Electrochemical Deposition in Vertical Tubes : An Hydrodynamic and Mass Transfer Study

Abstract

The present work is part of the research project NXCHROME led by IRT M2P. It concerns the chemical engineering of metallic hard chromium coatings on internal surface of large tubular parts. These parts are intended to be used in several sectors of activity (aeronautics, armament, energy...). Thus, taking into account severe specifications related to these applications, the parts will be very strongly solicited thermomechanically during their use, which requested high performances to increase substrate's life. In the current process, tubes are treated vertically, by electrodeposition from a hexavalent chromium salt based (chromic acid) electrolyte. These electrolytes have the advantage of being efficient, easy to use, with well known functions. However, they have a low faradic yield (<20%), leading to an important production of bubbles (cathodic and anodic).These bubbles create the fluid motion, and eventually govern hydrodynamic, liquid flow and mass transfer of the species in the tube.It is important to note that the use of CrVI salts has been prohibited since 2017 (except for authorizations for applications without alternatives) by the European regulation REACh (Registration, Evaluation, Authorization and restriction of Chemicals). Indeed, hexavalent chromium is classified in Appendix XIV among SVHC (substances of very high concern), due to its high toxicity (CMR compound : Carcinogenic, Mutagenic and Reprotoxic) and the high danger it represents for coworkers using this compound. The objective of NXCHROME is to find a alternative process with a "REACh compatible" electrolyte based on an innovative trivalent chromium. The modus operandi and the electrochemical mechanisms involved are completely different and less well known, compared to the former electrolyte. Nevertheless, it keeps this low faradic yield and the liquid agitation mode in tubular parts will remains identical.Nevertheless, this change is a good opportunity for a deep investigation in the "bubble induced hydrodynamic" which occurs during plating time, and to collect all reliable information on fluids behavior. Then, two main research axis have been considered. The first one is done via the numerical simulation of the secondary current distribution, i.e. taking into account the electrical phenomena in the electrolyte (ohmic drop), the geometry of the system and the kinetics of the electrochemical reactions. This part is in collaboration with HIVELIX company. The second one trhought the design and elaboration of a large size experimental set-up in stainless steel and PMMA (φ = 40mm, L = 3000mm) Figure 1. This ste-up includes electrochemically active cathode parts (production of bubbles by hydrolysis), and transparent parts (windows of observations of the bubbles, according to the height of the tube). As it is the bubble production which puts in movement the fluid with their vertical displacement towards the surface, informations on their size and behavior are of primary importance. Their speed is determined by their size (depending on the distance covered and the coalescence) and the liquid physical properties.To mimic the chromium electrolyte, a transparent saline solution has been chosen and prepared by adjusting its parameters to be identical to the industrial electrolyte (viscosity, conductivity...), while allowing bubble production on the central electrode surface.Data collection and order of magnitude are done by various means, namely flow rate measurements, gas retention within the liquid and high frequency camera shots (bubble size, bubble velocity, coalescence...). Figure 1