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Chapter 19: Problem 40

"Galvanized iron" is steel sheet that has been coated with zinc; "tin" cans are made of steel sheet coated with tin. Discuss the functions of these coatings and the electrochemistry of the corrosion reactions that occur if an electrolyte contacts the scratched surface of a galvanized iron sheet or a tin can.

Short Answer

Expert verified
The zinc or tin coatings on galvanized iron or tin cans serve to protect the base metal from corrosion. If an electrolyte contacts a scratched surface, it sparks a corrosion process. For galvanized iron, the more reactive zinc layer corrodes first, creating a sacrificial layer of zinc oxide that slows the overall corrosion process of the iron. For tin cans, the less reactive tin layer serves as a physical barrier, and if this layer gets scratched, the more reactive iron in the steel corrodes first, accelerating the rusting process.

Step by step solution

01

Understanding Coating Functions

First, let's understand what galvanized iron and tin cans are, as well as the role of their coatings. Galvanization is the process of applying protective zinc coatings to iron or steel, to prevent rusting. 'Tin' cans, on the other hand, are made from steel that has been coated with thin layer of tin. These layers serve to protect the metal underneath from corrosion, by serving as a physical barrier keeping water and oxygen from reaching the steel surface.
02

Electrolyte Contact and Corrosion Reaction

Electrolytes can stimulate corrosion reaction. If an electrolyte comes in contact with the scratched surface of a galvanized iron sheet or a tin can, it might instigate a corrosion process. This is since electrolytes help facilitate the flow of electric current, accelerating the corrosion process.
03

Electrochemistry of Galvanized Iron

For galvanized iron, if the protective zinc layer is damaged or scratched, the underlying metal surface would be exposed. And because Zinc is more reactive than iron, the zinc layer would first corrode or oxidize, creating a sacrificial layer of zinc oxide \(ZnO\). The formulation can be written as: \(Zn + O_2 \rightarrow ZnO\). This process slows down the overall corrosion of the iron.
04

Electrochemistry of tin cans

In the case of 'tin' cans, if the tin coating is damaged, the steel underneath could rust because it is in contact with the electrolyte and air. Because tin is less reactive than iron, the iron in the steel underneath would corrode first. The reaction could be written as: \(2Fe + 2H_2O + O_2 \rightarrow 2Fe(OH)_2\), which then dehydrates to form rust: \(4Fe(OH)_2 + O_2 + 2H_2O \rightarrow 4Fe(OH)_3\), the commonly known rust. The tin layer unlike zinc doesn't sacrificially corrode, rather it provides a physical barrier to elements.

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