In my copending application, I describe an electrolysis cell having particular utility as a means for generating hypochlorites and chlorine in swimming pool waters circulated through the cell. The waters contain a low concentration of a chloride salt such as sodium chloride. My application Ser. No. 525,882 is hereby incorporated by reference into this disclosure. In the accompanying drawings, FIGS. 1, 2, 3, and 4 depict the apparatus of my copending application.
As described in my copending application, a combination of steel negative electrode faces and rapid velocities was discovered to be adequate to avoid the accumulation of hardness deposits on the steel negative electrode faces. Moreover, as described in my copending application, baffles, or shields, 40 (FIG. 4a) were found necessary. It was determined that these baffles should extend parallel to the flow path of said liquid and for a distance on either side of each bipolar electrode face 38 or 39 (FIG. 3a) equal to at least 10 times the distance between opposing faces of adjacent electrodes. To accomplish this important but difficult arrangement, I disclose the filling of slots 37a and 37b with inner and outer conducting clips 42 and 43 respectively and that the resulting joints could be sealed by caps 44 with sealing cement 45. Examples I and II of my original application show how this structure worked 34 and 64 days, respectively, with no visible indications of problems at these joints. However, the required perfection of these joints was found difficult to retain with the further production of large numbers of cell assemblies. Joints which appeared to be perfect initially, subsequently would reveal a weak point or imperfection. At these points of imperfection, which were perhaps only a microscopic opening at the start, anodic corrosion of the conduction clip 42 and/or 44 would begin. As attack at these points continued, small areas of high resistance formed and tended to reduce the current going through all bipolar electrodes in series. The voltage drops existing at these imperfections increased and accelerated the deterioration of the imperfect joint. Towards the end of this process, nearly all of the rectified 120 ac line voltage might appear across just one or two joints. An electrical arcing, or scintillations, could be observed as current dropped toward zero and the cell assembly failed. Subsequent examination would show only one or a few electrodes to be bad. All other electrodes and joints would appear still to be in perfect condition.
Continued experience with the apparatus of my copending application revealed new and unexpected problems. One problem appeared where the tab from extreme positive electrode 30 and extreme negative electrode 31 come through cover 22. In my original specification, I disclose the use of "soft sealants," "bathtub sealants," and "silicone sealants." These common sealing materials were subsequently found not to be completely satisfactory because they tended to leak after an extended time of use. The metal electrodes and surrounding plastic expand and contract at different rates, thereby causing a leak over a period of time during which ambient or water temperatures varied. The physical insertion and removal of connectors to the tabs on electrodes 30 and 31 sometimes also helped to create small leaks by the physical flexing of these critical joints. Solvent cements are too brittle. Thick pastes and putties are too bulky for the precision of assembly needed and would not always adhere to the metal. Consultation with sales representatives from companies making sealants quickly showed there was no obvious answer to this problem with terminal electrodes. Numbers of sealants were suggested for trial and were subsequently tested for this invention.
Another problem appeared where the unshielded bottoms of terminal electrodes 30 and 31 sometimes became exposed to electrolyte. Because those bottome edges of these electrodes were cut and uncoated, corrosive attack began to eat away at the bottom of the terminal electrodes. The mere painting of bottom joints between terminal electrodes and bipolar electrodes was not satisfactory.
Another problem appeared which is known to the prior art of electrolysis of sea water and other dilute or concentrated brines. It is well known that hardness deposits will build up on most negative electrode surfaces. Those working in the art have reported these hardness deposits can easily be removed by periodically reversing the direction of current flow. This current reversal technique seems to work satisfactorily with carbon electrodes but when hydrogen is cathodically evolved on titanium, or evolved on porous coatings on titanium, some of the hydrogen tends to enter the titanium metal lattice forming titanium hydride. Titanium hydride is brittle and its lattice is about 13 percent expanded beyond that of the titanium lattice. When the current is reversed back to its original anodic direction, this titanium hydride surface is oxidized, which further deteriorates the titanium structure and tends to remove the conductive coatings which were so carefully placed there to permit the use of coated titanium as positive electrodes in the first place. These hydriding and oxidizing effects with coated titanium electrodes were so well established and well known that, when I approached the major producers of coated titanium electrodes for their recommendation on materials to be used in my electrode assembly, I was told no known electrodes would withstand current reversals under the conditions planned and desired for the utilization of my invention. One supplier said he had seen coated titanium electrodes become brittle and start to lose their coating in as short a time as one-half hour. Another said I could expect to see hydrides and surface deterioration within 24 hours. Another supplier of coated titanium electrodes said there was no way to tell whether his electrodes would withstand current reversals under the conditions I proposed. Thus, some experts in the art of electrolyzing hard brines reported that I should get rid of hardness deposits on coated titanium electrodes by some other way than by current reversals. Other experts were not sure as to how to eliminate hardness deposits.