In an ideal scenario where one half of a steel pipe is exposed to water and oxygen, while the other half is completely isolated from any water and oxygen, the rust formed on the exposed half would not directly spread to the isolated half. Rusting is a process that requires the presence of both oxygen and water to occur.
Rust is the result of an electrochemical reaction known as corrosion. In the presence of water and oxygen, iron in the steel reacts with oxygen to form iron oxide (rust). The reaction involves the transfer of electrons, and it occurs at the interface between the metal and the surrounding environment.
If one half of the steel pipe is exposed to water and oxygen, that half will be susceptible to corrosion and rust formation. However, the isolated half that is shielded from water and oxygen will not participate in this electrochemical reaction and, therefore, will not rust.
It's important to note that if the rusting process progresses extensively on the exposed half, the corrosion products (including rust particles) can potentially detach and be carried by water or air currents to other parts of the pipe, including the isolated half. This external transportation of rust particles could indirectly result in rust appearing on the isolated portion. However, direct rust spread through the steel material itself would not occur.
To prevent rust formation on the exposed half of the pipe, protective measures such as coatings, paints, or corrosion inhibitors can be applied to inhibit or slow down the electrochemical reaction and protect the metal surface from contact with water and oxygen.