Oxidation is a chemical reaction between a metal and oxygen, resulting in the formation of an oxide layer on the metal's surface. It is not a metallurgical defect in the same way that porosity, inclusions, and grain boundary cracking are.

Porosity in castings can be caused by entrapped air, shrinkage during solidification, and gas evolution during solidification. Entrapped air can be introduced during the pouring process, while shrinkage and gas evolution can occur as the metal solidifies.

Carbides are not inclusions in steel, but rather a type of second phase that can form during the solidification and heat treatment of steel. Oxides, sulfides, and nitrides are all common types of inclusions in steel.

Grain boundary cracking can be caused by stress concentration at grain boundaries, impurities at grain boundaries, and brittle grain boundary phases. Stress concentration can occur due to the different orientations of adjacent grains, while impurities and brittle phases can weaken the grain boundaries and make them more susceptible to cracking.

Adding alloying elements is not a common method for preventing metallurgical defects. Proper melting and casting practices, heat treatment, and mechanical working are all common methods for preventing metallurgical defects.

Heat treatment is used in metallurgy to improve the mechanical properties of a metal or alloy, change the microstructure of a metal or alloy, and remove impurities from a metal or alloy.

Carburizing is not a common type of heat treatment. Annealing, normalizing, and tempering are all common types of heat treatment.

Mechanical working is used in metallurgy to improve the mechanical properties of a metal or alloy, change the microstructure of a metal or alloy, and remove impurities from a metal or alloy.

Casting is not a common type of mechanical working. Rolling, forging, and extrusion are all common types of mechanical working.

Alloying elements are added to metals and alloys to improve their mechanical properties, change their microstructure, and remove impurities.

Oxygen is not a common type of alloying element. Carbon, manganese, and silicon are all common types of alloying elements.

Corrosion is caused by exposure to oxygen, water, and acids or bases. Oxygen and water can cause oxidation, while acids and bases can cause dissolution.

Heat treatment is not a common method for preventing corrosion. Protective coatings, cathodic protection, and anodic protection are all common methods for preventing corrosion.

The primary purpose of a metallurgical investigation is to determine the cause of a metallurgical failure, recommend corrective actions to prevent future failures, and improve the quality of a metallurgical product or process.

Mechanical testing is not a common type of metallurgical investigation. Fractography, metallography, and chemical analysis are all common types of metallurgical investigations.