(a) From the galvanic series (Table 17.2 ), cite three metals/alloys that may be used to galvanically protect cast iron. (b) As Concept Check \(17.4(\mathrm{b})\) notes, galvanic corrosion is prevented by making an electrical contact between the two metals in the couple and a third metal that is anodic to the other two. Using the galvanic series name one metal that could be used to protect a nickel-steel galvanic couple

Understanding the galvanic series is critical when trying to protect metals from corrosion. In essence, the galvanic series is a list that ranks metals and alloys by their electrochemical activity in seawater. Metals that are more reactive, known as anodic, are placed towards the top of the list, while less reactive, cathodic metals, are found towards the bottom. When two metals form a galvanic couple, the more anodic one will corrode faster than the more cathodic, effectively sacrificing itself to protect the other.

For example, in the textbook exercise, students are asked to identify metals that can protect cast iron by referring to the galvanic series. With magnesium, zinc, and aluminum higher up in the series compared to cast iron, they will act as anodes and corrode preferentially, thus shielding the cast iron, which becomes cathodic in this context. It's important for students to consult the galvanic series when choosing materials in applications to ensure the longevity of the less anodic metal.

The concepts of anodic and cathodic metals are at the heart of understanding galvanic corrosion. An 'anodic' metal has the tendency to lose electrons and corrode when connected to a 'cathodic' metal in the presence of an electrolyte. This is due to a potential difference between them, causing an electric current to flow from the anode to the cathode. The anodic metal is considered the 'sacrificial' one, as it will degrade over time to protect the cathodic metal from corrosion.

For instance, when magnesium (anodic) is paired with cast iron (cathodic), the magnesium corrodes but prevents the cast iron from rusting. This principle is widely used in marine environments and infrastructure projects where corrosion is a significant concern. By selecting an appropriate anodic material to pair with a valuable cathodic component, the system's overall integrity is preserved. Students should be well-versed in these terms to properly design corrosion protection measures.

A 'galvanic couple' arises when two different metals or alloys are electrically connected in an electrolytic environment, allowing for the flow of electric current between them. This couple consists of an anode, the metal that corrodes, and a cathode, the metal that is protected. Galvanic corrosion is most aggressive at the site closest to the electrical contact between the metals.

In the textbook exercise, students learn to identify a third metal that could protect a nickel-steel couple by serving as a more anodic material. For a nickel-steel couple, zinc is chosen for its position in the galvanic series, where it readily acts as the anode, ensuring that the nickel-steel material acts as cathode and thus remains corrosion-free. Recognizing how to create a beneficial galvanic couple is critical for engineers and designers in preventing costly and dangerous corrosion-related failures.