0620 · 9.5Corrosion of metals

Conditions needed for rusting

0:00A scratch can leave painted steel exposed to rusting, while a similar scratch in galvanised steel can still leave it protected. The difference is not simply the thickness of the coating. Zinc offers a kind of protection that paint cannot give. By the end of this video, you will understand the conditions for rusting and how different protective methods work. Corrosion is the gradual destruction of a metal through chemical reactions with its surroundings.

0:22When the metal is iron, including the iron in steel, this corrosion is called rusting. Other metals can corrode, but their corrosion products are not called rust. Keeping that distinction clear helps us describe both the material being damaged and the substance being formed. We need two ideas from earlier chemistry. The reactivity series compares how readily metals react, with zinc above iron because it is more reactive. Oxidation means loss of electrons.

0:47When a metal atom loses electrons, it becomes a positive ion, so corrosion involves a chemical change rather than simply dirt collecting on a surface. Iron and steel rust when both water and oxygen are present. The substance formed is called hydrated iron(III) oxide. Hydrated means that the compound contains water. This product is chemically different from the original metal, so rusting changes the material rather than just covering it.

1:10The crucial word is both. Oxygen alone is not enough if the surroundings are completely dry, and water without oxygen does not cause rusting. An object does not need to be submerged because moisture can provide the necessary water. Oxygen from the air can then reach the same exposed surface, allowing the reaction to occur.

Testing rusting and barrier methods

1:28We can test these conditions using three tubes containing similar iron nails. The first contains dry air, with a drying agent removing moisture. In the second, boiling removes dissolved oxygen from water, and an oil layer prevents air from entering it again. The third contains water with air above it, providing both substances needed for rusting. Reason through each prediction by checking which conditions are available. The nail in dry air should not rust because water is absent.

1:54Boiled water beneath oil lacks the oxygen needed for the reaction. Rusting is expected in the third tube because both requirements are met. These comparisons support a conclusion about conditions, not simply a description of one nail. For a fair comparison, use similar nails and leave them for the same length of time under otherwise similar conditions. Each arrangement is designed to remove one requirement or provide both. In practical or alternative to practical questions, you may need to explain the purpose of a drying agent or an oil layer.

2:22Link each component to the condition it controls. Barrier methods prevent rusting by excluding oxygen or water from the metal surface. Painting, greasing and coating with plastic are common examples. Think of the coating as a physical cover between the steel and its surroundings. Removing either required substance prevents rusting, even if the other is still present outside the barrier. Paint is useful for large exposed surfaces, such as steel gates.

2:46Grease suits moving metal parts, where a rigid coating would crack or wear away. Steel items such as wire racks are often given a plastic coating. The materials differ, but their protective principle is the same: they prevent the required substances from reaching the metal underneath. A barrier must remain effective to keep protecting the surface. A scratch through paint can expose steel to water and oxygen. Rusting can then occur at that exposed area, even though the surrounding coating remains intact.

3:12Paint does not change the reactivity of iron, so an uncovered patch still behaves like ordinary exposed metal.

Galvanising and sacrificial protection

3:19This next part is for the Extended paper. Galvanising means coating iron or steel with zinc. When the coating is intact, it acts as a barrier that excludes oxygen or water. Zinc also provides sacrificial protection, which explains why galvanised steel can remain protected even when a scratch exposes the metal underneath. In sacrificial protection, a more reactive metal is attached to a less reactive one. Zinc is more reactive than iron, so it is oxidised in preference to the iron.

3:45The protective metal is gradually used up in the process. The word sacrificial describes this trade-off: zinc undergoes corrosion so that the steel is protected. At a scratch, the remaining zinc is still in electrical contact with the steel. In wet conditions, zinc loses electrons more readily than iron. Those electrons pass through the connected metal and help prevent the iron from losing its own electrons. The protection therefore involves a chemical process, not just a coating covering the surface.

4:12The electron loss equation is shown on the board. A zinc atom loses two electrons and becomes a zinc ion with a 2+ charge. Losing negatively charged particles leaves the ion positively charged. This is oxidation, so an electron explanation must identify zinc as the substance losing electrons, rather than merely saying that it is stronger. Sacrificial protection depends on choosing a metal above iron in the reactivity series. Magnesium can also protect iron when it is attached in suitable conditions.

4:42A less reactive metal cannot provide the same protection by losing electrons preferentially. The sacrificial metal also needs electrical contact with the iron; simply placing it nearby is not enough. Consider this worked example involving three steel strips exposed to water and oxygen. The first has scratched paint, the second has a scratched zinc coating, and the third has intact plastic. We need to predict which surfaces are protected and explain why.

5:05Start by deciding whether the coating still forms a complete barrier over the steel. The scratch in the paint exposes steel, so both water and oxygen can reach that patch. Rusting can therefore occur there. The intact plastic prevents contact with the required substances, so its strip remains protected. These predictions follow directly from the barrier principle, without needing any comparison between the reactivities of different metals. The scratched zinc coating no longer provides a complete barrier at the gap.

5:32However, the remaining zinc is more reactive than iron and loses electrons preferentially. It is oxidised instead of the steel, providing sacrificial protection. This protection continues while enough zinc remains in electrical contact with the steel. It ends once the zinc has been used up.

Exam answers and recap

5:48A common exam mistake is saying that water alone causes rusting. That misses the need for oxygen, including oxygen dissolved in water. Another mistake is naming the product only as iron oxide. The precise name is hydrated iron(III) oxide, which identifies the rusting product rather than a different oxide of the same metal. Students also sometimes explain galvanising only by saying that zinc keeps water out. That describes its barrier action, but does not explain protection at a scratch.

6:14For sacrificial protection, link the greater reactivity of zinc to its electron loss, so zinc is oxidised instead of iron. Do not reverse the electron direction: zinc supplies electrons rather than taking them from the steel. An examination may ask you to state the conditions required for rusting. A direct answer is that both water and oxygen are required. If asked to describe how painting prevents rusting, identify it as a barrier method.

6:39State that the coating excludes oxygen or water from the iron surface, rather than simply saying it protects the metal. To explain why scratched galvanised steel remains protected, give the chemical reasoning in separate steps. Zinc is more reactive than iron. It loses electrons more readily and is oxidised instead of the iron. The zinc therefore provides sacrificial protection even where the scratch has exposed the steel, provided the metals remain in electrical contact.

7:04The whole topic connects conditions to prevention. Iron and steel need both water and oxygen to form hydrated iron(III) oxide. Paint, grease and plastic coatings prevent contact with at least one requirement. Galvanising adds a zinc barrier and sacrificial protection, because the more reactive coating loses electrons and is oxidised instead of the iron.