Inside the case are electrodes including cathodes and anodes where chemical reactions occur. The sacrificial anodes and cathodes can be of the same or of different materials. There are three main metals used as galvanic anodes, magnesium, aluminum and zinc. Galvanic corrosion is often prevented by the use of sacrificial anodes. Ruthenium(IV) oxide coated titanium anodes are used for the industrial production of chlorine. Automated robotic vehicles place the prepared anodes in cells containing an acidic electrolyte. An electrolytic method of anti-fowling involves running electrical current through two anodes (Scardino, 2009). Hence these zinc anodes require periodic inspection and replacement. ASTM International publishes standards on the composition and manufacturing of galvanic anodes. It is sometimes more economically viable to protect a pipeline using galvanic (sacrificial) anodes. In particular, migration of oxygen through the slag accelerates corrosion of metallic anodes. These zinc plates will get corroded first being in the lowest range of anodes. A simpler option is to use galvanic anodes, which are self-limiting and need no control. Vacuum tubes with silica envelopes allowed for radiation cooling by incandescent anodes. The rectifier output DC positive cable is connected to the anodes.