WEBVTT 00:00:01.830 --> 00:00:10.580 A steel oil pipeline is protected via an electrical connection to a buried block of sacrificial metal. 00:00:11.440 --> 00:00:19.320 Which diagram correctly illustrates the composition of the sacrificial metal and the direction of the electrical current? 00:00:20.770 --> 00:00:24.750 We are told that an oil pipeline is made from steel. 00:00:25.780 --> 00:00:31.570 Steel is a ferrous metal, in other words an iron-containing alloy. 00:00:32.460 --> 00:00:33.830 Steel is strong. 00:00:34.530 --> 00:00:42.130 But in the presence of oxygen and water or water vapor from the air, steel turns to red rust. 00:00:42.770 --> 00:00:44.830 Rust is weak and flaky. 00:00:45.530 --> 00:00:50.670 So without protection, a steel pipe would lose its strength and integrity over time. 00:00:51.590 --> 00:01:15.230 We are asked to identify which diagram correctly illustrates the composition of the sacrificial metal, either zinc, copper, tin, again copper, or lead, and to identify the direction of the electrical current. 00:01:16.000 --> 00:01:18.610 Let’s clear some space to answer this question. 00:01:19.310 --> 00:01:27.870 We’ve already seen that a steel pipe as it is exposed to oxygen and water or water vapor would begin to rust. 00:01:28.320 --> 00:01:37.150 Continual exposure will lead to a loss of integrity and the oil that has been carried by the pipeline may indeed leak out. 00:01:38.540 --> 00:01:41.140 Rusting is a specific type of corrosion. 00:01:41.630 --> 00:01:50.870 Corrosion is an irreversible, destructive process where a metal reacts with other substances to form more stable compounds. 00:01:51.480 --> 00:01:58.230 This degradation of steel pipes can be slowed down and prevented by using a sacrificial metal. 00:01:59.420 --> 00:02:06.420 We need to decide which metal is the sacrificial metal in this example, zinc, copper, tin, or lead. 00:02:07.430 --> 00:02:15.040 Sacrificial protection is when a more reactive metal protects a less reactive metal from corrosion. 00:02:15.840 --> 00:02:21.500 A portion of the reactivity series of metals is shown with the more reactive metals at the top. 00:02:22.370 --> 00:02:24.250 We know iron is in steel. 00:02:25.160 --> 00:02:32.260 And any metal above it in the series is more reactive than iron or will be oxidized more easily. 00:02:34.210 --> 00:02:45.510 And any metal below iron in the series is less reactive than iron and will be oxidized less easily than iron; in other words, iron is oxidized more readily. 00:02:46.450 --> 00:02:56.070 If we looked at our definition, we can see that sacrificial protection uses a more reactive metal to protect a less reactive metal from corrosion. 00:02:56.980 --> 00:03:07.260 So if we want to protect iron from corrosion, we need to use a metal that is more reactive than iron as the sacrificial metal. 00:03:08.090 --> 00:03:25.710 From our possible sacrificial metals zinc, copper, tin, and lead, we can see that zinc is higher up on the reactivity series than iron or more reactive than iron, while tin, lead, and copper are less reactive than iron. 00:03:26.900 --> 00:03:35.000 So, tin, lead, and copper are not suitable sacrificial metals to protect iron from corrosion. 00:03:36.530 --> 00:03:42.770 So, let’s erase lead, tin, and copper as possible answers. 00:03:43.530 --> 00:03:52.390 We now know that the sacrificial metal used to protect a steel pipe would be zinc, but what is the direction of the electrical current flow? 00:03:53.580 --> 00:03:57.380 Don’t be confused; it must be a closed circuit for current to flow. 00:03:57.710 --> 00:04:01.850 But which is the source of electrons, the zinc or the steel? 00:04:02.710 --> 00:04:05.510 We know that zinc is more reactive than iron. 00:04:06.000 --> 00:04:08.890 This means that zinc is oxidized more easily. 00:04:09.710 --> 00:04:17.850 So, the loss of electrons from zinc instead of from iron will be preferred; this is more energetically favorable. 00:04:18.780 --> 00:04:25.710 So, we know that zinc will donate electrons into the wire, and these will flow to the iron in the steel. 00:04:26.570 --> 00:04:33.990 Iron in the steel is prevented from being oxidized and is kept in the reduced state; it does not corrode. 00:04:34.920 --> 00:04:42.750 We say that zinc sacrifices its valence electrons to iron and in this way protects the iron in the steel from rusting. 00:04:43.680 --> 00:04:49.760 Now that we know the direction of the electrical current, we can identify the correct diagram. 00:04:51.280 --> 00:04:58.610 Which diagram correctly illustrates the composition of the sacrificial metal and the direction of the electrical current? 00:04:59.480 --> 00:05:00.980 The answer is (A).