Global challenges Study Guide

Corrosion is the destructive attack of a metal by reaction with its environment. For iron and steel, this specific type of corrosion is called rusting, which occurs when iron reacts with both oxygen and water. To prevent or mitigate corrosion, particularly rusting, several methods are employed, primarily focusing on separating the metal from these essential reactants. One common approach is to create a physical barrier. This involves coating the metal surface with a material that is impervious to oxygen and water. Examples include painting, greasing, oiling, or coating with plastic. Each of these creates a layer that physically prevents the oxygen molecules and water molecules from coming into direct contact with the iron surface, thereby stopping the electrochemical reactions that lead to rusting. For instance, a layer of paint on a car body forms a protective seal. If this layer is scratched or compromised, the exposed metal can then begin to rust as oxygen and water gain access.

Another highly effective method, particularly for iron and steel, is sacrificial protection. This technique involves connecting the metal to be protected to a more reactive metal. The more reactive metal will corrode in preference to the protected metal, hence 'sacrificing' itself. For example, zinc is more reactive than iron. When iron is coated with zinc (a process known as galvanising), the zinc acts as the sacrificial metal. If the surface is scratched, exposing both metals to oxygen and water, the zinc will preferentially oxidise (corrode), preventing the iron from rusting. This protection continues as long as some of the sacrificial metal remains. Even if the zinc coating is damaged, the iron is still protected, unlike with simple barrier methods like paint. Magnesium and aluminium are also more reactive than iron and can be used for sacrificial protection, often as blocks attached to structures like pipelines or ship hulls. The key principle is the difference in reactivity; the more reactive metal loses electrons more readily and therefore corrodes first.

A Life Cycle Assessment (LCA) is a thorough method for evaluating the environmental impacts associated with all stages of a product's life, from raw material extraction through material processing, manufacture, distribution, use, repair and maintenance, and disposal or recycling. The basic principles involve considering the entire 'cradle-to-grave' journey of an item to gain a complete picture of its environmental footprint. This holistic approach helps to identify where the most significant environmental impacts occur, allowing designers and manufacturers to make informed decisions to reduce these impacts. For instance, a product might have a low environmental impact during its manufacturing phase, but a very high impact due to the energy consumed during its use, or the difficulty and environmental cost of its disposal. Conversely, a product might require a lot of energy to produce, but be extremely durable and recyclable, leading to a lower overall impact. When performing an LCA, certain assumptions and values must be made, which can lead to different LCAs for the same product, depending on who conducts the assessment and what factors they prioritise. For example, the impact of transporting raw materials might be weighted differently based on the distance or method of transport. Furthermore, the overall benefit of a product or material must be considered alongside its environmental impact. A lifesaving medical device, for example, might have a high environmental footprint but its benefit to human health outweighs this impact. Therefore, an LCA should not only identify environmental burdens but also balance these against the utility of the product to society and economic factors.