that except for SO42-, other ions can participate in the cathodic reaction. The study of the chromate chrome plating process by the tracer atomic method shows that the chrome plating layer is obtained by reduction of hexavalent chromium instead of trivalent chromium.
Chromium plating bath (with or without sulfuric acid) as determined by the potentiostatic method.
1 is obtained from 250g/L Cr03 chrome plating solution; 2 is obtained from 250g/L CrO3, 5g/L H2S04 standard chrome plating solution.
As shown in the cathodic polarization curve, when sulfuric acid is not contained in the plating solution (curve 1), only hydrogen is evolved on the cathode, and no other reduction reaction occurs. When the plating solution contains a small amount of sulfuric acid (curve 2), the cathodic polarization curve consists of several line segments, and different reduction reactions occur on different curve segments.
In the ab segment, as the cathode current increases, the electrode potential gradually shifts negatively, and there is no hydrogen evolution and chromium formation on the cathode. The pH of the plating solution in the cathode region is less than 1, and the form of ions is mainly Cr2072-, and the cathode reaction at this time is Cr2O72-+14H++6e_2Cr3++7H20.
As the electrode potential continues to shift negatively, point b reaches a maximum. After point b, in addition to the reduction of Cr2072- to Cr3+, a large amount of bubbles were observed on the surface of the cathode, indicating that the H+ ions were reduced to hydrogen.
In the bc segment, two reactions of Cr2072-reduction to cr3+ and H+ reduction to H2 were carried out simultaneously, but in this segment, as the electrode potential was negatively shifted, the current gradually decreased, indicating that the surface state of the electrode changed. The colloidal membrane theory explains that, due to the above two reactions, H+ near the surface of the cathode is consumed in a large amount, and the pH value is rapidly increased (when the pH value is greater than 3), when the amount of Cr3+ ions generated reaches the dissolution of Cr(OH)3. When the product is formed, an orange-colored basic chromium chromate colloidal film Cr(OH)3•Cr(OH)Cr04 is formed on the surface of the cathode, which hinders the progress of the electrode reaction and makes the reaction speed. Significantly decreased; and because the sulfuric acid in the plating solution has a certain dissolution effect on the cathode colloid film, the formation and dissolution of the colloidal film are alternately alternated, so that the curve appears as a segment shape.
Due to the continuous precipitation of hydrogen, the pH value of the plating solution in the cathode region is gradually increased, and the conversion of Cr2072- into HCr04- ions is promoted, so that the concentration of HCrO4- ions is rapidly increased. When the electrode potential is negatively shifted to point C, CrO42- begins to be reduced to metal chromium and precipitates on the cathode. The reaction equation is CrO42-+4H++6e-===Cr+4OH.
It can be seen that metal chromium can be reduced and precipitated only after the potential of the cathode electrode reaches c. At this time, three reactions are simultaneously performed on the cathode, and as the potential of the cathode electrode is negatively shifted, the cathode current rapidly rises, the reaction speed increases, and the proportion of the main reaction for generating chromium metal gradually increases, that is, with the cathode current density. Increasing, the cathode current efficiency increases. (2) Anode process.
The anode used for chrome plating is an insoluble anode such as lead, lead bismuth or lead tin (containing 6% to 8% tin) alloy, which is one of the characteristics of chrome plating different from general plating. Because in the chromic acid electrolyte, the metal chrome plating is obtained by direct reduction of hexavalent chromium. When the metal chromium anode is dissolved, it exists in the form of ions of different valence states, mainly the ion form of trivalent chromium enters the solution, and the current efficiency of the anode is close to 100%, which will cause the trivalent chromium content to increase rapidly. The cathode current efficiency is only 10% to 25%, which makes the composition of the plating solution unstable. In addition, the metal chromium is hard and brittle and is not easily processed into various shapes.
In normal production, a dark brown lead dioxide film is formed on the surface of the lead or lead alloy anode. Pb+2H2O-4e-PbO2+4H+
This film does not affect the conductivity, the anode reaction can still proceed normally, and the electrode reaction is 2Cr3++7H2O-6e-Cr2O; One +14H+2H2O-4e-O2↑+4H+
It can be seen from the above reaction that the cr3+ ions generated on the cathode are reoxidized to Cr2072- ions on the anode, so that the Cr3+ concentration in the electrolyte is maintained at a certain level to ensure the normal progress of chrome plating production. When the trivalent chromium content in the plating solution is too high, a large-area anode and a small-area cathode may be used for electrolytic treatment to reduce the content of trivalent chromium in the plating solution. Generally controlling the anode area in production: cathode area = (2:1) ~ (3:2), so that the Cr3+ concentration can be kept within the allowable range of the process.
When no electricity is applied, the lead or lead alloy anode suspended in the plating solution forms a poorly conductive yellow lead chromate (PbCr04) film on the surface thereof due to chromic acid etching, causing the cell voltage to rise. The anode is not electrically conductive, so when not in production, the anode should be taken out of the plating tank, immersed in clean water, and should be washed frequently to remove the yellow lead chromate film. If the yellow film is very strong, it can be immersed in the lye for several days. After the film is softened, it is washed and removed. In addition, the dispersing ability and covering ability of the chrome plating solution are poor, and the shape and arrangement of the anode must be paid attention to. When plating complex parts, a pictographic anode and an auxiliary cathode should be used.