346 Section 4 Nonferrous Metallurgy shiny, corrosion-resistant surface, Figure 15-29. Both nickel (Ni) and chromium (Cr) are plated to protect steel or brass from corroding. One way to get a desirable finish is to treat the surface chemically. For example, a weak acid solution produces a patina, or thin layer of corrosion product, on bronze castings. This patina minimizes pitting corrosion in outdoor applications. Some nonferrous alloys can be anodized. In this process, a uniform, adherent, hard oxide is developed on a metal surface that protects the part from further corrosion, as well as from scratches and abrasion. Of course, paint or other polymer coatings are always options to protect clean metal. 15.6 Applications That Use Multiple Metals Some applications must use two or more metals to make a useful product. Many home thermostats control the temperature in a house. The control switch is usually a single blade of two roll-bonded metals that have different coefficients of expansion. As the temperature drops, the blade warps to close an electrical contact, turning on the heat. When the temperature rises again, the blade bends the other way, opening the contact. Copper (Cu), nickel (Ni), and gold (Au) are used to make reliable, removable electrical contacts in low-voltage circuits, such as for chipped credit cards, Figure 15-30, and computer components. Tin (Sn) and lead (Pb) are used to solder copper connections into electrical and electronic circuits. Biological Hazards Some metals are biologically hazardous, so operators and users must be alert to the hazard. For example, ho t- and cold-working of alloys with large percentages of beryllium puts beryllium oxide (BeO) dust into the air. BeO dust causes berylliosis, sapping lung capacity like silicosis, the “brick layers’ disease.” The long-term effects of lead exposure, particularly to employees in circuit board manufacturing and radiator repair shops, are well known. Cadmium (Cd) is no longer used to plate small bolts and nuts because the amount of cadmium in people, notably detected in some European countries, was increasing toward toxic levels. Metals in plating solutions can be extremely hazardous biologically. Spent chrome plating solutions contain hexavalent chromium, a major biohazard. If these plating solutions are put into a municipal wastewater system, the bacteria that clean a city’s wastewater die, interrupting the treatment process. Chromium solutions must be handled carefully and neutralized before any liquid leaves the plating facility. SAFETY NOTE SAFETY NOTE Ali_Cobanoglu/Shutterstock.com Figure 15-29. Chrome plating gives the parts of this motorcycle engine a nice shine, along with added corrosion resistance. nobeastsofierce/Shutterstock.com Figure 15-30. Most new credit cards are chipped, meaning they have an embedded microchip, which provides greater protection for the consumer. Copyright Goodheart-Willcox Co., Inc.