PREVENTION OF CORROSION AND GALVANIZATION
Cathodic protection Cathodic protection Cathodic protection (CP) is a technique used to control the corrosion of a metal surface by making it the cathode of an electrochemical cell. The simplest method to apply CP is by connecting the metal to be protected with another more easily corroded sacrificial metal to act as the anode of the electrochemical cell. For structures where passive galvanic CP is not adequate, including long pipelines, an external power source provides the current. Cathodic protection systems are used to protect a wide range of metallic structures in various environments. Common applications are steel water or fuel pipelines and storage tanks, steel pier pipes, ships and boats, offshore oil platforms and onshore oil well casings and metal reinforcement bars in concrete buildings and structures.

Prevention of corrosion

Corrosion of metals such as iron can be prevented by the following methods:
  • Cathodic protection
  • Using a sacrificial layer such as Zn, known as galvanizing
  • Electroplating
  • Painting
  • Alloying
The rusting of unprotected iron in the presence of air and water is inevitable because it is driven by an electrochemical process. However, other electrochemical processes can offer some protection against corrosion. Magnesium rods can be used to protect underground steel pipes by a process called cathodic protection. Connecting the steel pipe to a more active metal like magnesium makes an iron–magnesium voltaic cell which oxidizes the magnesium in preference to the iron. Replacing the magnesium rod is easier than digging up the pipe.
Underground steel pipes offer the strength to transport fluids at high pressures, but they are vulnerable to corrosion driven by electrochemical processes. A measure of protection can be offered by driving a magnesium rod into the ground near the pipe and providing an electrical connection to the pipe. Since magnesium has a standard potential of – 2.38 volts compared to – 0.41 volts for iron, it can act as the anode of a voltaic cell with the steel pipe acting as the cathode. With damp soil serving as the electrolyte, a small current can flow in the wire connected to the pipe. The magnesium rod will be eventually consumed by the reaction while the steel pipe as the cathode will be protected.
Mg(s) Mg2+(aq) + 2e−
O2(g) + 2H2O(I) + 4e− 4OH−(aq)

Galvanization

A protective water insoluble oxidized coat is the principle underlying a process called galvanization. Zinc has a slightly greater tendency to oxidize than iron. For this reason, many iron articles such as nails are galvanized by coating them with a thin layer of zinc. The zinc oxidizes to zinc oxide, an inert, insoluble substance that protects the inner iron from rusting. The galvanized nail is protected from rusting by the sacrificial oxidation of zinc.

Stainless steel Stainless steel Stainless steel, also known as inox (meaning "inoxydable" in French) is defined as a steel alloy with a minimum of 10.5% to 11% chromium content by mass.
Painted iron tubes to get protection from corrosion
Painted iron tubes to get protection from corrosion Rust formation can be controlled with coatings, such as paint, lacquer, or varnish that isolate the iron from the environment.

Yet another way to protect iron and other metals from oxidation is to coat them with a corrosion–resistant metal, such as chromium, platinum, or gold. Electroplating is the operation of coating one metal with another by electrolysis. The object to be electroplated is connected to a negative battery terminal and then submerged in a solution containing ions of the metal to be used as the coating.

The positive terminal of the battery is connected to an electrode made of the coating metal. The circuit is completed when this electrode is submerged in the solution. Dissolved metal ions are attracted to the negatively charged object, where they pick up electrons and are deposited as metal atoms. The ions in the solution are replenished by the forced oxidation of the coating metal at the positive electrode. Iron is generally electroplated with less oxidizing metals like chromium or tin.

Painting a corroding surface with an oxidizing element, so that the underlying metal is protected, is routinely done. The paint generally contains lead oxide, calcium oxide and other compounds – already oxidized compounds.

Another technique to avoid corrosion so that the life of the metal is made longer is to alloy that metal with some elements so that the corrosion is harnessed. Stainless steel, which is an alloy of iron, carbon, nickel, etc., (there are various types of steel, which vary in their elemental compositions) does not rust at all.