Patent EP-1870497-A1 (EP)
Inventor: JENSEN JENS DAHL DR
IPC: C25F5/00
Filed: 2006-06-23
Abstract: The method for electrochemical stripping of a metal coating of turbine blade/propulsion unit blade, comprises dipping blade in electrolyte solution, passing electric current through the blade and contacting auxiliary electrode with solution. The electric current is pulsed with a routine, which has a duty cycle of 50%, two current densities of 150-200 mA/cm 2>and a frequency of 250 Hz. The electrolyte solution contains less than 6 %wt. of hydrochloric acid, mixture of organic/inorganic acids and/or bases and inhibitors. A mechanical cleaning step is carried out by sand blasting. The method for electrochemical stripping of a metal coating of turbine blade/propulsion unit blade, comprises dipping blade in electrolyte solution, passing electric current through the blade and contacting auxiliary electrode with solution. The electric current is pulsed with a routine, which has a duty cycle of 50%, two current densities of 150-200 mA/cm 2>and a frequency of 250 Hz. The electrolyte solution contains less than 6 wt.% of hydrochloric acid, mixture of organic/inorganic acids and/or bases and inhibitors. A mechanical cleaning step is carried out by sand blasting before the electrochemical de-coating step. An insoluble coating mass per unit area of the turbine- or propulsion unit blade is 34-51 mg/cm 2>and is mechanically stripped off from the turbine- or propulsion unit blade surface after the electrochemical de-coating step. Another electrochemical de-coating step is carried out with pulsed current or direct current for 8 minutes to strip off the coating. The direct current has a current density of 16 mA/cm 2>where the processing time is 8 minutes. The additional pulsed current has the duty cycle of 50-99%, two current densities of 1-16 mA/cm 2>and a frequency of 10 -2>Hz. A visible residual mass of the coating per unit area of the turbine- or propulsion unit blade is 5-34 mg/cm 2>and is mechanically stripped off by sand blasting after the additional electrochemical de-coating step. The parts of the turbine blade are masked, impinged on the coating region with a blasting agent, subjected to an electrochemical primary de-coating step, impinged again on the coating region with a blasting agent, and subjected to secondary electrochemical de-coating step. The inner surfaces of the turbine blade are covered with wax. The base of the turbine blade is covered by a cap before the blasting process to protect it from the impact of the blasting agent. The cap is removed again after the blasting process. Corundum is used as the blasting agent, which has a grain size of 46 mesh or less. The first blasting step is carried out at an impact pressure of maximum 5 bar and other blasting steps are carried out at the impact pressure of maximum 3 bar. The primary and the secondary de-coating steps are carried out at 18-22[deg] C. The metal ion concentration in the electrolyte solution is monitored, where the electrolyte solution is exchanged at an iron ion concentration of greater than 100 ppm. The primary and secondary electrochemical de-coating steps are carried out for maximum 60 minutes and 30 minutes respectively. The turbine blade is heat-tinted at 550-650[deg] C for 30 minutes. The remaining metal coatings are stripped off by abrasive grinding. The turbine blade is marked after the removal of the coating.