CORROSION BEHAVIOR OF LEAD AND LEAD ALLOYS

28.2 CORROSION BEHAVIOR OF LEAD AND LEAD ALLOYS

Being amphoteric, lead is corroded by alkalies at moderate or high rates, depend- ing on aeration, temperature, and concentration. Nevertheless, lead resists corro- sion in many environments because the products of corrosion are insoluble and form self - healing protective fi lms. Because of these protective fi lms, the corrosion rate of lead is usually under anodic control [2] .

It is attacked, for example, by calcium hydroxide solutions at room tempera- ture, including waters that have been in contact with fresh Portland cement. Organic acids harmful to lead can be leached from wood structures, particularly damp timber including western cedar, oak, and Douglas fi r. This cause of corro- sion can be prevented by fully drying the wood, treating it to prevent contact with moist air, or inserting a moisture barrier between wood and lead [3] .

An alloy of 2% Ag – Pb is used as a corrosion - resistant anode in impressed current systems for cathodic protection of structures in seawater (see Section

13.6 ) [4] . Alloying with 6 – 12% Sb increases strength [only at temperatures < 120 ° C ( < 250 ° F)] of the otherwise weak metal, but corrosion resistance of the alloy in some media is below that of pure lead.

Lead is resistant to seawater. It is also durable for use in contact with fresh waters; however, the toxic properties of trace amounts of lead salts make it man-

CORROSION BEHAVIOR OF LEAD AND LEAD ALLOYS

datory to exclude its use, and the use of its alloys, for soft potable waters, carbon- ated beverages, and all food products. The U.S. Environmental Protection Agency (EPA) Lead and Copper Rule, implemented in 1991, limits lead in potable water to a maximum of 15 ppb [5] . The rate of corrosion in aerated distilled water is high (approximately 9 gmd) and the rate increases with concentration of dis- solved oxygen [6] . In the absence of dissolved oxygen, the corrosion rate in waters or dilute acids is either negligible or very low.

Lead is resistant to atmospheric exposures, particularly to industrial atmo- spheres in which a protective fi lm of lead sulfate forms. Buried underground, the corrosion rate may exceed that of steel in some soils (e.g., those containing organic acids), but in soils high in sulfates the rate is low. Soluble silicates, which are components of many soils and natural waters, also act as effective corrosion inhibitors.

Lead is used in solder alloys (typically with 5%tin, and a small amount of silver to increase strength) for joints of automobile radiators [7] . In applications where thermal cycling occurs, the high coeffi cient of expan- sion (30 × 10 −6 / ° C) of lead may cause intergranular cracking due to fatigue or corrosion fatigue.

28.2.1 Lead – Acid Battery

Lead – acid batteries are a principal use of lead. Following are the reactions that take place during discharge of a lead – acid battery [8] :

At the anode: Pb → Pb 2 + + 2e − (28.1) At the cathode: PbO 2 + 4 H + + 2 e − → Pb 2 + + 2 HO 2 (28.2)

→ 2 Pb + 2 HO 2 (28.3) Corrosion of lead in Eq. (28.1) is part of the overall electrochemical reaction by

The net reaction: Pb PbO 4 H +

which electrical energy is generated using the lead – acid battery. In addition to this positive and benefi cial aspect of corrosion in energy generation, corrosion can also lead to shorter battery lifetime and reduced capacity to produce energy. For example, corrosion during battery storage reduces the quantity of lead avail- able for energy production and may also produce corrosion products that inhibit the corrosion that takes place by Eq. (28.1) . Passivation of the lead anode increases the anodic overvoltage, decreases the current, and causes a deterioration of battery performance. The lead – acid battery uses lead grids at both the anode and the cathode. Corrosion of lead at the cathode has been reported to cause loss of capacity of batteries [9] . For corrosion by Eq. (28.1) , which results in energy production, the overvoltage should be as low as possible. For the destructive forms of corrosion, which lead to reduced battery lifetime and capacity, the over- voltage should be as high as possible [10] .

LEAD

Various alloys of lead are used as support grids for positive and negative plates of lead – acid batteries. Alloying elements are added to improve mechani- cal properties, but anodic corrosion of these alloys can limit battery lifetime

[11, 12] . Lead – antimony alloys, containing typically about 5% antimony, have been used for many years [13] . These alloys develop an adherent protective corrosion product layer; however, the deposition of antimony on the negative plate lowers the hydrogen overpotential [13, 14] . Furthermore, on overcharge,

toxic stibine (SbH 3 ) is produced. For these reasons, alloys of lead with calcium, tin, and aluminum are also being used [13, 14] . Other possible alloying elements that have been studied include magnesium, titanium, and bismuth [14, 15] . Lead – calcium – tin alloys used as grid material for maintenance - free batteries have been reported to be susceptible to intergranular corrosion at large grain sizes [16] .