The Study Of Electrodeposited Chromium With Composited-legends In Sulfate Salt

In recent years, with the rising of people’s environmental protection awareness andwith the strengthening of the law and regulations, to comply with the requirements ofscientific development, trivalent chromium plating has become one of the mostimportant technology to replace the heavy pollution of hexavalent chromium plating.The low concentration of trivalent chromium sulfate system chrome platingcomposition is simple, stable, reliable, and environmentally friendly technology. In thispaper, four kinds of low concentrations of sulfate composite plating solutioncoordination system, and the effects of various components on Electrodeposition wereinvestigated, we get four kinds of fast chrome system., the restore process of glycinesystem were decided through the electrochemical method.First, the effects of main salt(NaSO4), first ligand(glycine), second ligand(succinicacid,1,6-hexylene glycol, urea, ethylene diamine) and rare earth metal salt(ammoniumceric sulfate) concentration on the thickness and brightness of coating were investigatedrespectively by orthogonal test. The results showed that the optimum bath formula ofglycine trivalent chromium plating are as follows:Succinic acid system: Cr3+0.10mol/L, H3BO30.70mol/L, Na2SO40.60mol/L,glycine0.15mol/L, succinic acid0.10mol/L, ammonium ceric sulfate0.002mol/L,thiourea0.05mol/L, sodium dodecyl sulfate,0.0001mol/L.1,6-Hexylene glycol system: Cr3+0.10mol/L, H3BO30.70mol/L, Na2SO40.60mol/L, glycine0.10mol/L, hexylene glycol0.10mol/L, ammonium ceric sulfate0.001mol/L, ammonium ceric sulfate0.002mol/L, thiourea0.05mol/L, sodium dodecylsulfate0.0001mol/L.Urea system: Cr3+0.10mol/L, H3BO30.70mol/L, Na2SO40.60mol/L, glycine0.15mol/L, urea0.15mol/L, ammonium ceric sulfate0.001mol/L, ammonium cericsulfate0.002mol/L, thiourea0.05mol/L, sodium dodecyl sulfate0.0001mol/L.1,2-Ethylenediamine system: Cr3+0.10mol/L, H3BO30.70mol/L, Na2SO40.60mol/L, glycine0.10mol/L, ethylenediamine0.10mol/L, ammonium ceric sulfate0.001mol/L, ammonium ceric sulfate0.002mol/L, thiourea0.05mol/L, sodium dodecylsulfate0.0001mol/L.We also investigated the effects of plating time, current density, pH value and otherprocess conditions and optimized them. The results showed that four kinds of plating systems can all be carried out at room temperature. A large number of tests showed thatthe stability of bath is good, the plating bright area was more than50%, the thicknesscan up to10μm under the four system of prescription and process conditions.Second, a bright chromium coating with good corrosion resistant performance wasobtained by DC electroplating in sulfate electrolyte with different second ligand. Theadhesion strength of coating were measured by belt-traction experiment and scratchtester. The crystalline structure and the micro-morphology of the chromium coating wascharacterized by X-ray diffraction(XRD) and scanning electron microscopy(SEM)respectively. And the corrosion resistance of coating was tested by Tafel. Comparedwith other bath system, the chromium coating prepared in urea and ethylenediaminesystem have better adhesion strength.The chromium coating prepared inethylenediamine system has the strongest corrosion resistance, and the corrosionpotential is-0.353V, the corrosion current density is8.381×107A/cm2.Third, the mechanism of trivalent chromium plating in the sulfate-glycinecomposite system is studied by linear sweep voltammetry(LSV) andchronoamperometry(CA). The results showed that trivalent chromium electrodepositionwas not by direct reduction from Cr3+into Cr in glycine system, its a two-step andirreversible process, the first step is to get an electron, the second step is to get twoelectrons.Combined with the bath formula and Faraday’s second law, we inferred that thereaction process is:Cr3++eâ†'Cr2+,Cr2++2eâ†'Cr.And the cathode polarization wasimproved with the addition of succinic acid, hexamethylene glycol, urea,ethylenediamine, but does not change the three-dimensional instantaneous nucleationmechanism, and a uniform color, good corrosion resistance chromium coating can beobtained by trivalent chromium plating in the four kinds of complete system, and a bluemembrane formated in the urea and ethylenediamine system.